The paleo-Etel River incised valley on the Southern Brittany inner shelf (Atlantic coast, France): Preservation of Holocene transgression within the remnant of a middle Pleistocene incision?
Résumé
The study of a dense and regular grid of 59 High Resolution seismic profiles supplemented by 23 superficial vibro-cores has led to propose a three-dimensional detailed sedimentary model of the paleo-Etel River buried valley located on the inner shelf of South Brittany (Atlantic Coast, France). This fluvial valley (S2) lies at a mean depth of − 37 m below mean sea level above Paleozoic meta-sediments (U1) and middle to late Eocene formations (U2). No fluvial deposits seem to have been preserved. Its sedimentary record reflects the retrogradation of estuarine environments (tidal flats (U3), Tidal Ravinement Surface (S3), and intra to pro-estuarine sand bars (U4/S4/U5)) during the Holocene transgression (ca. 9000 to 7500 cal yr BP). Open marine environments seal up this estuary with the onset of a flat Wave Ravinement Surface (S5) overlain by thin muddy-sands deposits (U6-F2) interpreted as low-energy open marine conditions (4250 to 1800 cal yr BP). Shoreface sands, made up of coarse sands bodies migrating landwards (U6-F3, F4), cover this sedimentary sequence since 1800 cal yr BP. A numerical interpolation of the picked seimic horizons was proceed to produce seismic isopach maps of sedimentary units and isobath maps of their boundary surfaces. Furthermore, the isobath map of the basal channel topography has enabled us to calculate some geomorphological features (W/D ratio, mean slope, sinuosity index) that characterized the type of channel pattern. The Etel River geomorphological indexes correspond to a straight channel that involves bed-load transportation processes, potentially associated to braided deposits if the sediment load is sufficient. Compared to the Vilaine River, which braided regime is dated at middle Pleistocene in several zones, we assume a possible mainly climatic control for river incisions in Southern Brittany linked to drastical changes in climate regimes occurring during the "Middle Pleistocene Transition".