Sédimentologie, paléontologie et paléoenvironnements côtiers de la région de Porrentruy (Sud-Rhénan, Paléogène, Jura, Suisse): implications géodynamiques

Abstract.: Located in the distal part of the Swiss Molasse Basin and in the southern extension of the Rhine Graben, the conglomeratic deposits belonging to the Gompholithes & Conglomérats stratigraphic group have been the object of detailed sedimentological and paleontological studies. The great number of outcrops that came into sight during the building works of the Transjurane highway in the vicinity of Porrentruy (Swiss Jura) lead to a better understanding of Rupelian paleoenvironments (Early Oligocene). The sedimentological and paleontological studies reveal the existence of coastal environments with Mesozoic limestone cliffs notched by canyons with torrential rivers. Those rivers eroding the Mesozoic series create pebbles deposits forming marine deltas prograding towards North. In protected areas, some lacustrine environments can develop. These conglomeratic deposits are strongly bound to the Rupelian tectonic activity. The rhenish distension and the activity of the transform faults located between the Rhine Graben and the Bresse basin divide the Mesozoic blocks in horst and graben structures, thus allowing the erosion of sediments in higher regions (horst) and their transport in lower zones (graben). The discovery of rare pebbles made of endogene and effusive rocks in those conglomeratic deposits shows a transport coming from the Vosges massifs towards south to the Porrentruy region, probably with the support of a littoral drift. Although the surrection of the Vosges and Schwarzwald massifs (and the beginning of their erosion) is normally attributed to the base of the Miocene, the presence of those pebbles attests the existence of faults putting the basement of the Vosges massif to erosion since the base of Rupelia