Architecture and sequence stratigraphy of a late Neogene incised valley atthe shelf margin, southern Celtic Sea

Valleys on the outer Celtic Sea shelf were cut and filled during the late P liocene/early Pleistocene. The Kaiser valley is one of several valleys form ing an anastomosed network. The main valley, directed 20 degrees (Azimuth t rue N) (.) 50 m deep and > 10 km wide. It is connected to the parallel Domp aire and Parsons valleys by several 120-140 degrees directed incisions of l esser width and depth. Analyzed by mean of very high-resolution seismic dat a, the Kaiser valley is interpreted as containing a compound fill consistin g of eight erosionally based depositional sequences. A typical sequence com prises two facies: (1) fluvial channels at the base, which represent lowsta nd to early transgressive deposits; and (2) onlapping transgressive bay-fil l deposits that are locally interbedded at the top with isolated small chan nels attributed to hood tidal deltas. The erosional bases of the fluvial fa cies correspond to sequence boundaries, These are interpreted to result fro m relative sea-level falls. Successive fluvial and bay fill facies are sepa rated by flat erosional surfaces of high acoustic amplitude, which extend l aterally across the entire composite valley, locally beveling sequence boun daries and creating terraces on sic walls. These flat facies contacts are i nterpreted as bay ravinement surfaces produced by waves in an estuarine set ting. The larger-scale stacking pattern of the depositional sequences defin es a progradational-retrogradational trend, in which the lowest sequence is mainly constituted by fluvial channel deposits, whereas upper sequences di splay mostly bay-fill facies, The sequences are related to fifth order glac ioeustatic fluctuations, whereas their progradational-retrogradational tren d reflects fourth order eustatic variations and/or rapid tectonic tilting o f the area, as indicated by the presence of two incision orientations. The preservation of the system took place during a third-order sea-level rise, and was favored by subsidence of the margin, leading to its present occurre nce down to - 240 m below present sea level.