3D geometry and palaeogeography of the Paris basin chalk (cenomanian to campanian) - Tectonics and eustatic implications
Résumé
The Paris Basin is an intracratonic (sag) basin initiated in the Permian times by a period of tectonic extension. A regime of thermal subsidence then took place from the Triassic to the early Late Cretaceous. Its sedimentary infilling records the main geodynamic events affecting the western part of the Eurasian plate, from the opening and closure of the Tethyan oceans to the different opening stages of the Atlantic Ocean. Nevertheless, its Upper Cretaceous sedimentary record, consisting mainly of chalk deposits, still remains poorly known. As a consequence, reconstructing the Paris Basin evolution during this period has strong geodynamic implications, especially on the understanding of the intraplate response of western Europe to the Alpine compressional stress. Indeed, cretaceous compressional deformations (strong graben inversions) are only well recorded in eastern Europe, whereas only poor evidence is known west of the Brabant Massif in western Europe. The aim of this study is therefore to decipher the Paris Basin evolution during the Late Cretaceous, through the first multidisciplinary detailed study on its chalk deposits. A facies model of the Chalk sediments, based on field and drill core observations, enable us to identify its various depositional palaeoenvironments. Using sequence stratigraphy, we combine our facies model with well-log data to perform high resolution correlations at the 1Ma time scale. In so doing, we determined the 3D geometry and palaeogeography of the chalk deposits during the late Cretaceous, and identified the majors controls (tectonics and eustasy) on the Paris Basin Chalk deposition. Two main periods of geodynamic evolution were recognized. The first one (Cenomanian to Mid-Turonian) is characterized by weak E-W deformations related to a late stage of the Biscaye Bay opening. A major change in the deformation regime then occured during the Late Turonian. Palaeogeography and 3D geometry evidence a strong reorganisation of the stress regime, as well as NW-SE and NE-SW deformations. We attribute this change to the onset of the intraplate deformation linked to the alpine cycle, previously not identified in this part of Europe.