Sediments quaternaires a diatomees du Sahara nord-occidental

Alimen, Henriette; Beucher, Francoise; Casta, Lucien; Ehrlich, Aline

doi:10.2113/gssgfbull.s7-xii.1.103

Cited by 5 publications

(2 citation statements)

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“…Enormous volumes of sedimentary rock debris have been removed from the Qattara Depression by deflation [Said, 1960;Squyres and Bradley, 1964] [Norris, 1969], southern Africa [Logan, 1960], North Africa [Alimen, 1957], and Australia [Twidale, 1972] have suggested that relative redness of sand increases with age of the dunes. The progressive reddening of desert sands, due to the weathering of heavy mineral grains and ferruginous clay particles, first to yellow limonite and then to red hematite, has been described by Folk [1976].…”

Section: In Egypt the Western Desert Extends From The Libyan Border Tmentioning

confidence: 99%

Eolian features in the western desert of Egypt and some applications to Mars

El-Baz¹,

Breed²,

Grolier³

et al. 1979

J. Geophys. Res.

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Relations of landform types to wind regimes, bedrock composition, sediment supply, and topography are shown by field studies and satellite photographs of 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 features. These include sand dunes, alternating light and dark streaks, knob 'shadows,' and yardangs. Surface particles have been segregated by 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 topographic variations such as scarps and depressions strongly influence the zones of deposition and dune morphology. Sand from the longitudinal dunes on the plains is commonly redistributed into barchans in the depressions. These barchans are generally simple crescents that are morphologically similar to many of the dunes seen on Viking orbiter pictures of the north polar sand sea on mars. Light streaks are depositional features consisting of dune belts and elongate sheets of coarse to medium sand and granules. Intervening dark streaks are erosional features consisting of strips of desert-varnished bedrock and lag gravel surfaces exposed between the sand deposits. The shape of both light and dark streaks is controlled by wind flow around topographic highs. Dark zones (shadows) in the lee of mountains, hills, and knobs are erosional products from the topographic highs; they change shape only in response to movement of the adjacent lighter-colored sand deposits. Streamlined yardangs carved in crystalline limestone constitute one of the largest yardang fields on earth. Yardangs occur also in sandstone of the Nubian Series and in lacustrine sediments. The variables that affect the patterns of wind erosion and deposition in the Western Desert are topographic effects on wind velocities and directions, resistance of the bedrock, sand supply, and climatic change with time; vegetation is essentially absent and is not a controlling factor.

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Section: In Egypt the Western Desert Extends From The Libyan Border Tmentioning

confidence: 99%

Eolian features in the western desert of Egypt and some applications to Mars

El-Baz¹,

Breed²,

Grolier³

et al. 1979

J. Geophys. Res.

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“…Earlier studies of Upper Pleistocene sediments along the northern edge of the Sahara desert focused on the interior eastern valleys (Alimen et al, 1966;Conrad, 1969;Williams, 1970;Littmann & Schmitt, 1989) or on dune coastal areas (Rognon & Coudé-Gaussen, 1987, 1996Petit-Maire et al, 1987). These studies did not focus exclusively on fluvial morphodynamics, generally did not consider alluvial deposits older than the limit of the 14 C method, and came to varying conclusions.…”

Section: -Introductionmentioning

confidence: 99%

Upper Pleistocene comparative OSL, U/Th and 14C datings of sedimentary sequences and correlative morphodynamical implications in

Weisrock¹,

Wengler²,

Mathieu³

et al. 2006

quaternaire

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The lower Oued Noun valley, in the arid region of the Atlantic SW Anti-Atlas, contains an extensive Soltanian (= Upper Pleistocene proparte) terrace. The paper presents dates for these deposits and an interpretation of their fluvial dynamic and morpho-climatic geneses in this distal part of the valley. The Soltanian terrace, about 30 m thick at Fort Oued Noun, consists of 3 units: a basal deposit of coarse gravel buried by slope deposits and travertine (Unit 1); a main unit, more than 20 m thick (Unit 2) that consists of at least 7 repeated sequences each comprising basal fine gravels and sands, associated with detrital travertine, overlain by stromatolithic tufa and finally by a thick accumulation of sandy silts. These silts were deposited by suspension and decantation in shallow non-turbulent water bodies. The gravels, sands and travertines are more prominent at the bottom and in the upper parts of Unit 2 whilst silts dominate the mid members. Middle Paleolithic artefacts and bones of large mammals are found throughout this Unit. Unit 3, at the top of the formation, comprises red silts that differ from those of Unit 2, containing more aeolian grains and more palygorskyte and being spatially associated with adjacent tributaries fans. Radiometric dates were obtained on travertine (U/Th), on quartz grains extracted from sediments (OSL) and on Melanopsis and Charcoal (14 C). U/Th results show three travertine constructions at ca 90, 55-50 and 25-20 ka B.P. The 90 ka dates, however, are not in correct stratigraphic position. The OSL dates suggest that the period of silt aggradation of Unit 2 occured between ca. 50 and 25 ka B.P., the main part of them being deposited between 40 and 30 ka B.P. 14 C dates from the upper part of Unit 2 and the base of Unit 3, range between ca 28 and 18 ka B.P. These dates, together with geomorphological and sedimentological analyses, indicate that the valley floor had been lowered to its present depth before ca. 90 ka B.P. However, the slope deposits, older travertines and the basal gravel (Unit 1), classic fluvial and colluvial deposits, are not yet securely dated but they possibly correspond with wetter episodes in O.I.S.stages 5b, 5a and 4, or even later. The silts that form the bulk of Unit 2 were deposited into shallow swamps during biostasic episodes of O.I.S. 3 and were associated with high groundwater levels. Then, large mammals found grass and water along the valley and were hunted by Middle Paleolithic people. The water table remained high after 30 ka B.P. and the gravelbed channels of the local tributaries were active during the 30-20 ka B.P. period. After ca. 20 ka, sediments suggest more varied conditions in this part of the valley, vegetation disappeared and soils were reworked into local colluvial fans, with concomitant aeolian deposits (Unit 3, O.I.S. 2). Finally, deep linear incision occured, dissecting the Soltanian aggraded valley floor during the early Holocene.

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