Structural evolution and sedimentary record of the Stephano-Permian basins occurring beneath the Mesozoic sedimentary cover in the southwestern Paris basin (France)
Résumé
Stephanian to Permian post-orogenic basins (SPB) outcrop in several limited locations in and around the present-day French Variscan basement. Little is known about their subsurface occurrences beneath the post-depositional sedimentary cover. Our work intends to decipher the structural evolution of the hidden SPB in the southwestern Paris basin, where only a few regional studies have aimed to determine their location beneath the Mesozoic sedimentary cover.
Our approach is based on the reprocessing and interpretation of 36 seismic lines (1480 km) acquired by the oil industry in the 1980s in the southwestern Paris basin. We first obtain a comprehensive view of the geometry of the SPB, based on (i) the interpretation of the base of the Stephano-Permian surface (lower limit) and the erosional base of the surface of the Triassic layers (upper limit) and (ii) the recognition of specific internal geometries and seismic facies. The interpreted faults are grouped into different categories according to their period of activity, with a focus on synsedimentary faults related to thickness variations of the Stephano-Permian deposits. We then propose a structural scheme containing faults that were active during the Stephano-Permian period, in relation to the late-Variscan structural pattern which has led to the recognition of five sets of faults: N140-trending faults associated with secondary N155- and N055-trending faults (Arpheuilles basin); N115-trending faults (Contres basin); N030-trending faults (Brécy basin); NS-trending faults (transition between the Contres and Brécy basins). Based on the seismic interpretation, thickness maps are calculated both in time and in meters, allowing a pseudo-3D view of the three identified SPB, with thicknesses up to 3000 m (Contres basin); these maps indicate that the preserved extents and thicknesses of the basins in the subsurface are systematically greater than those observed at outcrop.
Finally, we show that the SPB were filled during two different tectonic phases: (i) an initial period of opening of the Arpheuilles, Contres and Brécy basins, during which Stephanian conglomeratic/coal facies were deposited under a strong structural control (normal faulting with certainly a strike-slip component, wedge-shaped geometry of the sediments); (ii) a consecutive pre-Triassic tectonic activity (N155-trending strike-slip in the Arpheuilles basin, uplift of the margins of the three basins), at the origin of a significant part of the sedimentary filling of the basins; this vertical uplift may have reached 2000 m.