Paleorivers and geoarchaeology in the southern Egyptian Sahara

Abstract

AbstractInterpretation of Shuttle Imaging Radar (SIR) images acquired during the November 1981 flight of Columbia led to the idea that a paleodrainage system of regional and perhaps transcontinental proportions crossed southern Egypt and northern Sudan prior to the onset of Quaternary aridity. Three seasons of field investigations in two locales (Wadi Arid and Wadi Safsaf, some 70‐80 km apart) near the Sudan border of southern Egypt confirmed initial interpretations of the radar images and produced geologic evidence of fluvial deposition in broad “radar river” valleys (designated RR‐1) whose presence is now obscured by eolian sand sheets. Development of an integrated, regional river system probably occurred in the late Paleogene or early Neogene. the river courses eventually were disrupted by tectonism, volcanism, and stream piracy. Their broad valleys were almost fully aggraded long before the middle Pleistocene appearance of man in the area. Some of the large paleovalleys on the radar images contain narrow (0.05‐2.0 km wide), braided inset channels (designated RR‐2). the RR‐2 channels, we believe, represent the last episodes of running water in the valleys during the Quaternary pluvials.In Wadi Arid (the type area for the RR‐1 valleys) 36 backhoe trenches and manual excavations yielded Acheulian handaxes, flakes, and cores from nearshore alluvial sediments and surface and shallow subsurface locales. These assemblages are typologically Middle to Late Acheulian and date from ca. 0.15th 0.5 million years ago. the unabraded, unrolled, buried artifacts and their geological contexts indicate that human groups were widely present in a subhumid riparian environment along the edges of the ancient valleys, while these localities were undergoing episodic, local aggradation. Scarce Middle Paleolithic and abundant Neolithic assemblages are widespread along the edges of Wadi Arid and bordering interfluves.In Wadi Safsaf, another broad RR‐1 valley, geologic evidence from 20 backhoe excavations demonstrates that an RR‐2 channel complex is inset into the alluvial fill in the central part of the valley. No artifacts were found in the alluvium excavated there, but Late Acheulian, Middle Paleolithic, and Neolithic surface assemblages are present nearby. Stratigraphic relations indicate the RR‐2 channels are late Pleistocene in age; they were already fully aggraded and their surfaces deflated before an eolian sand sheet of Holocene age was deposited on them. Bir Safsaf, on the northern edge of Wadi Safsaf, was a magnet for Late Acheulian populations, and the paleovalley just south of the bir is bordered with extensive alignments of artifacts, mainly handaxes. Some of these handaxes were found in situ, embedded in alluvial sediments, while others appear to have been exposed on the surface by deflation of the valley fill.The accumulated data support a provisional archaeo‐geochronological framework for the last half‐million years in the Wadi Arid‐Wadi Safsaf area. This framework provides a new geomorphic explanation for the widespread distribution of buried and exposed Middle Acheulian and Late Acheulian assemblages. the interpretation emphasizes the importance of exploring the non‐oasis zones for their archaeological and geochronological potential. Although our research has literally only scratched the surface, it demonstrates the potential of radar imaging for defining ancient drainage patterns in arid regions and their associated habitats for human occupation.