Submarine Geomorphology of Incised Valleys on the Northern English Channel Shelf

Late Quaternary sea-level fluctuations exposed large expanses of the English Channel shelf during glacial periods leading to the development of complex incised fluvial valley systems carved into the bedrock. These submerged remnants of the formerly extensive English Channel River provide a critical interface between the onshore fluvial systems of NW Europe and sediment sinks at the Celtic Sea shelf margin. Here we investigate the detailed morphology of the submerged River Arun, a tributary of the English Channel River, with a view to understanding the processes governing valley evolution. We conducted a high-resolution swath bathymetry survey of the Arun palaeovalley using a portable Reson 8101 240 kHz Multibeam Echosounder owned by the UK Universities High-Resolution Geophysics Consortium. An 8km x 17km area was surveyed in water depths ranging from 18m to 55m. The Reson 8101 was mobilised on a vessel of opportunity using an over-the-side pole mounting. Narrow focused beams (1.5° x 1.5° ) produced a footprint 0.5m² to 3.5m² and coverage over a 150° angular sector of seafloor. The sonar range resolution of 5cm combined with 2cm accuracy tidal measurements meant a very high vertical accuracy was achieved. DGPS positioning using a POS MV 220 was accurate to within 1m, and the system also provided satellite aided inertial heading and motion data. Co-registered acoustic backscatter data was merged with a Digital Elevation Model to aid in classifying the seabed. A dense grid of complementary commercial boomer profiles together with extensive cores have been used to constrain the subsurface stratigraphy. Our newly defined shelf morphology reveals a topographically complex submarine landscape comprising bedrock-incised valleys and embayments. The main Palaeo-Arun valley axis trends northwest to southeast and is bounded by bedrock on one margin and fluvial gravels on the other. The width of the valley narrows downstream from a maximum of 2km to 300m and the depth of incision increases from 2m to 20m. Integration of multibeam data with boomer seismic profiles permits a geological interpretation of the bathymetry data to be drawn. Upstream of the topographic valley, the valley form can be correlated into a buried valley system that is observed in seismic data. The geometry of the valley is controlled partly by the presence of a monocline formed of resistant bedrock lithologies and partly by incision into older fluvial gravels that now form submerged terrace remnants. Downstream valley narrowing is the result of incision of the river through a resistant sandstone cuesta. At this point we have identified a prominent knickpoint and plungepool where the gorge opens out into the northern branch of the English Channel River. Preliminary interpretation of the submarine geomorphology suggests that initial valley formation was governed by enhanced fluvial erosion of weaker lithologies between resistant sandstone cuestas. Fluvial aggradation in these topographic lows may have occurred partly due to ponding behind structural barriers. Subsequent incision to produce the observed valley form was probably a consequence of baselevel fall coupled with breaching of the resistant barrier prior to shelf drowning during the Holocene transgression. Our analysis indicates that bedrock structure can significantly influence the morphology of incised shelf valley systems.