Isabelle Cojan

Seismically induced deformation structures in Oligocene shallow-marine and aeolian coastal sands (Paris Basin)

Abstract Soft-sediment deformation structures have been identified in the Oligocene coastal sand body in the southern part of the Paris Basin (Stampian, Sables de Fontainebleau Formation). These coastal barrier sediments are characterized by pure, fine-grained sands. The deformed beds are observed over some 60 × 110 km2 and are present either in foreshore sands or aeolian sand dunes. Convoluted laminations of an average amplitude around 0.70 m were identified in the foreshore or interdune sediments. These are interpreted as the result of liquefaction. Conversely, deformed sets of aeolian dune sand often involve large amounts of sediment. These large-scale, contorted laminae (several metres thick) with minor internal faulting suggest deformation by slope failure under wet conditions. Such large-scale soft-sediment deformations may have been generated by storm surges and earthquakes which have similar liquefaction potentials. We propose that the deformation structures were created by a single triggering event because of the absence of superposition of deformed layers over the study area. The earthquake-triggering hypothesis is supported by the large amount of sand involved in the slope-failure features and the persistence of the contorted laminations over a wide area.