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Dynamique sédimentaire et diagenétique d'un lac interdunaire holocène en milieu subaride (N'Guigmi, Lac Tchad, Niger)
Africa Geosciences Review
In the Lake Chad basin, the studies of Quaternary lacustrine deposits combine lithological and paleobiological (pollen, diatoms) analyses and radiometric dating. This approach provides information about the water-level, the open water area, even the water quality (salinity, pH). On the other hand, the petrographic approaches are scarcely used, whereas they make it possible to precisely describe the past sedimentary dynamics. In this work, we continue the study of the N'Guigmi Holocene lake that constitutes a reference section around the Lake Chad (Southern Niger). The sedimentary sequence includes sandy deposits, clayey deposits and clayey calcareous diatomites, and modern wind-reworked sandy material. The clayey diatomites are finely laminated and related to a lacustrine environment that was periodically dried. The lamination reflects the variations of the lake level, each doublet clays/quartz corresponding to a sequence flooding/drying. The calcareous diatomites are linked to a palustrine environment, that was characterized by a low water-depth and frequent drying and desiccation episodes. The sedimentological analyses (granulometry, mineralogy) allow to distinguish three major sedimentary dynamics: decantation of eolian dust, biogenic silica and clay neogenesis, physicochemical precipitations. These processes are related to the nature of waters (surface or phreatic) and climate. This model makes it possible to reconstruct the evolution of depositional environments. On the section scale, the facies changes reflect the regional long-time climatic evolution. On the sampling scale, the sedimentological data reflect the local hydro-climatic changes. On the laminae scale, the microscopic observations highlight short-time (secular or decennial) climatic fluctuations. These laminated sequences are the smallest that we can investigate with traditional methods (microscopic petrography). However, on a millimetric scale, alignments of quartz constitute clues of a shorter time climatic fluctuation (decennial episodes or seasonal events?). Thus, the studied sedimentary record reflects the aridification of the Sahelian climate during Middle and Late Holocene.
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