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Le Précambrien supérieur et le Paléozoïque inférieur de l'Adrar de Mauritanie (bordure occidentale du bassin de Taoudeni, Afrique de l'Ouest), un exemple de sédimentation de craton. Étude stratigraphique et sédimentologique - TOMES 1 (Série 1) et 3 (Annexes)

Abstract : "Adrar of Mauritania" is the western edge of the Taoudeni Basin (1,500,000 km2, Western Africa). The full stratigraphic succession, lying unconformably over an Archean basement, is 1 billion years old (1 Ga) at its base and its top, not studied here, is of Devonian age. Three series have been distinguished. SERIES 1 or Lower Series is 1750 meters thick. Its lower part is made of fine-grained detrital material, its middle part of alternating stromatolitic magnesian limestones and fine-grained detrital material and its upper part of fine- to very fine-grained greywackes. Rb/Sr and K/Ar radiometric data coupled with stromatolite succession, when compared to well-studied USSR Riphean stromatolite sequences, suggest that Series 1 was deposited between 1000 and 650 Ma ago (Upper Proterozoic). As a whole, Series 1 was deposited in shallow marine to continental conditions; the main currents, probably tidal, NNE-SSW in direction, might have controlled the elongation of the stromatolite reefs; two glacial episodes have also been recorded, that were not fully documented yet. SERIES 2 or Median Series, 1200 meters thick, is slightly unconformable on Series 1. It begins with a characteristic sequence, called the Triad: a glacial tillite unit, followed by a calcareous unit, and a chert unit. The Triad is a lithostratigraphic marker, widespread throughout the whole Taoudeni basin. Series 2 deposits turn upwards into fine- to medium-grained, reddish, continental, detrital formations. At its upper part, Scolithus sandstones (Cambrian to Ordovician in age) were deposited during a marine transgression. The basal tillite of Series 2, possibly of Marinoan age, that was deposited in a low energy environment, is homogenous throughout the Taoudeni Basin. SERIES 3 or Upper Series is 175 meters thick. It begins with a detrital group of glacial origin, dated Caradoc in Morocco, equivalent to the Hoggar "Unité IV", characterized by numerous U-shaped valleys and paleo-eskers. Icecap was moving from ESE towards WNW. These glacial deposits are lithologically very similar to those of Quaternary age in Northern Hemisphere and strongly differ from the tillite of Series 2. The upper argillaceous group was deposited during an eustatic marine transgression. It is Silurian in age with Graptolitic fauna of Llandoverian, Gala-Taronnonian, Wenlockian and Ludlowian (eß1) ages. The overlying Devonian strata are not detailed in the present work. AS A WHOLE in Adrar of Mauritania, Upper Proterozoic and Lower Paleozoic series are made of fine-grained detrital deposits with, between 1000 and 400 Ma, several stromatolite limestone intercalations and two to four glacial levels, the first two of them poorly documented. Such protracted, shallow marine to continental deposition patterns, with unchanged sequences over vast regions, separated by very-low angle disconformities, are typical of a cratonic environment.
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Submitted on : Wednesday, May 9, 2012 - 10:14:03 PM
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Roland Trompette. Le Précambrien supérieur et le Paléozoïque inférieur de l'Adrar de Mauritanie (bordure occidentale du bassin de Taoudeni, Afrique de l'Ouest), un exemple de sédimentation de craton. Étude stratigraphique et sédimentologique - TOMES 1 (Série 1) et 3 (Annexes). Stratigraphie. Université de Provence - Aix-Marseille I, 1973. Français. ⟨tel-00695504⟩

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