Marine Sabkahs

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Marine Sabkhas

Marine sabkhas develop in arid environments on flat featureless coastal plains whose evaporite-bearing muddy or sandy sediment is subjected to coverage by tides. They represent transitional environments between land and sea be it island or continent. Many marine sabkha environments are intimately associated with carbonate depositional systems but they also can develop in clastic depositional systems (e.g. Ali and West 1983, Amdurer 1982, Saleh et al. 1999, Shinn 1973). One of the most intensively studied and best-known modern sabkhas is the carbonate flat of the Trucial Coast (Curtis et.al. 1963, Shearman 1963, 1966, Kendall and Skipwith 1969, Evans et al. 1969, Kinsman 1969, Butler 1970, 1982, Kirkham 1997, Wood 2002). This modern sabkha forms a narrow strip up to 8 km in width (Kendall et al. 1994). Two depositional settings characterize the salt flat 1) an intertidal zone whose sediments are covered and uncovered by diurnal tides, and 2) a supratidal zone whose inundation occurs only during spring and storm-driven tides. Sbkhas are largely barren plains with plant and animal life restricted to narrow zones. Vegetation includes blue-green algal mats and mangrove growths localized in intertidal settings and halophytes are known to occupy the landward fringes of the supratidal zone. Animal life is even more restricted in its distribution because of the extreme environmental stresses associated with sabkhas. Resident animal life includes Cerithid gastropods, burrowing crabs, Milliolid foraminifera and ostracods whose landward advance is limited to the intertidal zone.

Evaporite deposition is an integral part of marine sabkhas and the evolution of evaporitic brine is in part a function of location.

Marine sabkhas attached to islands develop evaporitic brines derived mostly from seawater whereas those attached to continental margins inherit a blend of marine and continetal fluids. Brine evolution and the type of evaporite precipitated subdivides the supratidal zone into three subzones (Kendall, et al. 1994, Warren 1989) The lower supratidal forms a narrow strip above the intertidal zone and is the site of gypsum mush precipitation. The appearance of nodular anhydrite within the buried gypsum mush characterizes the middle supratidal. It lies between the lower and upper supratidal, the later being defined by the distribution of mosaic anhydrite that completely replaced the gypsum mush horizon.

Upper Intertidal zone

Gypsum is known to precipitate in interstidal pore space of intertidal sediments. Piston cores taken of algal mats in the high intertidal zone of the Abu Dhabi marine sabkha reveal a smattering of discoidal gypsum growth within the algal mat. Gypsum growth is both displacive and incorporative with most crystals having a subvertical orientation. Sectioned discoidal gypsum shown in slabbed core have the typical "axhead" shape. Other piston cores show a high concentration of discoidal gypsum is also present just below the algal mat. All these early diagentic growths of calcium sulphate consist of centimeter-size crystals.

Discoid or lenticular gypsum crystals ("axhead") that precipitate from interstitial brines in the upper intertidal zone and the middle supratidal sabkha, Abu Dhabi, U.A.E. Larger discoidal gypsum crystals are known from the supratidal zone.

Slabbed piston core through algal mat (peat) documents the growth of lenticular gypsum in the upper intertidal zone.

Slabbed piston core from lower supertidal zone shows abundant small lenticular gypsum crystals below thin algal mat and globular anhydrite nodules above it. Photo courtesy of E. Shinn.

Lower Supratidal Zone

Gypsum 'mush' domes and cumulates form in the upper intertidal splash zone of the Abu Dhabi sabkha in 1960's. A combination of trapped brine pools and seawater spray source the prismatic gypsum growth. The domes are hollow structures that coalesce to form a set of intersecting ridges and intervening shallow brine pools. Photos from Shell Oil Company archives.

Gypsum 'mush' today at Qanatir, U.A.E. forms a layer of sediment above the algal peat. Note, the water table aproximatley coincides with the stratigraphic horizon of agal mat growth in the high intertidal zone.

Middle Supratidal Zone

The middle supratidal zone shows considerable variation in the form and crystal habit of calcium sulphate. The photo above shows gypsum precipitates from interstitial brines at the water table in the the middle supratidal of the sabkha at Abqaiq in Saudi Arabia. Note that the intimate association between gypsum roses and the water table leads to a "bedded appearance". Furthermore, gypsum precipitation is not limited to gypsum roses, as shown by the sheet of gypsum crystals collected from the same area.

Laguna madre, a clastic marine sabkha, features both gypsum rosetts and "bladed" gypsum at the water table.

The presence of nodular anhydrite defines the middle supratidal zone of the Abu Dahbi Sabkha. The anhydrite represents the inclompete transformation from gypsum mush and is accompanied by primary anhydrite precipitation to form a felted nodular anhydrite fabric. Anhydrite occurs above the water table.

Thin section of incorporative gypsum rosette from Laguna Madre, Texas, U.S.A. Gypsum growth in grainy sediment (quartz sand in this case) is typically poikilotopic whereas that in muddy sediment is displacive (image on left).

Upper Supratidal Zone

Extensive anhydrite precipitation occurs within the sediment of the upper supratidal zone. It is characterized by coalesced anhydrite nodules, tigmatic banhydrite folds and displacement of carbonate. Note deflation of sabkha exposes and truncates tigmatic anhydrite.

Locally the influx of continental meteoric water rehydrates anhydrite to form secondary gypsum where the upper supratidal merges with continental deposits.

References

ALI, Y.A., and WEST, I.M., 1983, Relationship of modern gypsum nodules in sabkhas of loses to compositions of brines and sediments in northern Egypt: Journal of Sedimentary Petrology, v. 53, p. 1151-1168.
AMDURER, M., and LAND, L.S., 1982, Geochemistry, hydrology and mineralogy of the Sand Bulge area, Laguna Madre Flats, south Texas: Journal of Sedimentary Petrology, v. 52, p. 703-716.
BUTLER, G.P., 1970, Holocene gypsum and anhydrite of the Abu Dhabi Sabkha, Trucial Coast: an alternative explanation of origin: Third Symposium on Salt, p. 120-152.
BUTLER, G.P., HARRIS, P.M., and KENDALL, C.G.S.C., 1982, Recent evaporites from Abu Dhabi coastal flats, in Handford, C.R., Loucks, R.G., and Davies, G.R., eds., Depositional and Diagenetic Spectra of Evaporites — A Core Workshop: SEPM Core Workshop No. 3: Calgary, Society of Economic Paleontologists and Mineralogists, p. 33-64.
CURTIS, R., EVANS, G., KINSMAN, D.J.J., and SHEARMAN, D.J., 1963, Association of dolomite and anhydrite in the recent sediments of the Persian Gulf: Nature, v. 197, p. 679-680.
EVANS, G., SCHMIDT, V., BUSH, R.P., and NELSON, H., 1969, Stratigraphy and geological history of the sabkha, Abu Dhabi, Persian Gulf: Sedimentology, v. 12, p. 145-159.
KENDALL, C.G.G.S.C., ALSHARHAN, A.S., and WHITTLE, G., 1994, Field Guidebook to Examine the Holocene Carbonates/Evaporites of Abu Dhabi, United Arab Emirates: for the International Geological Correlation Program (IGPCP-349): Abu Dhabi, U.A.E. University Publication Department, 46 p.
KENDALL, C.G.S.C., and SKIPWITH, P.A., 1969, Holocene shallow water carbonate and evaporite sediments of Khor Al Bazam, Abu Dhabi, Southwest Persian Gulf: American Association of Petroleum Geologists Bulletin, v. 53, p. 841-869.
KIRKHAM, A., 1997, Shoreline evolution, aeolian deflation and anhydrite distribution of the Holocene, Abu Dhabi: GeoArabia, v. 2, p. 403-416.
SHEARMAN, D.J., 1963, Recent anhydrite, gypsum, dolomite, and halite from the coastal flats of the Persian Gulf: Proceedings Geological Society of London, v. 1607, p. 63-65.
SHEARMAN, D.J., 1966, Origin of marine evaporites by diagenesis: Transactions of Institute of Mineralogy Metall. Section B, v. 75, p. 208-215.
SHINN, E.A., 1973, Sedimentary accretion along the leeward, SE coast of Qatar Peninsula, Persian Gulf, in Purser, B.H., ed., The Persian Gulf: Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea: Berlin Heidelberg New York, Springer-Verlag, p. 199-209.
WARREN, J.K., 1989, Evaporite sedimentology : importance in hydrocarbon accumulation: Englewood Cliffs, N.J., Prentice Hall, xv, 285 p.
WOOD, W.W., and SANFORD, W.E., 2002, Hydrology and solute chemistry of the coastal-sabkhas aquifer in the Emirate of Abu Dhabi, in Barth, A.P., and Boer, eds., Sabkha Echosystems: Amsterdam, Netherlands, Kluwer Academic Publishers, p. 173-185.

Site design June 2001. Updated January 2004, January 2005 - F.O. Meyer