Abstract
The Saint-Affrique basin is one of the major Permian continental basins of the southern French Massif Central. Two main sedimentary cycles, separated by an angular unconformity corresponding to an erosion surface, have been distinguished. Cycle 1 is formed by three successive formations, from bottom to top : formation Fl consists of alluvial deposits ; formation F2 is represented by lacustrine to fluvio- lacustrine deposits; formation F3 is represented by fluvial deposits. Palynological analyses indicate a late Autunian to lower Thuringian age for this cycle. Cycle 2 is constituted by fanglomerates (formation F4) and playa deposits (formation F5).Both sedimentological and structural evidence show that the structure of the basin and the depositional patterns were controlled by the development of roll over structures. These roll-overs were associated with north dipping listric normal faults which are responsible for the asymmetrical shape of the basin. The faults which controlled the subsidence of the basin floor formed in response to an extensional reactivation of pre-existing Hercynian thrusts in the basement (“negative tectonic inversion”). Taken together structural and sedimentological evidence indicate that the basin propagated southwards by successive incorporation of previously undeformed parts of the footwall. This mode of propagation can be ascribed to an extensional piggy-back sequence.The model for the development of the Saint-Affrique basin includes 2 extension stages separated by an erosion episode. The first extensional event, which corresponds to Cycle I, deformed the post-Stephanian erosional surface into a smooth hanging wall syn-cline with a large, but shallow deposition area. Well-marked fault activity with local subsidence is observed only during the deposition of formation F3 and denotes an increase in extension rate. The second extensional event is characterized by higher extension and subsidence rates and resulted in deeper individual sub-basins with a southward progression of both margin fault and depocentre.
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