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Paleoecology and biogeochemistry of brachiopods from the Devonian-Carboniferous boundary interval of the Griotte Formation, La Serre, Montagne Noire, France
Most studied brachiopod shells of the Griotte Formation are microstructurally and geochemically well preserved. Typical microstructural features are bladed calcite fibres and prismatic calcite of the secondary and primary layers, respectively. Microstructural alteration, where evident, is accompanied by geochemical changes in trace element contents and isotopic compositions.
Modelling of geochemical data suggests that the latest Famennian seas were characterized by restricted to mud-bottom brachiopod faunas in generally dysaerobic-aerobic, normal salinity, warm temperature waters with high to reduced productivity. Water depth increased significantly concomitantly with a decrease in temperature during deposition of the middle siliciclastic calcarenite relative to the subjacent lower oolite and the superjacent upper oolite of the Griotte Formation. Oceanographic conditions of the Tournaisian sea were significantly different from those of the Famennian at Montagne Noire. These seas were highlighted by an open, shallow-water brachiopod fauna, which concurs with the slight dysaerobia of the water. Furthermore, the seas experienced only a slight decrease in salinity but marked increases in productivity, temperature and a general shallowing of the water.