Pre-Mesozoic Basement of the SW Paris Basin (France) - Focus on The Stephano-Permian Basins
Résumé
Stephano-permian basins are found in several rather small places in and around the present-day French Variscan outcrops. They are mostly considered as intramontane post-orogenic basins, developing in close relationships with crustal to lithospheric geological events (such as extensive strike-slip, exhumation of metamorphic rocks, and widespread magmatism). On the other hand, little is known about their occurrences under post depositional sedimentary covers. This is for instance the case in the Paris Basin, where significant but scarce studies aimed to find their location under the Mesozoic sedimentary cover. There, the questions of their extension, thickness, sedimentary filling, internal geometry and structural control still remain open. In the same way, their differential conservation compared with the basins known in the field, or the possible structural decoupling between the pre-Mesozoic and Mesozoic units, has still to be debated. The geology of these hidden basins may only be reached through subsurface tools (boreholes, seismic lines, aeromagnetic and gravimetric maps), and correlation with known lateral outcrops. Our study focuses on the Stephano-permian basins located in the south-western part of the Paris Basin. The first step of our work deals with the reprocessing and interpretation of a selection of seismic lines; about 950 km of seismic profiles, acquired by the petroleum companies in the 80', were reprocessed using modern algorithms (leading to a better display of the horizons and faults). The picked horizons are the base and top Stephanian (i.e. Base Permian) and the Top Permian, giving the envelop of the Stephano-permian basins along the seismic profiles. About 25 boreholes (including 9 with checkshot surveys) were used to calibrate and tie well data to the corresponding seismic horizons. The interpretation highlights the major faults controlling the deposition of the Stephano-permian sediments, as well as the strong faults density at the basement/Stephanian interface. At the same time, a new sedimentological description of the Permian deposits of the Sancerre-Couy core has been done, showing that the lacustrine facies were more important than previously expected. We also present an attempt of (sequential) correlation of the Sancerre-Couy borehole with those used for the seismic study. In order to laterally extend the previous geometries (sedimentary basins, structural pattern...), all the above data/results are compared to geophysical equipotential data (aeromagnetic and gravimetric) recently acquired or processed in the region.