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Origine pédo-diagénétique de la calcite fibreuse pseudosphérulitique (pseudo-Microcodium) du Paléogène continental palustre dans le bassin d'Anzal (Anti Atlas, Maroc)
Africa Geoscience Review
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Microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols, especially in early Paleogene continental successions of the peri-Tethyan realm. Massive, decimetric-thick intervals formed by mosaics of pseudospherulitic fibrous calcite (PFC) crystals displaying an intracrystalline fibrous microfabric have been found associated with plant roots and paleosols (rhizoliths) in the Paleogene lacustrine deposits of Anzal basin. The fibrous microfabric is marked by inclusion-defined fibers that radiate from the central parts of the crystals, which can be termed pseudospherulites, because they are single crystals and not polycrystalline aggregates. These "spherulites" present a strong resemblance to Microcodium colonies in morphology, internal structure, and environment of formation. Microcodium is almost invariably associated with calcic paleosols. In contrast, the PFC, described in this paper, are developed within dolomicritic host sediments, in part by pedogenic alteration and in part by palaeo-groundwater precipitation. Cathodoluminescent microscopy, morphology and structure of the PFC indicate that, first, these aggregates grew with plant roots in paleosols, and second, at or near the water table, under periodically fluctuating geochemical conditions (calcite saturation and redox) caused by fluctuations of the water-table during periods of subaerial exposure. The analogy between PFC and Microcodium could be explained interpreting PFC as calcified root cells of other vascular plant group, which were able to colonize dolomitic substrates during early phases of subaerial exposure. However, the presence of gypsum inclusions within the PFC crystals, makes the hypothesis of a gypsum precursor as a possible solution for the PFC origin.
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