Geometry of the sedimentary filling and role of present-day - The border fault during the Cenozoic rifting phase of the Upper-Rhine graben of Western Europe
Résumé
The Upper Rhine Graben (URG) of NW Europe is a Cenozoic wrench rift basin about 300 km long and 30-40 km wide, with an Eocene to Quaternary sedimentary fill up to 4 km thick. The finite extension is about 5 km with a transtensional sinistral component of about 30-40 km. These Cenozoic deposits unconformably rest on Triassic to Jurassic sediments representing the eastward extension of the Mesozoic Paris Basin. We focus on the Bartonian-Priabonian to Rupelian syn-rift stage of the URG, which ends with an intra-Rupelian regional-scale transgression (Foraminifera Marls and Fish Shale Formation). We use subsurface data (well-log and seismic records) to discuss the geometry of the syn-rift deposits, the role of the present-day border fault during the rifting phase, and hence the subsidence history of the initial URG. The correlation along transects of well-log data using sequence stratigraphy show that the syn-rift deposits result of the stacking of 3rd order transgressive-regressive cycles, characterized by prominent transgressive phases. The sedimentary facies are organized according to a proximal-distal present-day W-E profile; the facies range from syn-tectonic conglomerates (alluvial fans environments) from the W, to eastward lagunal-lacustrine evaporites (e.g. halite), marls and shales. The transects also illustrate the lateral facies transition between the conglomerates and the halite deposits during the transgressive half-cycles. We also reprocessed and interpreted industrial seismic profiles cutting through the present-day border fault, in order to understand its relations with the Eo-Oligocene syn-rift deposits. Only some of the profiles show conglomeratic-type seismic facies, wedge shape and fanning geometries; this suggests that the present-day border fault is not the syn-rift fault along its complete length. The seismic and well data have been locally harmonized at the scale of the 3rd order sequences. Additional observations also point out the post-rift origin of the present-day border fault (at least locally) and the role of flexuration processes during the rifting stage. This work gives an up-to-date view of the role of the border fault during the rifting phase and its genetic relationships with the syn-rift sediments.