Mineralogy, geochemistry and origin of Quaternary Sabkhas in the Qatar Peninsula, Arabian Gulf

Abstract

This thesis represents the first comprehensive study of Holocene sabkhas in Qatar. It presents detailed analysis of surface morphology, recent evaporite and clastic sediments, and groundwater nature and flow, for two large sabkhas: the inland Dukhan Sabkha and the coastal Umm Said Sabkha. The fieldwork involved the taking of shallow pit and core samples, as well as brine samples from various location within the sabkhas. In addition, sand dune samples were obtained from selected locations within and between the two sabkhas. Laboratory analyses of the mineralogy and geochemistry of the sediments included: hand specimen study, smear slide and thin section petrography, XRD, XRF, organic carbon and SEM analyses. Chemical analyses of brines were also undertaken.

The dominant sediment facies in both sabkhas are sands with variable amounts of evaporite precipitates. Of secondary importance are sandy silts, muds and algal/microbial mat deposits. Most of the evaporites occur within these detrital facies. The surface is covered with a firm duricrust of halite and gypsum. A large hypersaline lake covers part of the northeastern sector of Dukhan Sabkha. Evaporite minerals present in the sabkhas are gypsum, halite, anhydrite, and a small amount of bassanite. Carbonate minerals are dolomite, calcite, Mg-calcite and aragonite. Siliciclastic minerals are quartz, K-feldspars and plagioclase, with minor heavy minerals and clays. The major oxides found in sediment samples throughout are SiO₂, CaO, SO₃, MgO and Al₂O₃. Of trace elements: Sr, Cr, Ba and Zr. V, Rb and Ni are high in comparison with other trace elements.

The relative abundance of gypsum and anhydrite is greater in Dukhan Sabkha than in Umm Said Sabkha, whereas halite is more abundant in Umm Said Sabkha. These differences relate primarily to the greater salinity of Umm Said Sabkha brines. The relative distribution of gypsum and halite within the upper meter of sediment is influenced both by grain size and marine flooding. The sandier sediments of Umm Said Sabkha tend to have gypsum above halite, whereas the reverse is true of the generally finer-grained Dukhan Sabkha. Anhydrite nodules are most common in the uppermost capillary zone in the salt lake area, where they are formed as a result of gypsum dehydration. Minor authigenic dolomite also occurs in this area because of high salinities and a high Mg:Ca²⁺ ratio. Elsewhere, the dolomite is of detrital origin. Clay minerals are present only in small quantities in both sabkhas, with palygorskite as the dominant clay, followed by chlorite and illite. Palygorskite is believed to be at least partly authigenic in origin, whereas the other clays are detrital. Organic carbon is only present in small amounts (mean 0.06-0.14%), except in few samples from the algal/microbial mats.

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