oa Comparative Sedimentology, Geomorphology and Hydrochemistry of Continental and Coastal Sabkha Systems, Qatar

Holocene sabkhas in the Arabian Gulf are important as analogues to ancient evaporitic hydrocarbon reservoirs. Sabkha, an Arabic term for salt flat, refers to marine coastal sediments which have been modified by precipitation of evaporitic minerals from groundwaters. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework for understanding hydrological and geochemical processes controlling the origins of evaporites, their spatial distribution and likely evolution of burial diagenesis through time. A range of hydrological models have been proposed to account for the sources of solutes forming the evaporites in different sabkhas, including discharge of continental groundwaters, evaporative pumping of seawater, seawater flooding and free convection. Comparison of two hydrological end-member regimes in Qatar, a coastal sabkha in continuity with marine water (Mesaieed) and a continental sabkha within a closed basin (Dukhan), offers new insights into the variability and complexity of sabkha hydrological systems. Mesaieed sabkha, located in the southeast of Qatar is part of a prograding coastal plain and consists of an onlap wedge of Holocene sediments with a basal coarse transgressive lag, overlying Eocene bedrock. The Holocene wedge is in hydraulic continuity with both marine and continental waters, although tidal head differences are observed only a short distance inland from the coast. Dukhan, a large inland sabkha in the Arabian Gulf, has formed in a syncline between the Qatar arch and the Dukhan anticline, which separate it from the west coast. The sabkha surface is below sea level, separated from a marine embayment to the north by an Eocene sill of unknown transmissibility. Sea-level reconstructions reveal marine communication in the Early Holocene, suggesting the basin was flooded during an earlier highstand, with evaporitic lakes or salinas developed within restricted areas. Gypsum is the most common diagenetic evaporite mineral in both sabkhas and is pervasive above and below the water table, with minor calcite, dolomite, anhydrite and halite. In Mesaieed such cementation is extensive in the proximal sabkha (in sediments dated c. 6000 yr BP), whilst in the central part (c. 4000 yr BP) gypsum is restricted to surface crusts and water table cements, and is largely absent in the distal (coastal) sabkha (≤ 2000 yr BP). In Dukhan gypsum occurs as surface crusts, detrital grains and coarse crystal mushes, formed in the sediment as well as in standing water. Halite and anhydrite occur near the surface. Geochemical studies suggest that evaporites are forming today. Both sabkhas act as sinks for brackish groundwater from the underlying Eocene aquifers of Qatar. Evaporative concentration of discharging groundwaters in the sabkha at and above the shallow water table generates dense fluids which reflux into the underlying Eocene, increasing solute concentration with distance from the Eocene/Holocene contact. In Mesaieed, studies indicate rare episodic inundation of the sabkha surface and subsurface intrusion of seawater may be significant in the distal part of the system and could explain the paucity of gypsum cementation.