Buried fluvial incisions as a record of Middle–Late Miocene eustasy fall on the Armorican Shelf (Bay of Biscay, France)

Abstract : High-resolution seismic data have been acquired in June 2008 on the Armorican Shelf (AS), in the northern Bay of Biscay, in order to reassess its stratigraphic architecture in detail and to study the impact of eustasy, tectonic and sediment delivery on the margin sedimentary record. Several profiles show fluvial-type incisions of several tens of meters (up to 54 m) associated to a widespread erosion surface. Several hypotheses are proposed for the stratigraphic position of this surface and incisions. We suggest Middle to Late Miocene age. Considering the relatively quiescent tectonic activity of the margin, we infer that the relative sea-level fall responsible of aerial incision on the Miocene shelf is eustasy-related. We propose the attested Serravallian–Tortonian eustatic lowstand (c. 11.6 Ma) as the key event responsible of such erosion and incision. This event marks the early beginning of the high-amplitude sea-level fluctuations that culminated during the Pleistocene and significantly controlled the present day AS morphology. The variability of vertical incision observed along single reaches can be explained by the confluence of several tributaries, the sinuosity of the channel and can be amplified by the unconsolidated nature of the Miocene substratum. The main pathways of the fluvial network corresponding to these buried valleys have been reconstructed and connections to other existing networks are proposed.