Origins of Evaporites in a Holocene Mixed Clastic and Carbonate Coastal Sabkha: Preliminary Hydrological and Geochemical data from Mesaieed Sabkha, Qatar

Modern sabkhas are recognized as analogues to ancient evaporitic reservoirs and as Earth analogues to Martian paleoenvironments. Sabkhas are normal marine coastal sediments modified by groundwater precipitation of evaporites and carbonates. Previous work on Holocene sabkhas has focused largely on dolomitisation in carbonate-evaporite systems. Little attention has been given to understanding the origins of evaporites in mixed clastic-carbonate systems and their influence on reservoir quality. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework within which to understand processes controlling the origins of evaporites, their spatial distribution and likely evolution through time. Mesaieed sabkha is a 4-6 km wide coastal plain which consists of an onlap wedge of Holocene sediments some 3-6 m thick reaching a maximum of 15 m, which onlaps onto Eocene bedrock. Within the sabkha, gypsum is the most abundant diagenetic mineral, reaching 20-50% of the sediment volume over several square kilometres, with minor calcite, dolomite, anhydrite and halite. Gypsum cementation is pervasive above and below the water table in the proximal sabkha, in sediments dated c.6,000 years before present (yr BP), whilst in the central part (c. 4,000 yr BP) gypsum is restricted to surface crusts and water table cements, and is largely absent in the distal (coastal) sabkha (≤ 2,000 yr BP). Preliminary analysis of hydrological and geochemical data suggests evaporative pumping of groundwater from the underlying aquifer is an important source of solutes in the upper part of the sabkha, whilst seawater recharges the lower sabkha via the porous and permeable Eocene carbonates. Evaporation close to the water table results in fluids reaching gypsum saturation, and active precipitation of gypsum is evidenced by depletion of calcium and sulphate in the shallow brines. This is most marked in the middle part of the sabkha where salinity is highest. These increased density fluids reflux downwards from the Holocene, to mix within the Eocene aquifer, where reaction with the Eocene carbonates results in relative enrichment of calcium. Introduction Many ancient sedimentary systems, particularly those deposited at low latitude, include units deposited in non-evaporitic settings but within which evaporitic minerals occlude significant volumes of porosity such as the Permian Zechstein Formation, the Permo-Triassic Khuff Formation, and the Jurassic Arab Formation. Much of the pore-filling or nodular anhydrite that is common at a wide range of burial depths may be secondary, precipitated from pore fluids rich in Ca 2+ and SO4 2(Kendall and Walters, 1978). Anhydrite is the product of dehydration of a gypsum precursor, which is the most abundant primary CaSO4 mineral (Warren, 2006). However, the sources of solutes forming these evaporites, flow pathways of fluids transporting these solutes and the stability of resulting evaporite precipitates has received little attention. Studies of ancient rock seldom attempt to distinguish between CaSO4 that was precipitated as gypsum prior to dehydration and CaSO4 precipitated at depth as primary anhydrite. Hence by using a modern arid environment to develop a better understanding of the distribution of early diagenetic CaSO4, and thus contribute to reconciling potential sources of secondary anhydrite formed during burial. Sabkhas are normal marine coastal sediments modified by groundwater precipitation of evaporites and carbonates. They are characterized by a low relief topography, elevation being determined by the upper limit of the capillary fringe. Precipitation of evaporite minerals occurs within sabkhas as a result of rates of evaporation which exceed rates of rainfall