Document Type

Article

Publication Date

7-4-1986

Abstract

complex history of Cenozolc filuvial activity in the pres- catly byperarid Easterm Sabara is inferred from SIR data and post- Bight feld in vestigatioas in southwest Egy pt and northwest Sudan. SIR bmages were coregistered with Laadsat and existing maps as a guide to exploration of the buried palodrainages (radar rivers) first dlscovered by SIR-A. Field obeervstions explsin the radar res ponses of three types of rsdar rivers, RR-1, RR-2, and RR-3. RR-1 (broad, aggraded valleys or basins flled with slluvium) generally give a dark response relative to the surrounding radar-bright tone of the wind-roughened, discon- tintously sand-covered bedrock of the remnant interfluves. The allu- vial I in some of the RR-1 ralleys gives a locally intermediate to bright response where cemented by diagenetic CaCO, (callche). The RR-2 type (braided channels inset in tbe RR-1 valleys) are filled with noncalichi. fed, leose sediment, and are visible on SIR because of the ""caliche effect""--the dark response of unconsolidated alluvium in the inset channels coatrasts with the brighter response of calichified alluvium in the bread valleys. The third type, RR-3 (narrow, long, bedrock-lncised channels), which are partly visible on the ground and on Landsat in areas where the sand sheet is patchy, are conspicuous on SIR because the dark response of their unconsolidated channel fill contrasts with the brighter response of the surrounding bedrock. Wadi Safsaf, reveal sedimentary and fossil evldence for running water in the uppermost (Quaternary) valley fll, Preliminary seis mic stu dies suggest that the alluvial fill (whose deeper parts have not yet been in- vestigated) extends to depths of several hundred meters Im Wadl Arid. Combined SIR-A/B corerage allows delineation of both sbores of Wadi Arid, where detailed trenching studles were carried out in 1984, and where Paleolithic tools of Acbeulean culture < ~ 230 000 years old) were excavated from shallow depths. Fresh handaxes were the first Acbeu- lean tools discovered in sits in a recognized paleoriverine environment in this part of the Sahara. Large surfsce concentrations of Acheulean handaxes along the shores of the RR-1 valleys are defating out of ter- race overbank deposits, in which they were buried before tbe sedi- ments ("*crystal sands'") were calichifled. Experlmental U/Th dating of the CaCO, suggests multiple episodes of carbonate infltration from groundwater, roughly coincident with the Quaternary pluvials and with Paleolithic ocupstions. A reglonal eolian disconformity marks the contact of the deflated river sediment with the overlying eolian sand sheets. A generalized model of the radar rivers, based on field studies and regional geologic relations, shows inferred changes in river regimen since the large valleys were established during the late Paleogene-early Neogene. Streamflow in the RR-l valleys was severely disrupted by tectonic and ¥olcanic events later in the Neogene, beheaded by the in- cision of the Eonile during the Messinlan event, and deteriorated to the point of extinction following the onset of Quatelnary hyperaridity. Rolled caliche nodules in the RR-2 channels on the'aggraded RR-1 val- ley floors, and rolled, streamworn Paleolithic artifacts in the incised RR-3 channels suggest that these rejuvenated watercourses carriea episodic runoff during pluvials. Disconformably overlying and concealing the RR-1 valleys, the RR- 2 channels, and parts of the RR-3 channels are eolian sand-sheet de- posits, which in turn are locally overridden by trains of barchan dunes. The sand sheets, if less than 1.5-m thiek, are invisible to the SIR sen- sor. but can often be identified on Landsat by thelr giant-ripple bed- forms. The sand sheets are thin, tobular bodies of loose sand, silt, and granules that extend over tens of thousands of square kilometers, and the processes that produce these widespread deposits are poorly under- stood. Sand dunes are rare in this part of the Sahara; although dunes higher (thicker) than about 3 m were not penetrated by SIR, these local features do not, by themselves, significantly obscure the underlying geology. SIR-based mapping of these paleodrainages, although incomplete, reveals missing links in an area once thought to be devoid of master streams. A former Trans-African master stream system may have flowed from hendwaters in the Red Sea Hills southwestward across North Africa, discharging into the Atlàntic at the Paleo-Niger delta, prior to the Neogene domal uplifts and building of voleanic edifices across the paths of these ancient watercourses.

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