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).

Salt ablation indicators; how flowing salt dissolves at the surface

John Warren - Tuesday, March 10, 2015

When a salt sequence is in contact with undersaturated waters it dissolves to leave behind layers of salt dissolution residues and breccias. The process of salt dissolution is most obvious and most visually stunning is where solution collapse dolines or stoping karst chimneys daylight, as was discussed in an earlier blog dealing with salt collapse features forming today in the vicinity of Uralkali’s Solikamsk 2 mine. Although spectacularly catastrophic, such features are not typical of an area where a halokinetic salt mass is flowing and ebbing across the landsurface. The continual resupply of salt prevents such features forming. Instead, stratiform or stratabound dissolution residue layers and perhaps breccia horizons, along with salt welds, are the more typical indicators of the former presence of a flowing and dissolving salt mass. In this essay I illustrate this by focusing on the geological development of dissolution features within and about three current or former halokinetic salt masses (namakiers), currently outcropping at the surface in central Iran. The three salt features are Kuh-e-Namak (Qom), Kuh-e-Namak (Shurab) and Kuh-e-Gach (Saveh). Listed in this order they illustrate a sequence of dissolution from an initial salt fountain or namakier, through supra-salt overburden foundering, to a time when all at-surface halite has dissolved and only a carapace of gypsum and clay residues remain. Similar features are seen in the rock record, wherever salt ablation breccias occur.

Namakier is a term suggested by Martin Jackson utilising the comparison to an ice glacier, where namak is the Persian (Farsi) word for salt. Gach is Farsi for gypsum. while kuh means mountain. So, the term kuh-e-namak when translated means “mountain of salt.” There are many such mountains of salt (namakiers) composed of Oligo-Miocene salt in Central Iran and mountains of Neoproterozoic/Cambrian salt in the Zagros fold belt, located further to the south and southeast of Qom. This means it is unwise to describe any salt glacier as kuh-e-namak, without a geographic suffix to better define its location. Likewise there are a number of mountains of gypsum, which are typically the remnants of a namakier, at the stage where all the halite has dissolved to leave behind a carapace of gypsum residues, which outline the former extent of the salt tongue. Gypsum residues too will ultimately dissolve, as is occurring about the current edges of Kuh-e-Gach (Saveh) in central Iran (Figure 1).

 

Qom Kuh is one of a number of salt diapirs currently emergent along releasing bends in the Qom-Saveh basin, which are regions that have been pulled apart along major dextral transpressive faults, crossing and defining the margin of the Qom-Saveh Basin in the central plateau of Iran. The other two features discussed in this essay, Kuh-e-Namak (Shurab) and Kuh-e-Gach Saveh are located on the current thrust margin of the basin and so have been cut-off from the mother salt supply, rather than directly above the current salt area (Figure 1). Interestingly, Kuh-e-Namak (Qom) is only ten or so kilometres away from the site of the famous 1956 blowout site at the Alborz 5 well atop the Alborz anticline (Figure 1). With an estimated volume of 5 million barrels of oil escaping at Alborz 5 over 82 days (until the blowout self-bridged), the salt-sealed and salt-cored fractured Qom carbonate (Oligo-Miocene) reservoir remains the feeder to one of the world’s largest blowouts. For comparison, Macondo in the Gulf of Mexico (another salt-related blowout) had an estimated total volume of escaped fluids of around 4.9 million barrels. We shall discuss salt-related blowouts and salt seal integrity in a future blog. For now, we shall focus on what typifies the geology where a namakier flows over the landsurface and then starts to dissolve.

 

Kuh-Qom, is still located in a supplying salt mass and so is actively flowing or fountaining salt to the surface, while the other two salt features are in varying stages of collapse and dissolution (Figure 1). Qom-Kuh is located on a releasing bend of an offset in the Qom Fault that crosscuts the plunging nose of the Alborz Anticline (Figure 2). The Qom Fault intersects a number of normal faults in the vicinity of Kuh Qom which are exposed to the east of Qom Kuh, where it extends beneath alluvium to the northwest of the diapir (Figures 1, 3). Splays of oblique-reverse and oblique-normal faults offset the nearby asymmetric salt-cored thrust anticline, known as the Alborz Anticline (where the Alborz 5 blowout occurred). These faults are well exposed in the 60m-high ephemeral-stream eroded cliff that defines the eastern flank of Qom Kuh. The adjacent ephemeral-river bench has bevelled into the Upper Red Formation marls at the 920m above sea level (Figure 3b). Faults are clearly seen in the zone separating the diapir from the hill created by the northern limb outcrop of the anticline. The largest of these faults has an irregular trace that was superimposed on a ductile dextral strike-slip shear zone before it offset the river bench some 1.5 m down to the east (Figure 3a). Today the crest of the salt fountain of Qom Kuh rises to 1235 m asl and is some 315 m above the surrounding plateau (Figure 3d).

 

The Qom Kuh namakier can be divided into two main components: (1) a smooth hemispheric dome or summit with a diameter of 2.5 km and surrounded by, (2) a dissected and dissolving apron of salt allochthonous flowing atop Upper Red Formation and Recent gravels (Figure 3b, d). The margin of this rim is defined by large blocks of salt-buoyed exotics, mostly Eocene volcanics, that have slide down the salt mass to accumulate as a pile of blocks about the salt edge (Figure 3a-c). Steps in the salt topography indicate that the extruded salt overflows a collar of Upper Red Formation up-tilted around the inferred diapiric vent. This raised collar is highest beneath the NE shoulder of Qom Kuh, where it is exposed in windows up to 1100 m asl and lowest in its SW corner (Figure 3; Talbot and Atabi, 2004; Cosgrove et al., 2009). At the Qom-Kuh summit the salt mass is characterised by diffusion karst and variably covered by up to a metre of clay-dominant residuals, along with rare cm-sized clasts of anhydrite and limestone.

The river cliff truncates the eastern end of a sedimentary bedrock collar (Figure 3a, b). The bench and cliff exposes the apron of allochthonous salt, which is up to 200 m thick and about 0.75 km long to the west of the summit, and some 2 km to the south (Figure 3d). The underlying rhombic vent, through which Qum Kuh extrudes, is interpreted to be a pull-apart between normal faults that hard-link a releasing offset along a regional transpressive strike-slip fault, which trends west–east (Figure 2b). In profiles though the summit, Qum Kuh can be considered as a pile of recumbent fold nappes of gneiss-like and mylonite-like Oligocene and Miocene salts that have extruded from depth and the gravity spread over recent alluvial gravels (Figure 3c).

An ephemeral stream still flows anticlockwise around most of Qom Kuh, and is still eroding the cliff on the east side of Qom Kuh. Broken pottery lying on the river-bevelled bench to the east, and exposed by wind-deflation of the soils overridden by the southern namakier, suggest that the river eroded the salt of Qom Kuh in historical times (≈ 10,000 years ago according to Talbot and Atabi, 2004). Salts dissolved from the Qom Kuh namakier are now being re-precipitated as Holocene evaporite beds in the adjacent playa low, which is possibly situated atop an active salt withdrawal basin (Figures 2, 3b).

 

The rhombic transtensional nature of the salt neck to this pull-apart graben structure that allows the ongoing salt supply to the Kuh-Qom salt fountain is seen more clearly in a now inactive rhomboid neck , which now outcrops at Kuh-e-Gach, Saveh, a highly dissolved namakier remnant (Figures 1, 4). Only the nodular gypsum carapace and the underlying reducing brine halo remain to define the position of the former namakier (Figure 4a-d). The leaching of the namakier salt created dense plumes of reducing brines that coloured the underlying redbeds grey, as it converted iron in the redbeds from its ferrous to ferric state (Warren, 2008). Clearly, at the other end of the dissolution spectrum from an active salt fountain, the actual evaporite mass (both halite and gypsum residues) will disappear, as at Kuh-e-Gach (Saveh). Once the reducing brine source is gone, it is also likely the greybeds will transition back into redbeds in this semiarid climate. All in all, not much evaporite evidence of a former salt fountain will remain as mineral salts in the namakier-influenced stratigraphy.  


As a general rule, the upper portion of an actively flowing salt mound is covered by residual soils, along with a variety of gypsum textures growing in the insoluble components (suffusion karst). Small ephemeral features, including shallow diffusion caves, are typical of the salt glacier surface, but in an active namakier the salt is flowing too fast to preserve them for more than a few hundred to a few thousand years depending on the lateral extent and rate of extrusion of the diapir. The karstic carapace atop the moving and dissolving namakier is dominated by soils, typically composed of a combination of dissolution residues, water-carried deposits and wind transported dust (loess). As the moving salt dissolves, dolines develop in the surface of these soils, they enlarge with time as the soils thicken. These non-halite features remain in place and can even accrete as they are carried downslope by underlying flowing salt evan as it dissolves. Dolines below the surface of these soils enlarge and the soils thicken as the moving salt dissolves in its passage away from the vent. Ultimately, once the supply of mother salt is depleted the at-surface namakier salt dissolves to leave behind a gypsum-encrusted surficial layer, no more than a metre or two thick (Figure 5; Warren, 2008).

 

Once the supply from the mother salt layer ceases, an at-surface namakier stops its outward expansion and begins to shrink to ultimately subside and disappear. This occurs over time scales of hundreds to thousands of years post-flow, to leave behind a salt-ablation carapace. As the salt dissolves, soils that previously had accumulated about to namakier edge, now touch down over the whole extent of the former salt glacier. Ultimately, karstification and salt core collapse becomes so pervasive all the way across the former extent of the namakier core so that only the edges of the relict plug still show any positive relief (Figure 6). Salt plug ruins and overburden blocks once buoyed by the salt mass are  broken and disturbed, while a halo of salt-buoyed exotic blocks defines the former extent of the salt tongue. Overall, in the local Iranian landscape underlain by salt, inactive and shrunken plugs (as at Shurab - Figure 7) tend to be negative landscape features (depressions). 

 

Although not leaving behind much in the way of salt mineral remnants, the salt-buoyed transport of exotic blocks, which can be hundreds of metres in diameter, and the remnants of lithologies that were plucked and buoyed by the salt as it made its way to the surface are the best evidence for the former presences of a namakier mass. Such a horizon or layer of exotic blocks can also be accompanied by, and typically is immediately underlain by, a disturbed layer of scattered and rotated overburden blocks. This style of mega-breccia creation is seen in the foundering and collapse blocks of Qom Limestone in the salt-cored breached anticline that defines the Shurab diapir (Figure 7).


Today, to the south of the Qom Basin, in the Persian (Arabian) Gulf there are numerous salt-cored islands in various stages of namakier dissolution, such as Hormuz, Das and Yas islands (Figure 8). These once fountaining extrusive salt masses are now largely inactive, with only minor at-surface evaporite residues present, mostly as gypcretes and caprock remnants. Highest parts of the diapir-cored islands are typically covered by dissolution breccias that cap the still dissolving salt core below. Halokinetic Precambrian (Hormuz) salt beneath the breccia carapace is pervasively karstified and where relatively shallow, as at Hormuz Island, is covered by suffusion karst and crosscut by tube caves. Active flow of the squeezed salt in the various island cores seems to have ceased sometime in the Miocene, so that terrains of exotic blocks of meta-igneous and dolomite litologies now outline much of the surface expression of former outcropping salt masses, Across the Gulf region this stratiform complex of residues and large salt-buoyed blocks is termed the "Hormuz complex." But what it actually is a series of salt ablation breccias. Today these regions of outcropping Hormuz Complex are heavily eroded and partially covered by Neogene shoal water carbonates and marine-cut platforms, as in Das and Hormuz islands (Figure 8a, b)


Texturally near-identical Neoproterozoic megabreccias, breccias and breccia trains crop out in the Flinders Ranges of South Australia (Figure 9; Dalgarno and Johnson, 1968; Lemon, 1985). The salt no longer remains in these features, but they are clearly remnants of what were once diapiric structures (Hearon IV et al, 2015). Many breccias in anticlinal cores of the this halokinetic terrane are still located at or near the level of the former mother salt bed (Callanna Beds), with current outcrop patterns largely indicative of the positions of deeper basement faults (Backé et al., 2010). Today, the breccias define the polytectonic remnants of former autochthonous salt pillows or glide planes and inversion structures, rather than true salt-cored diapirs. Some structures, such as the Oratunga and Wirrealpa diapirs, still preserve evidence of their earlier formative extension phase and show subcircular patterns of outcrop and breccia wings at allochthonous stratigraphic positions well above the level of the original mother salt bed. Almost all the transtratal (halokinetic) breccias line up along major regional shears and faults (welds) surrounded by jostling depopods or minibasins, indicating that the mother salt bed was flowing as the basin was under extension. The salt was then remobilised during the inversion phase and the transition from haselgebirge to rauhwacke textures. Lemon (1985, 1988, 2000), Dyson (2004) and Rowan and Vendeville (2006) clearly illustrate synkinematic controls on sedimentary facies and thicknesses adjacent to many of these breccia masses in the Flinders Ranges (Warren, 2015).

In summary, when a namakier shrinks it does so via at-surface dissolution of the salt mass, much in the same way a glacier retreats as it melts). Likewise, the outer edge of a salt glacier expands and contracts like the outer edge of an ice glacier. When the salt mass retreats it leaves behind a jumbled mass of material it once buoyed, which can include megabreccia blocks tens of metres across, as well as the insolubles of its carapace and insoluble intrasalt layers. These chaotic breccias are similar to a diapiric collapse breccia, but contain a much more polymict assemblage of clasts than an evaporite collapse breccia (after bedded salt) and often show evidence of mechanical reworking of portions of the breccia material by waves or currents. A separate term is probably needed to distinguish them from the more general term diapiric breccia; I call them salt ablation or salt-retreat breccias (Table 1; Warren, 2015).

 

As we saw in the successive stages of diapir dissolution and retreat exposed in the Qom-Saveh Basin of Central Iran, wherever an active but shrinking salt tongue subcrops or lies beneath a dissolution-derived gypcrete carapace, adjacent larger clasts accumulate by breakup of the soft overburden and by stacking of fragments and rafts formerly held within the now dissolved salt matrix. The resulting namakier breccia is composed of a coarse, unsorted, heterolithic (polymict) rubble supported by a fine-grained calcareous matrix dominated by solution flour. This material can be mixed with fluvial and alluvial material (note the fans fed by alluvial streams that cross the ablation zone in Figure 2a) or marine carbonate debris from times when the sea encroached on the salt tongue (as at Hormuz Island; Figure 8).

Ongoing dissolution of the namakier leaves behind a rubble moraine at the level of the former salt allochthon. Thus, salt ablation breccias associated with salt cores are jumbles of exotic material carried to the surface by upwelling salt, which is then combined with the disrupted remnants of the any brittle overburden that once covered an earlier now-dissolved salt tongues. The mixing of contemporary sediment (alluvial or marine) with salt-buoyed megabreccia blocks and insolubles in the salt, and the stratiform circum-stem or circum-weld nature of its occurrence, is what distinguishes a salt ablation breccia from a diapiric breccia formed by subsurface salt dissolution and from breccias that result from the dissolution of a salt bed (Table 1; Warren 2015). 

 

References

Backé, G., G. Baines, D. Giles, W. Preiss, and A. Alesci, 2010, Basin geometry and salt diapirs in the Flinders Ranges, South Australia: Insights gained from geologically-constrained modelling of potential field data: Marine and Petroleum Geology, v. 27, p. 650-665.

Cosgrove, J. W., C. J. Talbot, and P. Aftabi, 2009, A train of kink folds in the surficial salt of Qom Kuh, Central Iran: Journal of Structural Geology, v. 31, p. 1212-1222.

Dalgarno, C. R., and J. E. Johnson, 1968, Diapiric structures and late Precambrian-early Cambrian sedimentation in Flinders ranges, South Australia: American Association Petroleum Geologists, Memoir, v. 8, p. 301 -314.

Dyson, I. A., 2004, Christmas tree diapirs and the development of hydrocarbon reservoirs; A model from the Adelaide Geosyncline, South Australia: Salt-sediment interactions and hydrocarbon prospectivity: concepts, applications and case studies for the 21st Century. Papers presented at the 24th Annual Gulf Coast Section SEPM Foundation Bob F. Perkins Research Conference, Houston Tx, December 5-8, 2004 (CD publication), p. 133-165.

Hearon IV, T. E., M. G. Rowan, K. A. Giles, R. A. Kernen, C. E. Gannaway, T. F. Lawton, and J. C. Fiduk, 2015, Allochthonous salt initiation and advance in the northern Flinders and eastern Willouran ranges, South Australia: Using outcrops to test subsurface-based models from the northern Gulf of Mexico: Bulletin American Association Petroleum Geologists, v. 99, p. 293-331.

Hurford, A. J., H. R. Grunau, and J. Stöcklin, 1984, Fission track dating of an apatite crystal from Hormoz Island, Iran: Journal of Petroleum Geology, v. 7, p. 365-380.

Kent, P. E., 1987, Island salt plugs in the Middle East and their tectonic implications, in I. Lerche, and J. J. O'Brien, eds., Dynamical geology of salt and related structures, v. 3-37, Academic Press, New York.

Lemon, N. M., 1985, Physical Modelling of Sedimentation Adjacent to Diapirs and Comparison with Late Precambrian Oratunga Breccia Body in Central Flinders Ranges, South Australia: American Association Petroleum Geologists Bulletin, v. 69, p. 1327 - 1328.

Lemon, N. M., 1988, Diapir recognition and modelling with examples from the Late Proterozoic Adelaide Geosyncline, Central Flinders Ranges, South Australia: Doctoral thesis, Univesity of Adelaide.

Lemon, N. M., 2000, A Neoproterozoic fringing stromatolite reef complex, Flinders Ranges, South Australia: Precambrian Research, v. 100, p. 109-120.

Morley, C. K., B. Kongwung, A. A. Julapour, M. Abdolghafourian, M. Hajian, D. Waples, J. Warren, H. Otterdoom, K. Srisuriyon, and H. Kazemi, 2009, Structural development of a major late Cenozoic basin and transpressional belt in central Iran: The Central Basin in the Qom-Saveh area: Geosphere, v. 5, p. 325-362.

Rowan, M. G., and B. C. Vendeville, 2006, Foldbelts with early salt withdrawal and diapirism: Physical model and examples from the northern Gulf of Mexico and the Flinders Ranges, Australia: Marine and Petroleum Geology, v. 23, p. 871-891.

Talbot, C. J., and P. Aftabi, 2004, Geology and models of Qum Kuh central Iran: Journal of Geological Society of London, v. 161, p. 1-14.

Warren, J. K., 2008, Salt as sediment in the Central European Basin system as seen from a deep time perspective (Chapter 5.1), in R. Littke, ed., Dynamics of complex intracontinental basins: The Central European Basin System, Springer-Verlag, Berlin-Heidelberg, p. 249-276.

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

 


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