LITHOSTRATIGRAPHY, SEDIMENTOLOGY AND GEOCHEMISTRY OF PLEISTOCENE LAKES IN SOUTHERN EGYPT: HYDROLOGY AND LANDSCAPE EVOLUTION IN THE BIR SAHARA REGION

In the southern Bir Sahara area of Egypt’s Western Desert, isolated sedimentary sequences document environmental conditions during the Pleistocene, providing an opportunity to make comparisons to the Holocene African Humid Period (HAFP) as well as to examine changes in the region’s hydrologic system (e.g. size and extent of water bodies, the relationship to groundwater levels and local moisture conditions) and landscape evolution. At the BS-14 locality, sand with Acheulian artifacts is overlain by a carbonate spring deposit associated with a wet Middle Pleistocene climate event. At E-88-15, where Middle Paleolithic artifacts are present, sand is overlain by calcareous mud and mudstone (dominated by silt) which is in turn overlain by a cemented limestone. At BS-16, a dark colored sand grades upward into a deposit with a Levallois artifact and is overlain by a marl or lacustrine chalk containing freshwater gastropods. In the E-88-2 area, sediments reveal a major climate cycle with a hydrogenic soil developed in a sand overlain by a marl, with carbonate exceeding 75% and clastic clay indicating two transgressive pulses. The east excavations at E-88-2 document a transgressive event; Middle Paleolithic artifacts are embedded in sands overlying a calcareous sandstone. The nearby BS-13 sequence (with Middle Paleolithic artifacts) also consists of clastic deposits, including basin wash sands representing the beginning of a wet cycle overlain by sands with redox stains, suggesting a shore and near shore facies of a small lake. A transgressive lake event is signaled by muds (dominated by silts) overlain by deposits high in carbonates (marls, or vesicular bedded carbonates). Close to the BS-13 excavations the marl contains freshwater gastropods and ostracods. These stratigraphic sequences appear to reflect changing hydrologic conditions with the onset of wet cycles leading to the deposition of lacustrine muds and carbonates. Sedimentary facies relationships and biostratigraphic evidence indicate 1) waterbodies were in deflational basins and formed small lakes or ponds resulting from changes in groundwater levels that were higher than during the HAFP, and 2) local rains resulted in vegetational landscapes sufficient to reduce siliciclastic deposition within watersheds and to support populations of large mammals.