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Eolian Features in the Western Desert of Egypt and Some Applications to Mars

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Eolian Features in the Western Desert of Egypt and Some Applications to Mars VOL. 84, NO. B14 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 30, 1979 Eolian Features in the Western Desert of Egypt and Some Applications to Mars FAROUK EL-BAZ National Air and Space Museum, SmithsonianInstitution, Washington,D.C. 20560 C. S. BREED, M. J. GROLIER, AND J. F. MCCAULEY U.S. GeologicalSurvey, Flagstaff,Arizona 86001 Relations of landform types to wind regimes, bedrock composition, sediment supply, and topography are shown by field studiesand satellite photographsof the Western Desert of Egypt. This desert, which lies at the core of the largest hyperarid region on earth, provides analogs of Martian wind-formed fea- tures. These include sand dunes, alternating light and dark streaks,knob 'shadows,'and yardangs. Sur- face particleshave been segregatedby wind into deposits(dunes, sand sheets,and light streaks)that can be differentiated by their grain size distributions, surface shapes, and colors. Throughgoing sand of mostly fine to medium grain size is migrating southward in longitudinal dune belts and barchan chains whose long axes lie parallel to the prevailing northerly winds, but topographicvariations such as scarps and depressionsstrongly influence the zones of deposition and dune morphology. Sand from the longitu- dinal duneson the plains is commonly redistributedinto barchansin the depressions.These barchansare generally simple crescentsthat are morphologicallysimilar to many of the dunesseen on Viking orbiter pictures of the north polar sand sea on mars. Light streaksare depositional features consistingof dune belts and elongate sheetsof coarseto medium sand and granules.Intervening dark streaksare erosional features consistingof strips of desert-varnishedbedrock and lag gravel surfacesexposed between the sand deposits.The shape of both light and dark streaksis controlled by wind flow around topographic highs. Dark zones (shadows)in the lee of mountains,hills, and knobs are erosionalproducts from the topographichighs; they changeshape only in responseto movement of the adjacent lighter-colored sand deposits.Streamlined yardangs carved in crystallinelimestone constitute one of the largestyardang fields on earth. Yardangs occur also in sandstoneof the Nubian Seriesand in lacustrinesediments. The vari- ables that affect the patternsof wind erosionand depositionin the Western Desert are topographiceffects on wind velocitiesand directions,resistance of the bedrock, sand supply, and climatic changewith time; vegetation is essentiallyabsent and is not a controlling factor. INTRODUCTION summarized by Said [1962] and Smith [1963]. However, the In Egypt the Western Desert extends from the Libyan bor- work of the wind in hyperarid regions such as western Egypt der to the Nile River and from the Mediterranean Sea to the remains largely unrecognizedamong geologistswhose experi- ence has been limited to the semiarid deserts of the American border of Sudan at latitude 22øN. The desertoccupies 681,000 km2, more than two thirds of the total area of Egypt. The re- southwest[McCauley et al., 1977a, p. 1.4]. Although the West- gion consistsof gently northward dipping sandstoneand lime- ern Desert of Egypt is still relatively unexplored, it has re- stone that have been eroded to an eolian peneplain and ve- cently been the subject of comparative planetology studies neered with windblown debris. The remarkably fiat terrain is through the use of earth orbital photographsin combination broken by numerous escarpmentsthat bound depressions,by with multidisciplina• field investigations[El-Baz et al., 1979]. scattered inselbergs,and in the southwestcorner, by the Gilf Despite recurrentepisodes of lesseraridity than the present, Kebir Plateau and the isolated, circular mountains centered as evidenced by Paleolithic and Neolithic artifacts found in around Uweinat (Figures 1 and 2). parts of the Western Desert [Murray, 1951;Haynes, 1978, also Much of the basic research on dune classification and sand oral communication, 1978], wind excavation of the surface movement by wind has been carried out in the Western may have begun as early as Pliocene or even post-middle- Desert of Egypt, where 'the free interplay of sand and wind Miocene time [Murray, 1951; Said, 1960, p. 14, 1962, pp. 43- has been allowed to continue for a vast period of time, and 44]. 'Although the frequency of rain must have been greater than today, rainfall of the Pleistocenefluvial phaseswas inef- [where] if anywhere,it should be possiblein the future to dis- cover the laws of sand movement and growth of dunes' [Bag- fective in producing drainage lines or vegetativecover' [Said, nold, 1933, p. 121]. Early geomorphic descriptions include 1962, p. 43]. The Western Desert is a 'desertpeneplain' [Sand- ford, 1933] in which most of the geomorphicfeatures are pri- those of Beadnell [1910], King [1918], Ball [1927], Bagnold marily due to wind action [Said, 1960, p. 12]. Vegetation is es- [1933, 1939], Kdddr [1934], Sandford [1935], Peel [1939], and sentially absent. Fluvial processesin that part of Egypt no Mitwally [1953]. Bagnold's [1941] classictreatise on the phys- longer play an effective role in modifying the landscape. As ics of sand movement by wind is based largely on observa- on Mars, wind is the dominant agent of erosion, transporta- tions made in the Western Desert. Sand flow patterns in the Sahara,including western Egypt, have been mapped by Wilson tion, and deposition. [1971, Figure 5] from the 'sand storm resultants' of Dubief The Landsat mosaicof Egypt (Figure 1) showsthe regional [1952] and from barchan dune orientationsas shown on topo- patterns of large-scaleeolian featuresin the Western Desert, graphic maps [Wilson, 1971, p. 187]. Recent work in Egypt is including fields of sand dunes, alternating light and dark streaks, dark knob 'shadows,' and yardang fields. Many of This paperis not subjectto U.S. copyright.Published in 1979by these features are morphologically similar to wind forms ob- the American GeophysicalUnion. servedon Mariner 9 and Viking orbiter picturesof Mars. Paper number 9B 1264. 8205 8206 EL-BAZ ET AL.' SECONDMARS COLLOQUIUM ß'.: ..... -:.... -,.-- : •..,•: .. • %. ½.-f•;-,?,"""'"•-• /•X'•s•-•...,...•.•n{•..•x......... •::c.... '.•:.... .½•*- .;:X•?";q•::.., .;. ' '•,• .......•'•'•a• --:t•..-'-.-'-,....• •...... •½•.•.•m'-'*:*•,.......*:W•f:•-•'.•?"•-•:• ....•.-•A•** "'.... '"-'"•'" '.?..•, ........ •:•,'...C'•$•:,.:.., .... • •, '.......................... •••-••••••••,• ........... ,•... .....,:•,•. ..-,•,-z:,:•.•..•...-•.•.••.: ....... ,.. ... •..................... •.....•.•.• ...--•,.• :,...,:•-:•. :.•'•.• .... :•-.':" ,? -•:..:....::..-:• ....-,./,- ...... ....... ;::• ,......:.':':.:• ..... .. :•.-..-, ............ ........... {..... ":a:'-... ... • ..• .................................... ....... ..... .. •.;•?" ..,:,,..,; .... *•'•.•'..•;{. 7*½? ...... :::•-.•.............. :•:• .*•... ..,•:.... "' ...... :.• ,-....... •:, .... .:,..;-. •:-=} .. " :..•:':E..,:, ..::: . -*'...:½?EL" ..... .•: ':.. :....., ... :.... .7;.•. ;-:{-- '>;: -:---...,;. .....'......... "..:•9/7..-. --•,-.,.,......•:...,; .-.?:.:?, .... .....:.. .j../• .'.. .. .--.. .. .. .., ..... • •, ,• . .. •:-•-,;..:..-;....{- ':..... •;7:'- .--.::.• .-..,.:":.•'•-•- .':::''. ':' '}' ,:*' '?:.?;.;-,. -: ',.;.:--.-.:.'.•.--•, *"::'%{.:.-' '•... .•,' . :.-. ..-:-.• ..... -- "2".'... '*" . :•: , :. -..- .. ß.. 4 - '•,..,-'-.:-:.-j-':..;.' ?'•"":"•,,.;;..,,.:.*::•,,'..,.•,-0...... ,..•-,.,. ....,•'-:':",'&•?,,•,;:--• .......... ... -c-.-*-..',,, -'.,;....-.::,--:'*•½•c•-.:,:- .t -- ß Fig. 1. Mosaic of 65 Landsat imagesof Egypt, showingthe distinct differenceof the landforms in the Western Desert from those in the Eastern Desert (east of the Nile, Figure 2). Light-colored eolian depositsin the form of dunes, sand sheets, and streaks cover most of the Western Desert. In contrast to the Eastern Desert, few traces of fluvial processesre- main in the Western Desert [Haynes, 1978], and the landscapeis dominantly wind formed. SAND DUNES basin [Ball, 1927; Said, 1962], the largestdepression in Egypt, occupiesan area of approximately19,500 km: below sealevel. One of the largest accumulationsof dune sand in North The depression'snorthern and western margins are marked Africa is the Great Sand Sea of Libya and Egypt. In Egypt by a steep scarp capped by limestone of the Mamarican For- the sand sea occupiesthe west central part of the Western mation of Miocene age, which are underlain by the t¾iable Desert(Figure 2), beginningjust southof the Siwa Depression sand and silt of the Moghra Formation of Miocene age [Said, and continuing uninterruptedfor 600 km to the Gilf Kebir 1960].The southernand easternmargins of the depressionrise Plateau. The main sand massconsists of subparallel arrays of gradually into the fiat plain that stretchesunbroken to the Ba- enormouscompound linear dunes.These are broad whale- hariya Depression(Figure 2). Nearly 5800 km: of the floor of backs ('plinths') surmounted by sharp-crestedlongitudinal the Qattara Depressionis coveredby sebkha (water-saturated ('seif') dunes [Bagnold,1941, pp. 230-232, Plate 13; EI-Baz, salt that is thinly mantied with sand). The remainder consists 1976]. The whalebacks are relatively static, whereas the longi- of exposedbedrock and depositsof sand,pebbles, and clay. tudinal dunes are actively migrating southwardunder the pre- Enormous volumes of sedimentary rock debris have been vailing winds. removed from the Qattara Depression by deflation [Said, East of the Great Sand Sea, numerous other belts of longi- 1960; Squyresand Bradley,
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