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

摘要

Modern sabkhas are recognized as analogues to ancient evaporitic reservoirs and as Earth analogues to Martian paleo- environments. 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.