Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Institut des Sciences de la Terre de Paris (iSTeP), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Alex Ekwueme Federal University Ndufu-Alike, Ikwo (AE-FUNAI), Géoazur (GEOAZUR 7329), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de la Côte d'Azur, COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Université Côte d'Azur (UCA)-COMUE Université Côte d'Azur (2015-2019) (COMUE UCA)-Université Côte d'Azur (UCA)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD [France-Sud]), Ruhr University Bochum (RUB), Petroleum Development Oman (PDO), Brunei Shell Petroleum, Laboratoire Géosciences Océan (LGO), Université de Bretagne Sud (UBS)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), Université de Bretagne Sud (UBS), Université de Brest (UBO), Centre National de la Recherche Scientifique (CNRS), IFP Energies nouvelles (IFPEN), Laboratoire d’Océanologie et de Géosciences (LOG) - UMR 8187 (LOG), Institut national des sciences de l'Univers (INSU - CNRS)-Université du Littoral Côte d'Opale (ULCO)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD [France-Nord])
International audience; The offshore Niger Delta provides a spectacular example of gravity collapse tectonics, but the timing of shale mobilisation remains poorly understood. Here we present new information from the western Niger Delta, based on a detailed interpretation of a 3D seismic volume, calibrated with biostratigraphic data from exploration wells. The study area is underlain by mobile shales containing thrust-fold anticlines, overlain by >5 km thick Late Eocene to present succession of folded and faulted sediments with fluid migration features. Sedimentation rates estimated at one deep well increased during two phases, more than doubling during the Late Eocene to Serravallian (39.5–12.5 Ma), and by up to ten times during the Tortonian (9.5 Ma) to present. Thinning and onlapping geometries within the lowermost, Late Eocene-Burdigalian (39.5–18.5 Ma) interval are interpreted to record syn-depositional deformation. This indicates that shale tectonics in the offshore western Niger Delta initiated early in the evolution of the region, supporting previous interpretations that the active compressional zone of the Niger Delta was in the present continental slope before prograding to the outer-fold-thrust belt in the Pliocene. Stratal thinning above the crests of thrust-fold anticlines in the northeast and southwest of the study area suggests that shale tectonics persisted throughout the Neogene and Quaternary. A chaotic column that rises to seafloor from the crest of thrust-fold anticline, interfingering with the stratified succession is interpreted as a mud volcano edifice that has been active since at least the Burdigalian. This supports early shale mobilisation in response to generation of overpressure and also the fact that mud volcano system once formed, may act as conduits for pressure release throughout the history of a gravity-driven collapse system. The spatial association of seafloor mud volcanoes and pockmarks with deeper thrust-fold anticlines and normal faults suggests fluid migration from depth. In contrast, pockmarks overlying mass-transport deposits and submarine channels suggest fluid-flow from dewatering of more recent sediments. Our findings provide new insights into the Cenozoic tectono-stratigraphic evolution of the offshore western Niger Delta and the relation of shale mobility to fluid migration and escape during continental margin collapse.
