Ages of Quaternary pluvial episodes determined by uranium-series and radiocarbon dating of lacustrine deposits of Eastern Sahara

Sand sheet dynamics and Quaternary landscape evolution of the Selima Sand Sheet, southern Egypt

“…The time of formation and phreatic modification of the CPG is based on Uranium-series dating of the carbonates. The extensive exposures of kunkar that surround Bir Tarfawi and elsewhere in scattered locations have ages ranging from > 350 to 78 kyr (Szabo et al, 1995), but have yet to be traced unequivocally into the more fragmental subsurface sediments of the CPG in the survey line area. Stratigraphically it seems reasonable that the two sediments are equivalent, because nowhere have we found sand sheet units beneath the kunkar, and the CPG sediments where present are always the basal units of our shallow pits.…”

“…used the term "phreatogenic crusts" to describe calcareous crusts that occur in many shallow depressions in southern Egypt and northern Sudan, and interpreted them as resulting from a regional rise in the groundwater table. Because of the remobilization by later groundwater, such material may not always yield consistent dates except in local depositional environments (Roper, 1993;Szabo et al, 1995). The fact that CPG is not ubiquitous in the sand sheet could be due to local erosion of pre-existing deposits as well as non-deposition (supported by the undulatory top contacts of the CPG and FPA in our survey lines), leaving open the question of a middle Pleistocene lacustrine or high water-table environment.…”

“…Both archaeological evidence for the lack of Upper Paleolithic habitation (Wendorf et al, 1993) and radiometric dating of carbonates (Szabo et al, 1995) indicate that the period from 65 to --12kyr was extremely arid in this part of the Sahara. It is this time period that likely was responsible for extensive deposition of sand sheet (dated at 21-16kyr, Stokes et al, 1998).…”

“…1). The shallow depression surrounding Bir Tarfawi is the location of several inset lacustrine deposits that range in age from > 500 kyr to 4000 bp (Wendorf et al, 1993;Szabo et al, 1995). Pollen records at Selima Oasis indicate a fluctuating water table, with the latest high stand at 8.7 to < 6 kyr (Ritchie et al, 1985;Haynes et al, 1989).…”

“…Whether the same process can be applied to the remainder of the sand sheet is more difficult to determine. The extensive surficial limestone and kunkar (fragmented phreatogenic carbonate) deposits -- that extend for 20 km west of Bir Tarfawi suggests the presence of an extensive lake or high water table at > 350kyr, and at --277 kyr, the oldest distinct moist periods determined via Uranium-series dating of sediments from the sand sheet (Szabo et al, 1995). It is possible that a water-table-controlled planation surface (Stokes, 1968) could have been the penultimate control on base level erosion, but evidence for such a surface is lacking.…”