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Datations OSL, U/Th et14C croisées de séquences sédimentaires du Pléistocène supérieur et leurs implications morphodynamiques (Oued Noun, sud-ouest de l'Anti-Atlas, 29° N, Maroc)

Abstract : The lower Oued Noun valley, in the arid region of the Atlantic SW Anti-Atlas, contains an extensive Soltanian (= Upper Pleistocene pro-parte) 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 gravel-bed 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. RÉSUMÉ DATATIONS OSL, U/Th ET 14 C CROISÉES DE SÉQUENCES SÉDIMENTAIRES DU PLÉISTOCÈNE SUPÉRIEUR ET LEURS IMPLICATIONS MORPHODYNAMIQUES (OUED NOUN, SUD-OUEST DE L'ANTI-ATLAS, 29 o N, MAROC) La basse vallée de l'oued Noun, aujourd'hui située dans le domaine aride du versant sud de l'Anti-Atlas, montre une terrasse soltanienne (Pléistocène supérieur pro-parte) bien développée. Les objectifs de cet article sont de déterminer avec précision les âges des sédiments et de com-prendre les changements de la dynamique fluviale, avec ses phases successives d'aggradation et d'érosion, à la fin du Pléistocène supérieur. La formation soltanienne, épaisse de 30 m à Fort Oued Noun, se compose de 3 unités : un dépôt conglomératique de base surmonté par des dépôts de pente et des travertins (Unité 1) ; un dépôt principal de plus de 20 m (Unité 2), qui consiste en au moins 7 séquences répétitives mon-trant chacune à la base des petits galets et des sables associés à des travertins détritiques, surmontés de travertins stromatolithiques construits finale-ment recouverts d'accumulations épaisses de limons. Ces limons ont été déposés par suspension et décantation dans des eaux calmes et peu profondes. Les galets, sables et travertins sont surtout fréquents à la base et au sommet de l'Unité 2, tandis que les limons dominent dans sa partie
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André Weisrock, Luc Wengler, Julien Mathieu, Abderrahmane Ouammou, Michel Fontugne, et al.. Datations OSL, U/Th et14C croisées de séquences sédimentaires du Pléistocène supérieur et leurs implications morphodynamiques (Oued Noun, sud-ouest de l'Anti-Atlas, 29° N, Maroc). Quaternaire, Centre National de la Recherche Scientifique, 2006, 17 (1), pp.45-59. ⟨10.4000/quaternaire.645⟩. ⟨hal-02470559⟩

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