Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

Silica mobility and replaced evaporites: 3 - Archean cherts

John Warren - Sunday, August 28, 2016

Introduction

The two previous articles on silica mobility in evaporitic settings emphasised Phanerozoic examples and discussed silica textures largely tied to the replacement of sulphate evaporite nodules. This article will extend the time frame back to the Archean and also discuss scale controls on massive marine-derived evaporite beds in the early earth. The next article after this focuses on the Proterozoic. In order to extend our discussion into saline Precambrian successions, we must consider changes in ionic proportions and temperatures of the world’s oceans that this involves, and also include the background context of biological evolution of silica-extracting organisms.

Chert deposits clearly preserve a record of secular change in the oceanic silica cycle cross the Precambrian and the Phanerozoic (Maliva et al., 2005), with the chert nodule-evaporite association most obvious in alkaline brine-flushed areas in Phanerozoic sediments (previous 2 articles). Many silicified Phanerozoic evaporite examples co-occur with significant volumes of salts deposited in marine-fed megahalite and megasulphate basins. The evolutionary radiation of silica-secreting organisms across a deep time background is reflected in the transition from abiogenic silica deposition, characteristic of marine and nonmarine settings in the Archean and Proterozoic eons, to the predominantly biologically-controlled marine silica deposits of the Phanerozoic.

Silica levels in the Archean ocean

Estimated silica concentration in Precambrian seawater is 60 ppm SiO2 or more, while silica concentration of much of the modern ocean is controlled by silica-secreting organisms at values of 1 ppm or less to a maximum of 15 ppm (Perry and Lefticariu, 2014). There is no conclusive fossil evidence that such organisms were present in the Precambrian in sufficient abundance to have had a significant influence on the silica cycle, although some later Neoproterozoic protists likely had scales that were siliceous, and Ediacaran sponges certainly produced siliceous spicules. This contrasts with the Phanerozoic, during which the appearance of radiolaria and diatoms changed the locus of silica precipitation (both primary and replacement) from the peritidal and shallow shelf deposits characteristic of the Neoproterozoic, Mesoproterozoic, and much of the Paleoproterozoic, to the deep ocean biogenic deposits since the mid to late Phanerozoic. Comparative petrography of Phanerozoic and Precambrian chert shows an additional early change in nonbiogenic chert deposition occurred toward the end of the Paleoproterozoic era and was marked by the end to widespread primary and early diagenetic silica precipitation in normal marine subtidal environments (Table 1; ca. 1.8 Ga Maliva et al., 2005). Interestingly, the Precambrian transition corresponds to the onset of a plate tectonic regime resembling that of today (Stern, 2007). It was also the time when sulphate levels in the world’s oceans had risen to where gypsum became a primary marine evaporite, as evidenced by large silicified anhydrite nodules (with anhydrite relics) in the late Paleoproterozoic Mallapunyah Fm in the McArthur Basin, Australia (Warren, 2016). Paleoproterozoic early diagenetic “normal marine” cherts generally formed nodules or discontinuous beds within carbonate deposits with similar depositional textures. It seems these “normal marine” cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation. In saline settings cauliflower cherts are also obvious from this time onwards.

 

Some of these Paleoproterozoic peritidal cherts were associated with iron formations and are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed. These primary cherts lack ghosts or inclusions of carbonate precursors, have fine-scale grain fracturing (possibly from syneresis), exhibit low grain-packing densities, and are not associated with unsilicified carbonate deposits of similar depositional composition (Perry and Lefticariu, 2014). Cherts in some Paleoproterozoic iron formations (e.g., the Gunflint Formation, northwestern Lake Superior region) are composed of silica types similar to those in Phanerozoic sinters (e.g., the Devonian Rhynie and Windyfield chert sinters, Scotland, both of which preserved fine-scale cellular detail of Devonian plants, fungi and cyanobacteria, as well as elevated gold levels in the fault feeder system). Such “normal marine cherts lie outside the evaporite focus of this series of articles and for more detail the reader is referred to Perry and Lefticariu, 2014 and references therein.

Archean crustal tectonics and silicification of world-scale evaporites

Archean evaporites were not deposited as saline giants within subsealevel restricted basins created by sialic continent-to-continent proximity setting. In the greenstone terranes that typified the early Archean these tectonic settings simply could not yet exist (Warren, 2016, Chapter 2). Stern (2007) defines plate tectonics as the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere), and so it is a world-scale system or set of processes mostly driven by lithosphere sinking (subduction pull). He argues that the complete set of processes and metamorphic indicators, associated with modern subduction zones, only became active at the beginning of the Neoproterozoic (≈ 1 Ga). Stern interprets the older record to indicate a progression of tectonic styles from active Archaean tectonics and magmatism (greenstone belts), to something akin to modern plate tectonics at around 1.9 Ga (Figure 1). If so, then modern world-scale plate tectonics only began in the early Neoproterozoic, with the advent of deep subduction zones (blueschists) and associated powerful slab pull mechanisms. Flament et al. (2008) argue that the world’s continents were mostly flooded (mostly covered with shallow ocean waters) until the end of the Archaean and that only 2–3 % of the Earth’s area consisted of emerged continental crust by around 2.5 Ga (aka “water-world”).


It is very likely that the Archaean Earth’s surface was broken up into many smaller plates with volcanic islands and arcs in great abundance (greenstone terranes). Small protocontinents (cratons) formed as crustal rock was melted and remelted by hot spots and recycled in subduction zones. There were no large continents in the Early Archaean, and small protocontinents were probably the norm by the MesoArchaean, when the higher rate of geologic activity (hotter core and mantle) prevented crustal segregations from coalescing into larger units (Figures 1 and 3 ). During the Early-Middle Archaean, Earth’s heat flow was almost three times higher than it is today, because of the greater concentration of radioactive isotopes and the residual heat from the Earth’s accretion, hence the higher ocean temperatures (Figure 2; Eriksson et al. 2004). At that time of a younger cooling earth there was considerably greater tectonic and volcanic activity; the mantle was more fluid and the crust much thinner. This resulted in rapid formation of oceanic crust at ridges and hot spots, and rapid recycling of oceanic crust at subduction zones with oceanic water cycling through hydrothermally active zones somewhat more intensely than today (Zegers and van Keken 2001; Ernst 2009; Flament et al. 2008).


In the Pilbara craton region of Australia significant crustal-scale delamination occurred ≈ 3.49 Ga, just before the production of voluminous TTG (tonalite, trondhjemite, and granodiorite) melts between 3.48 and 3.42 Ga and the accumulation sonic evaporites (Figure 3; Zegers and van Keken 2001). Delamination resulted in rapid uplift, extension, and voluminous magmatism, which are all features of the 3.48–3.42 Ga Pilbara succession. As the delaminated portion was replaced by hot, depleted mantle, melts were produced by both decompressional melting of the mantle, resulting in high-MgO basalts (this is the Salgash Subgroup in the Pilbara craton), and melting of the gabbroic and amphibolitic lower crust, so producing TTG melts. Partial melting of the protocrust to higher levels can be envisaged as a multistep process in which heat was conducted to higher levels and advection of heat occurs by intrusion of partial melts in subsequently higher levels (indicated by purple arrows in Figure 3). TTG melt products that were first intruded were subsequently metamorphosed and possibly partially melted, as can be inferred from the migmatitic gneisses of the Pilbara. This multistep history explains the complex pattern of U-Pb zircon ages of gneisses and granodiorites found within the Pilbara batholiths and the range in geochemical compositions of the Pilbara TTG suite.


Key to the formation of early Archaean evaporites, which indicate a sodium bicarbonate ocean at that time (see next section), is the observation that crustal delamination and the creation of TTG melts led to up to 2 km of crustal uplift (Figure 3). This would have driven some regions of what were submarine sedimentary systems into suprasealevel positions in the Archean waterworld, so creating the potential for hydrographically-isolated subsealevel marine seepage sumps in those portions of the uplifted crust above the zones of delamination. It also explains the centripetal nature of much shallow marine sedimentation of that time. This is cardinal at the broad tectonic scale when comparing the distribution of Archaean and Phanerozoic evaporites (Warren, 2016). Most Archaean evaporite are remnants that are pervasively silicified and underlain by layered igneous complexes, which were dominant across the greenstone seafloor and are associated with bottom-nucleated baryte beds tied to hydrothermal seeps.

Felsic protocontinents (suprasealevel cratons) hosting silicified evaporite remnants probably formed atop Archaean hot spots from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks. Although the first continents formed during the Archaean, rock of this age makes up only 7% of the world’s current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5–40 % of the present volume continental crust formed during the Archaean. 

Archean oceans and silicified sodic evaporites 

Chert styles and occurrences in saline settings across deep time clearly show that we cannot carry Phanerozoic silica mobility models in saline lacustrine or CaSO4 evaporite associations directly across time into the deep Precambrian. Rather, comparisons must be made in a context of the evolution of the earth’s atmosphere and associated ocean chemistry, both of which are in part related to the earth's tectonic evolution.

Levels of early Archaean sulphate in the world ocean were probably less than a few percent of the current levels and probably remained so until the evolution of an oxygen-reducing biota into the Proterozoic (Habicht and Canfield 1996; Kah et al. 2004; Warren, 2016). Grotzinger and Kasting (1993) argue that high levels of atmospheric CO2 meant HCO3/Ca ratios were much higher in the Archaean and the Paleoproterozoic oceans than today. All the calcium in seawater was deposited as marine cement-stones and other alkaline earth precipitates well before bicarbonate was depleted and there was no Ca left over to precipitate as gypsum. The early Archaean waterworld ocean was likely a Na–Cl–HCO3 sea, and not the Na–Cl ocean of today (Kempe and Degens 1985; Maisonneuve 1982). This early Archaean hydrosphere had a chemistry similar to that found in modern soda lakes like Lake Magadi and Lake Natron (pathway I brines) and hence the term “soda-lake oceans.” This rather different marine brine chemistry would have precipitated halite and trona/nahcolite, not halite/gypsum. It probably meant that if gypsum/anhydrite did ever precipitate directly from evaporating Archaean seawater it did so only in minor amounts well after the onset of halite precipitation.

 

The case for nahcolite (NaHCO3) as a primary evaporite (Figure 4a-d), along with halite, in the 3.42 Ga rocks of the Barberton greenstone belt was first documented by Lowe and Fisher-Worrell,1999), both the nahcolite and the halite are silicified. Beds of these silicified sodic evaporite define 5 types of precipitates: (1) large, pseudohexagonal prismatic crystals as much as 20 cm long that increase in diameter upward; (2) small isolated microscopic pseudohexagonal crystals; (3) small, tapering-upward prismatic crystals as much as 5 cm long; (4) small acicular crystallites forming halos around type 1 crystals; and (5) tightly packed, subvertical crystal aggregates within which individual crystals cannot be distinguished. Measurement of interfacial angles between prism and pinacoid faces on types 1 and 2 crystals show four interfacial angles of about 63° and two of about 53°. The morphologies and interfacial angles of these crystals correspond to those of nahcolite, NaHCO3 (Figure 4e). There is no clear evidence for the presence of gypsum in these beds. Sugitani et al. (2003) reported silicified nahcolite (the high CO2 form of sodium carbonate salts; see Warren, 2016, chapter 2) in ≈ 3.2 Ga rocks in the northern part of the Eastern Pilbara block, Western Australia (Figures 4, 5). Coarse, upward-radiating, silicified evaporite crystals in the ca. 3.47–3.46 Ga Strelley Pool Chert (Lowe 1983) show the same habit, geometry, and environmental setting as silicified nahcolite pseudomorphs in the Kromberg Fm. in the Barberton belt, South Africa, and also probably represent silicified NaHCO3 precipitates (Lowe and Tice 2004). Depositional reconstructions in both regions imply a strong hydrothermal association to the silicification of the evaporites in both regions as do bottom-nucleated baryte layers that define seafloor seeps fed by hydrothermal waters moving up faults (Figure 4f; Nijman et al., 1999; van den Boorn et al., 2007).

The pervasive presence of type 1 brines as ocean waters in the early Archean, along with elevated silica levels in most surface ocean waters, compared to the Phanerozoic, implies a significant portion of Archean cherts may also have had a volcanogenic sodium silicate precursor, much like the silicification seen in the modern African rift valley lakes (Eugster and Jones, 1968 and article 1 in this series of articles on silica mobilisation). So in order to decipher possible evaporite-silicification associations we must include aspects of hydrothermal fluid inherent to the Archean, as well as the likely higher surface temperatures that typified highly reducing (anoxic) waters of the early Archean ocean (Figure 3).


Archean evaporite deposition and silicification

Worldwide, the most widespread Archaean depositional environment, especially in early Archaean greenstone terranes, was the mafic plain environment (Condie 2016; Lowe 1994). In this setting, large volumes of basalt and komatiite were erupted to form widespread mostly submarine mafic plains characteristic by ubiquitous pillow structures in the lava interlayers. A second significant sedimentary environment was a deepwater, nonvolcanic setting, where chemical and biochemical cherts, banded iron formation, and carbonate laminites were deposited. The typical lack of evaporite indications in these mostly deepwater sediments indicates an ongoing lack of hydrologic restriction while the sediments were accumulating (waterworld association). The third association, a greywacke-volcanic association becomes more widespread in later Archaean greenstones, which typically sit stratigraphically atop mafic plain units. This association is composed chiefly of greywackes and interbedded calc-alkaline volcanics, hydrothermal precipitates and, in some shallower parts, silicified evaporites. It was perhaps mostly an island arc system and dominantly more open marine as it typically lacks widespread indicators of former marine evaporites. However, more locally it also preserves fluvial and shallow-marine detrital sediments, that were probably deposited locally in Archaean pull-apart basins, and associated with mineralogically mature sediments (quartzarenites, etc.). These more continental associations typified the shallowest to emergent parts of these continental rifts.

Unlike the other two early Archean  greenstone terranes this third terrane type can in places, such as the Pilbara, be tied to sedimentary indicators of a surfacing seafloor, indicated by particular chert and volcaniclastic layers showing mud cracks, wave ripples, tidalites interbedded with hyaloclastics, vuggy cherts, banded iron formations, carbonates and thick now-dissolved and altered type 1 evaporite masses (breccias), perhaps residues of beds formerly dominated by sodium carbonate and halite salts (Figure 5). The Warrawoona Group, preserves many such silicified examples that retain fine detail of primary textures such as mud cracks, oolites, and evaporite crystal casts and pseudomorphs, all indicating shallow-water to emergent deposition atop the mafic plain. In terms of crystal outlines there few if any casts of possible gypsum crystals, more typically, they indicate bladed pseudo-hexagonal, bottom-nucleated nahcolite, trona and in some instances, halite pseudomorphs (Figure 4).

Depositionally, to acquire the needed high salinities, these cherty evaporite units must have risen, at least locally, to shallow near-sealevel depths and at time become emergent, allowing local hydrographically-isolated lacustrine/rift evaporite subaqueous deposition or precipitation of local seepage drawdown salts. Associated primary-textured carbonate and baryte layers interbedded with the cherts are typically minor, bottom-nucleated baryte textures that may likely indicate hydrothermal vent deposits (Figure 4f; Nijman et al., 1999).

Inherent high solubility of any sodium bicarbonate and/or halite salts in what was a hotter burial system, more strongly influenced by hydrothermal circulation than today, meant most of the original sodic evaporite salts were not preserved, unless silicified in early burial. But their presence as silicified pseudomorphs in less-altered greenschist terranes intercalated with volcanics (Figure 4), such as in the Yilgarn, Pilbara and Kaapvaal cratons, clearly shows two things; (1) at times in the early Archaean waterworld there was sufficient hydrographic restriction to allow marine sodian carbonate and sodian chloride evaporites to form and (2) this marine restriction/seepage inflow was probably driven by ongoing volcanism and associated uplift, with evaporites restricted to particular basinwide stratigraphic indicator levels. In the East Pilbara, the early Archaean evaporite stratigraphic level is the Strelley Pool chert, in the Warrawoona group (Figure 5). This is also the level with some of the earliest indications of cellular life-forms (Wacey 2009).

For the original sodic evaporites, it marks the hydrological transition from open marine seafloor to a restricted hydrographically-isolated marine-fed sump basin, surrounded by granite-cored highs with the required uplift likely driven by delamination at the level of the mantle transition (Figures 1 and 3). Given the intimate association of chemical sediments to volcanism in early Archaean greenstone basins, and the sodium bicarbonate ocean chemistry then, compared to the Phanerozoic evaporite hydrochemistries, we can expect a higher proportion of CO2 volatilisation, a higher boron content (tourmalinites) in early Archaean, and a higher level of silicification.

Is the present the key to the past?

The study of silicified evaporites and associated sediments, formed in the early stages of the Earth’s 3.5 Ga sedimentary record, shows that not only has ocean chemistry evolved (see August 24, 2014 blog), the earth’s lithosphere/ plate tectonic character has also evolved (Eriksson et al. 2013). The further back in time, the less reliable is the application of the current plate tectonic paradigm with its strongly lateral movements of crustal blocks and associated plate-scale evaporite basin controls. Phanerozoic evaporites, and the associated silicified sulphate nodules, define a marine-fed seep system where subsealevel continental rifts and continent-continent collision belts favour the formation of mega-evaporite basins (Warren, 2010). Instead, in a substantial portion of the earlier part of the 2 billion year earth history that is the Archaean, shows early-earth evaporite deposition was favored by hydrographic isolation created by strong vertical movement of earth’s crust related to upwelling mantle plumes and crustal delamination with more intense hydrothermal circulation and silicification. There is still no real consensus as to actual time when plate tectonics, as it operates today, actually began, but there is consensus that the present, in terms of plate tectonics, plate-edge collision and evaporite distribution, is not the key to much of the Archaean (Stern 2007; Rollinson 2007).

Uplift and the local accumulation of sodium carbonate Archean evaporites occurred in a depositional setting that was dominated by volcaniclastics,hydrothermal vents and extensional tectonics. Tectonic patterns in these settings have a strongly vertical flavor. In contrast, Phanerozoic salts formed from marine waters with a NaCl dominance with minor bicarbonate compared to calcium, and located mostly in subsealevel sumps formed at interacting sialic plate margins where the dominant tectonic flavor is driven the lateral movement of plates atop a laterally moving asthenosphere and the relative proportion of vilified salts is lower.

Whatever and wherever the onset of Archaean evaporite deposition, all agree that the mechanisms and aerial proportions world-scale plate tectonics were different in early earth history compared to the Phanerozoic. The current argument as to how different is mostly centred on when earth-scale plate tectonic processes became similar to those of today. Given much higher crustal heat flows, it is likely that hydrographically isolated subsealevel depressions, required to form widespread marine evaporites were more localized in the Archaean than today and were more susceptible to hydrothermal alteration, metamorphism and silicification. Appropriate restricted brine sumps would have tended to occur in magmatically-induced uplift zones atop incipient sialic segregations, with crestal subsealevel grabens, which were hydrographically isolated by their surrounds created by supra-sealevel uplift. Once deposited, the higher heat flow in Archaean crust and mantle would also have meant any volumetrically significant evaporites masses were more rapidly recycled, silicified and replaced via diagenetic and metamorphic processes than today.

Some authors have noted that there are no widespread marine evaporites in the Archaean and in the sense of actual preserved salts, this is true. But when one considers that the Archaean crust was much hotter than today and hydrothermal circulation was more active and pervasive, then widespread burial preservation of the primary salts seems highly unlikely. Even in the Neoproterozoic, lesser volumes of the original salt masses remain (Hay et al. 2006). The lack of preserved salts in earlier Precambrian strata is perhaps more a matter of great age, polycyclic metamorphic alteration and the typical proximity to shallow hydrothermal fluids in emergent evaporite forming regions of the Archean waterworld. However we must also ask if the onset of modern styles of plate tectonics also played a role in the relative absence of preserved saline giants in strata older than 1Ga, In the next article we shall look how cooling and the onset of sialic plate tectonics similar to today, altered the types, styles and distributions of silicified and other evaporite salts as the world's oceans moved toward a chemistry more akin to that of today.

References

 

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Salt as a Fluid Seal: Article 4 of 4: When and where it leaks - Implications for waste storage

John Warren - Thursday, March 24, 2016

 

In the three preceding articles on salt leakage, we have seen that most subsurface salt in the diagenetic realm is a highly efficient seal that holds back large volumes of hydrocarbons in salt basins worldwide (Article 3). When salt does leak or transmit fluid, it does so in one of two ways: 1) by the entry of undersaturated waters (Article 1 in this series) and; 2) by temperature and pressure-induced changes in its dihedral angle, which in the diagenetic realm is often tied to the development of significant overpressure and hydrocarbon migration (Article 2). The other implication linked to the two dominant modes of salt leakage is the source of the fluid entering the leaking salt. In the first case, the fluid source is external to the salt ("outside the salt"). In the second case, it can be internal to the main salt mass ("inside the salt"). However, due to dihedral angle changes at greater depths and pressures, a significant portion of leaking fluid passing through more deeply-buried altering salt is external. By the onset of greenschist facies metamorphism, this is certainly the case (Chapter 14 in Warren, 2016) 

Diagenetic fluids driving salt leakage are external to the salt mass

Within a framework of fluids breaching a subsurface salt body, the breached salt can be a bed of varying thickness, or it can have flowed into a variety of autochthonous and allochthonous salt masses. Autochthonous salt structures are still firmly rooted in the stratigraphic level of the primary salt bed. Allochthonous salt structurally overlies parts of its (stratigraphically younger) overburden and is often no longer connected to the primary salt bed (mother-salt level).

Breaches in bedded (non-halokinetic) salt

The principal documented mechanism enabling leakage across bedded salt in the diagenetic realm is dissolution, leading to breaks or terminations in salt bed continuity. Less often, leakage across a salt unit can occur where bedded salt has responded in a brittle fashion and fractured or faulted (Davison, 2009). In hydrocarbon-producing basins with widespread evaporite seals, significant fluid leakage tends to occur near the edges of the salt bed. For example, in the Middle East, the laterally continuous Hith Anhydrite (Jurassic) acts as a regional seal to underlying Arab Cycle reservoirs and carbonate-mudstone source rock. The high efficiency of the Hith seal creates many of the regions giant and supergiant fields, including Ghawar in Saudi Arabia, which is the largest single oil-filled structure in the world. Inherent maintenance of the evaporite's seal capacity also prevents vertical migration from mature sub-Hith source rocks into potential reservoirs in the overlying Mesozoic section across much of Saudi Arabia and the western Emirates. However, toward the Hith seal edge are a number of large fields supra-Hith fields, hosted in Cretaceous carbonates, and a significant portion of the hydrocarbons are sourced in Jurassic carbonate muds that lie stratigraphically below the anhydrite level (Figure 1).

 

The modern Hith Anhydrite edge is not the depositional margin of the laterally extensive evaporite bed. Rather, it is a dissolution edge, where rising basinal brines moving up and out of the basin have thinned and altered the past continuity of this effective seal.

The process of ongoing dissolution allowing vertical leakage near the edge of a subsurface evaporite interval, typifies not just the edge of bedded salts but also the basinward edges of salt units that are also halokinetic. The dissolution edge effect of the Ara Salt and its basinward retreat over time are clearly seen along the eastern edge of the South Oman Salt Basin where the time of filling of the Permian-hosted reservoir structures youngs toward the west (Figure 2).


 

Leakages associated with the margins of discrete diapiric structures

Once formed, salt diapirs tend to focused upward escape of basinal fluid flows: as evidenced by: (1) localized development of mud mounds and chemosynthetic seeps at depopod edges above diapirs in the Gulf of Mexico (Figure 3a); (2) shallow gas anomalies clustered around and above salt diapirs in the North Sea and (Figure 3b); (3) localized salinity anomalies around salt diapirs, offshore Louisiana and with large pockmarks above diapir margins in West Africa (Cartwright et al., 2007). Likewise, in the eastern Mediterranean region, gas chimneys in the Tertiary overburden are common above regions of thinned Messinian Salt, as in the vicinity of the Latakia Ridge (Figure 4).



Leakage of sub-salt fluids associated with salt welds and halokinetic touchdowns

Whenever a salt weld or touchdown occurs, fluids can migrate vertically across the level of a now flow-thinned or no-longer-present salt level. Such touchdowns or salt welds can be in basin positions located well away from the diapir edge and are a significant feature in the formation of many larger base-metal and copper traps, as well as many depopod-hosted siliciclastic oil and gas reservoirs (Figure 5: Warren 2016).


Caprocks are leaky

Any caprock indicates leakage and fractional dissolution have occurred along the evaporite boundary (Figure 5). Passage of an undersaturated fluid at or near the edge of a salt mass creates a zone of evaporite dissolution residues, which in the case of diapiric occurrences is called usually called a “caprock,” although such diagenetic units do not only form a “cap” or top to a salt structure.

Historically, in the 1920s and 30s, shallow vuggy and fractured caprocks to salt diapirs were early onshore exploration targets about topographic highs in the Gulf of Mexico (e.g. Spindletop). Even today, the density of drilling and geological data derived from these onshore diapiric features means many models of caprock formation are mostly based on examples in Texas and Louisiana. Onshore in the Gulf of Mexico, caprocks form best in dissolution zones at the outer, upper, edges of salt structures, where active cross-flows of meteoric waters are fractionally dissolving the salt. However, rocks composed of fractional dissolution residues, with many of the same textural and mineralogical association as classic Gulf of Mexico caprocks, are now known to mantle the deep sides of subvertical-diapirs in the North Sea (e. g., lateral caprock in the Epsilon Diapir) and define the basal anhydrite (basal caprock) that defines the underbelly of the Cretaceous Maha Sarakham halite across the Khorat Plateau in NE Thailand (Figure 5; Warren, 2016).


All “caprocks” are fractionally-dissolved accumulations of diapir dissolution products and form in zones of fluid-salt interaction and leakage, wherever a salt mass is in contact with undersaturated pore fluids (Figure 6). First to dissolve is halite, leaving behind anhydrite residues, that cross-flushing pore waters can then convert to gypsum and, in the presence of sulphate-reducing bacteria, to calcite. If the diapir experiences another growth pulse the caprock can be broken and penetrated by the rising salt. This helps explain fragments of caprock caught up in shale sheaths or anomalous dark-salt zones, as exemplified by less-pure salt-edge intersection units described as dark and anomalous salt zones in the Gulf of Mexico diapirs (as documented in Article 1).

2. Fluids that are internal to the salt mass

Fluid entry in relation to changes in the dihedral angle of halite is well documented (Article 2). It was first recorded by Lewis and Holness (1996) who postulated, based on their static-salt laboratory experiments;

"In sedimentary basins with normal geothermal gradients, halite bodies at depths exceeding 3 km will contain a stable interconnected brine-filled porosity, resulting in permeabilities comparable to those of sandstones". Extrapolating from their static halite pressure experiments they inferred that halite, occurring at depths of more than ≈3 km and temperatures above 200 °C, has a uniform intrasalt pore system filled with brine, and therefore relatively high permeabilities.

In the real world of the subsurface, salt seals can hold back significant hydrocarbon columns down to depths of more than 6-7 km (see case studies in Chapter 10 in Warren, 2016 and additional documentation the SaltWork database). Based on a compilation of salt-sealed hydrocarbon reservoirs, trans-salt leakage across 75-100 metres or more of pure salt does not occur at depths less than 7-8 km, or temperatures of less than 150°C. In their work on the Haselbirge Formation in the Alps, Leitner et al. (2001) use a temperature range >100 °C and pressures >70 MPa as defining the onset of the dihedral transition.

It seems that across much of the mesogenetic realm, a flowing and compacting salt mass or bed can maintain seal integrity to much greater depths than postulated by static halite percolation experiments. In the subsurface, there may be local pressured-induced changes in the halite dihedral angle within the salt mass, as seen in the Ara Salt in Oman, but even there, there is no evidence of the total km-scale salt mass transitioning into a leaky aquifer via changes in the halite dihedral angle (Kukla et al., 2011). But certainly, as we move from the diagenetic into the metamorphic realm, even thick pure salt bodies become permeable across the whole salt mass. Deeply buried and pressured salt ultimately dissolves as it transitions into various meta-evaporite indicator minerals and zones (Chapter 14, Warren, 2016).

When increasing pressure and temperature changes the halite dihedral angle in the diagenetic realm, then supersaturated hydrocarbon-bearing brines can enter salt formations to create naturally-hydrofractured "dark-salt". As we discussed in Article 2, pressure-induced changes in dihedral angle in the Ara Salt of Oman create black salt haloes that penetrate, from the overpressured salt-encased carbonate sliver source, up to 50 or more meters into the adjacent halite (Schoenherr et al. 2007). Likewise, Kettanah, 2013 argues Argo Salt of eastern Canada also has leaked, based on the presence of petroleum-fluid inclusions (PFI) and mixed aqueous and fluid inclusions (MFI) in the recrystallised halite (Figure 7 - see also Ara “black salt” core photos in Article 2 of this series).


Both these cases of dark-salt leakage (Ara and Argo salts) occur well within the salt mass, indicating the halokinetic salt has leaked or transmitted fluids within zones well away from the salt edge. In the case of the Argo salt, the study is based on drill cuttings collected across 1500 meters of intersected salt at depths of 3-4 km. Yet, at the three km+ depths in the Argo Salt where salt contains oil and bitumen, the total salt mass still acts a seal, implying it must have regained or retained seal integrity, after it leaked. Not knowing the internal fold geometries in any deeply buried salt mass, but knowing that all flowing salt masses are internally complex (as seen in salt mines and namakiers), means we cannot assume how far the hydrocarbon inclusions have moved within the salt mass, post-leakage. Nor can we know if, or when, any salt contact occurred with a possible externally derived hydrocarbon-bearing fluid source, or whether subsequent salt flow lifted the hydrocarbon-inclusion-rich salt off the contact surface as salt flowed back into the interior of the salt mass.

Thus, with any hydrocarbon-rich occurrence in a halokinetic salt mass, we must ask the question; did the salt mass once hydrofracture (leak) in its entirety, or did the hydrocarbons enter locally and then as the salt continued to flow, that same hydrocarbon-inclusion-rich interval moved into internal drag and drape folds? In the case of the Ara Salt, the thickness of the black salt penetration away from its overpressured source is known as it is a core-based set of observations. In the Ara Salt at current depths of 3500-4000 m, the fluid migration zones extend 50 -70 meters out from the sliver source in salt masses that are hundreds of metres thick (Kukla et al., 2011; Schoenherr et al., 2007).

So how do we characterize leakage extent in a buried salt mass without core?

Dark salt, especially if it contains hydrocarbons, clearly indicates fluid entry into a salt body in the diagenetic realm. Key to considerations of hydrocarbon trapping and long-term waste storage is how pervasive is the fluid entry, where did the fluid come from, and what are the likely transmission zones in the salt body (bedded versus halokinetic)?

In an interesting recent paper documenting and discussing salt leakage, Ghanbarzadeh et al., 2015 conclude:

“The observed hydrocarbon distributions in rock salt require that percolation occurred at porosities considerably below the static threshold due to deformation-assisted percolation. Therefore, the design of nuclear waste repositories in salt should guard against deformation-driven fluid percolation. In general, static percolation thresholds may not always limit fluid flow in deforming environments.”

Their conclusions are based on lab experiments on static salt and extrapolation to a combination of mud log and wireline data collected from a number of wells that intersected salt allochthons in Louann Salt in the Gulf of Mexico. Their lab data on changing dihedral angles inducing leakage or percolation in static salt confirms the experiments of Holness and Lewis (1996 – See Article 2). But they took the implications of dihedral angle change further, using CT imagery to document creation of interconnected polyhedral porosity in static salt at higher temperatures and pressures (Figure 8). They utilise Archies Law and resistivity measures to calculate inferred porosity, although it would be interesting what values they utilise for cementation exponent (depends on pore tortuosity) Sw and saturation exponent. Assuming the standard default values of m = 2 and n =2 when applying Archies Law to back calculate porosity spreads in halite of assumed Sw are likely incorrect.  


They then relate their experimental observations to wireline measurements and infer the occurrence of interconnected pores in Gulf of Mexico salt based on this wireline data. Key to their interpretation is the deepwater well GC8 (Figure 9), where they use a combination of a resistivity, gas chromatograms, and mud log observations to infer that hydrocarbons have entered the lower one km of a 4 km thick salt section, via dihedral-induced percolation.

 

I have a problem in accepting this leap of faith from laboratory experiments on pure salt observed at the static decimeter-scale of the lab to the dynamic km-scale of wireline-inferred observations in a salt allochthon in the real world of the offshore in deepwater salt Gulf of Mexico. According to Ghanbarzadeh et al., 2015, the three-part gray background in Figure 9 corresponds to an upper no-percolation zone (dark grey), a transition zone (moderate grey) and a lower percolation zone (light grey). This they then infer to be related to changes in dihedral angle in the halite sampled in the well (right side column). Across the data columns, what the data in the GC8 well show is:  A) Gamma log; allochthon salt has somewhat higher API values at depths shallower than 5000 m; B) Resistivity log, a change in resistivity to higher values (i.e., lower conductivity) with a change in the same cross-salt depth range as seen in the gamma log, beginning around 5100 m; C) Gas (from sniffer), shows a trend of decreasing gas content from the base of salt (around 6200 m) up to a depth around 4700 m, then relatively low values to top salt, with an interval that is possibly shalier interval (perhaps a suture - see below)  that also has a somewhat higher gas content ; D) Gas chromatography, the methane (CH4) content mirrors the total gas trends, as do the other gas phases, where measured; E) Mud Log (fluorescence response), dead oil is variably present from base of salt up to 5000 m, oil staining, oil cut and fluorescence (UV) are variably present from base salt up to a depth of 4400 m.

On the basis of the presented log data, one can infer the lower kilometer of the 4 km salt section contains more methane, more liquid hydrocarbons, and more organic material/kerogen compared to the upper 3 km of salt. Thus, the lower section of the salt intersected in the GC8 well is likely to be locally rich in zones of dark or anomalous salt, compared with the overlying 3 km of salt. What is not given in figure 9 is any information on likely levels of non-organic impurities in the salt, yet this information would have been noted in the same mud log report that listed hydrocarbon levels in the well. In my opinion, there is a lack of lithological information on the Gulf of Mexico salt in the Ghanbarzadeh et al. paper, so one must ask; "does the lower kilometer of salt sampled in the GC8 well, as well as containing hydrocarbons also contain other impurities like shale, pyrite, anhydrite, etc. If so, potentially leaky intervals could be present that were emplaced by sedimentological processes unrelated to changes in the dihedral angle of the halite (see next section).


Giving information that is standard in any mud-log cuttings description (such as the amount of anhydrite, shale, etc that occur in drill chips across the salt mass), would have added a greater level of scientific validity to to Ghanbarzadeh et al.'s inference that observed changes in hydrocarbon content up section, was solely facilitated by changes in dihedral angle of halite facilitating ongoing leakage from below the base of salt and not due to the dynamic nature of salt low as the allochthon or fused allochthons formed.  Lithological information on salt purity is widespread in the Gulf of Mexico public domain data. For example, Figure 10 shows a seismic section through the Mahogany field and the intersection of the salt by the Phillips No. 1 discovery well (drilled in 1991). This interpreted section, tied to wireline and cuttings information, was first published back in 1995 and re-published in 2010. It shows intrasalt complexity, which we now know typifies many sutured salt allochthon and canopy terrains across the Gulf of Mexico salt province. Internally, Gulf of Mexico salt allochthons, like others worldwide, are not composed of pure halite, just as is the case in the onshore structures discussed in the context of dark salt zones in article 1. Likely, a similar lack of purity and significant structural and lithological variation typifies most if not all of the salt masses sampled by the Gulf of Mexico wells listed in the Ghanbarzadeh et al. paper, including the key GC8 well (Figure 9). This variation in salt purity and varying degrees of local leakage is inherent to the emplacement stage of all salt allochthons world-wide. It is set up as the salt flow (both gravity spreading and gravity gliding) occurs at, or just below the seafloor, fed by varying combinations of extrusion or thrusting, which moves salt out and over the seabed (Figure 11).

 


 

Salt, when it is flowing laterally and creating a salt allochthon, is in a period of rapid breakout (Figure 11; Hudec and Jackson, 2006, 2007; Warren 2016). This describes the situation when a rising salt sheet rolls out over its base, much in the same way a military tank moves out over its track belt. As the salt spreads, the basal and lateral salt in the expanding allochthon mass, is subject to dissolution, episodic retreat, collapse and mixing with seafloor sediment, along with the entry of compactional fluids derived from the sediments beneath. Increased impurity levels are particularly obvious in disturbed basal shear zones that transition downward into a gumbo zone (Figure 12a), but also mantle the sides of subvertical salt structures, and can evolve by further salt dissolution into lateral caprocks and shale sheaths (Figure 6).

In expanding allochthon provinces, zones of non-halite sediment typically define sutures within (autosutures; Figure 12b) or between salt canopies (allosutures; Figure 12c). These sutures are encased in halite as locally leaky, dark salt intervals, and they tend to be able to contribute greater volumes of fluid and ongoing intrasalt dissolution intensity and alteration where the suture sediment is in contact with outside-the-salt fluids. Allochthon rollout, with simultaneous diagenesis and leakage, occurs across intrasalt shear zones, or along deforming basal zones. In the basal part of an expanding allochthon sheet the combination of shearing, sealing, and periodic leakage creates what is known as “gumbo,” a term that describes a complex, variably-pressured, shale-rich transition along the basal margin of most salt allochthons in the Gulf of Mexico (Figure 12a). Away from suture zones, as more allochthon salt rolls out over the top of earlier foot-zones to the spreading salt mass, the inner parts of the expanding and spreading allochthon body tend toward greater internal salt purity (less non-salt and dissolution residue sediment, as well as less salt-entrained hydrocarbons and fluid inclusion).

At the salt's upper contact, the spreading salt mass may carry its overburden with it, or it may be bare topped (aka open-toed; Figure 11). In either case, once salt movement slows and stops, a caprock carapace starts to form that is best developed wherever the salt edge is flushed by undersaturated pore waters (Figure 6). Soon after its emplacement, the basal zone of a salt allochthon acts a focus for rising compactional fluids coming from sediments beneath. So, even as it is still spreading, the lower side of the salt sheet is subject to dissolution, and hydrocarbon entry, often with remnants of the same hydrocarbon-entraining brines leaking to seafloor about the salt sheet edge. As the laterally-focused subsalt brines escape to the seafloor across zones of thinned and leaky salt or at the allochthon edge, they can pond to form chemosynthetic DHAL (Deepsea Hypersaline Anoxic Lake) brine pools (Figure 3a). Such seep-fed brine lakes typify the deep sea floor in the salt allochthon region of continental slope and rise in the Gulf of Mexico and the compressional salt ridge terrain in the central and eastern Mediterranean. If an allochthon sheet continues to expand, organic-rich DHAL sediments and fluids become part of the basal shear to the salt sheet (Figure 12a).


Unfortunately, Ghanbarzadeh et al., 2015 did not consider the likely geological implications of salt allochthon emplacement mechanisms and how this likely explains much of the geological character seen in wireline signatures across wells intersecting salt in the Gulf of Mexico. Rather, they assume the salt system and the geological character they infer as existing in the lower portions of Gulf of Mexico salt masses, are tied to post-emplacement changes in salt's dihedral angle in what they consider as relatively homogenous and pure salt masses. They modeled the various salt masses in the Gulf of Mexico as static, with upward changes in the salt purity indicative of concurrent hydrocarbon leakage into salt and facilitated by altered dihedral angles in the halite. A basic tenet of science is "similarity does not mean equivalence." Without a core from this zone, one cannot assume hydrocarbon occurrence in the lower portions of Gulf of Mexico salt sheets is due to changes in dihedral angle. Equally, if not more likely, is that the wireline signatures they present in their paper indicate the manner in which the lower part of a salt allochthon has spread. To me, it seems that the Ghanbarzadeh et al. paper argues for caution in the use of salt cavities for nuclear waste storage for the wrong reasons.

Is nuclear waste storage in salt a safe, viable long-term option?

Worldwide, subsurface salt is an excellent seal, but we also know that salt does fail, that salt does leak, and that salt does dissolve, especially in intrasalt zones in contact with "outside" fluids. Within the zone of anthropogenic access for salt-encased waste storage (depths of 1-2km subsurface) the weakest points for potential leakage in a salt mass, both natural and anthropogenic, are related to intersection with, or unplanned creation of, unexpected fluid transmission zones and associated entry of undersaturated fluids that are sourced outside the salt (see case histories in Chapter 7 and 13 in Warren, 2016). This intersection with zones of undersaturated fluid creates zones of weakened seal capacity and increases the possibility of exchange and mixing of fluids derived both within and outside the salt mass. In the 1-2 km depth range, the key factor to be discussed in relation to dihedral angle change inducing percolation in the salt, will only be expressed as local heating and fluid haloes in the salt about the storage cavity. Such angle changes are tied to a thermal regime induced by long-term storage of medium to high-level radioactive waste.  

I use an ideal depth range of 1-2 km for storage cavities in salt as cavities located much deeper than 2 km are subject to compressional closure or salt creep during the active life of the cavity (active = time of waste emplacement into the cavity). Cavities shallower than 1 km are subject to the effects of deep phreatic circulation. Salt-creep-induced partial cavity closure, in a salt diapir host, plagued the initial stages of use of the purpose-built gas storage cavity known as Eminence in Mississippi. In the early 1970s, this cavity was subject to a creep-induced reduction in cavity volume until gas storage pressures were increased and the cavern shape re-stabilised. Cavities in salt shallower than 1 km are likely to be located in salt intervals that at times have been altered by cross flows of deeply-circulating meteoric or marine-derived phreatic waters. Problematic percolation or leakage zones (aka anomalous salt zones), which can occur in some places in salt masses in the 1-2 km depth range, are usually tied to varying combinations of salt thinning, salt dissolution or intersection with unexpected regions of impure salt (relative aquifers). In addition to such natural process sets, cross-salt leakage can be related to local zones of mechanical damage, tied to processes involved in excavating a mine shaft, or in the drilling and casing of wells used to create a purpose-built salt-solution cavity. Many potential areas of leakage in existing mines or brine wells are the result of poorly completed or maintained access wells, or intersections with zones of “dark salt,” or with proximity to a thinned salt cavity wall in a diapir, as documented in articles 1 and 2 (and detailed in various case studies in Chapters 7 and 13 in Warren 2016).

In my opinion, the history of extraction, and intersections with leakage zones, during the life of most of the world’s existing salt mines means conventional mines in salt are probably not appropriate sites for long-term radioactive waste storage. Existing salt mines were not designed for waste storage, but to extract salt or potash with mining operations often continuing in a particular direction along an ore seam until the edge of the salt was approached or even intersected. When high fluid transmission zones are unexpectedly intersected during the lifetime of a salt mine, two things happen; 1) the mine floods and operations cease, or the flooded mine is converted to a brine extraction facility (Patience Lake) or, 2) the zone of leakage is successfully grouted and in the short term (tens of years) mining continues (Warren, 2016).

For example, in the period 1906 to 1988, when Asse II was an operational salt mine, there were 29 documented water breaches that were grouted or retreated from. Over the long term, these same water-entry driven dissolution zones indicate a set of natural seep processes that continued behind the grout job. This is true in any salt mine that has come “out of the salt” and outside fluid has leaked into the mine. “Out-of-salt” intersections are typically related to fluids entering the salt mass via dark-salt or brecciated zones or shale sheath intersections (these all forms of anomalous salt discussed in article 1 and documented in the case studies discussed in Chapter 13 in Warren 2016).

I distinguish such “out-of-salt” fluid intersections from “in-salt” fluid-filled cavities. When the latter is cut, entrained fluids drain into the mine and then flow stops. Such intersections can be dangerous during the operation of a mine as there is often nitrogen, methane or CO2 in an "in-the-salt” cavity, so there is potential for explosion and fatalities. But, in terms of long-term and ongoing fluid leakage “in-salt” cavities are not a problem.

Ultimately, because “out-of-salt” fluid intersections are part of the working life of any salt mine, seal integrity in any mine converted to a storage facility will fail. Such failures are evidenced by current water entry problems in Asse II Mine, Germany (low-medium level radioactive waste storage) and the removal of the oil formerly stored in the Weeks Island strategic hydrocarbon facility, Texas. Weeks Island was a salt mine converted to oil storage. After the mine was filled with oil, expanding karst cavities were noticed forming at the surface above the storage area. Recovery required a very expensive renovation program that ultimately removed more than 95% of the stored hydrocarbons. And yet, during the active life of the Weeks Island Salt Mine, the mine geologists had mapped “black salt” occurrences and tied them to unwanted fluid entries that were then grouted. Operations to block or control the entry of fluids were successful, and salt extraction continued apace.  This information on fluid entry was available well before the salt mine was purchased and converted to a federal oil storage facility. However, in the 1970s when the mine was converted, our knowledge of salt properties and salt's stability over the longer term was less refined than today.

Worldwide, the biggest problem with converting existing salt mines to low to medium level nuclear waste storage facilities is that all salt mines are relatively shallow, with operating mine depth controlled by temperatures where humans can work (typically 300-700 m and always less than 1.1 km). This relatively shallow depth range, especially at depths above 500 m, is also where slowly-circulating subsurface or phreatic waters are dissolving halite to varying degrees, This is where fluids can enter the salt from outside and so create problematic dark-salt and collapse breccia zones within the salt. In the long-term (hundreds to thousands of years) these same fluid access regions have the potential to allow stored waste fluids to escape the salt mass,

Another potential problem with long-term waste storage in many salt mines, and in some salt cavity hydrocarbon storage facilities excavated in bedded (non-diapiric) salt, is the limited thickness of a halite beds across the depth range of such conventional salt mines and storage facilities. Worldwide, bedded ancient salt tends to be either lacustrine or intracratonic, and individual halite units are no more than 10-50 m thick in stacks of various saline lithologies. That is, intracratonic halite is usually interlayered with laterally extensive carbonate, anhydrite or shale beds, that together pile into bedded saline successions up to a few hundred metres thick (Warren 2010). The non-halite interlayers may act as potential long-term intrasalt aquifers, especially if connected to non-salt sediments outside the halite (Figure 13). This is particularly true if the non-salt beds remain intact and hydraulically connected to up-dip or down-dip zones where the encasing halite is dissolutionally thinned or lost. Connection to such a dolomite bed above the main salt bed, in combination with damaged casing in an access well, explains the Hutchison gas explosion (Warren, 2016). Also, if there is significant local heating associated with longer term nuclear waste storage in such relatively thin (<10-50 m) salt beds, then percolation, related to heat-induced dihedral angle changes, may also become relevant over the long-term (tens of thousands of years), even in bedded storage facilities in 1-2 km depth range.


Now what?

Creating a purpose-built mine for the storage of low-level waste in a salt diapir within the appropriate depth range of 1-2 km is the preferred approach and a much safer option, compared to the conversion of existing mines in diapiric salt, but is likely to be prohibitively expensive. To minimise the potential of unwanted fluid ingress, the entry shaft should be vertical, not inclined. The freeze-stabilised “best practice” vertical shaft currently being constructed by BHP in Canada for its new Jansen potash mine (bedded salt) is expected to cost more than $1.3 billion. If a purpose-built mine storage facility were to be constructed for low to medium level waste storage in a salt diapir, then the facility should operate at a depth of 800-1000m. Ideally, such a purpose-built mine should also be located hundreds of metres away from the edges of salt mass in a region that is not part of an area of older historical salt extraction operations. At current costings, such a conventionally-mined purpose-built storage facility for low to medium level radioactive waste is not economically feasible.

This leaves purpose-built salt-solution cavities excavated within thick salt domes at depths of 1-2 km; such purpose-built cavities should be located well away from the salt edge and in zones with no nearby pre-existing brine-extraction cavities or oil-field exploration wells. This precludes most of the onshore salt diapir provinces of Europe and North America as repositories for high-level nuclear waste, all possible sites are located in high population areas and can have century-long histories of poorly documented salt and brine extraction and petroleum wells. Staying "in-the-salt" over the long-term would an ongoing problem in these regions (see case histories in chapters 7 and 13 in Warren, 2016 for a summary of some problems areas).

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Leitner, C., F. Neubauer, J. L. Urai, and J. Schoenherr, 2011, Structure and evolution of a rocksalt-mudrock-tectonite: The haselgebirge in the Northern Calcareous Alps: Journal of Structural Geology, v. 33, p. 970-984.

Lewis, S., and M. Holness, 1996, Equilibrium halite-H2O dihedral angles: High rock salt permeability in the shallow crust: Geology, v. 24, p. 431-434.

Schoenherr, J., J. L. Urai, P. A. Kukla, R. Littke, Z. Schleder, J.-M. Larroque, M. J. Newall, N. Al-Abry, H. A. Al-Siyabi, and Z. Rawahi, 2007, Limits to the sealing capacity of rock salt: A case study of the infra-Cambrian Ara Salt from the South Oman salt basin: Bulletin American Association Petroleum Geologists, v. 91, p. 1541-1557.

Terken, J. M. J., N. L. Frewin, and S. L. Indrelid, 2001, Petroleum systems of Oman: Charge timing and risks: Bulletin-American Association of Petroleum Geologists, v. 85, p. 1817-1845.

Thrasher, J., A. J. Fleet, S. J. Hay, M. Hovland, and S. Düppenbecker, 1996, Understanding geology as the key to using seepage in exploration: the spectrum of seepage styles, in S. D., and M. A. Abrams, eds., Hydrocarbon migration and its near-surface expression, AAPG Memoir 66, p. 223-241.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2016, Evaporites: A Compendium (ISBN 978-3-319-13511-3) Released Feb. 22 2016: Berlin, Springer, 1854 p.


 

Salt as a Fluid Seal: Article 3 of 4: When it doesn't leak - Seals to hydrocarbons

John Warren - Saturday, March 12, 2016

This, the third article in the series of four on salt leakage, discusses how and when salt acts as a seal. The fourth article will place this discussion in real world situations where the various salt units (especially “dark” salt) have lost long-term seal integrity and what this means in terms of CO2 geosequestration and waste storage.

Evaporite seal character

Unlike thick shales, subsurface evaporites in the diagenetic realm better fit Hunt’s (1990) definition of an actual pressure seal, which he defined as an impermeable rock with zero transmissivity maintained over long periods of geologic time. Very little subsalt fluid can escape through a salt mass that, until breached, tends to hold back all the compactional and thermohaline waters, gases or liquid hydrocarbons rising from below. In contrast, shale-seals consistently leak all these fluids to varying degrees.


In the realm of subsurface hydrocarbon exploration and development, salts (especially halite) are second only to clathrates in ability to form an effective seal to circulating pore waters and hydrocarbons, including methane. (Figure 1; Warren, 2016). Natural methane clathrates (methane-ice mixtures) are more efficient seals, but in the diagenetic realm, clathrate occurrence is limited by the inherent low-temperature stability requirement. This means clathrates act as hydrocarbon seals onshore in permafrost regions, such as some Siberian gas fields, or below clathrate layers down to depths of a hundred meters or so beneath the modern deep-sea floor, as occurs below the cold waters of the slope and rise across the halokinetic Gulf of Mexico or the non-halokinetic sediment of offshore Brunei (Warren, 2016; Warren et al., 2011a).

Like clathrates, evaporite layers can generate overpressures at very shallow burial depths, unlike clathrates they do not dissolve and dissipate in response to rising temperatures of the diagenetic burial realm. Evaporites create the highest and sharpest depth-related pressure differentials known in sedimentary basins in both overpressured and underpressured settings (Fertl 1976). Salt-sealed overpressured intervals can be as shallow as a few hundred meters below the surface or deeper than 6,000 m.


Unlike the low temperatures requirements for a clathrate seal, evaporite seals, with their extremely high entry pressures, ductility, very low permeabilities and large lateral extents, can maintain seal integrity over wide areas, even when exposed to a wide range of subsurface temperature and pressure conditions. A typical shale seal has permeability ≈ 10-1 to 10-5 md, with extreme values as low as 10-8 md (Figure 2). Quantitative measurement of evaporite permeability is beyond the capacity of standard instruments used in the oil industry and is mostly a topic of study for engineers working with waste-storage caverns. Their work shows the permeability of undisturbed halite is a nanodarcy or less, that is, undamaged subsurface salt has measured permeabilities that are less than 10-21 m2 (10-6md) with some of the tighter halite permeabilities ≈10-7 to 10-9 md. In contrast, typical massive anhydrites ≈10-5 md (Beauheim and Roberts, 2002). This explains a general “rule of thumb” used in the oil industry that a halite bed should be at least 2 m thick to be considered a possible seal, while and anhydrite bed should be at least 10 m thick. Equally important is the reliability of the geological model of the evaporite that is used to extrapolate lateral continuity in the seal (Warren, 2016). Pore pressures in thick sealing halites can approach lithostatic (Ehgartner et al., 1998) and when exceeded salt can locally fracture and leak (as discussed in the previous article).

Massive thick bedded pure halite units in the diagenetic realm usually contain few, if any, interconnected pore throats. The distance between NaCl lattice units is 2.8 x 10-10 m, while the smallest molecular diameter of a hydrocarbon molecule (methane) is 3.8 x 10-10 m. The most frequent way that hydrocarbons migrate through an unfractured undissolved halite bed is if the halite contains impurities that render it locally porous and make it brittle during deformation.

 


 

Seal capacity in flowing pure salt

Halite’s very high ductility and its ability to flow, re-anneal and re-establish lattice bonding by solution creep when subject to stress give it a low susceptibility to fracturing even when it is deforming (Figure 3). This is why cross-salt fault and fracture patterns, as seen in most salt-entraining basins, make the industry considers salt a “crack-stopper” (Figure 4). Worldwide, seismic imaging of halokinetic realms shows that the salt has flowed, while adjacent carbonate and siliciclastics sequences fractured. Halite’s ability to maintain seal integrity under stress, and so prevent the escape of hydrocarbons, reflects a combination of an ability to flow and re-anneal, and the small size of molecular interspaces in its ionically-bonded NaCl crystal lattice (Figure 5; see detailed discussion see Warren, 2016, chapters 6 and 10).


 

This propensity to flow under stress (tendency toward Newtonian flow) is why many laboratory tests and measurements consistently under-represent salt’s flow and subsurface seal integrity responses. Inherently any lab experiment is tied to short time frames of up to weeks or a few years. However, such laboratory tests are likely more relevant to real-world subsurface situations where salt in the vicinity of any wellbore is damaged by the nearby passage of the drill bit and its associated fluids. The applicability of laboratory measurements to real-world subsurface situations is a philosophical quandary inherent to many natural science experiments with a time-related possible-error component. By putting equipment into a natural subsurface salt region, or by removing salt samples from their natural deep subsurface environment to take measures in the lab, or by growing salt crystals in the lab to work on, we always alter things and so get outcomes that can never be 100% accurate with respect to the original unaltered subsurface salt setting. That is, within observational errors, how do we quantify random versus systematic errors when we are always altering the samples and the surrounds via the process of gaining access?

Whether, during catagenesis, buried halite beds that enclose organic intrabeds can release volatiles to sediments outside the salt mass is still a matter of some discussion among organic geochemists. The long-term lack of fracture or pore throats in buried salt beds is why organic-rich intrasalt carbonate or shale laminites tend to be inefficient source rocks in style 1a source rocks (Warren et al., 2011a). Likewise, possible flushing and maturation effects are poorly understood in subsurface situations where encased organic-rich beds are in contact with hydrated salts converting to their anhydrous equivalents (such as gypsum to anhydrite or carnallite to sylvite, mirabilite to thenardite). Loss of water of crystallisation in shallow burial (<0.5km) has the potential to allow organic-rich fluids to escape early as the hydrated salts transform to their anhydrous forms. Usually, such burial transformations are near complete in the first kilometre of burial and so may only allow immature hydrocarbons to escape into adjacent more porous sediments (Hite and Anders, 1991). There they must be stored, mature and remigrate during later burial if they are to act as hydrocarbon source rocks (Warren, 1986). Many intrasalt organic-rich beds survive well into the metamorphic realm and evolve into graphitic quartzites and marbles encased in meta-evaporitic albitites and scapolites.

As a general rule, even as a halite bed fractures, its inherent lack of strength and the consequent ability to flow means any microscale intercrystalline fractures quickly re-anneal by a combination of flow and pressure-solution induced recrystallisation. (Figure 5). Current consensus in the oil and gas industry is that some thin impurity-rich salt beds, interlayered with carrier beds, do leak small amounts of volatiles, but much less efficiently than thicker organic-rich mudstones and shales; whereas organics encased in thicker salt beds probably cannot leak from the unit until the enclosing salt dissolves or natural hydrofracturing occurs (as in the Ara Salt of Oman). Evaporite beds and salt allochthons constitute some of the strongest long-term subsurface barriers to the vertical migration of hydrocarbons in a sedimentary basin both as a seal to hydrocarbons and in COsequestration.

The next and final article in this series on salt leakage will consider; how and where does a salt seal leak in the real world of the subsurface?

References

Beauheim, R. L., and R. M. Roberts, 2002, Hydrology and hydraulic properties of a bedded evaporite formation: Journal of Hydrology, v. 259, p. 66-88.

Downey, M. W., 1984, Evaluating seals for hydrocarbon accumulations: Bulletin American Association of Petroleum Geologists, v. 68, p. 1752-1763.

Ehgartner, B. L., J. T. Neal, and T. E. Hinkebein, 1998, Gas Releases from Salt: SAND98-1354, Sandia National Laboratories, Albuquerque, NM, June 1998.

Fertl, W. H., 1976, Abnormal Formation Pressures: Amsterdam, Elsevier Scientific, 382 p.

Hite, R. J., and D. E. Anders, 1991, Petroleum and evaporites, in J. L. Melvin, ed., Evaporites, petroleum and mineral resources, v. 50: Amsterdam, Elsevier Developments in Sedimentology, p. 477-533.

Hunt, J. M., 1990, Generation and migration of petroleum from abnormally pressured fluid compartments: Bulletin American Association of Petroleum Geologists, v. 74, p. 1-12.

Ter Heege, J. H., J. H. P. De Bresser, and C. J. Spiers, 2005, Dynamic recrystallisation of wet synthetic polycrystalline halite: dependence of grain size distribution on flow stress, temperature and strain: Tectonophysics, v. 396, p. 35-57.

Urai, J. L., Z. Schléder, C. J. Spiers, and P. A. Kukla, 2008, Flow and Transport Properties of Salt Rocks, in R. Littke, ed., Dynamics of complex intracontinental basins: The Central European Basin System, Elsevier, p. 277-290.

Warren, J. K., 1986, Shallow water evaporitic environments and their source rock potential: Journal Sedimentary Petrology, v. 56, p. 442-454.

Warren, J. K., 2011b, Evaporitic source rocks: mesohaline responses to cycles of “famine or feast” in layered brines, Doug Shearman Memorial Volume, (Wiley-Blackwell) IAS Special Publication Number 43, p. 315-392.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released Feb. 2016: Berlin, Springer, 1854 p.

Warren, J. K., A. Cheung, and I. Cartwright, 2011a, Organic Geochemical, Isotopic and Seismic Indicators of Fluid Flow in Pressurized Growth Anticlines and Mud Volcanoes in Modern Deepwater Slope and Rise Sediments of Offshore Brunei Darussalam; Implications for hydrocarbon exploration in other mud and salt diapir provinces (Chapter 10), in L. J. Wood, ed., Shale Tectonics, v. 93: Tulsa OK, AAPG Memoir 93 (Proceedings of Hedberg Conference), p. 163-196.

 

 

 

 

 

 

Seawater chemistry (2 of 2): Precambrian evolution of brine proportions

John Warren - Wednesday, August 26, 2015

We saw in the previous Salty Matters article (part 1 of 2) that ionic proportions of major ions in seawater and oceanic salinity have changed through the Phanerozoic and so influenced the make-up of bittern precipitates once the lower salinity salts (carbonates, gypsum and halite) had precipitated. In the Phanerozoic, seawater was dominantly a Na-K-Mg-Ca-Cl (Ca-rich) brine that changed periodically to a Na-K-Mg-Cl-SO4 (SO4-rich) type, as in the modern ocean. This oscillation across 600 million years forces  number of questions, for example, do similar oscillations in ocean chemistry extend back across the Precambrian? How consistent is the chemistry of the world’s oceans since the early Archean? Does the evaporite evidence in Precambrian sediments support a notion of a primordial reducing atmosphere and/or higher levels of bicarbonate in an early Archean ocean?

Some authors postulate that there have been no significant changes in the major ion proportions in seawater and hence the evaporation mineral series for the past 4 Ga (Morse and Mackenzie, 1998). Others assert that the Archean was dominantly a time of little or no atmospheric oxygen and that ocean waters were reducing anoxic fluids and so sulphate levels were low and sulphide levels high in evaporative marine waters (Krupp et al., 1994). Yet others propose that the bicarbonate to calcium ratio was so high in Archean and Palaeoproterozoic seawater compared to today that all the calcium was used up in widespread abiotic marine aragonite and Mg-calcite precipitates (Sumner and Grotzinger, 2000). In this case trona or nahcolite are likely marine evaporites in the early Archean bitterns (see Figure 1 in part 1). Still others have theorised cyclic changes in oceanic chemistry occurred across much of the Precambrian were similar to those of the Phanerozoic. Such changes were perhaps related to changes in styles and rates of sea floor spreading-hydrothermal circulation in midoceanic ridges (Channer et al., 1997) and the development of tonalitic continents (Knauth, 1998). 

Given that the world's oldest known halites occur in the Bitter Springs Formation in the Amadeus Basin of Australia and that they were deposited some 840 Ma, we can only extend a halite chevron inclusion-based study of ocean chemistry back to that time. These brines were sulphate-depleted, while recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545 Ma) in Pakistan, contains solitary inclusions indicating SO4-rich brines (Kovalevych et al., 2006). This supports a similar late Neoproterozoic ocean chemistry to today, as do proportions derived from primary fluid inclusions from the Neoproterozoic Ara Formation of Oman (ca. 545 Ma). It seems that  SO4-rich seawater existed during latest Neoproterozoic time. In contrast while recrystallised halite from the somehat older Bitter Springs Formation contains brine inclusions that are entirely Ca-rich, implying ambient basin brines and the mother seawater were Ca-rich some 830-840 Mas. These combined data, supported by the timing of aragonite and calcite seas, as preserved in various marine carbonates, suggest that during the Neoproterozoic, significant oscillations of the chemical composition of marine brines, and seawater occurred over the last 250 million years of the NeoProterozoic, and that the end-members were similar to those of the Phanerozoic oceans. It seems that Ca-rich seawater dominated for a substantial period of Late Precambrian time (more than 200 Ma) from 850 Ma, until some 650 Ma, this was replaced by SO4-rich seawater, returning to Ca-rich seawater at 530 Ma. 

The detail for much of the remaineder of the Precambrian back to 4 Ga is far less precise than when modelling inclusion chemistries based on actual halites. The oldest documented chevron halite is 850Ma and the oldest bedded anhydrite is 1.2Ga, beyond that, only evaporite pseudomorphs are available to study. So, beyond the 850 Ma record established by halite inclusions in the Bitter Springs Fm., can other Precambrian evaporites especially the calcium sulphates with a record that extends back patchily to the Mesoproterozoic, give indirect clues as to a chemical scenario for the world’s paleo-oceans and brine?

 

Pseudomorphs, especially of halite hoppers, occur in marine rocks as old as Archean, but are far more common, as are the actual salts, in Proterozoic strata (Figure 1; Warren, 2016). Halite or its pseudomorphs characterise areas of widespread marine chemical sedimentation from the Archean to the present. CaSO4 pseudomorph distribution is more enigmatic. In the 1980s and 1990s, the oldest documented CaSO4 pseudomorphs were thought to cm-sized growth-aligned barytes and cherts in 3.45 Ga metasediments in the Pilbara/North Poleregion of Western Australia. They were interpreted as replacing primary bottom-nucleated gypsum (Figure 2; Barley et al., 1979; Lowe, 1983; Buick and Dunlop, 1990). These barytes and cherts occur in volcaniclastics in association with what are possibly the world’s oldest stromatolites (Hofmann et al., 1999; Allwood et al., 2007). Similar growth-aligned baryte crystals, which initially were also interpreted as likely primary gypsum pseudomorphs, occur in the Nondweni greenstones in South Africa, some 3.4 Ga (Wilson and Versfeld, 1994).

 

Sequences in both regions are now completely silicified or barytised. At the time they were first documented, the recognition of what were considered shallow-water Early Archean gypsum pseudomorphs at North Pole, Pilbara Craton, caused a re-evaluation of models of a totally reducing Archean atmosphere (Dimroth and Kimberley, 1975; Clemmey and Badham, 1982). The presence of free sulphate in surface brines of the Archean world was thought to imply an at least locally oxygenated hydrosphere. Gypsum precipitating in Archean ocean waters also meant calcium levels in the ocean waters were in excess of bicarbonate, as is in the modern oceans. The presence of free-standing gypsum on the seafloor is incompatible with any model of the Early Archean ocean as a “soda lake.”

However, in both the Pilbara and the South African sequences there are no actual calcium sulphate evaporites preserved, only growth-aligned crystal textures, now preserved as baryte or chert. Textures in baryte ore from Frasnian sediments in Chaudfontaine, Belgium, are near identical to those observed at North Pole, Australia. The Belgian barytes are primary shallow subsea-bottom precipitates with no precursor mineral phase (Figure 2 inset; Dejonghe, 1990). Some workers in the Pilbara feel that the growth-aligned Archean baryte in this region is also a primary seafloor precipitate, formed in the vicinity of hydrothermal vents (Vearncombe et al., 1995; Nijman et al., 1999; Runnegar et al., 2001). As such, it is not secondary after gypsum. A similar hydrothermal discharge model has been developed for aligned barytes in the Barberton Greenstone belt (de Ronde et al., 1994, 1996). 

Based on this more recent analysis, levels of Archean sulphate in the world ocean were probably less than a few percent of the current levels and probably remained so until the evolution of a widespread oxygen-producing biota into the Proterozoic (Figures 3, 4; Habicht and Canfield, 1996; Kah et al., 2004). Barium sulphate is highly insoluble in modern oxygenated seawater. To carry large volumes of barium or sulphur (as sulphide) in seawater solution to the precipitation site required anoxic conditions. If the aligned baryte crystals are primary, their formation still requires sulphate to be locally present on the seafloor, at least in the vicinity of the depositional site. A possible source for local sulphate production in the shallow waters that characterised the North Pole site was shortwave ultraviolet photoxidation of volcanic SO2, indicating an inorganic association (Runnegar et al., 2001). Within barytes in the same 3.47-Ga-old barytes there are microscopic sulphides. These sulphide inclusions show a d34S of 11.6‰, possibly indicating microbial sulphate reduction with H2 as electron donor in what was an anoxic seafloor (Canfield et al. 2004; Shen et al., 2009).

According to Nijman et al. (1999) the occurrence of the North Pole baryte in sedimentary mounds atop growth faults meant sulphate was locally derived via boiling of escaping hydrothermal vent waters enriched in Ba, Si and sulphide. As these hydrothermal waters vented beneath marine water columns perhaps 50 metres deep, they boiled or violently degassed. Consequent mixing with normally stratified seawater, caused instantaneous oxidization of sulphide into sulphate that then, on cooling, combined with the Ba to precipitate as growth-aligned baryte crystals on the seafloor. Conflicting notions (replaced gypsum versus primary baryte) mean that at this stage of our understanding, the bedded baryte evidence cannot be reliably used to support an evaporite paragenesis of gypsum and so infer an Archean ocean with ionic proportions similar to those of today.

Archean and Proterozoic distributions of gypsum have been further complicated by the misidentification of primary aragonite splays and pinolitic siderite marbles as gypsum replacements (Warren 2016; Chapter 15). When these misidentifications are removed from the record it is obvious that calcium sulphate precipitating directly from Archean seawater to form widespread beds did not occur, and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth h istory. In contrast to gypsum, halite pseudomorphs are found throughout the Precambrian (Figure 1;e.g. Boulter and Glover, 1986). 

Grotzinger and Kasting (1993) argue that high levels of atmospheric CO2 meant HCO3/Ca ratios were much higher in the Archean and the Palaeoproterozoic oceans than today. All the calcium in seawater was deposited as marine cementstones and other alkaline earth precipitates well before bicarbonate was depleted and there was no Ca left over to precipitate as gypsum. The early Archean ocean was perhaps a Na–Cl–HCO3 sea, and not the Na–Cl ocean of today (Kempe and Degens, 1985; Maisonneuve, 1982). This early Archean hydrosphere had a chemistry similar to that found in modern soda lakes like Lake Magadi and Lake Natron (pathway I brines) and hence the term “soda-lake oceans” (see Figure 1 in part 1) This rather different marine brine chemistry would have precipitated halite and trona/nahcolite, not halite/gypsum. It probably meant that if gypsum did ever precipitate from Archean seawater it did so only in minor amounts well after the onset of halite precipitation. Excessive sodium in the ocean may help explain the ubiquity of stratiform albitites in much of the Archean. They would have formed throughout the marine realm as early diagenetic replacements of labile volcaniclastics/zeolites in volcanogenic/greenstone terranes).

A case for nahcolite (NaHCO3) as a primary evaporite, along with halite, in the 3.42 Ga rocks of the Barberton greenstone belt was documented by Lowe and Fisher-Worrell (1999). Sugitani et al. (2003) reported silicified nahcolite (the high CO2 form of sodium carbonate salts) in ≈3.2 Ga rocks in the northern part of the Eastern Pilbara block, Western Australia. Coarse, upward-radiating, silicified evaporite crystals in the ca. 3.47–3.46 Ga Strelley Pool Chert (Lowe, 1983) show the same habit, geometry, and environmental setting as nahcolite in the Barberton belt and also probably represent silicified NaHCO3 precipitates (Lowe and Tice, 2004).


Marine nahcolite in the 3.5-3.2 Ga sedimentary record is thought to be evidence of surface temperatures around 70±15°C (Figures 3b, c, 4; Lowe and Tice, 2004). Contemporary early Archean nahcolite (NaHCO3) as a primary evaporitic mineral in a very aggressive weathering regime, in the absence of land vegetation, is best explained by a mixed CH4 and CO2 atmospheric greenhouse. CH4/CO2 ratios were <<1 and pCO2 was at least 100-1000 times the present value, perhaps as high as several bars (Kaufman and Xiao, 2003). The formation of large areas of continental crust at 3.2-3.0 Ga, including the Kaapvaal and Pilbara cratons, resulted in the gradual depletion of atmospheric CO2 through weathering and a lack of marine nahcolite since the early Archean. By 2.9-2.7 Ga, declining pCO2 was associated with climatic cooling and siderite-free soils. 

Transitory CH4/CO2 ratios of ~1 may have resulted in the sporadic formation of organic haze from atmospheric CH4, and are reflected in one or more isotopic excursions involving global deposition of abnormally 13C-depleted organic carbon in sediments of this age. Surface temperatures of <60°C after 2.9 Ga may have allowed an increase in the distribution and productivity of oxygenic photosynthetic microbes (and a decrease in sulphur dependent thermophiles). Eventual lowering of newly formed continental blocks by erosion, reduced loss of atmospheric CO2 due to weathering, and continued long-term tectonic recycling of CO2 resulted in rising pCO2 and decreasing CH4/CO2 ratios in the later Archean and eventual re-establishment of a mainly CO2 greenhouse. Similar events may have been repeated in the latest Archean and earliest Proterozoic, but gradually rising production of O2 effectively kept CH4/CO2 ratios to <<1.

 

By 2.2-2.0 Ga and perhaps as early as 2.5 Ga, reliable examples of pseudomorphs after primary marine-sourced calcium sulphate first appear in the rock record, but aside from the Karelian beds associated with the Lomagundi Event (LE), widespread stratiform sulphate beds of anhydrite do not appear until 1.2 Ga (Figure 5a). Undeniable CaSO4 nodular and lenticular pseudomorphs are widespread in latest NeoArchean of South Africa and Palaeoproterozoic to Mesoproterozoic sediments of the McArthur Basin, Northern Territory, Australia, and in rocks of Great Slave Lake in northern Canada. For example, in the Malapunyah Formation (1.65 Ga) of the Northern Territory, Australia, the outer portions of numerous decimetre to metre-diameter silicified anhydrite nodules still retain outlines of felted anhydrite laths (pers. obs). The oldest reliable sulphate pseudomorphs after anhydrite and gypsum in Australia come from Palaeoproterozoic cherts in the 2.0-2.2 Ga Bartle Member of the Killara Formation, western Australia (Pirajno and Grey, 2002). These cherts locally retain small amounts of anhydrite (verified by XRD, as well as appearing as highly birefringent flecks in thin sections). Other widespread but younger sulphate pseudomorphs occur in the 1.2 Ga Amundsen Basin in the Canadian Arctic Archipelago. Actual CaSO4 beds outcrop in the 1.2 Ga Society Cliff Formation in Baffin and Bylot Islands of the Canadian Archipelago (Kah et al., 2001, 2004). Sulphate evaporite pseudomophs and nodules in all these Neoproterozoic basins are hosted in sedimentary layers up to tens of metres thick and with lateral extents measured in hundreds of square kilometres. All were laid down in shallow marine, coastal, and alluvial environments under an increasingly oxygenated Meso- to Neoproterozoic atmosphere (Jackson et al., 1987; Walker et al., 1977). After passing from the Archean, by the Mesoproterozoic the hydrosphere contained free sulphate and Ca/HCO3 ratios were lower, leading to a decrease in molar-tooth, herringbone and other carbonate textures indicative of widespread inorganic calcium carbonate saturation in shallow oceanic waters (Figure 6). However, oceanic mother brines for these now-widespread calcium-sulphate evaporites were largely H2S rich with only moderate levels of oxygen in the atmosphere until some 800 Ma (Figure 3a).

The work of Kah et al. (2004) shows that prior to 2.2 Ga, when oxygen began to accumulate in the Earth’s atmosphere, sulphate concentrations in the world’s oceans were low, <1 mM and possibly <200 μM (Figure 5). By 0.8 Ga, oxygen and thus sulphate levels had risen significantly. Sulphate levels were between 1.5 and 4.5 mM, or 5–15% of modern values, for more than a billion years after initial oxygenation of the Earth’s biosphere some 2.2-2.4 Ga and mid -ocean depth waters were anoxic for most of that time (Brocks et al., 2005). Marine sulphate concentrations probably remained low, no more than 35% of modern values, for nearly the entire Proterozoic. A significant rise in biospheric oxygen, and thus oceanic sulphate, may not have occurred until the latest Neoproterozoic (0.54 Ga), just before the Cambrian explosion, when sulphate levels may have reached 20.5 mM, or 75% of present day levels. This is a time when thick sulphate platforms first characterised the salt basins of Oman, prior to that most actual calcium sulphate is in the form of nodules or relatively thin beds.

In a refinement of the sulphate model, Bekker and Holland (2012) note that free sulphate bottom-nucleated sulphate evaporites and not just pseudomorphs were present during the Lomagundi Event (2.22 to 2.06 Ga), and then became relatively scarce once more until some 1.2 Ga. For example, there is a 200 m thick stratigraphic interval of sulphate evaporites of Lomagundi-age, preserved in a shallow-water open-marine siliciclastic and carbonate succession (Lower Jatuli informal group) of Karelia, Russia (Morozov et al., 2010). The Lomagundi Event defines the most extreme and longest lasting isotope excursion of carbon in the world’s marine carbonate record. Bedded gypsum pseudomorphs in the Malmani Group some 2.5 Ga (Gandin and Wright, 2007; Eriksson and Warren, 1983) implies that elevated oceanic sulphate levels that typify the Lomagundi Event may have extended a little further back in time, at least locally (Figure 5).

At the same time as the Lomagundi event, the average ferric iron to total iron (expressed as Fe2O3/Fe|Fe2O3|) ratio of shales increased dramatically. At the end of the Lomagundi Event (LE), the first economic sedimentary phosphorites were deposited, and the carbon isotope values of marine carbonates returned to ≈0.0‰VPDB (Figure 2.50). Thereafter marine sulphate evaporites and phosphorites again became scarce, while the average Fe2O3/Fe|Fe2O3| ratio of shales decreased to values intermediate between those of the Archean and Lomagundi-age shales.

In support of this notion of an “oxygen overshoot,” sulphur isotope work by Reuschel et al. (2012) on the 2.1 Ga dolomitic Tulomozero Fm, which entrains abundant CaSO4 pseudomorphs, concluded that there was a minimum level of 2.5 mM sulphate in the world ocean at that time (Figure 5).

Bekker and Holland (2012) argue the short appearance of sulphate evaporites in Logamundi and the other associated events can be regarded as a ca. 200 Ma “glitch” in the gradual oxidation of the atmosphere–ocean system. It was driven by a positive feedback between the rise in atmospheric O2, the oxidation of pyrite in rocks undergoing weathering, a decrease in the pH of soil and ground water, and an increase in the phosphate flux to the oceans. This sequence led to a major increase in the rate of organic matter burial, a rise in atmospheric oxygen, a large increase in the 13C value for marine carbonates, the deposition of marine evaporites containing gypsum and anhydrite, and the formation of the first commercially important phosphorites. The end of the LE was probably brought about by the weathering of sediments deposited during the LE.

In yet another proposal of hydrosphere-atmosphere evolution, Huston and Logan (2004) argue that the presence of relatively abundant bedded sulphate deposits before 3.2 Ga (as the contentious Archean barytes and chert mentioned earlier) and after 1.8 Ga (as CaSO4 salts), and the peak in banded iron formation abundance between 3.2 and 1.8 Ga, and the aqueous geochemistry of sulphur and iron, when taken together suggest that the redox state and the abundances of sulphur and iron in the hydrosphere varied widely during the Archean and Proterozoic. They propose a layered hydrosphere prior to 3.2 Ga in which sulphate was enriched in an upper oceanic layer, whereas the underlying layer was reduced and sulphur-poor. The sulphate was produced by atmospheric photolytic reactions with volcanic gases in a reducing atmosphere. Mixing of the upper and lower water masses allowed the banded barytes to form prior to 3.2 Ga and created an ocean chemistry where nahcolite was a marine evaporite. Between 3.2 and 2.4 Ga, decreasing volcanogenesis and sulphate reduction removed sulphate from the upper layer, producing broadly uniform, reduced, sulphur-poor and iron-rich oceans.

Whatever the origin of the early Archean baryte and chert, around 2.2 - 2.4 Ga, as a result of increasing atmospheric oxygenation, the flux of sulphate into the hydrosphere by oxidative weathering was greatly enhanced, producing layered oceans, with sulphate-enriched, iron-poor surface waters and reduced, sulphur-poor and iron-rich bottom waters. Gypsum evaporites were increasingly likely as marine precipitates. The rate at which this process proceeded varied between basins depending on the size and local environment of the basin. By 1.8 Ga, the hydrosphere was relatively sulphate-rich and iron-poor throughout. Gypsum was now a widespread marine evaporite. Variations in sulphur and iron abundances suggest that the redox state of the oceans was buffered by iron before 2.4 Ga and by sulphur after 1.6 to 1.8 Ga (Figure 1).

Gypsum in combination with halite was the marine evaporite association from then until now. Seawater was predominantly a Na-Cl±SO4 ocean. Neoproterozoic stratiform sulphates along with widespread halokinetic halite, occur in the Bitter Springs Formation of the Amadeus basin, central Australia (0.8 Ga), its equivalents in the Officer Basin, the Callana beds of the Flinders Ranges and the younger Infracambrian salt basins of the Arabian (Persian) Gulf (≈0.545 Ga; Wells, 1980; Cooper, 1991; Mattes and Conway-Morris, 1990; Edgell, 1991).


The transition to calcium sulphate textures in evaporite pseudomorphs mirrors a marked change in the style of marine carbonates that began around 2.2 to 2.3 Ga when herringbone calcite and precipitated carbonate beds become much less common and the precipitation mode shifted from the seafloor to the water column (Figure 6; Sumner and Grotzinger, 1996, 2000). The boundary also corresponds to the “rusting” of the oceans when oxygen levels became high enough to precipitate widespread banded iron deposits on the seafloor. Microdigitate stromatolites cross this boundary with little effect, suggesting the marked decrease in dissolved iron exerted little influence on them.

The relative scarcity of actual Pre-Phanerozoic salts, not pseudomorphs, especially in the Archean has been used by some to argue that conditions were less favourable for widespread evaporite deposition in the early Precambrian (Cloud, 1972). Others, myself included, feel that the relative scarcity of preserved evaporites in older sequences reflects the greater likelihood of fluid flushing, evaporite dissolution and metasomatism in progressively older rocks. It is likely that oceanic calcium-sulphate evaporites were less common in the Archean, and that sodium carbonates mixed with halite were dominant evaporite salts in the seawater-fed saline giants in appropriate tectonic seepage depressions of the Early Archean. But widespread evaporite deposition from sodium-dominated brines did occur throughout the Archean in large drawdown basins isolated from a surface connection with the ocean. A paucity of preserved bedded evaporite salts in the Precambrian reflects an increased probability of partial or complete evaporite dissolution, remobilization and metasomatism with increasing geological age (see meta-evaporite).

In what is an inclusion study of oldest actual halite, Spear et al., (2014) characterised marine brine chemistry using brine inclusions in the 830 Ma salt of the Browne Formation, Officer Basin, Australia (equiv. to Bitter Springs Fm.). It seems that concentrations of the major ions in these inclusions, except K+ and possibly SO42−, fall within the known range of Phanerozoic seawaters. This ananlysis suggests that mid-Neoproterozoic marine sulphate concentrations were lower (≈90%) than modern values. By the terminal Neoproterozoic, fluid inclusions in halite and evaporite mineralogy from the Khewra Salt of Pakistan and the Ara salt in Oman indicate seawater sulphate levels had risen significantly, to 50%-80% of modern concentrations, which parallels increases in atmospheric and oceanic oxygen.

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Seawater chemistry (1 of 2): Potash bitterns and Phanerozoic marine brine evolution

John Warren - Tuesday, August 11, 2015

The significance of evaporites as indicators of the chemical evolution of seawater across time and in relation to potash bitterns is considered in the next two Salty Matters articles. This article focuses on Phanerozoic seawater chemistry, where actual salts are widespread and the proportions of potash bittern salts are a useful pointer to the chemical makeup of the mother brine. Throughout both articles, the term “lower salinity” refers to marine brines with salinities between one and ten times that of ambient seawater. The second article considers seawater chemistry based on Precambrian evaporites, where much of the evidence of mother brine composition comes from salt pseudomorphs, rather than remnants of actual salts. In the second article we shall see that atmospheric conditions in the Early Precambrian were reducing and hotter than today, so that seawater was more saline, warmer, anoxic, with higher levels of calcium and bicarbonate compared to Phanerozoic seawater. Gypsum (CaSO4.2H2O), which requires free sulphate, was a rare precipitate during concentration of Archean seawater. Changing atmospheric proportions of CO2, CH4 and O2 meant sodium carbonate salts were significant lower-salinity early Archean marine-brine precipitates. Yet today, sodium carbonate salts, such as trona (NaHCO3.Na2CO3), nahcolite (NaHCO3) and shortite (2CaCO3.Na2CO3) cannot precipitate from a brine with the ionic proportions of modern seawater. The presence of sodium carbonate salts in any evaporite succession across the Phanerozoic is a reliable indicator of a nonmarine mother brine (Figure 1).


A Phanerozoic dichotomy: evolving marine potash bitterns

Consistently across the last 550 million years, halite and gypsum (mostly converted to anhydrite in the subsurface) are the dominant lower-salinity marine salts. But potash-bittern evaporite associations plotted across the same time framework define two end-members (Figure 1):

1) Sulphate-enriched potash deposits, with ores typically composed of halite (NaCl) with carnallite (MgCl2.KCl.6H2O) and lesser sylvite (KCl), along with varying combinations of MgSO4 salts, such as polyhalite (2CaSO4.MgSO4.K2SO4.H2O), kieserite (MgSO4.H2O) kainite (4MgSO4.4KCl.11H2O) and langbeinite (2MgSO4. K2SO4); and

2) Sulphate-depleted potash deposits are composed of halite with sylvite and carnallite, and entirely free or very poor in the magnesium-sulphate salts. The sulphate-depleted association typifies more than 65% of the world’s exploited Phanerozoic potash deposits. Sylvite ores with this association have properties that are easier to process cheaply (Warren 2016; see also blog 4 of 4 in the Salty Matters Danakhil articles).

The sulphate-enriched group of ancient potash salts contains a bittern mineral suite predicted by the evaporation and backreaction of seawater with proportions similar to modern marine brine. In contrast, the sulphate-depleted group of bittern salts must have precipitated from Na-Ca-Mg-K-Cl brines with ionic proportions quite different from that of concentrated modern seawater. The separation between the two bittern associations is defined by brine evolution across the gypsum divide. That is, once gypsum (CaSO4.2H2O) and halite (NaCl) have precipitated in the lower salinity spectrum, are the remaining brines enriched in sulphate or calcium (Figure 1)? The greater suitability for potash utilisation of the sulphate depleted bitterns makes understanding and hence predicting occurrences of the sulphate-depleted association in time and space a useful first-order potash exploration tool.

Why the dichotomy?

In the older literature dealing with Phanerozoic salt chemistry, MgSO4-depleted potash evaporites were often explained as diagenetically-modified marine evaporite brines, thought to result from backreactions during burial diagenesis of normal marine waters (Borchert, 1977; Dean, 1978; Wilson and Long, 1993). If so, then the mother seawater source across the Phanerozoic had ionic proportions like those of today, but diagenetically altered via; a) dolomitisation, b) sulphate-reducing bacterial action, c) mixing of brines with calcium bicarbonate-rich river water, or d) rock-fluid interaction during deep burial diagenesis. As another option, Hardie (1990) suggested MgSO4-depleted potash bitterns formed by the evaporative concentration of sulphate-depleted nonmarine inflow waters seeping into an evaporite basin via springs and faults. Such springs were sourced either from CaCl2-rich hot hydrothermal brines or via cooling of deep basinal brines. Such fault-fed deeply-circulating CaCl2 brines source the various springs feeding the Dead Sea, the Qaidam Basin, the Salton Sea and the Danakil Depression. In all these cases, the elevated salinities of inflow waters are related, at least in part, to the dissolution of buried evaporites. Upwelling of brines in these regions is driven either by thermally-induced density instabilities, related to magma emplacement, or by the creation of tectonically-induced topographic gradients that force deeply-circulated basinal brines to the surface. Ayora et al. (1994) demonstrated that such a deeply-circulating continental Ca–Cl brine system operated during deposition of sylvite and carnallite in the upper Eocene basin of Navarra, southern Pyrenees, Spain.

Today, a more widely accepted explanation for SO4-enriched versus SO4-depleted Phanerozoic potash bitterns, is that seawater chemistry has evolved across deep time. Background chemistry of the marine potash dichotomy is simple and can be related to brine evolution models published by Hardie more than 30 years ago (Hardie, 1984). He found that the constituent chemical proportions in the early stages of concentration of any marine brine largely controls the chemical makeup of the subsequent bittern stages. These ionic proportions control how a brine passes through the lower salinity CaCO3 and gypsum divides (Figure 1). That is, a marine brine’s bittern make-up is determined by the ionic proportions in the ambient seawater source. It determines the carbonate mineralogy during the precipitation of relatively insoluble evaporitic carbonates (aragonite, high-magnesium calcite, or low-magnesium calcite) which in turn controls its constituent chemistry as it attains gypsum saturation. These two stages are called the CaCO3 and gypsum divides. Hence, the chemical passage of a bittern is controlled by the ionic proportions in the original ambient seawater. The CaCO3 divide kicks in when a concentrating seawater brine attains a salinity around twice that of normal seawater (60‰). The gypsum divide occurs when brine concentrations are around 4-5 times that of normal seawater (140-160‰). Normal seawater has a salinity around 35-35‰ and the various potash bittern salts precipitate when concentrations are around 40-60 times that of the original seawater (Figure 2 – lower part).

As seawater concentrates and calcium carbonate mineral(s) begin to precipitate at the CaCO3 divided then, depending on the relative proportions of Ca and HCO3 in the mother seawater, either Ca is used up, or the HCO3 is used up. If the Ca is used up first, an alkaline brine (pH>10) forms, with residual CO3, along with Na, K, SO4 and Cl, but no remaining Ca (Figure 1). With ongoing concentration this brine chemistry will then form sodium bicarbonate minerals, it cannot form gypsum as all the Ca is already used up. Such an ionic proportion chemistry likely defined oceanic waters in the early Archaean but is not relevant to seawater evolution in the Proterozoic and Phanerozoic, as evidenced by widespread gypsum (anhydrite) or pseudomorphs in numerous post-Archean marine-evaporite basins. At higher concentrations, early Archean marine brines would have produced halite and sylvite bittern suites, but with no gypsum or anhydrite (Figure 1).

If, instead, HCO3 is used up during initial evaporitic carbonate precipitation, as is the case for all Phanerozoic seawaters, the concentrating brine becomes enriched in Ca and Mg, and a neutral brine, depleted in carbonate, is formed. Then the ambient Mg/Ca ratio in a concentrating Phanerozoic seawater will control whether the first-formed carbonate at the CaCO3 divide is aragonite (Mg/Ca>5) or high-magnesium calcite (2>Mg/Ca>5), or low-Mg calcite(Mg/Ca>2). The latter Mg/Ca ratio is so low it is only relevant to concentrating Cretaceous seawaters. Elevated Mg/Ca ratios favouring the precipitation of aragonite over high Mg-calcite typify modern marine seawater brines, which have Mg/Ca ratios that are always >5 (Figure 2). At lower salinities, modern marine brines are Na-Cl waters that with further concentration and removal of Na as halite evolve in Mg-SO4-Cl bitterns (Figure 2)

 

The next chemical divide reached by concentrating marine Phanerozoic brines (always depleted in HCO3 at the carbonate divide) occurs when gypsum precipitates at around 4-5 times the concentration of the original seawater (Gypsum divide in Figure 1). As gypsum continues to precipitate, either the Ca in the brine is used up, or the SO4 in the brine is used up. If the Ca is depleted, a calcium-free brine rich in Na, K, Mg, Cl, and SO4 will be the final product and the diagnostic bittern minerals will include magnesium sulphate minerals. This is the pathway followed by modern seawater bitterns. If, however, the sulphate is used up via gypsum precipitation, the final brine will be rich in Na, K, Mg, Ca, and Cl. Such a sulphate-depleted brine precipitates diagnostic potassium and magnesium chloride minerals such as sylvite and carnallite. If calcium-chloride levels are very high, then diagnostic (but uncommon) minerals such as tachyhydrite (CaCl2.2MgCl2.12H2O), and antarcticite (CaCl2.6H2O) can precipitate from this brine. But both these assemblages contain no sulphate bittern minerals, making potash processing relatively straightforward (Warren, 2016). In Phanerozoic marine salt assemblages, tachyhydrite, which is highly hygroscopic, is present in moderate quantities only in Cretaceous (Aptian) marine sylvite-carnallite associations in the circum-Atlantic potash basins and the Cretaceous (Albian) Maha Sarakham salts of Thailand, along with its equivalents in Laos and western China. The CaCl2-entraining bittern mineral assemblages of these deposits imply ionic proportions of Cretaceous seawater differ from those of today.

Inclusion evidence

Based on a study of brine inclusion chemistry preserved in halite chevrons, from the Early Cretaceous (Aptian, 121.0–112.2 Ma) of the Sergipe Basin, Brazil, the Congo Basin, Republic of the Congo, and the Early to Late Cretaceous (Albian to Cenomanian, 112.2–93.5 Ma) of the Khorat Plateau, Laos and Thailand, Timofeeff et al. (2006) defined a very different chemical makeup for Cretaceous seawater, compared to that of today. Brine proportions in the fluid inclusions in these halites indicate that Cretaceous seawaters were enriched several fold in Ca, depleted in Na and Mg, and had lower Na/Cl, Mg/Ca, and Mg/K ratios compared to modern seawater (Table 1). 


Elevated Ca concentrations, with Ca>SO4 at the gypsum divide, allowed Cretaceous seawater to evolve into Mg–Ca–Na–K–Cl brines lacking measurable sulphate. Aptian seawater was extreme in its Ca enrichment, more than three times higher than present day seawater, with a Mg/Ca ratio of 1.1–1.3. Younger, Albian-Cenomanian seawater had lower Ca concentrations, and a higher Mg/Ca ratio of 1.2–1.7. Cretaceous (Aptian) seawater has the lowest Mg/Ca ratios so far documented in any Phanerozoic seawater from fluid inclusions in halite, and lies well within the range chemically favourable for precipitation of low-Mg calcite ooids and cements in the marine realm.


Likewise, a detailed analysis of the ionic make-up of Silurian seawater using micro-inclusion analysis of more than 100 samples of chevron halite from various Silurian deposits around the world was published by Brennan and Lowenstein (2002), clearly supports the notion that ionic proportions in the world’s Silurian oceans were different from those of today (Figure 3). Samples were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and the Early Silurian in the Canning Basin (Australia) in the Mallowa Salt of the Carribuddy Group. The Silurian ocean had lower concentrations of Mg, Na, and SO4, and much higher concentrations of Ca relative to the ocean’s present-day composition (Table 1). Furthermore, Silurian seawater had Ca in excess of SO4. Bittern stage evaporation of Silurian seawater produced KCl-type potash minerals that lack the MgSO4-type late stage salts formed during the evaporation of present-day seawater and allowed sylvite as a primary precipitate. In a similar fashion, work by Kovalevych et al. (1998) on inclusions in primary-bedded halite from many evaporite formations of Northern Pangaea, and subsequent work using micro-analyses of fluid inclusions in numerous chevron halites (Lowenstein et al., 2001, 2003), shows that during the Phanerozoic the chemical composition of marine brines has oscillated between Na-K-Mg-Ca-Cl and Na-K-Mg-Cl-SO4 types. The former does not precipitate MgSO4 salts when concentrated, the latter does (Figure 3). A recent paper by Holt et al. (2014), focusing on chevron halite inclusions from various Carboniferous evaporite basins, further refined the transition from the Palaeozoic CaCl2 high Mg-calcite sea into a MgSO4-enriched aragonite ocean of the Permo-Carboniferous, so showing CaCl2 oceanic chemistry (and sylvite-dominant bitterns) extend somewhat further across the Palaeozoic than previously thought (Figure 4).

 

More recent work has shown varying sulphate levels in the Phanerozoic ocean rather than Mg/Ca variations are perhaps more significant in controlling aragonite versus calcite at the CaCO3 divide and the associated evolution of MgSO4-enriched versus MgSO4-depleted bittern suites in ancient evaporitic seaways than previously thought. Bots et al. (2011) found experimentally that an increase in dissolved SO4 decreases the Mg/Ca ratio at which calcite is destabilized and aragonite becomes the dominant CaCO3 polymorph in an ancient seaway (Figure 5). This suggests that the Mg/Ca and SO4 thresholds for the onset of ancient calcite seas are significantly lower than previous estimates and that Mg/Ca levels and SO4 levels in ancient seas are mutually dependent. Rather than variations in Mg/Ca ratio in seawater being the prime driver of the aragonite versus calcite ocean chemistries across the Phanerozoic, they conclude sulphate levels are an equally important control.


Mechanisms

There is now convincing inclusion-based evidence that the chemistry of seawater has varied across the Phanerozoic from sulphate-depleted to sulphate-enriched, what is not so well understood are the various worldscale processes driving the change (Figure 4). Spencer and Hardie (1990) and Hardie (1996) argued that the level of Mg in the Phanerozoic oceans has been relatively constant across time, but changes in the rate of seafloor spreading have changed the levels of Ca in seawater. This postulate is also supported in publications by Lowenstein et al. (2001, 2003). Timing of the increase of Ca in the world’s oceans was likely synchronous with a decrease in the SO4 ion concentration, which at times was as much as three times lower than the present.

Simple mixing models show that changes in the flux rate of mid-oceanic hydrothermal brines can generate significant changes in the Mg/Ca, Na/K and SO4/Cl ratios in seawater (Table 1). Changes of molal ratios in seawater have generated significant changes in the type and order of potash minerals at the bittern stage. For example, Spencer and Hardie’s (1990) model predicts that an increase of only 10% in the flux of mid-ocean ridge hydrothermal brine over today’s value would create a marine bittern that precipitates sylvite and calcium-chloride salts, as occurred in the Cretaceous instead of the Mg-sulphate minerals expected during bittern evaporation of modern seawater. Such Ca-Cl potash marine bitterns correspond to times of “calcite oceans” and contrast with the lower calcium, higher magnesium, higher sulphate “aragonite oceans” of the Permo-Triassic and the Neogene (Figure 3; Hardie, 1996; Demicco et al., 2005).

Ocean crust, through its interaction with hydrothermally circulated seawater, is a sink for Mg and a source of Ca, predominantly via the formation of smectite, chlorite, and saponite via alteration of pillow basalts, sheeted dykes, and gabbros (Müller et al., 2013). Additional removal of Mg and Ca occurs during the formation of vein and vesicle-filling carbonate and carbonate-cemented breccias in basalts via interaction with low-temperature hydrothermal fluids. Hence, changing rates of seafloor spreading and ridge length likely influenced ionic proportions in the Phanerozoic ocean and this in turn controlled marine bittern proportions.

According to Müller et al., 2013, hydrothermal ocean inputs are and the relevant ionic proportions in seawater are driven by supercontinent cycles and the associated gradual growth and destruction of mid-ocean ridges and their relatively cool flanks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge flank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fluid flux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcification from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their flanks along the Pacific rim.

Holland et al. (1996), while agreeing that there are changes in ionic proportion of Phanerozoic seawater and that halite micro-inclusions preserve evidence of these changes, recalculated the effects of changing seafloor spreading rates on global seawater chemistry used by Hardie and others. They concluded changes in ionic proportions from such changes in seafloor spreading rate were modest. Instead, they pointed out that the composition of seawater can be seriously affected by secular changes in the proportion of platform carbonate dolomitised during evaporative concentration, without the need to invoke hydrothermally driven changes in seawater composition. In a later paper, Holland and Zimmermann (2000) suggest changes in the level of Mg in seawater were such that the molar Mg/Ca ratio of the more saline Palaeozoic global seawater (based on dolomite volume) was twice the present value of 5.

Using micro-inclusion studies of halites of varying ages, Zimmermann (2000a, b) has proposed that the evolving chemistry of the Phanerozoic ocean is more indicative of changing volumes of dolomite than it is of changes in the rates of seafloor spreading . Using halite inclusions, she showed that the level of Mg in seawater has increased from ≈38 mmol/kg H2O to 55 mmol/kg H2O in the past 40 million years (Figure 6). This increase is accompanied by an equimolar increase in the level of oceanic sulphate. Over the longer time frame of the Palaeozoic to the present the decrease in Mg/Ca ratio corresponds to a shift in the locus of major marine calcium carbonate deposition from Palaeozoic shelves to the deep oceans, a change tied to the evolution of the nannoplankton. Prior to the evolution of foraminifera and coccoliths, some 150 Ma, the amount of calcium carbonate accumulating in the open ocean was minimal. Since then, a progressively larger portion of calcium carbonate has been deposited on the floor of the deep ocean. Dolomitization of these deepwater carbonates has been minor.

 

In a study of boron isotopes in inclusions in chevron halite, Paris et al. (2010) mapped out the changes in marine boron isotope compositions over the past 40 million years (Figure 7). They propose that the correlation between δ11BSW and Mg/Ca reflects the influence of riverine fluxes on the Cenozoic evolution of oceanic chemical composition. Himalayan uplift is a major tectonic set of events that probably led to a 2.5 times increase of sediment delivery by rivers to the ocean over the past 40 m.y. They argue that chemical weathering fluxes and mechanical erosion fluxes are coupled so that the formation of the Himalaya favoured chemical weathering and hence CO2 consumption. The increased siliciclastic flux and associated weathering products led to a concomitant increase in the influx levels of Mg and Ca into the mid to late Tertiary oceans. However the levels of Ca in the world’s ocean are largely biologically limited (mostly by calcareous nannoplankton and plankton), so leading to an increase in the Mg/Ca ratio in the Neogene ocean.

 

a study of CaCO3 veins in ocean basement, utilising 10 cored and documented drilled sites, Rausch et al. (2013) found for the period from 165 - 30 Ma the Mg/Ca and the Sr/Ca ratios were relatively constant (1.22-2.03 mol/mol and 4.46-6.62 mmol/mol respectively (Figure 8). From 30 Ma to 2.3 Ma there was a steady increase in the Mg/Ca ratio by a factor of 3, mimicking the brine inclusion results in chevron halite. The authors suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. They go on to note that additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate. However, world-scale factors driving the increase in Mg in the world’s oceans over the past 40 million years are still not clear and are even more nebulous the further back in time we look.

 

Changes in Phanerozoic ocean salinity

As well as changes in Mg/Ca and SO4, the salinity of the Phanerozoic oceans shows a fluctuating but overall general decrease from the earliest Cambrian to the Present (Figure 9; Hay et al. 2006). The greatest falls in salinity are related to major extractions of NaCl into a young ocean (extensional continent-continent proximity) or foreland (compressional continent-continent proximity) ocean basins (Chapter 5). Phanerozoic seas were at their freshest in the Late Cretaceous, some 80 Ma, not today. This is because a substantial part of the Mesozoic salt mass, deposited in the megahalites of the circum-Atlantic and circum-Tethyan basins, has since been recycled back into today’s ocean via a combination of dissolution and halokinesis. Periods characterised by marked decreases in salinity (Figure 9) define times of mega-evaporite precipitation, while periods of somewhat more gradual increases in salinity define times when portions of this salt were recycled back into the oceans (Chapter 5).


The last major extractions of salt from the ocean occurred during the late Miocene in the various Mediterranean Messinian basins created by the collision of Eurasia with North Africa. This was shortly after a large-scale extraction of ocean water from the ocean to the ice cap of Antarctica and the deposition of the Middle Miocene (Badenian) Red Sea rift evaporites. Accordingly, salinities in the early Miocene oceans were between 37‰ and 39‰ compared to the 35‰ of today (Figure 9). The preceding Mesozoic period was a time of generally declining salinity associated with the salt extractions in the opening North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous) and the earliest Cambrian oceans also had some of the highest salinities in the Phanerozoic. Recently, work by Blättler and Higgins (2014) utilising Ca isotopes studies of selected Phanerozoic evaporites has confirmed the dichotomous nature of Phanerozoic ocean chemistry that was previously defined by micro-inclusion studies of chevron halite (Figure 3).

So what?

In summary, based on a growing database of worldwide synchronous changes in brine chemistry in fluid inclusions in chevron halite, echinoid fragments, vein calcites at spreading centres and Ca isotope variations, most evaporite workers would now agree that there were secular changes in Phanerozoic seawater chemistry and salinity. Ocean chemistries ranged from MgSO4-enriched to MgSO4-depleted oceans, which in turn drove the two potash endmembers What is not yet clear is what is the dominant plate-scale driving mechanism (seafloor spreading versus dolomitisation versus uplift/weathering) that is driving these changes.

In terms of marine bitterns controlling favourable potash ore associations, it is now clear that the variation in ionic proportions in the original seawater controls whether or not potash-precipitating bitterns are sulphate enriched or sulphate depleted. A lack of MgSO4 minerals as co-precipitates in a sylvite ore makes the ore processing methodology cheaper and easier (Warren, 2016). Understanding the ionic proportion chemistry of Phanerozoic seawater is a useful first-order exploration tool in ranking potash-entraining evaporite basins across the Phanerozoic.

References

Ayora, C., J. Garciaveigas, and J. Pueyo, 1994, The chemical and hydrological evolution of an ancient potash-forming evaporite basin as constrained by mineral sequence, fluid inclusion composition, and numerical simulation: Geochimica et Cosmochimica Acta, v. 58, p. 3379-3394.

Blättler, C. L., and J. A. Higgins, 2014, Calcium isotopes in evaporites record variations in Phanerozoic seawater SO4 and Ca: Geology, v. 42, p. 711-714.

Borchert, H., 1977, On the formation of Lower Cretaceous potassium salts and tachyhydrite in the Sergipe Basin (Brazil) with some remarks on similar occurrences in West Africa (Gabon, Angola etc.), in D. D. Klemm, and H. J. Schneider, eds., Time and strata bound ore deposits.: Berlin, Germany, Springer-Verlag, p. 94-111.

Bots, P., L. G. Benning, R. E. M. Rickaby, and S. Shaw, 2011, The role of SO4 in the switch from calcite to aragonite seas: Geology, v. 39, p. 331-334.

Brennan, S. T., and T. K. Lowenstein, 2002, The major-ion composition of Silurian seawater: Geochimica et Cosmochimica Acta, v. 66, p. 2683-2700.

Dean, W. E., 1978, Theoretical versus observed successions from evaporation of seawater, in W. E. Dean, and B. C. Schreiber, eds., Marine evaporites., v. 4: Tulsa, OK, Soc. Econ. Paleontol. Mineral., Short Course Notes, p. 74-85.

Demicco, R. V., T. K. Lowenstein, L. A. Hardie, and R. J. Spencer, 2005, Model of seawater composition for the Phanerozoic: Geology, v. 33, p. 877-880.

Hardie, L. A., 1984, Evaporites: Marine or non-marine?: American Journal of Science, v. 284, p. 193-240.

Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

Hardie, L. A., 1996, Secular variation in seawater chemistry: an explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y.: Geology, v. 24, p. 279 - 283.

Hay, W. W., A. Migdisov, A. N. Balukhovsky, C. N. Wold, S. Flogel, and E. Soding, 2006, Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 240, p. 3-46.

Holland, H. D., J. Horita, and W. Seyfried, 1996, On the secular variations in the composition of Phanerozoic marine potash evaporites: Geology, v. 24, p. 993-996.

Holland, H. D., and H. Zimmermann, 2000, The Dolomite Problem Revisited: Int. Geol. Rev., v. 42, p. 481-490.

Holt, N. M., J. García-Veigas, T. K. Lowenstein, P. S. Giles, and S. Williams-Stroud, 2014, The major-ion composition of Carboniferous seawater: Geochimica et Cosmochimica Acta, v. 134, p. 317-334.

Kovalevych, V. M., T. M. Peryt, and O. I. Petrichenko, 1998, Secular variation in seawater chemistry during the Phanerozoic as indicated by brine inclusions in halite.: Journal of Geology, v. 106, p. 695-712.

Lowenstein, T. K., L. A. Hardie, M. N. Timofeeff, and R. V. Demicco, 2003, Secular variation in seawater chemistry and the origin of calcium chloride basinal brines: Geology, v. 31, p. 857-860.

Lowenstein, T. K., M. N. Timofeeff, S. T. Brennan, H. L. A., and R. V. Demicco, 2001, Oscillations in Phanerozoic seawater chemistry: Evidence from fluid inclusions: Science, v. 294, p. 1086-1088.

Müller, R. D., A. Dutkiewicz, M. Seton, and C. Gaina, 2013, Seawater chemistry driven by supercontinent assembly, breakup, and dispersal: Geology, v. 41, p. 907-910.

Paris, G., J. Gaillardet, and P. Louvat, 2010, Geological evolution of seawater boron isotopic composition recorded in evaporites: Geology, v. 38, p. 1035-1038.

Rausch, S., F. Böhm, W. Bach, A. Klügel, and A. Eisenhauer, 2013, Calcium carbonate veins in ocean crust record a threefold increase of seawater Mg/Ca in the past 30 million years: Earth and Planetary Science Letters, v. 362, p. 215-224.

Spencer, R. J., and L. A. Hardie, 1990, Contol of seawater composition by mixing of river waters and mid-ocean ridge hydrothermal brines, in R. J. Spencer, and I. M. Chou, eds., Fluid Mineral Interactions: A Tribute to H. P. Eugster, v. 2: San Antonio, Geochem. Soc. Spec. Publ., p. 409-419.

Timofeeff, M. N., T. K. Lowenstein, M. A. M. da Silva, and N. B. Harris, 2006, Secular variation in the major-ion chemistry of seawater: Evidence from fluid inclusions in Cretaceous halites: Geochimica et Cosmochimica Acta, v. 70, p. 1977-1994.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released November 2015: Berlin, Springer, 1600 p.

Wilson, T. P., and D. T. Long, 1993, Geochemistry and isotope chemistry of Ca-Na-Cl brines in Silurian Strata, Michigan Basin, USA: Applied Geochemistry, v. 8, p. 507-524.

Zimmermann, H., 2000a, On the origin of fluid inclusions in ancient halite - basic interpretation strategies, in R. M. Geertmann, ed., Salt 2000 - 8th World Salt Symposium Volume 1: Amsterdam, Elsevier, p. 199-203.

Zimmermann, H., 2000b, Tertiary seawater chemistry - Implications from primary fluid inclusions in marine halite: American Journal of Science, v. 300, p. 723-767.

Salt's uses across human history

John Warren - Wednesday, May 13, 2015

Salt's uses across human history

Until the 19th Century and the advent of refrigeration, salt's main uses was as a preservative and as a much sought after flavouring for foodstuffs. Even today, most people think of salt in terms of sprinkling it on their food, or in colder climates they may also think in terms of road de-icing. These are in fact lesser modern usages of halite; its main use is as a feedstock in the chemical industry, a topic we shall discuss in a later blog. This essay focuses on salt's importance to humanity prior to the chemical age.


Sodium chloride (halite), the most common industrial evaporite salt and is used in some form by virtually every person in the world. The human body contains about 110 gm of salt. Salt is essential to all living creatures and even many plants. Since the body cannot manufacture it, salt is an "essential" nutrients, and as an electrolyte, we lose it every time we sweat. Without enough salt, muscles won't contract, blood doesn’t circulate, food goes undigested, and ultimately the heart ceases to beat.

Halite, along with other salts, has long played a very important role in human affairs. Early hominids lived on the edge of the saline Lake Olduvai (Hay and Kyser, 2001) and salt was part of their diet. In ancient Greece it was so valuable that the slave trade involved an exchange of salt for a slave and gave rise to the expression, “not worth his salt.” Some 4,700 years ago the Peng-Tzao-Kan-Mu was published in China. It is probably the earliest known treatise on pharmacology, with detailed discussions of the palliative and curative powers of more than 40 kinds of salts, including descriptions of two methods of recovering usable salts from brine. There are more than 14,000 reported usages of halite and more than 30 references to salt in the Bible. Some 3,200 years ago, near Hallstatt in Austria, Bronze-Age miners were extracting salt, from a network of several kilometres of galleries up to 300 metres below the surface. Two thousand seven hundred years later some ten pages deal with salt in the “De Re Metallica” by Georgius Agricola (Georg Bauer), both mining it and producing it from seawater or brine springs. Published in 1556, Agricola’s was the first book on mining to be based on field research and observation.

 

Salt as a preservative

Historically, whether food was hunted, gathered, or grown and harvested, food supply was rarely available year-round to all members of a society. Yet, effective, year-round, reliable food storage was vital, especially for non-nomadic agricultural societies. Today, to maintain reliable food supplies to our ever-expanding urban populations, we refrigerate, freeze-dry or can our food. Food preservation problems seem trivial to most consumers in the developed world, outside of the world’s war zones. But, prior to the 19th century, effective food storage often made the difference between life and death to large segments of the world's human population.

In arid climates, food can effectively be stored by drying. But in more humid temperate climates, fungus and bacteria rapidly destroy stored and cellared food. Even where food can be stored in winter ice, it quickly rots when spring thaws set in. Documents from northern Europe, give some clues about the severity of the problem, and its solution. In medieval societies, with relatively poor transportation systems, villages and counties had to be close to self-sufficient in food. If a bad harvest occurred, to mitigate a potential disaster there had to be enough food stored to tide over until the next harvest. Medieval Europe offers an example of the way in which agricultural societies dealt with food security. Good-quality arable land was scarce, and had to be used for crops. That meant that grazing and foraging animals, mainly cattle and pigs, were turned out into the local woodlands for the summer to forage for grass, roots, and nuts. Any relative shortage of winter fodder in turn meant that surplus animals would best be butchered before cold weather drove them indoors. In medieval England the annual slaughter was traditionally around Martinmas, St Martin's Day (10 November), but it was earlier in the colder climes of Sweden. In turn, that meant that fresh meat was readily available only at that time, and that fresh protein in the form of milk and butter was only available in winter from cows kept in shelter. In addition, taxes were often paid in kind rather than money, and that meant that the landlord had to be given foodstuffs that could be stored.

The response of the Swedes and most northern Europeans, was to preserve almost all their food, and they used salt to do so. Beef and pork were salted and dried as joints, hams, and sausages. Butter was salted. Typically it took a pound of salt to preserve 10 pounds of butter (salt was sufficiently costly that housewives removed salt before they used stored butter). Fish, whether freshwater or from the sea, were salted and dried, and bread was salted and hung to dry. Surviving records from 1573 show the servants of King Gustavus Vasa of Sweden ate some 102 kg (224 pounds) of beef and pork, but 99 kg of it (218 pounds) was salted and dried. They almost never had fresh meat. The King issued an order to release 3-year-old butter from the tax stores for some men hired to work at the castle, and ordered the sale of 4-year-old (barley) malt because it was starting to get weevils in it. He ordered the peasants to store their butter and meat in the fall, after the annual slaughter, but he also ordered them not to eat any of it for 12 months (as they should be eating the previous year's food during that time).

 

Outside of salted food storage, the ancient Egyptians are famous for their perfection of the art of mummification. A key ingredient in the process was natrun, which is a natural mixture of halite, trona and sodium sulphate (Edwards et al., 2007). The ancients knew its preservative properties as it readily absorbs water, making it an excellent desiccant/preservative of organic material. Natrun is found in large quantities in the beds of several Egyptian playa lakes (e.g. Wadi Natrun and El Kab, as well as Behiera in the nearby Libyan desert and in Lake Natron in the African Rift valley; Figure 2). It has been mined and traded from such localities for thousands of years. Writings as old as the reign of Rameses III (1198-1166 B.C.) refer to natrun deposits. Its preservative qualities must have been immediately apparent to the ancient Egyptians from its effects on any wild life, which had died in these lakes (Figure 2). There is some evidence that the ancient Egyptians artificially precipitated natrun by isolating shallow basins of salt lake waters for faster evaporation, as is still done in parts of the Faiyum depression today. For purification and preservation, natrun was preferred over pure halite as it chemically attacks and destroys grease and fat, and so is a superior drying agent (as is sodium borate). Its residues are found not only in tombs and in pits, along with other discarded embalming materials, but also forms nodules and residues in the mummies themselves.

There is some popular debate over the method in which the natrun was used for mummification by the ancient Egyptians. Some argue it was used in a way similar to the contemporary method for “salting” fish. Dry natrun would be sprinkled over the body, perhaps with sawdust, or spread with linen cloth wraps. Others with a more starry-eyed bent, believe the body was immersed in vats containing a natrun solution. Such a wet method would have been odiferous and accelerated putrefaction, thus counterproductive to the preservation of the body, although it makes for good Hollywood images. A dry body is also more readily bandaged as well as being more amenable to the attachment of amulets and other jewellery. Although mummification has supernatural trappings in popular culture and ancient religions, its basis is rooted in simple chemistry and processes as mundane as salting fish.

 

The mummies of some Buddhist monks (Sokushinbutsu sect) in Japan resulted from the practise of nyūjō, which ultimately aimed to cause their own death and mummification by encasement in salt. This ritual took years to complete and involved starvation and dehydration. During the first three years, an ascetic monk significantly decreased his body fat by eating only nuts, seeds, and berries, while he increased his physical activity. Towards the end of the ritual the monk reduced his food intake even further by only consuming bark, roots, and sometimes stones. Post-mortem preservation was further aided by consumption of toxic herbs and tea that eliminated bodily fluids and killed bacteria that aid in decomposition. Japanese Sokushinbutsu monks were known to drink a tea made from the urushi tree, also known as the Chinese lacquer tree because it’s sap is used to lacquer tableware, instruments, and jewellery.

After years of starvation and dehydration, when the monk felt like he was close to death, his fellow monks arranged his body in the lotus position inside a coffin or a tomb. Then they surrounded the dying man with salt, wood, paper, or lime to pull more moisture away from the body and further prevent post-mortem decay. Only a small opening for air was allowed when the tomb was closed. The monk then chanted, meditated and occasionally rang a bell until he died.

Once his fellow monks heard silence they completely sealed the tomb. After several years, the monks exhumed the body to see if the self-mummification ritual was successful. Like some Eastern Orthodox religions, these Buddhists believed that an incorrupt body, a body having delayed decomposition, indicated a monk’s holiness. If the body was incorrupt after exhumation then the corpse was placed in a temple, adorned, and tended to by followers. However, if a tomb was opened and the body had decayed, then the corpse was left behind and the tomb was resealed. That monk’s efforts were respected, but his body was not given the deference of a religious relic. Japan banned unburying in 1879 and assisted suicide, including religious suicide is now illegal. In a similar vein, in 1933, the Dalai Lama was buried sitting up in a bed of salt.

Mummification can occur naturally if a body is encased in halite, and natrun is not necessary, although it improves preservation. In 1593 AD, and again in 1616 AD, several tombs encased in salt were exposed by natural salt weathering and collapse in the Hazel Mountains. When the coffins were opened by the local people of Hallein and Hallstatt, there was astonishment that the bodies inside had very well preserved soft tissues. It was the result of the hyperarid encasement in a Neolithic salt mine, but frightened religious locals, encouraged by the local clergy, insisted on prompt reburial, along with additional religious efforts to lessen ambient sin levels (in part in the form of alms to mother church) and hence more prayer to create more effective seals. There was a similar popular response in 1734 AD when the salt preserved body of a man wearing mountain clothing (likely a salt miner) was discovered. Fearful locals, once again encouraged by the local clergy, insisted on immediate reburial with no further scientific study or observations on the remains (Aufderheide, 2011). 

In Iran, first in the winter of 1993 and later in 2004, in the modern Chehr Abad Salt Mine, near Hamzehloo, Zanjan Province, a total five salt-preserved male bodies were found in a collapsed tunnel of a former salt mine, which was active around 400 BC. The first discovery in the winter of 1993 was a salt encased bearded head and some artefacts, the later discovery, beginning in November 2004, was of the remaining bodies. It is likely all five men died in earthquake induced collapses in the salt mine (Pollard et al., 2008). Encasement in the hyperarid atmosphere of the collapsed salt mine tunnel led to natural mummification of the bodies.

Salt and war

Salt’s historical use as a food preservative, along with its medicinal use, made it a valuable commodity with political and military significance. The earliest recorded war over access to a supply of salt was over a salt lake in China in 3000 BC. In 2200 BC the Chinese emperor Hsia Yua declared that Shandong Province must supply the Imperial Court with salt. An ancient Chinese philosopher once called salt “the sweetest thing on earth.” The words, “war” and “peace” originate from the words for salt and bread in ancient Hebrew and Arabic, while from the Latin "sal," came words such as "sauce" and “sausage."

As an example of salt’s military import let’s look at the significance of a reliable salt supply to the army of the Old South in the 1860s during the American Civil War. Each Confederate soldier was provided with starch (26 pounds of coarse meal, 7 pounds of flour or biscuit, 3 pounds of rice), protein (10 pounds of bacon), and salt (one and a half pounds). Bacon was the meat of the South, and every pound of it required salt. As well as military personnel, horses also need salt in their diet. The Confederacy also needed this precious mineral to treat wounds, tan leather and dye cloth for uniforms. Last century, the historian Ella Lonn (1933) devoted an entire book to the problem of reliable salt supply for the Confederacy during the Civil War. We know that the Confederate soldiers were hungrier than the Northerners throughout the war. We shall never know whether the hogs that were not slaughtered because there was no salt to preserve them took the edge off the Confederate troops, or whether the salt that was not available for the horses took the edge off the cavalry. "What hogs we have to make our meat, we can't get salt to salt it," wrote Mrs Sarah Brown to Governor Pettus of Mississippi in December 1861. In 1862, Governor Brown of Georgia wrote that only half of the meat of the State could be saved for the 1862-1863 season.

 

That most intelligent and brutally efficient of the Northern Generals, Sherman, had no doubt about salt’s importance to any army and its morale, he considered it as important as gunpowder, he declared. "Without salt they cannot make bacon and salt beef," and, "Salt is eminently contraband, because [of] its use in curing meats, without which armies cannot be subsisted." Sherman sent a captain for trial on a charge of aiding the enemy, because he had allowed salt through the lines to the Confederates. The Union forces were sent orders to destroy salt stores and salt works wherever they were found (Figure 4). Throughout the American Civil War the South’s salt production facilities in Saltville, Va., Virginia's Kanawha Valley and Avery Island, Louisiana, were targets of the Union Army. The North fought for 36 hours to capture Saltville, Va., where the salt works were considered so crucial that Confederate President Jefferson Davis offered to waive military service to anyone willing to tend coastal salt kettles and so supply the South's war effort.

In November 1863, General Burnside noted in a despatch to Grant that Lee had placed a strong defensive force in front of Saltville [Virginia]. Grant understood the significance of the deployment. In December 1863 he wrote to General Foster, "If your troops can get as far as Saltville and destroy the works there, it will be an immense loss to the enemy." In the event, the Confederates guarded the works so well that the Union Army did not take (and destroy) the salt works until December 1864. General Burbridge boasted that the loss of Saltville would be "more felt by the enemy than the loss of Richmond." Meanwhile the North, even with salt sources of its own, imported 86,208 tons of salt from England in 1864 alone.

Likewise, thousands of Napoleon’s troops died during his retreat from Moscow, because for lack of salt their wounds would not heal.

Ancient salt production and its taxable value

Around 6,000 BC on the margins of Lake Yun Cheng in Northern China’s we see the first evidence of an industry designed to harvest and produce salt, via the evaporation of lake brines in purpose-built salt pans. In Europe, the first recorded industrial production of pan salt took place in Italy some 2500 years ago when Ancus Martius, one of the early Roman kings, began letting sea water into an enclosed basin, then allowing the sun to evaporate the water to create a salt residue. The importance that Rome attached to the salt works and port at Ostia was such that the main highway along which the salt was carried to Rome was called the Via Salaria. Like Venice after it, the city of Rome based much of its early commerce on trading salt. Special salt rations paid to early Roman soldiers were known as “salarium argentum”, the forerunner of the English word “salary.” With a near monopoly on supply to Rome, the traders in the port of Ostia raised the salt price so high that the state was forced to take over the industry in 506 BC.

When Julius Caesar landed in Britain in 55 BC, he brought his salinators with him, but found that even the backward Britons were extracting salt by pouring brine on to hot stones. The Romans, however, used iron pans in which they boiled the brine, and Caesar established a brine-based salt works in Cheshire and subsequently in other localities where ancient salt occurred at shallow depths. The towns in Britain where salt was made from brines extracted from shallow buried ancient salt beds can be distinguished to this day by the termination “wich”, an Anglo-Saxon descriptor for a place where salt was made and includes towns like Greenwich, Ipswich, Droitwich, Northwich and Middlewich. Likewise, within regions of shallow salt and brines in Austria and Germany, names containing "salz" and "halle," such as Salzburg ("salt city"), Salzkammergut, Reichenhall, Halle, Hallein, and Hallstatt, as well as the old Austrian/Polish province of Galicia, identify some of the salt-bearing areas.

Merchants in 12th-Century Timbuktu in Africa, the gateway to the Sahara Desert was the seat of renowned scholars, who valued salt extracted from salt lakes to north in the vicinity of Taoudenni, Mali, as highly as books or gold. The Taoudenni mines are located on the bed of an ancient erg-edge salt lake and have been actively quarried for more than a 1,000 years. Today, the miners use crude axes to dig pits that usually measure 5 m by 5 m with a depth down to around 4 m. The miners first remove up to 1.5 m of red clay overburden, in contrast to the salt miners in the Danakil who work at the active pan surface (see Salty Matters, 19 April, 2015). Then several layers of poor quality salt are removed before reaching three layers of high quality salt. The salt is cut into slabs that are 110 cm x 45 cm by 5 cm in thickness and weigh around 30 kg. Two of the high quality layers are of sufficient thickness to be split in half, so that 5 slabs can be produced from the three layers. Having removed the salt from the base area of the pit, the miners excavate horizontally to create galleries from which additional slabs can be obtained.  As each pit is exhausted, another is dug so there are now thousands of pits spread over a wide area on the lake. Over the centuries salt has been extracted from three distinct areas of the lake depression, with each successive area located further to the southwest. The  areas can be clearly seen on satellite photographs (22.606519°E, 4.030660°S). Until recently salt was transported south by huge camel trains, now more and more salt is carried out by 4-wheel drive trucks, south to Timbucktu and on to the river port of Mopti (Figure 5).  Among the some of the nomadic tribes of the Sahara and Ethiopia's Danakil Plains, salt carried by camel trains and is still used occasionally as money or bartered for a cash equivalent. When the camel trains of Mali carried the salt, each animal typically carried 4 blocks of salt. On reaching the salt market, three blocks sold off the back of each animal went to the camel train owners, and the profit of the sale of the remaining block to the salt miner. 

 

In Tibet, Marco Polo noted that tiny cakes of salt, manufactured from salt lakes in the high plains of Tibet were pressed with images of the Grand Khan and used as coins.The ancient Maya made salt at Salinas de los Nueve Cerros, Guatemala, an area where natural salt springs flowed into a river gully, giving easy trading access to downstream customers (Figure 6). This site was the only large-scale source of salt for the interior Lowland Maya. Maya technology included solar evaporation and firing of brine from salt springs in special large ceramic bowls that are the largest receptacles ever found in any Maya sites.

The highly organized salt trade of China was observed by Marco Polo, who recorded that the major item of trade on the Yangtze River was salt, shipped upstream from the coast (especially from the city of Hangchou) to the interior cities. The Chinese produced salt by many methods: they evaporated it, boiled seawater, and pumped brine from wells drilled into salt beds. Modern oil-drilling traces its roots back to Chinese methods of bamboo-based drilling technology that originally evolved for salt production from ancient subsurface brine sources (this will be the subject of a later blog).

 

Salt production, politics and taxes

Salt's economic value has meant it has been taxed by governments from the ancient Chinese and Romans to governments of late medieval Europe to those of France, even up to the late 1940s. In 2200 BC the Chinese Emperor Hsia Yu levied a salt tax, which was one of the world's first documented state taxes.

The Mediterranean and the rise of Venice

The great trading ports of the Mediterranean dealt in salt as well as spices and textiles. Not surprisingly, the greater of them, Genoa and Venice not only traded in salt, but fought for supremacy over the trade. Because of the hot dry summers and mild wet winters, salt can be made in a saltern or pan in almost any suitable seashore flat or plain in the Mediterranean. So although it is possible to envisage a trader's cartel from a specific geological region of shallow buried salt in Austria or England, it is much more difficult to control the production of salt in coastal saltpans. So, in hindsight, it is surprising how effectively Genoa and especially Venice, managed to take control of Mediterranean salt production, as well as trading, across the 13th to 16th centuries. Genoa was positioned in the Western Mediterranean and Venice at the head of the Adriatic. Each used all its political and military strength to consolidate its local salt trade, and to encroach as far as possible on that of its rival. However, Venice was more organised politically, which translated into more ruthless and effective use of state power. And Venice made a conscious decision to concentrate on the salt trade, whereas to the Genoese it was just one of a set of potentially profitable cargoes. Where the two came into conflict over salt, the Venetians tended to win.

Venice managed to make a business out of control of the Adriatic salt trade. Venice owed some of its early wealth to the salt trade from salt works in its lagoon, and had a number of contracts with inland Italian cities in the 13th century to supply them with salt. The more that Venice came to control the salt trade in the Adriatic, the more the resulting profits were used by the city to subsidise other trading activities. Venetian traders delivering salt to the city were given bank credits, for example, allowing them to buy goods quickly. As the historian S. A. Adshead has written, "For the Venetians, salt was not a commodity among commodities... it greased the wheels of all the working parts and fuelled its motor". The salt trade allowed Venetian traders to compete very effectively with their rivals across the board. Salt was "il vero fondamento del nostro stato” (The true foundation of our state).


Always, from their beginnings in the 5th Century, the Venetians were willing to exercise raw power to foster their control of salt. Prior to the rise of the Venetian State, the Roman salt-making center in the Adriatic was at Comacchio, a little north of Ravenna. After the fall of Rome, records of the 8th-century Lombard King Luitpold show that Comacchian salt was being shipped to all the major inland cities of Lombardy, through Ferrara, at least as far inland as Parma, Lodi, and Brescia. By 523 AD Venice was producing salt and in 932 AD the Venetians destroyed Camacchio. They burned the citadel, massacred the inhabitants, and carried off the survivors to Venice, where they had to swear an oath of loyalty to the Doge before they were released. The Venetians began to construct salt works on their own lagoon, and around 1028, we find the Doge of Venice giving permission for Chioggia to build more salines on the Venetian lagoon. However, it turned out that it was not as easy to build salt works in the relatively exposed, storm-prone lagoon of Venice as it had been at Comacchio, and it took a long time before salt production became really successful at Chioggia. Meanwhile, the city of Cervia, south of Ravenna, filled the salt production vacuum left by the destruction of Comacchio and Cervia was in full production at least by 965- ­975 AD.

Around 1180, it was clear that Cervia and Chioggia were rivals for salt production, under the protection of Ravenna and Venice respectively. The Archbishop of Ravenna and the Doge of Venice now began exerting political pressure on the Adriatic salt market. Venice declared it illegal for Chioggia salt to be sold or shipped without a Venetian certificate, and Ravenna did the same for Cervia. The salt market was now out of the hands of merchants and in the hands of the politicians and the Catholic church. By 1234, war between Venice and Ravenna ended with a ban on any Ravenna (Cervia) salt being shipped northward, and Venetian galleys enforced the treaty.

Then, the Venetians went one logical step further: for all practical purposes they gave up trying to be salt producers, and instead concentrated on being (monopoly) salt traders. Between 1250 and 1280, they came more and more to be the dominant buyers of salt, which they then warehoused, shipped and sold (Figure 7). By the 1350s, no salt could move on a ship in the Adriatic unless it was a Venetian ship bound to or from Venice.

A golden rule of Venetian policy was that all trade goods under their control must pass through Venice. As late as 1590 they were making an 81% mark-up on salt sold inland. But that was not always the case, sometimes, if it would foster trade in higher-value goods that would yield more profit, Venice sold salt at less than normal rates. All this activity was planned and supervised by a special State body, the Collegio del Sal. The rewards were staggering, and help to justify the tenacity and ruthlessness with which the Venetians operated the salt business. Typically, Venetian merchants bought salt for 1 ducat a ton, and it cost them about 3 ducats a ton to ship it to Venice. There they received a State subsidy of 8 ducats a ton. The State collected a tax as the salt left Venice, and after shipping to the customer, the selling price was roughly 33 ducats a ton. That was a profit worth fighting for! And it was not only the merchants who profited. Some of the State profits went to the architecture, sculpture, and paintings that remain today and make Venice so magnificent (Figure 7).

The Venetians had different methods for maintaining their trading monopoly. On the island of Pag, they would buy up all the salt that was not needed locally. It would then be shipped to Venice, warehoused and sold (at very high prices) to customers. At Muggia and Capodistria, the Venetians were given a fraction (about 10%) of the salt produced (presumably as protection money), but the locals were allowed to sell the other 90% only as long as it was carried overland, effectively limiting its value and the sales area.

As late as 1578, the Venetians destroyed the salt works at Trieste, and in the following twenty years were making an 80% profit on salt sold inland on the Lombardy plain. But around 1600, paradoxically with the defeat of the Turks at sea, shipping intensity in the Adriatic became too great for the Venetians to be able to maintain their monopoly by force. Their source of riches in the spice trade had also been cut off as the trade routes to India now passed around Africa, and so their shipping power and wealth declined.

Salt and wealth in inland Europe and the UK

Much of the salt supply of inland central and northern Europe came from the mining of shallowly buried ancient salt (Permian) or associated brines. The great salt extraction centre at Reichenhall, in southern Bavaria, was first operated in Roman times, but was destroyed later, possibly by Attila the Hun, but more likely by the German Odoacer. It was rebuilt in the early 7th Century by Saint Rupert of Salzburg (Figure 8) and became the concession of the Bishop of Salzburg, who derived a great deal of power and money from the salt trade. So mother Church promoted the “salt bishops” to Archbishops. About 1190, however, a competing salt works had opened at nearby Berchtesgaden, without the Archbishop's approval, and a major quarrel between Church and State erupted, with the Archbishop and the Emperor in conflict. The Church lost, and in 1198 the Bavarian saltworks passed into the control of the Duke of Bavaria. Reichenhall's production peaked at about this time, and it later lost out in competition with a new salt works opened to the south by the persistent Archbishops of Salzburg. During that time it remained an important salt centre for several hundred more years and, even today, derives income from geotourism and from the therapeutic salt baths of Heilbaden.

 

Thwarted in Bavaria, the Archbishop of Salzburg turned to salt springs closer by, and so a new salt industry sprang up at Hallein, first mentioned in documents in 1232. By 1300 its production had outstripped that of Reichenhall, and as it was situated closer to the Danube, it was able to ship salt as far as Bohemia, as well as into Austria and Bavaria. The Archbishop gradually bought up shares in Hallein, and by the early 16th century he held them all. However, the crown of Bohemia passed into the Habsburg family, and from the early 1600s, the great market of Bohemia was closed to the Archbishop. The other Austrian salt works were small at first. In the Salzkammergut, salt springs emerged from horizontal tunnels in the valley sides, which, although the locals did not know it, were the ancient galleries into the old flooded salt mines that had been worked in prehistoric times. The salt works at Hall, in the Tyrol, provided a power base for its owners, who were the local Hapsburg Dukes from 1363. The Dukes would sell salt to the Swiss, then use the profits to pay for the Hapsburg campaigns against the Swiss!

Salt production was always limited in Austria by shortages of fuel needed to extract salt by boiling brines. As the boiling houses consumed the local timber, they had to be moved, and fuel was a problem in salt manufacture in this region until modern times and the advent of highly mechanised mining operations. In 1770 there were purpose-built flumes running down the mountain sides, used not for water supply but to float down billets of timber for the boiling houses. Since fuel ran out at Hallstatt very early, the Emperor built a wooden pipeline to take the brine from the ancient mines down the valley to Ischland, on the way it crossed the Gosau Valley via a purpose-built bridge. Salt continued to play a significant role in the politics of the region after 1600,     when it was produced by three major players, Austria, Bavaria, and the Archbishop of Salzburg. The Austrian Empire grew to include Bohemia and Moravia, and this salt-less region became a captive market for the Austrian salt producers, with substantial tax revenue accruing to the Habsburg Emperor. Salt production was considered a state monopoly and Salzmonopol was considered "the brightest jewel in the possession of the Hofkammer.” By1700 it provided some 10% of the total revenue of the state.

In times of military emergency the Habsburgs would regularly use the salt income as collateral for raising money quickly. They did it first when Bohemia revolted in 1618 in the Defenestration of Prague, and Protestant forces besieged Vienna. Emperor Ferdinand II mortgaged his salt revenues to pay for the Catholic army that saved Vienna and won the decisive battle of the White Mountain in 1620. Salt revenue from the Wieliczka salt mine paid the Polish army under King John Sobieski when it rescued Vienna from the Turkish siege of 1683. Interestingly, the Wieliczka salt revenue had earlier passed to the Habsburgs in return for their assistance to the Poles in the Swedish invasion of 1657. Salt was also a state monopoly in Bavaria. Both Austria and Bavaria sought to promote their own salt exports and protect their domestic markets from salt imports, hence there was a flourishing trade in contraband salt.

In 1611 the Archbishop of Salzburg was forced to market his salt through Bavaria, so the rivalry now had only two players. Given that Austria and Bavaria between them controlled all the major salt sources in Central Europe, it is difficult to understand why they did not cooperate to form a cartel. A brief agreement, the Rosenheimer Salt Trade Agreement, was set up in 1649, but lasted for only 40 years. The centrepiece of Bavarian foreign policy became a campaign to sell salt effectively to her western neighbours, given that Austria could sell hers throughout the growing Austro-Hungarian Empire. It is not a coincidence that Bavaria consistently fought on the French side against the Austrians in the War of the Spanish Succession in the early 1700s and during the Napoleonic Wars in the early 1800s.

On the British mainland, Mary Queen of Scots was perhaps the first head of state to have the idea of making salt a taxable source of governmental revenue. She granted a patent to an Italian to make salt in Scotland and then placed a heavy tax on it, which she appropriated to herself. Elizabeth, Queen of England, and Mary's lifelong “dear sister” and eventual executioner, thought this an excellent idea and likewise taxed English salt making. Salt tax was a source of great resentment to everyone, English and Scots alike, and smuggling grew to alarming proportions. In 1785, the Earl of Dundonald wrote that every year in England, 10,000 people were arrested for salt smuggling. During Queen Anne’s reign, the salt tax rose to £30 a ton, an enormous amount of money in those days. The whole of England arose in rioting protest, with the result that the salt tax was finally abolished.

In Burgundy in the 1700s, salt was taxed at more than 100% as it came from the salt-works. This tax was extended to the whole of France when Burgundy was absorbed and the notorious salt tax “la gabelle” became a necessary input to the government's finances. Cardinal Richelieu said that salt was as vital to France as American silver was to Spain. The repeal of the salt tax was a major goal of the revolutionaries of 1789. A few years later, as soon as he became Emperor, Napoleon restored the salt tax to pay for his foreign wars. The salt tax continued until 1945 to feed French government coffers.

It is said that income from a salt pan in southern Spain largely financed Columbus’ voyages. The Erie Canal, an engineering marvel that connected the Great Lakes to New York's Hudson River in 1825, was called “the ditch that salt built” as salt taxes paid for half of the cost of construction. The “Great Hedge of India,” the mid-18th century colonial equivalent of the Great Wall of China, stretched 3,700 km from the western border of Punjab down to the Bay of Bengal. It was manned by 12,000 men and planted by the British to minimise salt smuggling into Bengal and so enforce the collection of the Indian salt tax. As late as the 1940s the people of India under the leadership of Mahatma Gandhi protested British taxes on salt supply. In 1930 Gandhi led a 200-mile march to the Arabian Ocean to symbolically collect untaxed salt for India's poor.

Artisanal salt and culinary expectation

Today, halite is a cheaply-produced commodity extracted from the subsurface in mines, or salt solution plants, or produced at the surface in saltpans. In the production of table salt, processing, packaging and marketing are the major costs for most salt manufacturers. An interesting exception to the low sale price of modern table salt is the artisinal “Fleur de Sel de Geurande” a delicate gourmet form of white seasalt that is still hand-produced on fens along the coast of Brittany (Figure 9). It costs ≈US$40/kg and is produced by “paludiers” only on suitable summer days when halite rafts can be raked from the brine surface of specially maintained coastal salt pans, which are floored with grey clay. According to the local legend, salt flowers only form on hot days when the wind blows from the east (from the sea). It and the cheaper grey salt (sel gris), which is scraped from the pan floors and also prized by gourmands, has been produced this way in French coastal fens since Pre-Roman times. The flowers of salt is marketed as a “natural” product that contains all 84 trace elements and micronutrients found in the sea, and as being a natural source of potassium, calcium, copper, zinc and magnesium.

 

This halite product has an intense white color, with rigid crunchy crystalline structure and high moisture content giving it a distinct “feel on the tongue.” This is because “Fleur de Sel“ is composed of clusters of halite rafts. These rafts formed on the brine surface, as a thin layer of floating salt crystals, which are harvested daily via raking and then placed on plastic sheets to dry in the sun, making it a highly labor intensive product (Figure 9). The flower of salt product is packed with no other processing, unlike what happens to industrially-produced sea salt that undergoes a process that typically consists of varying combinations of washing, centrifugation and drying by the heat of combustion, grinding and sieving. While large saltwork companies need several square kilometres for salt pan installations, a flower of salt product can be obtained in ponds with total areas smaller than 0.1 hectare. There is a definite economic upside to the artisanal production of “flowers of salt.” Since it is a handmade product, small salterns can be constructed/operated by family groups, so offering a new or supplemental income source for low-income populations living in or near hypersaline strand line areas.

Impurities like clay are called grey spot or black spot in highly efficient mechanised salt production plants across the world and are considered undesirable in the processed end-product. To the cynical it says something about French marketing skills, and perhaps the gullibility of middle-class gourmands with too much money and time on their hands, that each year the gourmet industry successfully markets un-processed dirt-polluted salt (sel-gris) scraped from pan floors for top prices. We shall look deeper into the geological characterisitics of various gourmet salt styles in a later essay.

The various untreated salt products from France, the Himalayas and elsewhere are typically marketed as a "natural organic" product, "completed untreated" so it retains all its "essential nutrients." Such blanket claims from marketers targeting a moneyed, health-conscious and "new-age" mostly middle-class demographic, should at times be taken with a grain of salt. For example, some types of Himalayan "natural" salt produced from high altitude continental lakes in Tibet is iodine deficient. Its local use has led to high levels of cretinism and other thyroid problems in the local peasant population. Introduction of a "processed" iodised salt by the Chinese authorities is still met with resistance, yet such use of iodised salt in China has reduced goiter to 10% of previous levels in the Chinese population. For the similar reasons, "back to nature" and "organic" foods are increasingly popular in middle-class consumers of Australia. The associated resistance to the use of "iodised" salt and other processed products with iodine additives by urban new-age parents has lead to unhealthy levels of iodine depletion in preschool-age urban children, when tested in Melbourne and Sydney (Li et al., 2001; McDonnell et al., 2003). Likewise the use of natural "untreated" salt from Lake Magadi and Natron as a food additive has led to significant health problems (fluorosis) in the local population due to "naturally" high levels of fluorine in the harvested salt (Vuhahula et al., 2009).

Salt, social standing, and religious superstition

Salt, because of its high value in the ancient world, has maintained both cultural and religious significance over more than three millennia. For example, in Medieval and Renaissance European kingdoms, easy access to salt during meals assigned social status. Intricately carved salt cellars would be placed on select tables within easy reach of those deemed worthy. Accordingly at any noble table, to be seated “below the salt” was to be seen as unworthy of access to such luxury (Figure 10).

 

From its value in its use as a preservative and food additive in the ancient pre-rationalist world, salt became a religious symbol, representing immutability and incorruptible purity. In many religions, salt is still included on the altar to represent purity, and it is mixed into holy water of various sects for the same reason. Ancient Greek worshippers consecrated salt in their rituals, for example the Vestal Virgins sprinkled all sacrificial animals with salt and flour. Salt was a token of permanence to both Jews of the Old Testament and Christians of the New Testament. To the Jews it came to signify the eternal covenant between Jews and Israel. Jewish temple offerings still include salt on the Sabbath and orthodox Jews still dip their bread in salt as a remembrance of those sacrifices. Covenants in both the Old and New Testaments were often sealed with salt, explaining the origin of the word “salvation.” In the Catholic Church, salt is used in a variety of purifying rituals. Jesus called his disciples “the Salt of the Earth”, a statement that was commemorated by the Catholic church until Vatican II, by placing a small taste of salt on a baby's lip at his or her baptism. 

So to the religious, salt is supernatural symbol of the permanent sanctity of Jesus and offers supposed protection to the superstitious. For example, salt is still used to make holy water and also the more powerful exorcised water of the Roman Catholic Church. Salt is also used to make protective circles during exorcisms of demons. In the middle of the last millennia in Europe, salt was believed to provide defence against witches, witchcraft, demons, sprites, and the evil eye. It was a common belief that witches, and the animals they bewitched, could not eat anything salted. Inquisitors were advised by demonologists to protect themselves by wearing an amulet of salt, consecrated on Palm Sunday, along with other blessed herbs, pressed into a disk of blessed wax. Carrying a concealed packet of salt was said to ward off the evil eye as well. Another known talisman to ward off evil spirits was a jar of salt and a knife. Some people put salt and pepper in their left boot for good fortune. To ward off an evil witch, a peasant might throw salt outside the front door and lean a broom next to it. A passing witch would have to count the grains of salt and the blades of straw on the broom before she could do any harm.

Similarly, any waste of salt can be a portent of evil. In Leonardo Da Vinci's famous painting, “The Last Supper,” Judas Escariot has just spilled a bowl of salt - a portent of evil and bad luck. In Buddhist tradition, salt repels evil spirits. It is also why in many Asian cultures it's customary to throw salt over your shoulder before entering your house after a funeral: it scares off any evil spirits that may be clinging to your back. In the Christian tradition you should throw spilt salt over you left shoulder as, according to the Medieval Church, the devil or his demons reside behind or on your left shoulder, with your guardian angels on the right. In Hawaii and Samoa sea salt is used for protection, both by placing salt in each of the four corners of the house and by poring salt on the door threshold to prevent any spirits from crossing into one’s home. Shinto religion also uses salt to purify an area. Before sumo wrestlers enter the ring for a match - which is actually an elaborate Shinto rite - a handful of salt is thrown into the centre to drive off malevolent spirits (Figure 11). In the American Southwest, the Pueblo worship the Salt Mother. Other native American tribes had significant restrictions on who was permitted to eat salt. Hopi legend holds that the angry Warrior Twins punished mankind by placing valuable salt deposits far from civilization, requiring hard work and bravery to harvest the precious mineral.

 

Chinese folklore credits the Phoenix with the discovery of salt. In Norse mythology the gods first came from a salty ice-block over the course of four days as the sacred cow, Auðumbla brought Búri the first god in Norse mythology, and grandfather of Odin, out of the salty ice block. In another creation myth, Tiamat is the symbol of the chaos of primordial creation in Mesopotamian religion (Sumerian, Assyrian, Akkadian and Babylonian). She is a primordial salty goddess of the ocean, mating with Apsu (the god of fresh water) to produce the younger gods. Her husband, Apsu, later makes war upon their children and is killed. When she, too, wars upon her husband's murderers, she is then slain by Enki's son, the storm-god Marduk. and the arch of the heavens and the earth were formed from her divided body. Records from the Middle Euphrates Hittite kingdom of Mari attest to the veneration of Hatta, the god of salt, through the erection of a statue to him by the city’s ruler, Zimri-Lim (Stackert, 2010). Among Hittite rituals, perhaps the best-known use of salt is one that parallels its use in various Mesopotamian curses: the First Hittite Soldier’s Oath employs salt within an analogical curse ritual against that soldier who would commit sedition. Ancient Greek worshippers also consecrated salt in their rituals.

 

Outcrops of diapiric salt masses can also have superstitious significance (Genesis 19:26); Lot’s wife was noted in the journals of Fulcher of Chartres (Chaplain to King Baldwin) who accompanied the crusader Baldwin I across the Dead Sea valley in December 1100 AD. In reality, the apophenic feature described as Lot’s wife is a 12m-high column of diapiric salt lying at the foot of the much larger Mt Sedom (Usdum) on the edge of the Dead Sea (Figure 12). It is one of a number of dissolutional remnants along the gypsum-capped cavernous edge of an outcropping diapir composed of Miocene salt, which makes up to core of Mount Sedom.

References

Aufderheide, A. C., 2011, The Scientific Study of Mummies, Cambridge University Press, 634 p.

Hay, R. L., and T. K. Kyser, 2001, Chemical sedimentology and paleoenvironmental history of Lake Olduvai, a Pliocene lake in northern Tanzania: Geological Society of America Bulletin, v. 113, p. 1510-1521.

Li, M., G. Ma, K. Guttikonda, S. Boyages, and C. Eastman, 2001, The re-emergence of iodine deficiency in Sydney, Australia: Asia Pacific J of Clin. Nutr., v. 10, p. 200-203.

Lonn, E., 1933, Salt as a Factor in the Confederacy: New York, Walter Neale, 324 p.

McDonnell, C., M. Harris, and M. Zacharin, 2003, Iodine Deficiency and goitre in schoolchildren in Melbourne, 2001: Med. J. Aust., v. 178, p. 159-162.

Pollard, A. M., D. R. Brothwell, A. Aali, S. Buckley, H. Fazeli, M. H. Dehkordi, T. Holden, A. K. G. Jones, J. J. Shokouhi, R. Vatandoust, and A. S. Wilson, 2008, Below the salt: A Preliminary study of the dating and biology of five salt-preserved bodies from Zanjun Province, Iran: Iran, v. 46, p. 135-150.

Stackert, J., 2010, The variety of ritual application for salt and the Maqlu salt incantation, in T. Abusch, and D. Schwemer, eds., Corpus of Mesopotamian Anti-witchcraft Rituals: Volume One, Brill, p. 235-252.

Vuhahula, E. A. M., J. R. P. Masalu, L. Mabelya, and W. B. C. Wandwi, 2009, Dental fluorosis in Tanzania Great Rift Valley in relation to fluoride levels in water and in "Magadi" (Trona): Desalination, v. 248, p. 610-615.

Danakhil Potash, Ethiopia: Beds of Kainite/Carnallite, Part 2 of 4

John Warren - Wednesday, April 29, 2015

The modern Dallol saltflat described in the previous blog defines the upper part of more than 970 metres of halite-dominated Quaternary evaporites that have accumulated beneath the present salt pan of the Northern Danakhil. The total sequence is made up of interbeds of halite, gypsum, anhydrite and shale with a potash succession separating two thick sequences of halite (Figure 1; Holwerda and Hutchison, 1968; Augustithis, 1980). At depths of more than 35-40 meters, and deepening to the east, this km-thick subcropping Quaternary halite-dominated fill contains one, and perhaps two or more, potash beds. For a more detailed description of the upper part of the fill the reader is referred to the previous blog and Chapter 11 in Warren, 2015.


Bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series and alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series across much of the southern part of the depression beyond Lake Assale). Potash exploration drilling and core recovery is concentrated in the accessible parts of the northern Danakhil rift, where the saltflat facilitates vehicle access, compared with the lava-covered regions south of Lake Assale. The most recent volcanic activity affecting the known potash region was the emplacement of the Dallol Mound, which has driven local uplift of the otherwise subsurface potash section to where it approaches the surface in the immediate vicinity of the mound (Figure 2a).

Away from the Dallol volcanic mound the upper potash bed beneath the saltflat lies at a depth of 38-190 metres. A lower inferred potash bed likely occurs at depth along the eastern end of the saltflat, but this second bed is inferred from high API kicks in gamma logs run in deeper wells, no solid salt was recovered (Holwerda and Hutchison, 1968). The upper proven potash bed is now the target zone for a number of minerals companies currently exploring for potash in the region. Regionally, both potash units dip east, with the deepest indicators of the two units encountered by the drill in a single well on the eastern side of the saltflat at depths of 683 and 930 m, respectively (Figure 2: Holwerda and Hutchison, 1968). The likely Quaternary age of the potash units, the marine brine source, explains the high magnesium content of the potash bittern salts, as modern seawater contains high levels of Mg and SO4.


My study of core that intersected the potash interval and that is sandwiched between the Lower and Upper Halite units shows both the lower and the upper halite units retain pristine sedimentary textures, with features and vertical successions that indicate distinct hydrologies during their deposition (Figure 3). There is no textural evidence of halokinetic recrystallization in halites any of the studied cores and published seismic also indicates consistent dips in the evaporites . Most of the textures in the cored potash interval indicate a subaqueous density-stratified environment, with brine reworking of the upper part of primary kainitite, carnallitite units. Perennial subaqueous, density-stratified brines also typify the hydrology of the Lower Halite unit, albeit with somewhat lower salinities tan those precipitating the bitterns (Figure 3). The brine that precipitating the Upper Rocksalt Formation was shallower and more ephemeral. The following paragraphs summarise my core-based observations and interpretations that led to this interpretation of the evolving brine hydrology.

The Lower Rocksalt Formation (LRF) is dominated by bottom-growth-aligned subaqueous halite textures and lack of siliciclastic detritus, unlike the Upper Rocksalt Formation (Figure 3). Halite textures in the LRF lack porosity and dominated by coarsely crystalline beds made up of cm-scale NaCl-CaSO4 couplets dominated by upward-pointing halite chevrons and mantled by thin CaSO4 layers (Figure 3). This meromictic-holomictic textural association passes up into the upper part of the LRF with cm-scale proportions alternating of less-saline to more-saline episodes of evaporite precipitation decreasing, indicating an “on-average” increasingly shallow subaqueous depositional setting as one approaches the base of the kainitite unit. The combination of bottom-nucleated and cumulate textures in the LRF are near identical to those in the halites in the kainitite interval in the Messinian of Sicily (see later). 

The laminated Kainitite Member is also a subaqueous unit with layered cumulate textures (Figure 3), it was likely deposited on a pelagic bottom beneath a shallow body of marine-fed bittern waters, which never reached carnallite saturation. Above this are the variably present carnallitic Intermediate and Sylvinitite members and the overlying Halite marker beds in turn overlain by the Upper Halite unit. All retain pristine textures indicating mostly subaqueous deposition, soon followed by varying exposure and reaction with shallow phreatic brines moving across the top of Kainitite member. This shallow phreatic brine crossflow drove syndepositional mineral alteration and collapse in the upper part of the kainitite and carnallitite units.


The potash-entraining interval between the URF and LRF is called the Houston Formation has been drilled and cored extensively by explorers in the basin, showing it is consistently between 15 and 40 metres thick (Figure 1). Stratigraphically, it consists of lower Kainitite Member (4-14m thick) atop and in depositional continuity with the LRF (more than 500m thick) (Figure 3). The Kainitite Member is fine-grained, laminated, locally wavy-bedded, containing up to 50% kainite cumulates in a cumulate (non-chevron) halite background, along with small amounts of a white mineral that is likely epsomite. It is overlain by what older literature describes as the “Carnallitite Intermediate unit” (3-25 m thick). More recent potash exploration drilling has shown all the members that constitute the Intermediate Carnallitite Member is not always present within the Dallol depression. Mineralogically is at best considered as variably developed (Figure 3). Its lower part is a layered to laminated carnallite-halite mixture with some kieserite, anhydrite and epsomite. This can pass up or laterally into kainitite with sylvite. Above the Intermediate Member is the 0-10m thick Sylvinite Member containing 20-30% sylvite, along with polyhalite and anhydrite (up to 10%). Typically the sylvinite member shows primary layering disturbed by varying intensities of slumping and dissolution. Often the upper part of a carnallite unit (where present) also shows similar evidence of dissolution and reprecipitation.

Cores through the sylvinite member and parts of the upper carnallitite member sample a range of recrystallization/flow/slump textures, rather than primary horizontal-laminar textures. Beneath the sylvinite member, the variably-present upper carnallitite member contains a varied suite of non-commercial potash minerals that in addition to carnallite include, kieserite, kainite (up to 10% by volume) and polyhalite, along with minor amounts of sylvite. Minor anhydrite is common, while rinneite may occur locally, along with rust-red iron staining. Sylvite is more abundant near the top of the carnallitite member and its proportion decreases downward, perhaps reflecting its groundwater origin. Kainite is the reverse and its proportion increases downward. The sylvinite member and the carnallitite member also show an inverse thickness relationship. Bedding in the carnallitite member is commonly contorted with folded and brecciated horizons interpreted as slumps. The base of the carnallitite member is defined as the level where carnallite forms isolated patches in the kainite before disappearing entirely.

Drilling in the past few years has clearly show that in some parts of the evaporite unit, located nearer to the western side of the basin, the lower and upper carnallite units are separated by thick bischoftite intervals (Figures 2b, 3). The bischofite is layered at a mm-cm scale and with no obvious breaks related to freshening and exposure, implying it too was deposited in a perennially subaqueous or phreatic cavity setting (Pedley et al., in press).

The potash/bischofite interval passes up into a slumped and disturbed halite-dominated unit that is known as the “Marker Beds” because of the co-associated presence of clay lamina and bedded halite, along with traces of potash minerals (Figure 3). This unit then passes up into the massive Upper Rock Salt unit across an unconformity at the top of the halite “Marker Beds.” Bedded, and at times finely laminated cumulate textures in the various magnesian bittern units, are used by many to argue that the kainitite and the lower carnallitite members are primary or syndepositional precipitates.

Three types of potash-barren zones can occur within it and are possibly related to the effects of groundwaters and solution cavity cements within the carnallitite unit, perhaps precipitated before the deposition of the overlying marker halites. Barren zones in the Sylvinite member are regions where: a) the entire sylvinite bed is replaced by a relatively pure stratiform halite, along with dispersed nodules of anhydrite, b) zones up to 23 m thick and composed of pure crystalline halite (karst-fill cements?) that occur patchily within the sylvinite bed and, c) potash-depleted zones defined by coarsely crystalline halite instead of sylvinite. Bedding plane spacing and layering and some slumping styles in the halite in styles a and b are similar to that in the sylvite bed. Contact with throughflushing freshened nearsurface and at-surface waters perhaps created most of the barren zones in the sylvinite. Fluid crossflow may also have formed or reprecipitated sylvite of the upper member, via selective surface or nearsurface leaching of MgCl2 from its carnallite precursor (Holwerda and Hutchison, 1968; Warren 2015). Due to the secondary origin of much of the sylvite in the Sylvinite member, the proportion of sylvite decreases as the proportion of carnallite increases, along with secondary kieserite, polyhalite and kainite.

The kainite member is texturally distinctive and is composed of nearly pure, fine-grained, dense, relatively hard, amber-coloured kainite with ≈ 25% admixed halite (Figure 3). Core study shows the lamina style remained flat-laminar (that is, subaqueous density-stratified with periodic bottom freshening) as the mineralogy passes from the LRF up into the flat-laminated kainitite member (Figure 4: Warren, 2015; Pedley et al., in press). Throughout, the kainitite unit shows a cm-mm scale layering, with no evidence of microkarsting or any exposure of the kainitite depositional surface. That is, the Kainitite Member is a primary depositional unit, like the underlying halite and still retains pristine evidence of its dominantly subaqueous depositional hydrology. The decreased proportion of anhydrite in the Kainitite Member, compared to the underlying LRF, indicates a system that on-average was more saline than the brines that deposited the underlying halite. The preponderance of MgSO4 salts means the Kainitite unit like the underlying LRF formed by the evaporation of seep-supplied seawater.

This situation differs from the present “closed basin” hydrology of the Danakil Depression which typifies the URF and the overlying Holocene succession (Hardie, 1990; Warren, 2015).

Units atop the primary laminated textures of the kainitite, lower carnallitite and bischofite members (where present) tend to show various early-diagenetic secondary textures (Figure 4). It seems much of the sylvinite and upper carnallitite member deposition was in shallow subsurface or at-surface brine ponds subject to groundwater crossflows and floor collapse, possibly aided by seismically-induced pulses of brine crossflow. In addition, this perennial density-stratified brine hydrology was at times of holomixis subject to brine reflux and the brine-displacement backreactions that typify all evaporite deposition, past and present (Warren 2015).

The observation of early ionic mobility in potash zone brines in the Danakil depositional system is also not unusual in any modern or ancient potash deposit. It should not be considered necessarily detrimental to the possibility of an extensive economically exploitable potash zone being present in the Danakil Depression. Interestingly, all the world’s exploited potash deposits, including those in the Devonian of Canada and Belarus, the Perm of the Urals and the potash bed of west Texas, show evidence of syndepositional and shallow burial reworking of potash (Warren, 2015). Early potassium remobilization has created the ore distributions in these and other mined potash depositsTextures and mineralogies in the Upper Rocksalt Formation (URF) define a separate hydrological association to the marine-fed LRF and Houston Formation (Table 4). Compared to the LRF, the URF has much higher levels of depositional porosity, lacks high levels of CaSO4, and has high levels of detrital siliciclastics. This is especially so in its upper part, which shows textural evidence of periodic and ongoing clastic-rich sheetflooding and freshening (Figure 4). It was deposited in a hydrology that evolved up section to become very similar to that active on today’s halite pan surface. The URF contains no evidence of salinities or textures associated with a potash bittern event and is probably not a viable exploration target for solid potash salts.

Above the URF is a clastic unit with significant amounts of, and sometimes beds dominated by, lenticular gypsum and displacive halite. The unit thickens toward the margins of the depression (Figure 2). The widespread presence of diagenetic salts indicates high pore salinities as, or soon after, the saline beds that stack into the clastic unit were deposited. Some of these early diagenetic evaporite textures are spectacular, as seen in the displacive halite recovered in a core from the lower portion of the clastic overburden, some 45 m below the modern pan surface (Figure 3).

What is clear from the textures preserved in the potash-rich Houston formation and the immediately underlying and overlying halites is that they first formed in a subaqueous-dominated marine-fed hydrology (Figure 4), which evolves up section into more ephemeral saltpan hydrologies of today (see the previous blog). The potash interval encapsulated in the Houston formation has primary mineralogical associations that are derived by evaporation of Pleistocene seawater (kainitite, carnallitite). In contrast the sylvite section in the Houston tends to form when these primary mineralogies are altered diagenetically perhaps soon after deposition but, especially, when hydrothermal waters circulated through uplifted beds of the Houston Formation, as is still occurring in the vicinity of the Dallol Volcanic Mound. Or where the chemical/meteoric interface associated with the encroachment of the bajada sediment pile drove incongruent dissolution of carnallite along the updip edge of the Houston Fm (as we shall discuss in the next blog). 

References

Augustithis, S. S., 1980, On the textures and treatment of the sylvinite ore from the Danakili Depression, Salt Plain (Piano del Sale), Tigre, Ethiopia: Chemie der Erde, v. 39, p. 91-95.

Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

Pedley, H. M., J. Neubert, and J. K. Warren, in press, Potash deposits of Africa: African Mineral Deposits, 35TH International Geological Congress (IGC), Capetown (28 August to 4 September 2016).

Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p. 

Danakhil Potash, Ethiopia: Is the present geology the key? Part 1 of 4

John Warren - Sunday, April 19, 2015

Geology of potash in the Danakil Depression, Ethiopia: Is the present the key?

The Danakhil region, especially in the Dallol region of Ethiopia, is world renowned for significant accumulations of potash salts (both muriates and sulphates), and is often cited as a modern example of where potash accumulates today. What is not so well known are the depositional and hydrological dichotomies that control levels of bittern salts in the Pleistocene stratigraphy that is the Danakhil fill. Geological evolution of the potash occurrences in the Dallol saltflat and surrounds highlights the limited significance of Holocene models for potash, when compared to the broader depositional and hydrological spectra preserved in ancient (Pre-Quaternary) evaporite deposits (see Warren, 2010, 2015 for a more complete analysis across a variety of evaporite salts).

Across the next four blogs, I shall discuss the geological character of the Danakhil fill and the controls on potash in the depression via four time-related discussions; A) Current continental fan - saltflat hydrology that typifies present and immediate past deposition in the depression (Danakhil Blog1). B) A time in the latest Pleistocene when there was a marine hydrographic connection exemplified by a healthy coralgal rim facies (probably ≈ 100,000 years ago, and a subsequent drawndown gypsum rim facies. Both units are discussed in this blog, (Danakhil Blog1), and C) a somewhat older Pleistocene period when widespread potash salts were deposited via a marine seepage fed hydrology (Danakhil Blog2). Then, within this depositional frame, we will consider D) the influence of Holocene volcanism and uplift driving remobilisation of the somewhat older potash-rich evaporite source beds into the Holocene hydrology (Danakhil Blog3) and finally how this relates to models of Neogene marine potash deposition (Danakhil Blog4). These observations and interpretations are based in large part on a two-week visit to the Dallol, sponsored by BHP minerals, and focused on the potash geology of the region. 

Dallol Physiography

The Danakhil Depression of Ethiopia and Eritrea is an area of intense volcanic and hydrothermal activity, with potash occurrences related to rift magmatism, marine flooding, and deep brine cycling. The region is part of the broader Afar Triple Junction and located in the axial zone of the Afar rift, near the confluence of the East African, Red Sea and Carlsberg rifts (Figure 1a; Holwerda and Hutchison, 1968; Hutchinson and Engels, 1970; Hardie, 1990). The depression defines the northern part of the Afar depression and runs SSE parallel to the Red Sea coast, but lies some 50 to 80 km inland, and is separated from the Red Sea by the Danakil Mountains. The fault-defined Danakil Depression is 185 km long, up to 70 km wide, with a floor that in the deeper parts of the depression is more than 116 meters below sea level. It widens to the south, beginning with a 10 km width in the north and widening up to 70 km in the south (Figure 1a). In the vicinity of Lake Assele, the northern portion of the Danakil is known as the Dallol Depression and has been the focus for potash exploration for more than a century and is in the deepest region of the depression with elevations ranging between 50m to 120m below sealevel (Figure 1b, c). Shallow volcano-tectonic barriers, behind Mersa Fatma, Hawakil Bay and south of the Gulf of Zula, prevent hydrographic (surface) recharge to the depression. Marine seepage is not occurring at the present time, but likely did so at the time the main potash unit was precipitated. Lake Assele (aka Lake Kurum) with a water surface at -115m msl should not be confused with Lake Asal (-155 msl), located 350 km to the southeast of the Danakil. Asal an active marine-fed hydrographically isolated lacustrine drawdown system, which today is depositing a combination of pan halite and subaqueous gypsum in the deepest part of the Asal-Ghubbat al Kharab rift (Figure 1a; Warren, 2015).

Today the halite-floored elongate saltpan, known as the Dallol saltflat, occupies the deepest part of the northern Danakil Depression, extending over an area some 40 km long and 10 km wide (Figure 1b, c). The pan’s position is asymmetric within the Danakil Depression; it lies near the depression’s western edge, some 5km from the foot of the escarpment to the Balakia Mountains and the Ethiopian Highlands, but some 50 km from the eastern margin of the depression, which is in Eritrea. The Dallol saltpan and adjacent Lake Assele today constitute the deepest continental drainage sump in the Afar depression (Figure 1b, c). The area, located east and northeast of the main modern Dallol saltpan depression, is mostly an extensive gypsum plain (Bannert et al., 1970). As we shall see, the gypsum pavement, and its narrower equivalents on the western basin flank, defines a somewhat topographically higher (still sub-sealevel) less-saline, lacustrine episode in the Dallol depression history fill. To the south of the Dallol salt pan, bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series in combination with alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series in southern part of the depression beyond Lake Assele (Figures 1a).

Climate

In terms of daily and monthly temperatures, the Dallol region currently holds the official record for highest average, year-round, monthly temperatures; in winter the daily temperature on the saltflat is consistently above 34°C and in summer every day tops 40°C, with some days topping 50°C (Figure 2; Oliver, 2005). These high temperatures and a lack of rainfall, typically less than 200 mm each year, place the Dallol at the hyperarid end of the world desert spectrum and so it lies at the more arid end of the BWh Köppen climate zone (Kottek et al., 2006; Warren, 2015).


History of extraction of salt products and their transport (Table 1)

Using little-changed extraction and transport methods, salt (halite) has been quarried by local Afar tribesmen for hundreds of years. First, using axes, a crust of pan salt is chopped into large slabs (Figure 3a). Then workers fit a set of sticks into grooves made by the axes. Next, working the stick, workers lever slabs of bedded salt, which is cut into rectangular tiles of standard size and weight, called ganfur (about 4kg) or ghelao (about 8kg). Tiles are stacked, tied and prepared for transport out of the depression on the backs of camels and donkeys (Figure 3b). Around 2,000 camels and 1,000 donkeys come to the salt flat every day to transport salt tiles to Berahile, about 75 km away. Previously, salt tiles were carried via camel train to the city of Mekele, some 100 km from the Danakil. Mekele, located in the Ethiopian highlands is known as the hub of Ethiopia’s former “white gold” salt trade and still today is known as the “old” salt caravan city. Today, the salt caravans walk the extracted salt to Berahile, located some 60 km from Mekele. From there, trucks transport the salt to Mekele. Each truck can transport up to 350 camel salt loads. From the Mekele salt market, Dallol salt blocks are transported and sold to all parts of Ethiopia for use mainly as table salt or as an add-on in animal feed. The lifestyle of the miners and the camel trains is likely to change in the next few decades as sealed roads are now under construction that will link Mekele to Dallol.


Once potash (sylvite and carnallite) was discovered in the Dallol region in 1906, an Italian company by the name of Compagnia Mineraria Coloniale (CMC) established the first mining operation. In 1918 a railway was completed from the port of Mersa Fatma to a termination some 28 km from Dallol (Table 1). Rail construction took place from 1917-1918, using what was then the British and French “military-standard” 600 mm rail-gauge Decauville system. "Decauville" rail construction used ready-made sections of small-gauge track and so the trackway was rapidly assembled; <2 years to complete more than 50 km of track. Once completed, the railway transported extracted potash salt from the "Iron Point" rail terminal near Dallol, via Kululli to the port. Potash production is said to have reached some 50,000 metric tons in the 1920s, extracted from an area centred on the Crescent Deposit, which is located near the foot of uplifted lake beds on the southern flanks of Mt Dallol. However, significant salt production had ceased by the end of the 1920s, as large-scale mines in Germany, the USA, and the USSR began to supply the world market with cheaper product. Unsuccessful attempts to reopen potash production were made in the period 1920-1941. Between 1925-29 some 25,000 tons of sylvite were shipped by rail from the Dallol, with a product that averaged 70% KCl. After World War II, the British administration dismantled the railway and removed all traces of it. In 1951-1953, the Dallol Co. of Asmara sold a few tons of product from the Dallol.


The potash concession title was transferred to the American “Ralph M. Parsons Company” (Parsons) at the end of the 1950s. Parsons initiated the first systematic exploration for potash in the Danakil depression and drilled more than 250 exploration holes during their 9-year evaluation campaign. Major potash resources were confirmed a few km west of Mount Dallol, in a mineralized zone that was named the “Musley” Deposit. Following on from positive exploration results, they began an engineering study to investigate potential processing and mining methods for the Musley Deposit and subsequently in October 1965 sank a shaft into the orebody. They installed underground mine facilities and established a pilot processing plant on surface, to investigate recovery from the bulk samples collected from the underground workings. They envisaged developing the Musley Deposit as a conventional room-and-pillar operation and to this end developed six underground drifts totalling some 805 m in length. Unconfirmed reports suggest that an influx of water flooded the mine (possibly triggered by a seismic event) and after failed attempts to solve the water problem, the activities Parsons ceased activities in 1968. As of end 2014, some salt block buildings built by the Italian and other companies still partially stand as ruins, along with rusting equipment.

Based on the previous work conducted by Parsons, a German potash producer, Salzdetfurth AG (SAG), began a new exploration campaign in the Danakil Depression in 1968 and 1969. In addition to their work in the Dallol depression, SAG drilled a number of wells in a concession south of Lake Assale, and conducted a geological mapping campaign as far north as Lake Badda, on the border with Eritrea. SAG’s exploration work away from the known Dallol deposits did not prove fruitful as they drilled only one drill hole that reached the potash level. This drill hole, located approximately 25 km to the southeast of Mount Dallol, intersected a kainitite bed, with no sylvinite intersection. The SAG concession was returned to state authorities of Ethiopia. Subsequent drilling by other explorationists in this region has confirmed the deepening of the kainitite level to the southeast of Dallol and the lack of sylvinite at greater depths.

Since the dismantling of the railway, there has been no high-volume transport system to carry potash product the Red Sea coast. Currently, the Ethiopian Government is constructing all-weather roads from Dallol to Mekele and Afdera When complete this road system will facilitate transport of future potential potash product from the Dallol to Afdera, from where existing roads provide access to Serdo and from there to the seaport of Tadjoura in Djibouti (Figure 1a). This section requires an addition 30 km of all-weather road to be completed to the coast and will facilitate cost-effective transport of potash product to the large agricultural markets of India and China. The transport distance to the Eritrean coast from Dallol is much shorter, but political considerations mean such a route is not a viable option at the present time.

EVAPORITE DEPOSITIONAL PATTERNS IN OUTCROP

Surficial sediment distributions outline classic drawdown facies belts in the Dallol region, with a wadi-fed alluvial fan fringe passing down dip into sandflats (local dune fields), dry mudflats (with springs), saline mudflats and ephemeral to perennial brine pans of Lake Assele (Figure 1b). The fans, especially along the western margin of the depression are indented or locally covered by a mostly younger succession of constant-elevation marine, biochemical and evaporitic sediments fringes or “bathtub rim” facies (Figure 4).

 

Alluvial Fan fringe (Bajada) 

Pleistocene alluvial/fluvial beds, exposed by local uplift, deflation and ongoing watertable lowering, outcrop about updip edges to the salt-crusted parts of the northern Danakil, and form low flat-topped plateaus or mesas on the plain. These mesas define the tops of alluvial fans aprons, which are heavily dissected and eroded by occasional storm runoff and rainfall. This fan fringe contains relatively fresh water lenses in a desert setting that is one of the world’s harshest (Kebede, 2012). Most of the depositionally active fans line the western margin of the basin and many of the downdip fan edges occur slightly up dip a still-exposed gypsum pavement (Figure 5a), showing depositional equilibration largely with an earlier higher lake stage, while others, such as the Musley fan, have flowed across cut into the gypsum pavement level and now feed water and sediment directly into the edges of the saltflat that defines the lower parts of the depression (Figure 4). Watercourses of the fans that have dissected earlier wadi (bajada) deposits as well as the earlier lacustrine gypsum and limestone pavements so create excellent windows into the stratigraphy of these units. Fan avulsion is indicated by palaeosol layers exposed by downcutting of younger streams (Figure 5b, c).

 

The Musley fan characteristics are well documented by current and previous potash explorers in the basin as these permeable gravels and sands store a reliable water source for potential solution mining/ore processing in the Musley area and so has been cored by a number of proposed water wells. Internally, the fan is composed of interfingering layers and lenses of sand, gravel and clay (paleosols), with highly porous intervals in the sand and gravels (Figure 5b, c). Depth to the watertable varies from >2m to 60m, and salinities from 760 ppm to more than 23,400 ppm. The principal source of recharge is flash flooding, originating in Musley Canyon, which drains the Western Escarpment, along with minor inflows from the adjacent uplifted volcanic block and local highly intermittent rains (Figure 1a). Of six potential water wells drilled in the fan by the Ralph M. Parsons Company in the 1960s, four returned water of good quality (<2000 mg/l), while the other two had waters with salinities in excess of 20 g/l. Pumping test data indicate average transmissivity of the water-bearing beds around 870m2/day, with salinities in the fan increasing from west to east, approaching the saltflat.

Chemical sediments outcropping in the depression

Overall, surface sediment patterns in the Danakil depression define a depositional framework of brine drawdown, related to basin isolation from an earlier hydrographic (at surface) marine connection to the Red Sea, followed by stepped evaporative drawdown. This is indicated by fringing topographically-distinct belts or rims of now inactive coralgal carbonates and gypsum evaporites (aka “bathtub ring” patterns) that cover earlier Pleistocene and Neogene clastics (Figures 3a, e, Figure 4). These “rims” of marine limestone and subsequent gypsum were followed by today’s drawdown saline-pan halite-dominant hydrology (Figures 4, 7a-c). The current hydrological package of sediments encompassing the current drawdown episode lies atop and postdates the Pleistocene potash-hosting Lower Halite Formation in the depression and is probably equivalent to the uppermost part of the clastic overburden facies, as illustrated in the drilled and cored portions of the depression stratigraphy. As we shall discuss in the next blog, only the uppermost portion of the recovered core stratigraphy has equivalents in current depression hydrology (Figure 6). 


In earlier work, some authors interpreted the fringing belts, especially the exposed coralgal reef belt, as being possibly of Pliocene or even Miocene age. However, when one looks at the stratiform nature of the outcrop trace of both the reef belt and the gypsum belt, and the carapace nature of its depositional boundaries in the field, it is immediately apparent they must be younger (Figure 5a, c; Figure 7). Both the reefal and gypsum belts track horizontal hydrological intersections with the landscape, in what is an ongoing volcanogenic and tectonically active depression. When the reefal belt image is overlain by a DEM it shows the reef belt is consistently at sea level (Figure 1c). If the outcrops of the reef belt and the gypsum pavement were older than late Pleistocene or Holocene, then ongoing episodes of tectonism and volcanism would have modified the elevations of the two outcrop belts in the landscape, as is seen in Miocene redbed outcrops. These underlying and centripetal Miocene sections clearly show the influence of ongoing tilting and tectonism and hence why the flat-lying tops to the reef and gypsum belts imply a late Pleistocene-Holocene (Figure 5d).

That is, the topographic distribution of the top of the reef facies, which lies within a metre or two of current sea level, implies that the Danakil depression had a relatively recent connection to the Red Sea. The pristine preservation of aragonitic corals and sand dollars in the adjacent marls suggest the connection was either related to the penultimate interglacial (around 104, 000 years ago) or to an early Holocene transgression into the depression. Bannert et al. (1970) assign a C14 age of 25.4-34.5 ka to this formation. However, we consider this is unlikely as DEM overlay levelling shows the reef rim, wherever it outcrops, lies within a meter of current sealevel. World eustacy clearly shows that sealevel was more than 50-60m below its present level some 25,000-30,000 years ago. A 25-35 ka determination of the reef rim would require the whole basin was subject to a single basinwide wide vertical uplift event that did not fragment or disturb the lateral elevation of the rim.

The coralgal reef terrace indicates normal marine water were once present in the Dallol depression, while the occurrences of the stratiform gypsum pavement are consistent with a former arid lake hydrology at a somewhat lower elevation than the reef rim (Figures 1c, 5a). Like the reef rim, the gypsum pavement fringe defines a consistent elevation level or surface, most clearly visible along the western margin of the present salt flat. It is the result of gypsum deposition during a period of drawdown associated with brine level stability, subsequent to the isolation of the depression from its former “at-surface” marine connection. During this time gypsum accumulated as a stack of subaqueous aligned gypsum beds, along with a series of gypsiferous tufas and rhizoconcretions in zones about the more marginward spring-fed parts of the gypsiferous lake margin (Figure 7d-f). The evolution from marine waters that deposited the reefs and adjacent echinoid-rich lagoonal marls at a higher level in the depression (the lit zone) into a more saline seepage-fed system, with no ongoing marine surface connection to the Red Sea is indicated by the diagenetic growth of large lenticular (“bird’s-beak”) gypsum crystals within the marine marls and the dominant subqueous bottom-nucleated textures in the gypsum beds. In a similar way, the now-outcropping subaqueous-gypsum drawdown rim deposits, located at higher elevations than current saline pan levels typify other drawdown saline lakes in the Afar region, such as Lake Asal in Djibouti, all such occurrences indicate an earlier, somewhat less saline, hydrological equilibrium level (Warren, 2015).


Active today is the lowest parts of the Dallol saltflat is an ephemeral saltpan hydrology indicated by bedded salt crusts dominated by megapolygonal crusts made up of aligned-chevron halite stacks separated by mm-cm thick mud layers . This current pan hydrology is associated with even greater drawdown levels compared to the former gypsum-dominant hydrology (Figure 8). Current deposits, made up a series of stacked brine-pan salt sheets,  are still quarried as a renewable resource by the local tribesmen (Figure 3). These modern brine flats accumulate pan halite whenever the Lake Assele brine edge (strandline) is periodically blown back and forth over the modern brineflat. It driven by southerly winds, which are frequent in the annual weather cycle, and can move thin sheets of brine kilometres across the pan in a few hours (Figure 1a, Figure 8). Superimposed on this southerly supply of brine is an occasional land-derived sheetflood event, driven by rare rainstorms and the deposition of silt-mud layers from water sheets sourced from the adjacent wadi belt. This ephemeral brineflat hydrology is stable with respect to the current climate (groundwater inflow ≈ outflow). It means the current brineflat of the northern Danakil low is  accumulating bedded pan salt at an even lower topographic level in the basin than the surrounding gypsum pavement, so implying today’s halite-dominant pan beds form under more arid conditions (less humid, more drawndown) than that of the gypsum pavement.


This stepped (reef to gypsum to halite) late-Pleistocene-early Holocene hydrology, captured in the modern surficial geology of the Dallol Depression, likely postdates a somewhat wetter (humid) climatic period indicated by the widespread deposition of a clastic overburden unit, atop the Upper Halite Formation (UHF; Figure 6). That is, the modern hydrology in present-day Lake Assale, and the adjacent saline mud flats of the Dallol pan, is not the same hydrology as that which precipitated the massive salt of the Upper Halite Formation (UHF). A potash-free halite unit extensively cored beneath the present clastic-dominated saline pan (to be discussed in the next blog). Texturally and hydrologically the depositional system of stacked salt crusts, which dominates the upper part of the UHF in the cored wells, is similar to today's halite-dominated passage from the salt flat into the subaqueous Lake Assale. However, as we shall see, a wetter moister period, dominated by sheetfloods and higher amounts of clastics, separates the two hydrological events in all the cored wells. Today's outcrop geology of alternating saltpan and clastic beds are a different to marine-fed seepage hydrology formed the Lower Halite Formation (LHF), with its potash bittern cap (Houston Formation). 

Most importantly there is no evidence of primary potash deposition in the modern lake/ pan hydrology of the Dallol saltflat. It is clear that the world-famous bedded potash (mostly kainitite) units of the Danakhil accumulated in a bittern hydrology that is not present in today's Dallol depositional hydrology (Blog 2). As we shall see, Holocene potash only occurs in the vicinity of the Dallol Volcanic Mound, where uplift has moved older, formerly buried, potash beds into a more active hydrothermal hydrology (Blog 3).

References

Bannert, D., J. Brinckmann, K. C. Käding, G. Knetsch, M. KÜrsten, and H. Mayrhofer, 1970, Zur Geologie der Danakil-Senke: Geologische Rundschau, v. 59, p. 409-443.

Ercosplan, 2011, Resource Report for the Danakhil Potash Deposit, Afar State Ethiopia, comissioned by Allana Potash. Document EGB 11-008.

Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

Hutchinson, R. W., and G. G. Engels, 1970, Tectonic significance of regional geology and evaporite lithofacies in northeastern Ethiopia: Philosophical Transactions of the Royal Society, v. A 267, p. 313-329.

Kebede, S., 2012, Groundwater in Ethiopia: Features, numbers and opportunities, Springer.

Kottek, M., J. Grieser, C. Beck, B. Rudolf, and F. Rubel, 2006, World Map of the Köppen-Geiger climate classification updated: Meteorologische Zeitschrift, v. 15, p. 259-263.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p.

What is an evaporite? Solar versus cryogenic (freeze-dried) salts

John Warren - Tuesday, February 24, 2015

The term evaporite is usually used to describe sediment precipitated during the solar-driven desiccation of a standing water body in a saltern or salina, or a near-surface pore brine in an evaporitic mudflat or sabkha. Almost all the modern and ancient examples of bedded salts that we work with in the rock record are thought to have crystallised via this process of solar evaporation. The chemical dynamics of solar evaporation are simple; on average, water molecules within a standing at-surface brine lake or in near-surface pore spaces, near a water-table and its associated capillary zone, do not have enough kinetic energy to escape the liquid phase and so cross the surface tension barrier (figure 1). Otherwise, liquid water would turn to vapour spontaneously and any at-surface liquid phase would spontaneously disappear, while recharge to an underlying water-table would be an impossibility.

 

Every so often in this situation, the level of solar energy transfer (heat absorption) at the molecular collision site is sufficient to give a water molecule (near the water-air interface) the heat energy necessary to pass into the vapour phase and so exit the liquid water mass (figure 1). That is, for a water molecule to escape into the vapour phase it must absorb heat energy, be located near the liquid surface, be moving in the proper direction and have sufficient energy to overcome liquid-phase intermolecular forces and then pass through the surface tension interface. As the concentration of the residual brine increases, the specific heat capacity decreases, and the density increases (the effects of specific heat and density increases on evaporite mineralogy and distribution in the depositional setting will be the topic of a future blog).

But salts, some with the same mineralogy as solar evaporation salts, can also form as a water or brine body freezes to leave behind cryogenic salt layers (aka the "freeze-dried" salts). This is the process that forms significant volumes of the sodium sulphate salts in various cold-zone brine lakes and saline ice-sheets around the world. Unlike solar evaporites, cryogenic brines and associated salts require temperatures at or below the freezing point of the liquid phase. Cryogenic salts, such as mirabilite (Na2SO4.10H2O), hydrohalite (NaCl.2H2O), antarcticite (CaCl2.6H2O) and epsomite (MgSO4.7H2O), can then accumulate. These cryogenic salts crystallise in cold, near-freezing, residual brines as they concentrate via the loss of the liquid water phase as it converts/solidifies to ice. As the volume of ice grows, the various anions and cations are excluded from the expanding ice lattice. Hence, concentration of the residual brine increases until it reaches saturation with a salt phase that then precipitates (figure 2). There are a number of well-documented cryogenic salt beds in various Quaternary-age cold-continental lacustrine settings. Probably the best known are the sodium sulphate salts in Karabogazgol, Turkmenistan, where strand-zone stacks of cryogenic mirabilite form each winter. Beneath the lake centre there are subsurface beds (meters thick) of Quaternary-age cryogenic glauberite-halite.

 

In the Turkmen language, Karabogazgol means “lake of the black throat,” so named because the gulf is continually gulping down the waters of the Caspian Sea, via a narrow connecting natural channel (figure 3). Ongoing evaporation in Karabogazgol keeps the water surface in the perennial brine lake depression around a metre below that of the Caspian. It is one of our few natural examples of evaporative drawdown occurring via a hydrographic (surface) connection to the mother water body. Groundwater seepage connections with the mother water mass are more typical, especially in hot arid basins.

 

Only since the end of the Soviet era and the re-opening of the lake’s natural connection to the Caspian Sea in 1992, by a newly independent Turkmen government, did Karabogazgol re-fill with perennial brines. Since then, a natural cryogenic mirabilite winter cycle has returned the Karabogaz hydrology to its longterm natural state. Today, the main open water body in the centre of Karabogaz is a Na-Mg-Cl brine, sourced via gravitationally-driven inflow of Caspian Sea waters. Perennial Karabogaz brines today have a density of 1.2 g/cm3, and pH values that range between 7.2 and 9. Surface water temperatures in the lake centre range from around 4°C in December (winter) to 25°C in July (summer). Temperature fluctuations and the cool arid steppe climate (Koeppen BSk) of Karabogazgol combine to drive the precipitation of different mineral phases during the year. Calcite, aragonite and perhaps hydromagnesite usually precipitate from saturated lake surface waters in spring, gypsum and glauberite in summer (via solar evaporation), while rafts of cryogenic mirabilite form at the air brine interface in the winter. These winter rafts are then blown shoreward, to form stacked strand-zone-parallel accumulations of cryogenic mirabilite and halite. By the following summer much of the strand-zone mirabilite has deliquesced or converted to glauberite. In the 1920s and 30’s, prior to the damming of the connecting channel between Karabogaz and the Caspian Sea, the strand-zone salts were harvested by the local peasantry. From the 1950s to 1980s there was a significant Soviet chemical industry operational in the basin, focused on older buried salt bed targets.


Beneath Karabogaz bay there are 4 beds dominated by various NaSO4 salts (figure 4). These cryogenic beds are likely the result of cooler climatic periods over the last 10,000-20,000 years. Back then, under a glacial climate, large amounts of mirabilite formed each winter, much like today. But, unlike today, a cooler more-humid glacial climate meant that the bay was not as subject to as an intense summer desiccation as it is today. Dense residual bottom brines were perennially ponded and so preserved a summer-halite sealing bed atop the winter NaSO4 layer. This allowed the underlying mirabilite/epsomite    precipitates to be preserved across the lake floor. During the following winter the process was repeated as mirabilite/epsomite/halite beds stacked one atop the other to create a future NaSO4 bedded-ore horizon. In time, a combination of groundwater and exposure, especially nearer the Gulf’s strand-zone, converted most of the mirabilite, along with epsomite, to astrakanite, and then both phases to glauberite in the upper three beds. This explains the association of the richer glauberite zones with the lake edges (figure 4). Whenever water of crystallisation is released by a mirabilite to thenardite conversion, it then slightly dilutes any strong residual brine; in Korabogazgol this facilitated the high sodium-sulphate mineral and brine compositions seen in modern and ancient waters across the bay.

There are similar cryogenic salt beds preserved in perennial saline lakes across the cold arid portions of the Great Plains of Canada and there is also a mirabilite bed preserved beneath Holocene sediments in Great Salt Lake, Utah. Last century, some of the richer subaqueous salt beds in the Canadian lakes were sources of commercial NaSO4 salts (figure 5). However, extraction of brine and solid salts and an increasingly expensive product meant this area no longer competes with cheaper product from the solar evaporite NaSO4 lakes of Mexico and Turkey. Only one (Big Quill Lake) of the Canadian sites remains operational.


Hydrohalite is another common cryogenic salt, it quickly redissolves as brine temperatures rise above 0 degrees centigrade and so is said to indicate halite cryogenesis (figure 6). Hydrohalite crystals have distinctive pseudo-hexagonal cross sections (c.f. typical cubic forms of halite) and crystals or NaCl-filled pseudomorphs have been recognised in a number of modern cold saline lake settings. For example, hydrohalite has been extracted from the lake bottom sediments in saline Lake Bonney in Antarctica, where the bottom water temperatures vary between +2.0 and -2.0°C. It also can precipitate in winter in the Baskunchak salt lake, located some 300 km northwest of the Caspian Sea (48°N latitude). There hydrohalite was directly observed on two occasions when formative brine temperatures were between-3° and -23°C. In summer, halite precipitates via solar evaporation in the same saline lake. Hydrohalite also occurs in bottom sediments in salt-saturated cryogenic lakes in Saskatchewan, at about 51°N latitude, and has been observed in nearby saline springs sediments of the Northern Great Plains. Hydrohalite pseudomorphs occur as halite crystals with hexagonal cross sections in cores some 100-140m deep, in Death Valley, California, indicating NaCl cryogenesis occurred in the Pleistocene Death Valley Lake at a time when brine temperatures were less that 0°C.


When polar seawaters freeze on Earth, hydrohalite and mirabilite precipitate from the residual marine brines and accumulate in ice sheet fissures, or in load-induced fractures in the ice understory wherever an increasingly saline brine sinks into rock fractures beneath the growing ice sheets. For example, there are mirabilite layers on the ice floes of the Ross Ice Shelf near Black Island. Likewise, there are dense residual saline brines in interstitial waters extracted from deep cores in sediments of McMurdo Sound. It seems that when ice sheets retreat, the at-surface cryogenic salts dissolve in the freshened at-surface hydrology, but dense hypersaline brines can remain behind in deep fissures, held and preserved in the rock fractures. In the extreme setting of at-surface brine freezing in some of the small saline depressions in the Dry Valleys of Antarctica, a solid form of calcium chloride, antarcticite, grows cryogenically. Antarcticite precipitates today in Don Juan Pond, Antarctica, in what is probably the most saline perennial natural water mass on Earth (47% salinity). Although the Don Juan pond is often cited in the saline literature as a most impressive example of an extremely-hypersaline modern closed-basin cryogenic hydrology, it should be pointed out that this cryogenic salt pond measures some 100 by 300 metres across and is tens of centimetres deep (figure 7). 

 

On Earth, the volume of salt beds formed by cryogenesis is much less than the volumes that result from solar evaporation. Extraterrestially, in planets and moons of our solar system with liquid water and located further out than the earth’s orbit, there are likely, at least locally, volumes of cryogenic salts that are significant. Cryogenesis explains sulphate salt (epsomite-dominant) phases that typify ice crack fissures crisscrossing the surface of Europa (a moon of Jupiter). Sulphate salts also grow seasonally in soils of Mars where, for example, widespread gypsum forms via ice ablation in the circumpolar Martian dune-field. But, for now, I will leave the discussion of the significance of these extraterrestial cryogenic salts, it will be the topic of a future blog dealing with liquid water indications in and on a variety of planets and moons located beyond the earth’s orbit.

On Earth, as ground temperatures increase, cryogenic salts tend to deliquesce or convert to their higher temperature daughter salts (thenardite, glauberite and halite). But worldwide, in appropriate cold climatic settings, there are numerous examples of cryogenic salt beds; the volumes grow even larger if we include sediments containing cryogenic hydrated calcite (ikaite-glendonite; CaCO3.6H2O). Across deep time, the volume of cryogenic salts increases during glacial episodes, their susceptibility to deliquescence and conversion as temperatures increase means a low propensity for significant preservation other than as pseudomorphs. If such pseudomorphs are to be found across the rock record, then there will be a greater likelihood of their retention and recognition in sediments of the late Tertiary, the Permo-Carboniferous, the Ordovician and the late Neoproterozoic, which are all times of an icehouse climate.

Solikamsk sinkhole and potash ore

John Warren - Thursday, February 19, 2015

The Solikamsk-2 mine output currently accounts for a fifth of the Uralkali's potash capacity. The mine is one of Uralkali’s potash mines in the Kama district (western Urals) of Russia. The possible loss of the Solikamsk-2 mine in the near future may put upward pressure on the currently low world price of potash. Even if loss of production from the mine doesn't drive an increase in price, Uralkali as a company will continue to be profitable. All mines in Solikamsk basin benefit from low production costs, related to shallow ore depth. The collapse sinkhole is a classic example of what can happen if any salt or potash mine operates at a depth that is shallow enough to intersect the overlying  zone of active phreatic water crossflow.


In November 2014 the most recent example of an evaporite-related ground collapse or sinkhole, daylighted to the east of the Solikamsk-2 potash production site. It seems likely that halite-undersaturated waters, sourced from above, now flow into an area of the Solikamsk-2 workings that were no longer mined, but were still connected to the active extraction areas of the Solikamsk mine. Worldwide experience shows that flooding is difficult to control once a salt mass is breached. Increasing volumes of inflow waters into the mine workings will likely lead to the ultimate loss of the Solikamsk 2 mine, as it has in what are other now abandoned salt mines and solution brinefields across the world.

Recently the head of the Ural region's Mining Institute, Alexander Baryakh, was quoted as saying (Moscow Times Dec. 11 2014): ....."Based on our analysis and the world's experience in developing potassium mines, the risk of a negative scenario — the complete flooding on the mine — remains high. We are ready for this contingency, but we are doing everything possible to minimize related risks,"  adding that, "fortunately, the accident poses no danger to the residents of [the local town of] Solikamsk....”

When, in mid November, a stoping solution cavity above the abandoned section of the mine workings reached the surface, the resulting sinkhole diameter measured 30m by 40m. As of 6 February 2015, the at-surface sinkhole diameter had increased in size and measured 58 by 87 metres wide and was some 75 metres deep. Uralkali’s ongoing measurements show levels of brine inflow into the Solikamsk-2 mine continuously varied over this time frame. Between 11 December 2014 and 21 January 2015, the average brine inflow was around 200 cubic metres per hour. Between 22 January and 06 February 2015, the average brine inflow increased substantially, reaching approximately 820 cubic metres per hour. 

Accordingly, where underground equipment at Solikamsk 2 is not being used to mitigate the consequences of the collapse and water inflow, Uralkali has started to remove plant via the mine shaft. Three "Ural" continuous miners have already been dismantled and taken out. In line with their accident mitigation plan, Uralkali in a recent press release states: that the company continues to comprehensively monitor the situation both underground and at the surface; water inflows are monitored through brine level checks; groundwater levels are monitored by water monitoring wells and via the drilling of additional water monitoring wells currently in progress; gas levels are monitored around the sinkhole and in the mine; while the sinkhole is continuously monitored from a distance, using stationary cameras and air drones; and a seismologic network has been set up over the sinkhole area.

This latest collapse is one of a series of evaporite collapse dolines or sinkholes that have daylighted atop the mined regions in this part of Russia. Collapse cavities typically reach the surface some years after extraction operations below the collapse have ceased. For example, in 1995, a collapse sinkhole formed atop the Solikamsk-2 potash mine’s Verkhnekamsky deposit. The collapse on January 5th, 1995 resulted in a 4.7 magnitude seismic event on the Richter scale, with an associated initial 4.5 m of surface subsidence. Underground, the mine roof collapsed over an area measuring 600 m by 600 m. Across the period 1993 to 2005, several hundred earthquakes were recorded in the Berezniki-Solikamsk region with magnitudes varying from 2 to 5. These earthquakes were caused by collapsing underground tunnels of potash mines, mined out over the 70 continuous years of production. In October 2006, in order to prevent catastrophic outcomes of a sudden brine influx into the underground workings, Berezniki potash mine #1 was flooded by Uralkali. After that, three major sinkholes occurred in the region above the flooded workings.

On July 28th, 2007, a huge sinkhole appeared on the surface above the closed Berezniki mine #1, its creation likely aided by infiltration of undersaturated floodwaters water into the abandoned underground workings. At the surface this sinkhole had an initial size of 50 by 70 meters and was 15 meters deep. By November, 2008, the sinkhole had expanded into a crater measuring 437m by 323m and some 100 m deep.  The July 28th collapse released an estimated 900,000 cubic metres of gas (a mix of methane, hydrogen, carbon dioxide, carbon monoxide and other gases), which in turn led to gas explosions on the following day. Timely placement by the mine operators of a “significant volume” of backfill, prior to flooding, is credited with preventing further catastrophic collapses.

The mined potash ore level at both collapse sites (Solikamsk and Berezniki) is Middle Permian (Kungurian) in ag. The potash is halite-hosted and occurs in one of six evaporitic foreland sub-basins, extending from the Urals foreland to the Caspian basin. Sylvinite (potash) beneath the 1995 Verkhnekamsky collapse area was extracted from two to three halite-potash beds, with 10 to 16 metres of total extraction height. At that time the mine used a panel system of rooms and pillars under 200 to 400 m of overburden. Rooms were 13-16 m wide and pillars 11-14 m wide by 200 m long. Due to the relatively shallow nature of the Solikamsk and Berezniki potash ore levels, compared to other potash mines in the world, a “rule of thumb” used across the Upper Kama mining district is that surface subsidence typically reaches 50% of the subsurface excavation height around 48 months after excavation. 

The 1995 collapse event occurred 15 years after mining began, and 7 years after mining was completed in the area beneath the sinkhole. The delay before the main collapse doline surfaced implies there was a rigid bridging of overburden as a roof to the mine level. This is consistent with the uncommonly high release of seismic energy associated with the1995 collapse. The next largest collapses associated with published seismic measurements occurred in 1993 and 1997 with seismic magnitudes of 2.6 and 2.8, respectively.

An even earlier surface collapse occurred on July 25, 1986 atop a portion of the nearby 3rd Bereznki potash mine and is yet another case of a sinkhole forming atop a mine that was operating at relatively shallow depths. Potash extraction at Bereznki was active at depths of 235 and 425 m below surface. There, the targeted ore zone was overlain by a 100 m thick “salt complex” made up of halite and carnallite beds, overlain in turn by clays, carbonates, aquifers and sediments. Mining created “yield pillars,” with 5.3 m wide rooms, 3.8 m wide pillars and a 5.5 m mining height. After mining, conditions in mined-out areas were described as, “pillars crushed and roofs sagged.”

Observations of significant brine leakage into the 3rd Bereznki potash mine workings at a depth of 400m indicated a loss of hydraulic control as early as January of 1986. This was a prelude to the massive dissolution cavitation that was occurring in the 90 m interval of disturbed salt and clastics that overlay the potash level. Some 7 months later a large cavity formed in the sandstone/limestone overburden, which was nearly 200 m thick. In the mine it appeared the water inflow situation remained relatively stable, at least from January until July 1986. Failure of the mine head then occurred, the result of a cavity that had migrated vertically through more than 300m of limestones, mudstones and sandstones. 

Final cavity stoping was indicated by the near instantaneous appearance of a caprock sinkhole, which was 150 m deep and 40-80 m across and located at the top of a stoping breccia pipe or chimney of the same dimensions. Failure of this sequence began at 18:30 hours on July 25 with “clearly felt underground shocks” culminating with a final collapse at midnight, which was accompanied by an explosion with “flashes of light.” In the final stages of stoping by the rising solution pipe, before the sinkhole daylighted, it took only 12 days to migrate through the last 100 metres of mudstone. This very high rate of stoping was likely aided by structural weakness in a fracture zone along a local fold axis.

In all these cases of rapid sinkhole creation, the collapse occurred above what was formerly an active area of the mine  and took place some years after mining had ceased. In all cases, the ultimate cause of the size of the collapse was likely a combination of a significant cavity growth below what was a mechanically strong rock rock, likely a dolomite or a limestone bed. This unit had significant structural integrity and so allowed a solution cavity to expand prior to the ultimate brittle collapse of roof rock. Once collapse did occur, undersaturated groundwaters, sourced in the overburden, then reached the salt level in large volumes and further expanded the region of collapse. Likewise, once the upward stoping cavity reached the shallower unconsolidated sediment levels, the cavity's passage to the surface sped up so that it daylighted and expanded in a rapid fashion.  

Dissolving evaporites and solution dolines occur naturally in all parts of the world, wherever salt is within a few hundred metres of the landsurface, but mining of both salt and potash at depths shallower than 250-350 metres can exacerbate the speed and and intensity of what is an ongoing natural process of evaporite solution, surface collapse and sinkhole growth. While Uralkali's operations in this region continue to exploit a relatively shallow potash ore source, it will continue to supply the Company a relatively inexpensive product, but the company will have live with sinkholes breaking out above some areas of the mined region. That is, as long as Uralkali can continue to be a low cost supplier of potash, there will be likely be ongoing landsurface-stability problems. Some of the problems may not daylight until years after the extraction operation has ceased.


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hydrothermal potash perchlorate Realmonte potash halokinetic Weeks Island salt mine sulphur sedimentary copper ancient climate salt tectonics Stebnik Potash Lake Peigneur Dead Sea saltworks salt ablation breccia marine brine K2O from Gamma Log York (Whitehall) Mine nacholite flowing salt cauliflower chert DHAL vadose zone Proterozoic 13C enrichment well logs in evaporites African rift valley lakes freefight lake Archean Hell Kettle 18O supercontinent Zabuye Lake Muriate of potash potash ore collapse doline anomalous salt zones saline clay Evaporite-source rock association solar concentrator pans natural geohazard halophile Musley potash halotolerant climate control on salt sodium silicate endosymbiosis Warrawoona Group salt suture Schoenite High Magadi beds Dead Sea caves Ingebright Lake North Pole extrasalt Koppen climate hydrological indicator evaporite carnallitite salt karst anthropogenically enhanced salt dissolution Bathymodiolus childressi brine pan McArthur River Pb-Zn meta-evaporite CaCl2 brine Boulby Mine sulfur CO2: albedo allo-suture causes of glaciation oil gusher geohazard carbon cycle Corocoro copper bedded potash base metal hectorite mirabilite antarcticite Badenian magadiite Ure Terrace sinkhole NaSO4 salts methanogenesis Koeppen Climate 18O enrichment evaporite-metal association evaporite-hydrocarbon association auto-suture Deep water on Mars DHAB mass die-back Lop Nor Pangaea basinwide evaporite Platform evaporite hydrohalite intersalt Dead Sea karst collapse Mega-monsoon palygorskite Atlantis II Deep Sulphate of potash Great Salt Lake tachyhydrite Pilbara dissolution collapse doline Dallol saltpan subsidence basin water in modern-day Mars Salar de Atacama Red Sea Density log crocodile skin chert evaporite karst salt leakage, dihedral angle, halite, halokinesis, salt flow, vestimentiferan siboglinids potash ore price dihedral angle deep seafloor hypersaline anoxic basin Five Island salt dome trend mine stability Kara bogaz gol Gamma log hydrogen Neutron Log lithium battery SOP Paleoproterozoic Oxygenation Event namakier dark salt lithium carbonate salt mine Hyperarid Sumo well blowout NPHI log stevensite jadarite potash cryogenic salt gas in salt lapis lazuli anthropogenic potash nuclear waste storage vanished evaporite brine evolution black salt deep meteoric potash brine lake edge GR log mummifiction H2S circum-Atlantic Salt Basins knistersalz waste storage in salt cavity Hadley cell: RHOB salt periphery Turkmenistan Lomagundi Event solikamsk 2 Karabogazgol gem halite-hosted cave halite stable isotope salt trade Precambrian evaporites Messinian source rock zeolite gassy salt blowout 13C nitrogen lot's wife Clayton Valley playa: Quaternary climate Zaragoza silica solubility SedEx lazurite Neoproterozoic Oxygenation Event Lake Magadi gypsum dune lithium brine hydrothermal karst Ripon Mesoproterozoic sepiolite salt seal Lop Nur well log interpretation evaporite dissolution bischofite gas outburst Belle Isle salt mine Neoproterozoic carbon oxygen isotope cross plots venice alkaline lake recurring slope lines (RSL) organic matter wireline log interpretation silicified anhydrite nodules MVT deposit sinjarite authigenic silica HYC Pb-Zn Ethiopia Crescent potash Stebnyk potash kainitite eolian transport trona chert Magdalen's Road doline methane seal capacity Danakhil Depression, Afar sulfate capillary zone snake-skin chert Deep seafloor hypersaline anoxic lake CO2 Catalayud MOP Calyptogena ponderosa Thiotrphic symbionts Jefferson Island salt mine methanotrophic symbionts Seepiophila jonesi rockburst lunette Kalush Potash Lamellibrachia luymesi epsomite sulphate intrasalt astrakanite

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