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Textural Types of Evaporites in Holocene Sabkhas of Qatar and Their Geological Significance

Abstract

The growth habits and distribution of evaporite minerals in sabkhas of Qatar provide insights in their origins and models for the interpretation of ancient evaporites. The Holocene forms a 3–10 km wide coastal plain around the peninsula of Qatar. Rates of evaporation exceed precipitation by a factor of 1000 for six or more months of the year. In such an arid climate, bodies of standing water and evaporation in the capillary fringe promote rapid formation of evaporites. Evaporite minerals are common throughout the 3–10 meters of the Holocene. Gypsum dominantes (8–20%), followed by halite (6–10%), with minor anhydrite, calcite, dolomite and probably attapulgite. The most obvious textural differences occur between subaqueous mineral phases deposited in standing water, those formed in the capillary fringe, and precipitates near the water table. Subaqueous crystals are clear, devoid of much sediment, relatively large (mm-cm sized), laminated and may show compressional ridges(tepees) at the surface. Gypsum forms vertically-oriented, ‘fish-tail’ twins in vertical arrays. Halite occurs as hoppers and vertical prisms. Halite is an indicator of a marine water source. Thick subaqueous deposits only occur where there has been a marine source. Layers of crystals nucleated along chemical contacts in the water column and fish-tail twin habits are diagnostic. Evaporites formed in the capillary fringe are typically displacive, incorporate sediment, finely crystalline or poikilotopic, showing variable degree of crystallographic ordering. Micritic coatings are common. Halite is a common pore-filling, often leaving molds. Capillary fringe evaporites habits and morphology reflect seasonal changes in water chemistry. Evaporites formed at the water table are poikilotopic and may/may not be euhedral. This style of predominantly gypsum cementation has not been documented previously. Desert rose twins are the most well know water table morphology. Layers may extend over kilometers, reducing interparticle pore space by 20–30%. Crystals grow by interparticle cementation with little/no replacement. All styles of evaporite cementation are driven by extreme aridity of the climate. Millimeter sized crystals can grow in a few months, a geological instant. Limiting factors on growth rates of evaporites are seasonal water chemistry fluctuations and in some cases, limited supply of marine waters. Surface evaporites in Holocene export plumes into underlying bedrock.