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Towards Process-Based Understanding of Diagenesis in a Holocene Coastal Sabkha System, Qatar


Holocene sabkhas in the Arabian Gulf are important analogues to ancient evaporitic hydrocarbon reservoirs. Sabkha is an Arabic term for salt flat; a normal marine coastal sediment modified by precipitation of evaporitic minerals. A range of hydrological models have been proposed to account for the sources of solutes forming sabkha evaporites, including continental groundwater discharge, evaporative pumping of seawater, seawater flooding and free convection. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework for understanding processes forming the evaporites, their spatial distribution and likely evolution during shallow burial diagenesis. The Mesaieed sabkha is a prograding coastal plain in southeast Qatar, and consists of an onlap wedge of Holocene sediments with a basal coarse transgressive lag, infilling topographic lows in the underlying Eocene limestones and dolomites. The coastal plain is some 4-6 km wide, and typically 3-6m thick, reaching 15m towards the coast. This process-based study integrates hydrological and geochemical data from a suite of wells within a depositional and mineralogical model extending from the bedrock aquifer to the coast. Within the sabkha, gypsum is the most abundant diagenetic mineral, reaching 20-50% of the sediment volume over several km2. Calcite, dolomite, anhydrite and halite are minor. Gypsum cementation is pervasive at surface crusts and below the water table in the proximal sabkha (in sediments dated c. 6,000 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 data suggest evaporative pumping of groundwater from the confined Eocene aquifer is a major source of solutes for the overlying sabkha, with significant seawater contribution limited to a zone >1km from the coast. Shallow evaporation drives precipitation of pore-filling, poikilotopic and displacive gypsum within the sabkha, resulting in fluids depleted in calcium and sulphate but enriched in conservative elements. During periods of reduced evaporation, these fluids reflux downwards and mix vertically within the upper part of the Eocene aquifer. Fluid chemistry shows significant depletion in Mg2+ and enrichment in Ca2+ (after accounting for gypsum precipitation) providing evidence for formation of dolomite by replacement of calcite.