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

Abstract

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.

Palaeoclimate in the Saharan and Arabian Deserts during the Middle Palaeolithic and the potential for hominin dispersals

“…The evidence for this humid period comes from a range of humid proxies, with lacustrine sediments from the Chott el Djerid in southern Tunisia (Causse et al, 2003), tufa from Kharga Oasis (Blackwell et al, 2012) and Wadi Abu Had-Dib (Hamdan, 2000) in Egypt, calcrete from Ras el Wadi in northern Libya (Giraudi, 2005), travertine from Oued Noun in Morocco (Weisrock et al, 2006(Weisrock et al, , 2008, fluvial sediments from Wadi Zewana in northern Libya (Rowan et al, 2000), as well as Oued Noun (Weisrock et al, 2006) and Tassint in Morocco (Thorp et al, 2002). Thus there is evidence for humidity in the eastern Sahara and along its northern margins at this time, but no evidence in Arabia (Fig.…”

L’évolution hydromorphologique de la vallée du Yamé (Pays Dogon, Mali) : signal climatique et hydrosystème continental en Afrique de l’Ouest entre 50 et 4 ka cal. BP

“…Dans cet article, l'ensemble de l'évolution holocène, dont la résolution stratigraphique est exceptionnelle pour l'Afrique de l'Ouest, est exposée, ainsi que celle du Pléistocène supé-rieur, depuis le stade isotopique 3. Cette approche chronologique permet de confronter les résultats obtenus à ceux des travaux menés dans les régions voisines ou acquises selon des approches méthodologiques différentes (deMenocal et al, 2000 ;Gasse, 2000Gasse, , 2005Garcin et al, 2007 ;Jullien et al, 2007 ;Lézine, 1989 ;Lézine et Cazet, 2005 ;Thomas et Thorp, 2003 ;Shanahan et al, 2006 ;Weisrock et al, 2006b ;Zhao et al, 2003).…”

“…En effet, il s'agit souvent de recherches anciennes qui ne bénéficiaient pas de calage absolu irréprochable et/ou de campagnes de datation systématique, ou d'études plus ré-centes portant sur des formations qui sont datées par quelques échantillons seulement. Certes, ces derniers résultats sont souvent confirmés par la confrontation des diverses mé-thodes utilisées comme les datations radiocarbone, OSL et Uranium/Thorium (Weisrock et al, 2006b), mais très rares sont les séquences qui sont calées par plusieurs dizaines d'échantillons prélevés dans des stratigraphies cohérentes. Au contraire à Ounjougou les datations OSL calent avec une grande fiabilité les formations U3, U4 et U5 entre 55 et 30 ka cal.…”

Controls on dryland mountain landscape development along the NW Saharan desert margin: Insights from Quaternary river terrace sequences (Dadès River, south-central High Atlas, Morocco)

“…Quaternary sediments and landforms occur throughout the region (Plaziat et al, 2008) but river terrace research is limited, being geographically restricted to coastal areas of the Anti-Atlas (e.g. Weisrock et al, 2006) and localised parts of the northern High Atlas (Delcaillau et al, 2016). Coastal river terrace studies are either regional treatments, using river-marine terraces for numerical uplift modelling (e.g.…”