Filling sequence in Late Paleozoic continental basins: A chimera of climate change? A new light shed given by the Graissessac-Lodève basin (SE France)

摘要

The characteristics of sedimentary deposits in continental basins, especially those controlled by extension, are often used to infer global climatic changes. We question the relevance of such an approach because the stratigraphic pattern of those basins are mostly similar whatever their latitudes/paleolatitudes (i.e. their general climatic conditions), namely the transition from (1) an ancestral drainage network to (2) deep lake with a dominance of black and grey shale deposits, (3) shallow oxic lake with a dominance of red pelites deposits and to eventually (4) a new drainage network. Such transitions observed in Late Palaeozoic continental basins throughout Laurasia and Gondwana are generally interpreted as evidence of global change from a wet climate to an arid climate. From the sedimentological and structural analysis of a Late Palaeozoic basin, the Graissessac-Lodève basin, in the south of the Variscan French Massif Central, we proposed a new tectono-stratigraphic sequence of this basin from the Late Carboniferous to the Early Trias. We estimate the paleo-area (from 295-450km2 to 880-1150km2) of the basin and we deduce a subsidence curve from the Late Carboniferous to the Late Permian (from 600±200m/My to 100±10m/My). We conclude that the Graissessac-Lodève basin stratigraphic pattern corresponds to a classical continental rift sequence controlled by tectonics. In order to assess a possible control of the stratigraphic sequence by the climate, we have reconstructed the paleo-depth of the lake with (1) the morphometric analysis of the wave ripples and (2) with an estimation of the lacustrine slope considering the tectonic origin of the lake. We find that the maximum lacustrine depth has evolved from a range of 80-300m during the Late Carboniferous to 40-45m during the Late Permian. From our estimates of the different configurations of the basin, we show that the volume of the water in the lake has remained nearly constant between 10 and 30km3, from the Late Carboniferous (300My) to the Late Permian (250My). Eventually, we conclude that the sedimentological indicators of shallowing and episodic drying observed from the Late Carboniferous to Late Permian in this basin, and probably in all the Pangea basins of the same type, are the consequence of the basin enlargement by tectonics and its sedimentary filling rather than climatic change.