Salty Matters

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Calcium Chloride (CaCl2), Article 2 of 2: CaCl2 minerals in evaporites

John Warren - Wednesday, May 31, 2017



Found in association with some highly-saline calcium chloride brines are four naturally occurring CaCl2 minerals with evaporite associations, namely; 1) tachyhydrite CaMg2Cl6.12H2O), 2) antarcticite (CaCl2.6H2O), 3) sinjarite (CaCl2.2H2O) and 4) chlorocalcite (KCaCl3). For any CaCl2 salt to precipitate in a saline setting requires a Ca-enriched mother brine completely depleted in sulphate (Table 1). Otherwise, gypsum precipitates and in so doing removes all calcium from the concentrating brine, well before the bittern stage.


Of the four, tachyhydrite is perhaps the most common solid phase, sinjarite or chlorocalcite the least. Bischofite is sometimes a co-precipitate with tachyhydrite and indicative of a mother brine with an elevated MgCl2 content (Figure 1). Even so, tachyhydrite is still a rare bittern mineral phase across the Phanerozoic of Earth, although as we shall see later, the situation may be a little different on the martian surface. Along with carnallite and bischofite, tachyhydrite typifies highly saline marine bittern assemblages in only a few ancient potash-rich evaporite systems. These same systems are also MgSO4 poor, and tend to define times in Earth history of MgSO4-depleted seas. Occurrences include: Silurian potash in Michigan Basin; Carboniferous potash in the Canadian Maritimes and the Paradox Basin; Permian Zechstein-2 Stassfurt potash seams in Stassfurt, Germany; Triassic Saharan potash of North Africa; Cretaceous potash evaporites in Brazil, Gabon and Thailand and Oligocene potash in the Rhine Graben. Tachyhydrite-bearing successions are much less common within a broader MgSO4-poor group of potash deposits (see Warren 2016, chapters 2 and 11; Warren, 2017).

To form antarcticite (CaCl2).6H2O) at the bittern stage, a CaCl2 brine must be depleted in Mg, otherwise tachyhydrite forms, along with carnallite and bischofite. Carnallite, bischofite and tachyhydrite are not part of the bittern suite precipitated from today’s MgSO4-rich oceans, where sulphate and magnesium levels are too high relative to calcium. At times in the past, these three salts were more common precipitates in the bittern stages of ancient CaCl2 or MgSO4-depleted oceans. These tended to be at times when rates of seafloor spreading, and hence volumes of through-circulated seawater, were greater than today (Lowenstein et al., 2014). It seems that most the highly saline calcium chloride brines form in sedimentary basins with a thick evaporite near the base of the sediment fill (see article August 11, 2015). But, not all are basins with highly saline CaCl2 brines are co-associated with bedded tachyhydrite occurrences in the buried and dissolving evaporite units.


Antarcticite (CaCl2.6H2O) is extremely hygroscopic, colourless with perfect basal cleavage and good to perfect prismatic cleavage, is brittle with a hardness between 2 and 3, and has a specific gravity of 1.715 ± 0.010 (14 °C), and a density of 1.700 g/cm3 (Dunning and Cooper, 1969). The lowest temperature for antarcticite to precipitate is -50 °C and it’s melting point is 30 °C.

There are only two known modern hypersaline saline deposits where the pure calcium chloride mineral antarcticite (CaCl2.6H2O) is present as thin crystal beds and efflorescences; they are, Bristol Dry Lake (BWh) in California and Don Juan Pond (EF) in the Wright Valley, near Lake Vanda in Antarctica (Dunning and Cooper, 1969; Horita, 2009). Both saline lakes possess similar CaCl2-enriched, MgCl2-depleted chemistries and are fed in part by hydrothermal waters. In Bristol Dry Lake the concentration process is driven by solar evaporation, while cryogenesis is the primary driver in Don Juan Pond. Only in Bristol Dry Lake is a CaCl2 brine is commercially extracted from Quaternary sediments. There is also a minor occurrence as isolated small crystals in the Kunteyi Lake of the Tibetan Plateau (Kezao and Bowler, 1986; Zheng and Liu, 2009)

Don Juan Pond, Antarctica

Don Juan Pond is a perennial free-standing water body, approximately 300 m long and 100 m wide, surrounded by glacial moraine deposits in the Wright Valley, Antarctica (Figure 2). It defines the outcrop portion of a liquid water drainage sump that retains a 10-cm-deep CaCl2 brine at the surface (Torri and Ossaka, 1965; Marion, 1997; Burt et al., 2003) (Figure 2). This pool contains some of the saltiest free-standing perennial waters on the earth’s surface, with concentrations ranging up to 40% (by mass; Dickson et al., 2013). Mean annual precipitation the Don Juan Pond region is 5 to 10 cm and air temperatures range from –55°C to +10°C (Marion, 1997). The pond does not freeze because it is a eutectic brine of H2O, CaCl2, and NaCl, with a freezing point of -52°C and a density of approximately 1.4 (Marion, 1997; Burt et al., 2002). Crystals of antarcticite were first found sticking out of the brine at the bottom of the pond, with halite and gypsum fringes at the edges of the pond (Torri and Ossaka, 1965).

Precipitation of antarcticite in this cryogenic environment is the result of the arid climate, ongoing cold temperatures and a CaCl2 brine chemistry (Torri and Ossaka, 1965). The CaCl2 brine is in part derived from springs fed by a deep, confined, basaltic aquifer, with a groundwater similar in composition to the surface brine (Harris et al., 1979; Marion, 1997; Burt et al., 2002). The origin of the CaCl2 is still contentious, but is most likely be related to mineral weathering and periodic deliquescence of calcium chloride held in the sediments(Dickson et al., 2013).

Brine in the Don Juan Pond is predominantly from CaCl2 solutions (Figure 3; Marion, 1997). Only Na of the remaining cations contributes significantly to the overall charge balance. Both Mg and K are present at very low concentrations, and SO4 is usually not detectable. In contrast to low ionic variability in seawater composition, levels of the dominant ions (Ca, Na, and Cl) in the Pond are highly variable (Figure 3). The concentration of the brine varies seasonal, controlled mainly by a complex hydrologic system including groundwater discharge, melting of nearby permafrost, rare precipitation, and evaporation or sublimation (Harris et al., 1979; Dickson et al., 2013). Despite the changes in the absolute concentration of Ca and Cl across an annual cycle, the Ca/Cl molal ratio remained relatively constant over the 23-year sampling period; it varies between 0.432 and 0.506 with an average of 0.462 ± 0.018 (Figure 2; Marion, 1997).

Bristol Dry Lake, California

Bristol Dry Lake is a 155 km2 continental playa located within a closed Basin and Range valley in the Mojave Desert of southern California (Figure 4a). It is the second known location of natural antarcticite and is thought to precipitate from lake brines via a lowering of brine temperatures (Dunning and Cooper, 1969). No antarcticite was identified in the nearby Cadiz or Danby dry lakes. Lake brines first concentrate by solar evaporation beyond halite precipitation Mountains surrounding Bristol Dry Lake are composed mostly of pre-Cambrian and Mesozoic granite, some Palaeozoic carbonates, and Tertiary and Pleistocene volcanic rocks including Quaternary olivine basalts that erupted from nearby Amboy Crater. Alluvial fans are present between the mountains and the lowest parts of the valley (Dunning and Cooper, 1969; Handford, 1982).

Bristol Dry Lake playa contains more than 300 m of interbedded halite and salt-bearing sands, silts and clay. Halite and gypsum are currently deposited from ephemeral bodies of water under evaporative conditions, with a gypsum pavement surrounding more saline halite-saturated sumps in the central lake depression (Figure 4a; Dunning and Cooper, 1969; Handford, 1982; Rosen and Warren, 1990). The primary evaporite minerals at Bristol Dry Lake are gypsum, anhydrite, and halite with rare antarcticite, celestite (SrSO4), calcite, and possibly chlorocalcite (KCaCl3). Bristol Dry Lake is mined for its chloride minerals and has been since the 1900s, and so is covered with pits and trenches (Figure 4b).

Antarcticite was identified in one of these previously excavated trenches approximately 9 kilometres south of Amboy, near the road from Amboy to Twenty-nine Palms. Dunning and Cooper (1969) examined this and other trenches during the winters of 1961, 1962, and 1968. Only one trench had antarcticite. The trench was 3 m deep, 20 m long, and contained a 10 to 25 cm deep pool of CaCl2 brine extending along the length of the trench. At the base of the pit was a massive halite deposit, with acicular groups of antarcticite protruding outward from the halite and within the brine. Antarcticite is not present on the playa surface as no surface brines are present. The only CaCl2 brines exposed to solar radiation are subsurface brines that have seeped into, and are now exposed within, the excavated trenches.

Dunning and Cooper (1969) suggested that CaCl2 brines at Bristol Dry Lake originated from chlorine of volcanic origin interacting with calcium carbonate in surrounding alluvial deposits (Figure 5). Lowenstein et al. (2009) argue that the nearby active magma chamber drives the formation of Ca-Cl brines at elevated temperatures, as well as the thermally-driven transportation of these brines to the surface. Other evidence for the magma chamber is the Amboy crater and its associated recent lava flows, which occur directly North of Bristol Lake. Evaporative concentration of this calcium chloride brine, driven by an arid climate and typical surface temperatures below 30°C, encourage antarcticite precipitation (Dunning and Cooper, 1969).

Sinjarite (CaCl2.2H2O)

Sinjarite is a modern ephemeral precipitate in surficial alluvial fan sediments near Sinjar in Iraq (Figure 6). No natural occurrences of either antarcticite or sinjarite have been documented in ancient salts, except as a volumetrically insignificant mineral phase in brine inclusions in both sediments and igneous rocks (Polozov et al., 2016), even in sedimentary basins containing highly saline CaCl2-rich pore fluids.

Sinjarite is an extremely rare pink-tetragonal-hygroscopic mineral that is highly soluble with 117 g dissolving in 100 g of water at 20 °C (Table 1; Aljubouri and Aldabbagh, 1980). The lowest temperature for sinjarite to precipitate is 42.3 °C, while its melting point is 175.5 °C (Wardlaw, 1972). Theoretical composition of sinjarite is 27.26% Ca, 48.24% Cl, and 24.50% H2O but the actual chemistry for sinjarite present at its type locality in Iraq is 25.84% Ca, 46.64% Cl, 26.55% H2O, and 0.85% Na with 226 ppm K and trace amounts of Mg, Sr, and Fe (Aljubouri and Aldabbagh, 1980). Properties include; good cleavage, average density of 1.81 g/cm3, a very deliquescent habit, hardness of 1.5, vitreous to resinous lustre, white streak, and occurs naturally in massive to granular forms.

Little research has been performed on sinjarite and knowledge of the environment of formation comes from the only place it was discovered. Sinjarite occurs in detritus in a wadi cutting through the Sinjar anticline near the town of Sinjar, Iraq (west of Mosul). Sinjarite precipitates via slow evaporation of groundwater saturated with Ca and Cl ions. CaCl2 in the groundwater solution must be 3.5 times greater than NaCl for sinjarite to precipitate instead of halite. The extremely deliquescent nature of sinjarite means that the mineral is ephemeral and quickly dissolves in wet seasons or changes to the hexahydrite antarcticite when the temperature is less than 30°C (Aljubouri and Aldabbagh, 1980).

CaCl2 salts and liquid water flows on current-day Mars

On the current Martian surface it has been recently proposed that aqueous solutions form seasonally, via the deliquescence of hygroscopic salts in contact with atmospheric water vapor. Regions of these hygroscopic salts are thought to be indicated by occurrences of surface features known as recurring slope linae - RSL (Figure 7a, b; Chevrier and Rivera-Valentin, 2012). Older studies largely focused on perchlorate species as the most likely set of hydroscopic salts driving the periodic formation of RSLs, but another Mars-relevant set of salts, with similar low eutectic temperatures, relevant to ambient conditions with a significant deliquescence potential, are the calcium chloride hydrates, antarcticite and sinjarite. Gough et al. (2016) propose hydrated calcium chloride salts are linked to RSL formation on Mars. They also note deliquescence of CaCl2, not perchlorate, is also known to be responsible for the only terrestrial RSL analog known thus far: these are the seasonal water tracks in the McMurdo Dry Valleys, especially in the vicinity of the Don Juan Pond, as documented by Dickson et al., 2013 (Figure 8).

Soluble regolith salts like perchlorate and calcium chloride salts with low eutectic temperatures are likely to deliquesce at low relative humidity (RH) values at a wide range of temperatures (Gough et al., 2016). Deliquescence is the process by which a solid crystalline phase absorbs water vapor to form a saturated aqueous (liquid) solution. This phase transition from solid salt to liquid brine occurs at a deliquescence relative humidity (DRH), the value of which is specific for each hydration state of each salt and often varies with temperature. A salt is expected to be aqueous whenever the temperature is above the eutectic temperature of the salt and the relative humidity is above the DRH, although at much higher RH values ice may form. The low temperature deliquescence of perchlorates has been extensively studied (Gough et al., 2011, 2014; Nuding et al., 2014; Zorzano et al., 2009). Many perchlorate species have deliquescence relative humidity (DRH) values below 40% RH, and so should deliquesce and be stable or metastable liquids under Martian surface conditions (Chevrier et al., 2009), although slow reaction kinetics may limit their formation (Fischer et al., 2014). Not as much attention has been paid to other soluble, deliquescent salts, especially the calcium chloride salts, that may be present in the martian regolith.

Chlorine has been detected by rovers and landers in every Martian soil sample analyzed to date and is found at similar concentrations (≈0.2 to 1%) in all locations (Glavin et al., 2013). The form of the chlorine (i.e. chloride vs. perchlorate) is unknown at some sites, although measurements of the regolith at the Phoenix landing site confirmed chloride, perchlorate (Hecht et al., 2009) and probably chlorate (Hanley et al., 2012). The identity of the associated cation(s) is also generally unknown, although calcium, sodium, magnesium and potassium are the most likely candidates (Hecht et al., 2009). Regionally and globally widespread chloride deposits have also been detected by orbiting spectrometers (Figure 7c; Osterloo et al., 2010; Keller et al., 2006).

The global distribution of chloride deposits across the Martian surface is similar to that of recurring slope lineae (RSL), prompting hypotheses of a relationship between the two (Figure 7c; McEwen et al., 2011; Stillman et al., 2017). Chevrier and Rivera-Valentin (2012) suggest that CaCl2 is one of the best candidates for the formation of RSL, the narrow, dark features that appear and grow seasonally on Mars and appear to be caused by flowing liquid.

Two hydrated chloride salts, MgCl2 and CaCl2, may be the most appropriate salts because the seasonality of observed RSL formation best matches the seasonality of the melting of ice associated with these salts (Chevrier and Rivera-Valentin, 2012). Specifically, these chloride salts have eutectic temperatures less than or equal to the threshold temperature of 250 K that seems to mark the start of RSL activity (McEwen et al., 2011), but the eutectic temperatures of these salts are not so low that brines on Mars would be permanently liquid. Additionally, calculations show that CaCl2 in particular will cause seasonal melting of water ice throughout the top 20 cm of the martian regolith, therefore providing greater potential fluid flow than other salts (Chevrier and Rivera-Valentin, 2012). More recently, hydrated chloride salts (although not specifically calcium chloride) were observed to be present in RSL but not in surrounding regions (Ojha et al., 2015). It is not generally believed that deliquescence could be fully responsible for RSL formation because the small amount of water vapor in the martian atmosphere would limit the condensed phase water that could form. It is certainly possible, however, that salt deliquescence may be involved in the formation of RSL or in their appearance or behavior (McEwen et al., 2011; Ojha et al., 2015; Dickson et al., 2013; Stillman et al., 2017).

In summary, determining whether liquid water exists on the Martian surface is central to understanding the hydrologic cycle and potential for extant life on Mars (Ohja etal., 2015; Stillman et al., 2017). Recurring slope lineae, narrow streaks of low reflectance compared to the surrounding terrain, appear and grow incrementally in the downslope direction during warm seasons when temperatures reach about 250–300 K (-23°C to +26°C), a pattern consistent with the transient flow of a volatile material. Brine flows (or seeps) associated with seasonal deliquesence of hydrated salts (possibly hydrated calcium chloride salts or perchlorates) are proposed to explain the formation of recurring slope linea. As yet, no direct evidence for either liquid water or actual hydrated salt mineralogies has been found.


Tachyhydrite (CaMg2Cl6.12H2O) (occasionally spelled tachydrite or tachhydrite) is a yellow, transparent to translucent, trigonal-rhombohedral mineral with very high solubility. Pure tachyhydrite is composed of 7.74% Ca, 9.39% Mg, 41.10% Cl, 37.09% O, and 4.67% H (Wardlaw, 1972). Its chemical formula is most accurately expressed as [Mg(H2O)6]2[CaCl6] because the structure consists of Mg(H2O)6 octahedra and CaCl6 octahedra loosely linked together by hydrogen bonds. Physical properties include; good cleavage, vitreous to greasy lustre, white streak, massive habit, bitter taste, deliquescent habit (dissolves or liquefies upon exposure to air), a density of 1.66 g/cm3, a hardness of 2, and it is hygroscopic (Braitsch, 1971).

Tachyhydrite is extremely soluble with 160 g dissolving in 100 g of water at 20 °C, and its solubility increases with increasing temperature (D’Ans, 1961; Wardlaw, 1972). Its hygroscopic and extremely deliquescent nature means at earth surface temperatures a crystal becomes liquid by absorbing moisture in the air. On exposure, tachyhydrite quickly alters to a residue of bischofite (MgCl2.6H2O) and a CaCl2.nH2O phase (Wardlaw, 1972). This is why tachyhydrite is typically documented in natural occurences where a crystal is still encased in halite. Tachyhydrite has a wide thermal stability field from 21.95°C to at least 167°C (Braitsch, 1971; Clark et al., 1980). The lowest temperature for tachyhydrite precipitation is 21.95°C at a concentration of 92.7 mol CaCl2/1000 mol H2O (450 g CaCl2/liter (D’Ans, 1961; Braitsch, 1971; Wardlaw, 1972). Its lowest temperature of formation increases by 0.0162ºC for every atmosphere of pressure (D’Ans, 1961).

Modern tachyhydrite occurrences

Natural tachyhydrite is documented only in a few modern hypersaline settings and it never forms a bed with primary precipitation textures (Figure 9). It is found in greater quantities in some ancient potash deposits. Tachyhydrite occurs seasonally as a minor interstitial cements and efflorescences, along with antarcticite (the hexahydrate form of calcium chloride), in the modern ephemeral halite crusts, atop sabkhas, of the Gavkhoni Playa (BWk), southeast of Isfahan, Iran (Pakzad and Ajalloeian, 2004), in mine wates in the Salar de Pedenales in the andean Altiplano, and perhaps as minor salt effloresecnes in the uppermost parts of the Abu Dhabi sabkha (Wood et al., 2005).

Much of the elevated ionic content of various Iranian playas in the Great Kavir is due to salt dissolution of the crests of nearby at- or near-surface diapirs and namakiers, where the mother salt sourced in halokinetic Miocene marine salts (Warren, 2008). These outcropping diapirs have carried carnallite and sylvite remnants into the namakiers that now reside at the surface (Rahimpour-Bonab and Kalantarzadeh, 2005). Tachyhydrite is also found as white feathery efflorescences within waste piles at an abandoned borate working along the southwestern margin of Salar de Pedernales, Chile (a BWk Koeppen climate location very near the ET climate boundary; Ericksen et al., 1989).

It seems that tachyhydrite’s high solubility means it has little or no preservation potential as a solid salt in any modern at-surface depositional setting; in the Peruvian case, it is a winter precipitate that disappears with the end of winter. In Abu Dhabi it, along with other highly soluble salts in the salty surface efflorescences, can disappear in morning dew (Warren, pers. obs.). In both the Iranian and the Peruvian settings the climate is cool and elevated. It seems the arid desert environments where bedded subaqueous-textured tachyhydrite accumulated in the drawndown Aptian seepage basins of the opening Atlantic were very different to its occasional rare occurrence in the evaporite settings of today.

Cretaceous of Brazil & W. Africa

Lower Cretaceous (Aptian) evaporite deposits in Sergipe, Brazil, and Gabon and Congo in western Africa contain significant amounts of tachyhydrite in halite-carnallite beds, along with other SO4-poor bitterns (Figure 10; Wardlaw, 1972; Borchert, 1977; de Ruiter, 1979; Hardie, 1990; Garrett, 1995; Zhang et al., 2017). These evaporite basins formed during the early Cretaceous rifting of Africa and South America. In each basin, potash-rich halite-dominant evaporites are transitional between older continental pre-rift and rift siliciclastics and younger post-rift marine shales and carbonates (Wardlaw, 1972, Borchert, 1977; de Ruiter, 1979; Szatmari et al., 1979; Hardie, 1990; Garrett, 1996).

The basic salt cycle of the Gabon and Congo basins includes from bottom to top: (1) thin black shale, (2) halite, (3) combination of halite and carnallite (carnallitite, and (4) bischofite and/or tachyhydrite (Figure 10;de Ruiter, 1979; Zhang et al., 2017). Although variations exist, a similar sequence is present in Sergipe, Brazil comprising the Ibura Member of the Muribeca Formation. The Ibura Member includes from bottom to top: (1) halite, (2) carnallite, (3) tachyhydrite, (4) sylvite (KCl) and halite, and (5) anhydrite (CaSO4) (Wardlaw, 1972; Borchert, 1977; Garrett, 1996). Tachyhydrite is located within the central and deepest portions of the Sergipe basins (Wardlaw, 1972; Borchert, 1977; Szatmari et al., 1979).

The origin of the evaporite sequences in Sergipe, Gabon and Congo is controversial with some authors suggesting a marine origin (Wardlaw, 1972; de Ruiter, 1979; Szatmari et al., 1979) or diagenetic origin (Borchert, 1977). More recent literature discussing Sergipe, Gabon, Congo, and other similar tachyhydrite deposits cite a non-marine/hydrothermal origin based on geochemical and textural studies (Lowenstein et al., 1989; Hardie, 1990; Garrett, 1996; El Tabakh et al., 1999). Yet other recent authors indicate a marine source based on its dominance in beds with a CaCl2 oceanic timing (Warren, 2016; Zhang et al., 2017).

The brine that tachyhydrite precipitated from must have been high in Ca because in waters with more SO4 than Ca present, the Ca is used-up by gypsum or anhydrite. Only if the Ca level is greater than SO4 in the early stages of precipitation will Ca stay in the brine allowing for potential precipitation of tachyhydrite (Wardlaw, 1972; Hardie 1990). The CaCl2-rich brine that produced the tachyhydrite and other potash salts is thought to have formed either by the interaction of hot meteoric groundwater with rift-related sediment and bedrock, or an influx of seawater at a time of a MgSO4-depleted ocean. Based on figure 1, Hardie (1990) suggests heated seawater could also have interacted with sediment and/or bedrock to produce a CaCl2 brine. This hot CaCl2 brine was perhaps driven to the surface by thermal circulation (Hardie, 1990). Alternatively, without arguing that the proportions of major ions in seawater change over time, the fact that modern marine water cannot precipitate tachyhydrite precludes it being the dominant mother brine in the Sergipe, Congo, and Gabon Basin evaporite basins,  (Lowenstein et al., 2014; Warren  2016).

Textures recovered in core, and visible in mine walls in Brazil, indicate Sergipe tachyhydrite was a primary bedded precipitate in those circum-Atlantic Cretaceous evaporite basins with stratiform tachyhydrite units in potash beds (Figure 10; Wardlaw, 1972). The Cretaceous world ocean at the time this tachyhydrite precipitated was a MgSO4-free marine bittern. Figure 9 shows tachyhydrite does not occur as a bittern phases in many other potash sequences deposited in basinwide settings at other times of CaCl2 oceans (e.g. the Devonian of Canada, the Silurian of the Michigan Basin). This has led to the argument that stratabound tachyhydrite in potash beds indicates an additional hydrothermal/basinal source for the mother brine. If so, the presence of tachyhydrite is not solely indicative of a MgSO4-depleted marine feed to the basin (Hardie 1990).

Cretaceous occurrences in Thailand

Another significant tachyhydrite occurrence is within slightly older mid Cretaceous (Cenomanian) evaporites of the Maha Sarakham Formation on the Khorat Plateau of northeastern Thailand (Figure 11). The majority of the Maha Sarakham Formation consists of alternating anhydrite and halite, separated by mudstone and siltstone “redbeds”. A 10- to 75-m-thick salt unit overlies a lower halite unit and underlies mudstone red beds of the lower clastic unit. Carnallite is most abundant in this salt unit, but sylvite and halite are also locally abundant. Tachyhydrite is present with halite and carnallite, but not with sylvite and is perhaps concentrated more in former basin centre or sump positions (El Tabakh et al., 1999; Warren 2016). Hite and Japakasetr (1979) estimated that tachyhydrite comprises less than 30% of the total carnallitite deposit, but tachyhydrite can form pure layers up to 16 m thick. Tachyhydrite crystals are euhedral and average 1 mm in size (El Tabakh et al., 1999). Sylvite and halite are also associated with accessory amounts of hilgardite [Ca2BCl(OH)2] and boracite (Mg3ClB7O13).

The origin of the Maha Sarakham Formation evaporites is still contentious; Hite and Japakasetr (1979) advocate a purely marine origin (highly restricted and concentrated sea), Hardie (1990) advocating an exclusively non-marine origin (hydrothermal CaCl2 brines), and El Tabakh et al. (1999) advocate an alternating marine and non-marine origin. They envisage an inland basin that was periodically inundated by the rising sea and at the time of tachyhydrite deposition received an additional influx of hydrothermal CaCl2 brines. Hardie (1990) states that formation of Maha Sarakham evaporites may have been similar to that of tachyhydrite formation in the Cretaceous basins of Brazil and western Africa, although he offers no evidence for the origin of the hydrothermal CaCl2 brines. El Tabakh et al. (1999) suggested that hydrothermal CaCl2 waters entered the restricted marine basin and created the right conditions for tachyhydrite precipitation and they cite contemporaneous granitic intrusions as possible evidence of thermal activity during the time of tachyhydrite formation.

Hydrothermal oceanic CaCl2 brine

Today, large volumes of relatively dilute, oceanic CaCl2 brines form via hydrothermal circulation and thermally-driven alteration of mid-ocean ridge (MOR) lithosphere (see part 1). These CaCl2 waters occur in and near active fracture zones, wherever seawater interacts with labile basalt (oceanic crust) at elevated temperatures and converts the circulating fluid from a Na-Mg-Cl water into a low-salinity Na-Ca-Cl brine, depleted in Mg and SO4. Similar hydrothermally-driven alteration of continental basalts via deeply circulated seawater interactions forms modern CaCl2-rich brine seeps, for example, within the thermally active continental Danakil rift valley (Hardie, 1990).

Tachyhydrite is a 'Swift Water" mineral

There may be an additional factor at work in terms of our recognition of tachyhydrite’s basin-by-basin distribution across the Phanerozoic. That factor is tachyhydrite’s high solubility in most subsurface waters and drilling muds. Even the name, tachyhydrite, was chosen from the Greek tachy hydros for “swift water” to indicate its rapid deliquescence on exposure to atmospheric conditions. This property means that any ground-up remains (rock chips) of tachyhydrite in a return mudstream in a conventional drilling operation typically do not make it to the surface. So, as most oil companies prefer not to core evaporites when drilling for hydrocarbons trapped beneath salt, there may be more tachyhydrite occurrences in deeply buried basin evaporites than the few currently documented.

Ancient tachyhydrite has only been found encased in halite in recovered cores and mine walls, never in drill cuttings, and its presence or absence can be easily misinterpreted in wireline logs, especially when drilling through thick salt masses in the search for oil and gas (companies tend to run a more limited log suite in thick salt units).


Tachyhydrite’s lack of shear strength and extremely high solubility makes mining any associated potash beds difficult; it presence leads to head beam and floor stability problems in conventional underground mines in Brazil and West Africa. Holle, a West African potash mine in the 1970s, was never economical and was lost to flooding after a few years of conventional mining and the Brazilian mine has ongoing floor and wall stability problems ties to tachyhydrite-induced heave. Today, solution mining is proposed as a more efficient means to exploit areas of Cretaceous potash associated with tachyhydrite beds, as in Aptian halite-potash associations on both sides of the Atlantic. High levels of tropical humidity mean at-surface potash evaporation pans are not viable in either area, so brine concentration and processing will probably require a more expensive option involving motorised dryers.


Not only is tachyhydrite highly soluble, but it is also highly reactive whenever in comes into contact with subsurface bitterns. Based on its occurrence in the Permian Stassfurt series in Germany, D’Ans (1961) suggested that much of the widespread polyhalite found in the CaCl2 brine-rich parts of the Zechstein Basin was a reaction product indicating former tachyhydrite. He noted that, at temperatures higher than room, there is a reaction between gypsum/anhydrite, sylvite and tachyhydrite-bischofite, or their dissolution-related brines, which removes tachyhydrite to form diagenetic polyhalite and calcium chloride brines;

CaMg2Cl6.12H2O+4KCl+8CaSO4--> 2CaSO4Mg(SO4)4(H2O)2 + 5CaCl2 + 4H2O

This was the mechanism suggested by Manheim (1974) to explain the lack of tachyhydrite, and the formation CaCl2-rich brines that characterise hydrothermal pore waters circulating in sediments associated with buried Miocene evaporites along the deep axial trough of the Red Sea.

Terrestrial CaCl2 minerals across time

The high solubility of CaCl2 minerals means that, with the exception of some parts of the Aptian Atlantic Potash association, even in past times of likely CaCl2-rich oceans, calcium chloride minerals are relatively rare as primary-textured occurrences in solid salt beds. On the other hand, as we saw in part 1, CaCl2 brines are commonplace in basinal or formation waters in many Phanerozoic sedimentary basins entraining thick sequences of dissolving ancient salt. For example, since 1914, CaCl2 brines have been extracted from Silurian strata adjacent to Salina Salt of the Michigan Basin USA, yet there are no significant volumes of tachyhydrite documented in the Michigan Basin(Garrett, 2004).

Historically, before micro-inclusion studies of chevron halite showed that the ionic proportions of seawater likely varied across the Phanerozoic, the various CaCl2 basinal brines occurrences in Silurian and Cretaceous strata were explained as an indicator of widespread dolomitisation and other diagenetic reactions, which preferentially extracted magnesium from pore waters. Since then, some authors have argued that CaCl2 enrichments in many ancient basinal brines, including brines in the Detroit group are partial leftovers of primary seawater chemistries (Lowenstein and Timofeeff, 2008). That is, Ca-enriched (MgSO4-depleted) pore brines are indicators of ancient CaCl2 oceans, with the pore brines being remnants from time the enclosing evaporitic and marine sediments were deposited (relict or connate brines).

Others, such as Houston et al. (2011), conclude this is not necessarily so, they agree that there are two end-members typifying highly saline subsurface brines. But they conclude that end-member chemistries relate to either substantial subsurface halite dissolution, or to preservation of early reflux-related seawater. Houston et al. (2011) go on to argue that CaCl2-enriched formation water chemistries from many basins worldwide, including the Michigan Basin, do not support an interpretation of variation in ionic proportions in seawater across the Phanerozoic. They find that CaCl2-rich brines formed either by dissolving bittern salts in the subsurface, or simply lost water in the subsurface after significant rock-fluid interaction had taken place. Water loss might be achieved by interaction with a gas phase at the elevated temperatures of deep burial or, alternatively, water may have been lost to clays. Both these mechanisms would have the effect of dehydrating (concentrating) the brine.

However, whatever the mechanism, it does seem that there is a Quaternary continental/pre-Quaternary marine dichotomy in the nature and distribution of CaCl2 brines and the associated antarcticite/tachyhydrite minerals, much as there is for the world’s potash deposits (MgSO4-enriched versus MgSO4-depleted). On Earth, there are no documented examples of pre-Quaternary bedded antarcticite. The situation may be different on Mars.

Today terrestrial antarcticite either precipitates in the hot-arid Bristol Dry Lake or the icecap-polar setting of Don Juan Pond, neither is associated with brine sourced in seawater or derived by the recycling of older (“connate”) marine evaporite successions. Rather the occurrenceof the calcium chloride minerals is in part a reponse to low temperatures and regolith cycling. Other present-day examples of regions with somewhat enriched levels of CaCl2 surface waters, lack both tachyhydrite and antarcticite precipitates, and contain higher levels of Mg that are tied to deeply circulated marine/hydrothermal waters and variable interactions with MgSO4-enriched marine salts (e.g., Lake Asal and the Dallol Salt Pan). CaCl2 brines of the Dead Sea and the Red Sea show even more elevated levels of MgCl2 as they are derived, at least in part by the fractionated dissolution of bedded and halokinetic Miocene marine evaporites. The Mg-rich clay palygorskite is co-associated with these CaCl2 brines in the subsurface of the Red Sea and the Dallol.

In contrast, tachyhydrite in some ancient marine-fed basinwide evaporite associations is found as somewhat rare, but at times bedded, units in the bittern-rich portions of the halite succession, as in the Cretaceous basinwide evaporites of Brazil, the Congo and Thailand, and as a cementing phase in the Permian Stassfurt 2 in the Zechstein (where it is commercially extracted in association with MgCl2 brines).

All the ancient tachyhydrite examples mentioned above, are associated with the presence of widespread potash salts within adjacent salt beds. However, there are many other even larger and richer ancient potash deposits, such as the intracratonic Alberta basin (Devonian) and the Kama basins (Permian) where no tachyhydrite is documented. It seems that the terrestrial precipitation of bedded tachyhydrite is not just favoured by times of CaCl2 oceans, it also requires additional input from saline hydrothermal/basinal waters. Such settings are most likely in the transition phases in an actively-opening hydrothermally-influenced continental rift as passes into the marine seepage realm at a time when the adjacent ocean was a MgSO4-depleted system.


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Salt as a Fluid Seal: Article 2 of 4: Internal fluid source

John Warren - Wednesday, January 20, 2016


Black Salt: as an indicator of overpressure

The previous article in this series on salt leakage focused on black and dark salt created by ingress or interaction of undersaturated waters with relatively shallow halokinetic salt masses, with entry zones often tied to intervals of salt shear. The resulting black or dark salt textures are one style of “anomalous” salt. This article looks at fluid entry into salt in subsurface intervals of high pore pressure, exemplified by the “black salt” in the Ara salt seals of Oman. Such intervals are often tied to burial-pressure and temperature-related changes to the dihedral angle of salt (halite).

Dihedral angle changes and the permeability of salt

Permeability in intercrystalline pore networks in re-equilibrating and crystallising subsurface salt is tied to the dihedral angle  at solid-solid-liquid triple junctions (Figure 1; Lewis and Holness, 1996, Holness and Lewis, 1997). When the halite dihedral angle is higher than 60° under static laboratory conditions, this contact angle equates to the maintenance of closure of polyhedral grain boundaries by halite precipitation, and so at these lower temperatures both bedded and halokinetic recrystallized salt is impermeable (Schenk and Urai, 2004; Holness and Lewis, 1996). In this temperature range, the small amount of brine present in the salt is distributed in micrometer-sized isolated fluid inclusions at termini of salt crystal polygon apices. In contrast, when the solid-solid-liquid interfaces of increasingly heated and pressurised polyhedral halite attain dihedral angles that are less than 60° then the fluid-inclusion filled intercrystal cavities link up and the salt mass becomes permeable.

At burial temperatures >100°-150°C and pressures of 70 MPa or more, the dihedral angle has decreased to values <60°, driving a redistribution of the fluid into a thermodynamically stable network of connected, fluid-filled channels or fused fluid strings at grain boundary triple junctions. This transition may be related to the observation by Peach and Spiers (1996) that, during natural deformation of rocksalt at great depths, salt undergoes natural hydraulic fracturing or dilatancy. The dihedral angle is, therefore, a thermodynamic property that changes with pressure P and temperature T. Holness and Lewis’s experiments demonstrated that buried salt masses, subject to high pressures and elevated temperatures, can acquire intercrystalline or polyhedral permeability comparable to associated with intergranular permeability in sand.

This typically occurs at higher temperatures and pressures where intercrystal water positions link within flowing or static, but texturally re-equilibrated, salt and so creates continuous fluid strings along evolving intercrystalline junctions in the burial-recrystallised salt. The newly attained intercrystal configuration allows penetration and throughflow of hot, dense brines or hydrocarbons into and through the altered mass of salt polyhedrons. In Oman has created characteristic haloes of black salt about pressurised salt-encased carbonate slivers (next section).

At the same time as a recrytallising salt mass passes into the earlier stages of the greenschist facies, the salt is dissolving and altering to sodic scapolite (Warren, 2016; Chapter 13). Thus, through the later stages of diagenesis and into early to medium grades of metamorphism, the salt and its daughter products may act as sources and conduits for flow of chloride-rich metalliferous brines and salt slurries. This occurs as bedded and halokinetic salt evolves from a dense impermeable salt mass into permeable salt with higher dihedral angles and so explains salt’s significant role in the creation of many of massive base metal and IOCG deposits (Warren, 2016; Chapters 15, 16).

Black salt and overpressure in Oman

The transition in dihedral angle with increasing pressure and temperature explains the occurrence of black (bitumen-charged) haloes in salt encasing some carbonate-sliver reservoirs in the Ara Salt of Oman (Figure 2; Kukla et al., 2011a, b). Once this recrystallization occurs, the previous lower P&T mosaic halite loses its ability to act as an aquitard or aquiclude (seal) and can instead serve as a permeable conduit for escaping highly-pressurised and hydrocarbon-rich formation waters. According to Lewis and Holness, the depth at which the recrystallization occurs may begin as shallow as a few kilometres (Figure 1). But, their pressure bomb laboratory-based static-salt experiments did not completely encompass the ability of natural salt to pressure creep and self-seal by longer-term diffusion-controlled pressure solution (Warren 2016, Chapter 6). Even if the changing dihedral angles alter and open up permeability at such shallow depths, there is no guarantee that subsequent flowage associated with pressure solution will not re-anneal these new pores. The ability of salt to continue to act as a highly efficient hydrocarbon seal to depths of 6-10 km means, in my opinion, that bedded salt does may become a relative aquifer until attaining depths of 6-10 km or more. This occurs certainly at temperatures and pressures where the sequence is entering the greenschist realm. In extremely overpressured situations the transition of dihedral angles is much shallower, as in the 40-50m thick black salt rims that typify the salt-encased hydrocarbon-charged carbonate stringers in the Ara Salt of Oman (Kukla et al., 2011b). Once it does transform into polyhedral halite, a former aquiclude becomes an aquifer flushed by chloride-rich brines, likely carrying other volatiles.

A release of entrained inclusion (±intercrystalline) water at temperatures > 300-400°C (early greenschist) influences the textures of deeply buried halite. Most of the inclusions in chevron halite and other inclusion-rich cloudy primary salts are due to entrained brine inclusions and not mineral matter. Figure 3 plots the weight loss of various types of halite during heating. It clearly shows cloudy (inclusion-rich) halite releases up to 5 times more brine (0.2-0.5 wt%) than clear coarsely crystalline halite. An analysis of all fluids released during heating shows carbon dioxide and hydrogen contents are much lower than the water volumes: CO2/H2O < 0.01 and H2/H2O < 0.005. Organic compounds, with CH4, are always present (<0.05% H2O), and are twice as abundant in cloudy halite. There are also traces of nitrogen and, in some samples, hydrogen sulphide and sulphur dioxide (Zimmermann and Moretto, 1996).

The influence of overpressure driving changes in the dihedral angle of pressurised salt is most clearly seen in black-salt encased Late Neoproterozoic to early Cambrian intra-salt Ara (stringer) reservoirs of the South Oman Salt Basin (Figures 2, 4, 5; Kukla et al., 2011b). These carbonate bodies are isolated in salt and frequently contain low-permeability dolomites and are characterised by high initial hydrocarbon production rates due to overpressure. But not all stringers are overpressured, and a temporal relationship is observed defined by increasingly overpressured reservoirs within stratigraphically younger units. There are two separate pressure trends in the stringers; one is hydrostatic to slightly-above hydrostatic, and the other is overpressured from 17 to 22 kPa.m−1, almost at lithostatic pressures (Figure 4).

The black staining of the halite is caused by intragranular microcracks and grain boundaries filled with solid bitumen formed by the alteration of oil (Figures 2, 5). The same samples show evidence for crystal plastic deformation and dynamic recrystallization. Subgrain-size piezometry indicates a maximum differential paleostress of less than 2 MPa. Under such low shear stress, laboratory-calibrated dilatancy criteria suggest that oil can only enter the rock salt at near-zero effective stresses, where fluid pressures are very close to lithostatic. In Schoenherr et al.’s (2007b) model, the oil pressure in the carbonate stringer reservoirs reservoir increases until it is equal to the fluid pressure in the low, but interconnected, porosity of the Ara Salt, plus the capillary entry pressure (Figure 5). When this condition is met, oil is expelled into the rock salt, which dilates and increases its permeability by many orders of magnitude. Sealing capacity is lost, and fluid flow will continue until the fluid pressure drops below te minimal principal stress, at which point rock salt will reseal to maintain the fluid pressure at lithostatic values. Inclusion studies in the halite indicate ambient temperatures at the time of entry were more than 90°C, implying hydrocarbons could move into interconnected polyhedral tubes in the halite. These conduits were created in response to changes in the polyhedral angle in the halite in response to elevated temperatures (Lewis and Holness, 1996).

Hydrocarbon-stained “black salt” can extend up to 100 metres from the pressurised supplying stringer into the Ara salt of Oman (Figure 2, 5). It indicates a burial-mesogenetic pressure regime and is not the same process set as seen in the telogenetic “black salt” regions of the onshore Gulf of Mexico. The latter is created by dissolution, meteoric water entry, and clastic contamination, as in the crests of nearsurface diapirs such as Weeks Island (Warren 2015). An Ara stringer enclosed by oil-stained salt but now below the lithostatic gradient likely indicates a later deflation event that caused either complete (C) or partial (E) loss of overpressures. Alternatively, stringers showing overpressure, but below the lithostatic gradient (E), might be explained by regional cooling or some other hitherto unexplained mechanism (Figure 4a; Kukla et al., 2011a, b).

Structural, petrophysical and seismic data analysis suggests that overpressure generation in the Ara is driven initially by rapid burial of the stringers in salt, with a subsequent significant contribution to the overpressure from thermal fluid effects and kerogen conversion of organic-rich laminites with the stringer bodies. If the overpressured stringers come in contact with a siliciclastic minibasin, they will deflate and return to hydrostatic pressures (A) in Figure 4. When the connection between the minibasin and the stringers is lost, they can regain overpressures because of further oil generation and burial (A’). If hydrocarbon production in undeflated stringers stops relatively early, the fluid pressures do not reach lithostatic pressures (B). If hydrocarbon generation continues, the fluid pressures exceed the lithostatic pressure (red star), leading to dilation and oil expulsion into the rock salt to what is locally known as “black salt” (D and E).

As well as these examples of overpressure associated with older evaporites, overpressure readily develops in salt-sculpted Tertiary basins. For example, overpressure occurs in salt shear (gumbo) transitions beneath some, but not all, shallow salt allochthons in Green and Mahogany Canyon regions in the Gulf of Mexico (Beckman, 1999: Shaker 2008). Where salt allochthons are climbing the stratigraphy, subsalt sealing and associated overpressure can occur beneath the salt mass at shallower levels than is observed in overpressured shale basins.

In terms of extension and compression regimes within a single allochthon tongue, Shaker (2008) noted that in extensional regions in halokinetic basins the magnitude and direction of the principal stresses are controlled by sediment load, salt thickness, and salt emplacement-displacement history. Therefore, the maximum principal stress is not necessarily represented by the sheer weight of the overburden, as is usually assumed in quiescent terranes. Salt buoyancy often acts upward and has the tendency to accelerate and decelerate the principal stress above and below the salt, respectively. A distinctive shift of the pore pressure envelopes and normal compaction trends takes place across the salt body in several wells drilled trough salt below minibasins in the Mississippi Canyon, Green Canyon, and Garden Banks areas of the Gulf of Mexico. A lower pore pressure gradient has been observed below the salt and a higher gradient above the salt barrier. On the salt-rooted minibasin scale, a high-gradient was also observed in areas where the salt was emplaced and a lower gradient where the salt withdrew (Shaker and Smith, 2002). On the other hand, in the compressional portion of a salt allochthon system, lateral stress generated by the salt movement piling up salt at the foot of the slope acts as the maximum principal stress, whereas the load of sediment represents the minimum stress.

Extreme overpressuring is commonplace in subsalt settings in the Gulf of Mexico at depths of 3000-4000 m and its variability creates drilling problems, as evidenced by the BP Horizon spill and explosion on April 20, 2010. Gas generated at greater depths in these regions can be trapped under the salt seal at pressures approaching lithostatic. It means drilling under the allochthonous salt on the Gulf Coast slope can intersect undercompacted sediments that are moderately to extremely overpressured and friable (Hunt et al., 1998). The influence of highly effective Jurassic salt seals on pressure gradients in the Neogene stratigraphy of the Gulf of Mexico is seen in the increased mud weights typically required for safe drilling, once an evaporite allochthon is breached by the drill (Table 1). Many wells intersecting salt allochthons in the deepwater realm of the Gulf of Mexico and the circum-Atlantic Salt basins are overpressured at some depth below the base of salt with mud weights controlling pressures ranging from 14 to 17.5 ppg.


This and the previous article (Warren, 2015) demonstrate that black salt is a form of anomalous salt that indicates salt has leaked, however, the locations and conditions where leakage has occurred are distinct. The black salt encountered in the salt mines of the US Gulf Coast are indicative of meteoric water entry in relatively shallow conditions in regions where the salt is in contact with the surrounding shales of muds that enclose the diapir salt core. In other words, fluid entry is from the outside of the salt mass and fluids move into the salt from its edges and likely enhance  the porosity in the intercrystalline salt. In contrast, the black salt occurrences in the Ara Salt of Oman are indicative of overpressure haloes, generated internally via hydrocarbon and fluid expulsion in carbonate slivers, which are are fully encased in salt. This creates naturally hydrofractured envelopes in the salt mass in zones where pressure and temperature induced changes in the dihedral angle has generated intercrystalline fluid strings within the recrystallised polyhedral halite. The two settings of black salt formation are distinct.

There is not a single mechanism that creates black salt in a halokinetic salt mass. We shall discuss the implications of this in the next article which will include a look at leakage models in halokinetic salt systems both in terms of their seal integrity and the implications for short and  long term storage of hydrocarbons and nuclear waste. 


Beckman, J., 1999. Study reveals overpressure sources in deep-lying formations. Oil and Gas Journal, September: 137.

Holness, M.B. and Lewis, S., 1997. The structure of the halite-brine interface inferred from pressure and temperature variations of equilibrium dihedral angles in the halite-H2O-CO2 system. Geochimica et Cosmochimica Acta, 61(4): 795-804.

Hunt, J.M., Whelan, J.K., Eglinton, L.B. and Cathles III, L.M., 1998. Relation of shale porosities, gas generation, and compaction to deep overpressures in the US Gulf Coast. In: B.E. Law, G.F. Ulmishek and V.I. Slavin (Editors), Abnormal pressures in hydrocarbon environments. American Association Petroleum Geologists Memoir 70, Tulsa, OK, pp. 87-104.

Kukla, P., Urai, J., Warren, J.K., Reuning, L., Becker, S., Schoenherr, J., Mohr, M., van Gent, H., Abe, S., Li, S., Desbois, Zsolt Schléder, G. and de Keijzer, M., 2011a. An Integrated, Multi-scale Approach to Salt Dynamics and Internal Dynamics of Salt Structures. AAPG Search and Discovery Article #40703 (2011).

Kukla, P.A., Reuning, L., Becker, S., Urai, J.L. and Schoenherr, J., 2011b. Distribution and mechanisms of overpressure generation and deflation in the late Neoproterozoic to early Cambrian South Oman Salt Basin. Geofluids, 11(4): 349-361.

Lewis, S. and Holness, M., 1996. Equilibrium halite-H2O dihedral angles: High rock salt permeability in the shallow crust. Geology, 24(5): 431-434.

O'Brien, J. and Lerche, I., 1994. Understanding subsalt overpressure may reduce drilling risks. Oil and Gas Journal, 92(4): 28-29,32-34.

Peach, C. and Spiers, C.J., 1996. Influence of crystal plastic deformation on dilatancy and permeability development in synthetic salt rock. Tectonophysics, 256: 101-128.

Schenk, O. and Urai, J.L., 2004. Microstructural evolution and grain boundary structure during static recrystallization in synthetic polycrystals of sodium chloride containing saturated brine. Contributions to Mineralogy and Petrology, 146: 671-682.

Schoenherr, J., Littke, R., Urai, J.L., Kukla, P.A. and Rawahi, Z., 2007a. Polyphase thermal evolution in the Infra-Cambrian Ara Group (South Oman Salt Basin) as deduced by maturity of solid reservoir bitumen. Organic Geochemistry, 38(8): 1293-1318.

Schoenherr, J., Urai, J.L., Kukla, P.A., Littke, R., Schleder, Z., Larroque, J.-M., Newall, M.J., Al-Abry, N., Al-Siyabi, H.A. and Rawahi, Z., 2007b. 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, 91(11): 1541-1557.

Shaker, S., 2008. The double edged sword: The impact of the interaction between salt and sediment on subsalt exploration risk in deep water. Gulf Coast Association of Geological Societies Transactions, 58: 759-769.

Warren, J.K., 2015. Salt as a fluid seal: Article 1,  Salty Matters blog; First published on Dec 19, 2015;

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

Zimmermann, J.L. and Moretto, R., 1996. Release of water and gases from halite crystals. European Journal of Mineralogy, 8(2): 413-422.



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