QUATERNARY STRATIGRAPHY AND GEOCHRONOLOGY OF SEDIMENTARY DEPOSITS AT GEBEL RAMLAH IN SOUTHWEST EGYPT

Sedimentary sequences and geomorphic landforms in the vicinity of Gebel Ramlah in southwest Egypt provide evidence of changing environmental conditions during the Quaternary that significantly influenced human presence in the area. Sedimentary deposits reflect wetland and playa environments and also episodes of increased wadi activity in the presently hyperarid Eastern Sahara. The underlying bedrock and the gebel are composed of Cretaceous and Paleocene siliciclastics and limestones. Geomorphic mapping, documentation of natural outcrop exposures and stratigraphic sequences (revealed by coring, excavations and trenches), along with radiocarbon and luminescence dating indicate dynamic changes in geologic contexts during the Pleistocene and Holocene. The slope of the gebel is partially covered with dunes. Alluvial fans on the gebel slope coalesce, forming a bajada. Middle and Late Pleistocene luminescence dates are associated with alluvial, playa, and aeolian deposits. Artifacts observed on the landscape surface indicate the presence of Middle Paleolithic human groups. Radiocarbon dating provides evidence for changing Holocene geologic contexts. The best constraints on the dating of the Holocene strata are from charcoal recovered from in-situ archaeological features embedded within the sediments; land snails might only be useful for providing maximum age estimates. For example, stratigraphic exposures at Wadi Eagle situated in the southeast area of the Ramlah basin contain archaeological features (hearths) dated to about 6,000-6,400 14C years B.P. based on radiocarbon measurements on charcoal and are associated with the Neolithic presence. Holocene strata also contain molluscs that currently inhabit marsh settings. Trace-fossils are interpreted as root-casts or rhizoliths associated with palustrine wetland vegetation. On top of the gebel there is a basin with playa silts and clays (muds) and eolian sands to a depth of nearly 3 meters. The depositional sequence on top on the gebel is older than about 5,800 14C years B.P. based on a radiocarbon date on bone from an archaeological feature on the gebel playa surface. The sediments at Gebel Ramlah document intermittent erosional and depositional processes associated with localized rain events on the gebel and the larger watershed basin. The Quaternary geology in the Gebel Ramlah area is interpreted as indicating wetter environmental conditions within the two basins during the early and middle Holocene with episodic torrential rains during the late Holocene.