MID- TO LATE PLEISTOCENE PALAEOLAKES IN THE DAKHLEH OASIS, WESTERN DESERT, EGYPT

The presence of extensive permanent palaeolakes, increased rainfall, and surrounding productive palaeoenvironments in the central Western Desert of Egypt is attested by Pleistocene freshwater lacustrine deposits, spring-laid tufas and biota in Dakhleh Oasis. The modern oasis depression is divided into two principal basins by a ridge of resistant Cretaceous sandstone exposed along an anticlinal axis. Preserved lake bottom marls are found along the southern and eastern margins of both of these basins. Mollusc and vertebrate fossils and plant casts show a typical Pleistocene African plains fauna with a lakeside component. Earlier and Middle Stone Age artifacts occur in association with the sediments. The lake deposits are estimated to have attained thicknesses greater than 15-20 m, and water depths that could have reached 60 m. Lake margins in the western Palaeolake Kellis and eastern Palaeolake Teneida were at least 165 m asl in elevation, and would have been higher if united as Palaeolake Balat. Sections through surviving remnant ‘witness’ basin margin highs show that 20-35 m of Late Cretaceous bedrock and Quaternary lake deposits have since been removed, giving an average rate of erosion over 250,000-300,000 years of 30 m or 0.12-0.10 mm/yr. It is concluded that conditions in the Egyptian central Western Desert were sufficient to maintain large perennial lakes, related to runoff and groundwater inflow from the Nubian Sandstone Aquifer, and for the deposition of tufas along the Libyan Escarpment face in Dakhleh as well as in Kharga oases. Dakhleh tufas are isotopically similar to the more extensive Kharga tufas, suggesting comparable mechanisms and conditions of formation, though Dakhleh lacustrine carbonates have more positive oxygen isotope compositions than tufas from either region, suggesting the closed lake basins were subject to significant evaporative loss. As the relatively high-elevation tufas in both oases were likely precipitated from waters issuing from perched aquifers or from surficial drainage on the Libyan Plateau, rather than from the Nubian Sandstone Aquifer, examination of both the spring-laid tufas and lacustrine marls of the oasis depression floor is required to understand the complex response of oasis hydrology to fluctuating Pleistocene climates.