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

Abstract

ABSTRACT Valleys on the outer Celtic Sea shelf were cut and filled during the late Pliocene/early Pleistocene. The Kaiser valley is one of several valleys forming an anastomosed network. The main valley, directed 20° (Azimuth true N), is 50 m deep and > 10 km wide. It is connected to the parallel Dompaire and Parsons valleys by several 120-140° directed incisions of lesser width and depth. Analyzed by mean of very high-resolution seismic data, the Kaiser valley is interpreted as containing a compound fill consisting of eight erosionally based depositional sequences. A typical sequence comprises two facies: (1) fluvial channels at the base, which represent lowstand to early transgressive deposits; and (2) onlapping transgressive bay-fill deposits that are locally interbedded at the top with isolated small channels attributed to flood tidal deltas. The erosional bases of the fluvial facies correspond to sequence boundaries. These are interpreted to result from relative sea-level falls. Successive fluvial and bay-fill facies are separated by flat erosional surfaces of high acoustic amplitude, which extend laterally across the entire composite valley, locally beveling sequence boundaries and creating terraces on the valley walls. These flat facies contacts are interpreted as bay ravinement surfaces produced by waves in an estuarine setting. The larger-scale stacking pattern of the depositional sequences defines a progradational-retrogradational trend, in which the lowest sequence is mainly constituted by fluvial channel deposits, whereas upper sequences display mostly bay-fill facies. The sequences are related to fifth-order glacioeustatic fluctuations, whereas their progradational-retrogradational trend reflects fourth-order eustatic variations and/or rapid tectonic tilting of 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 occurrence down to -240 m below present sea level.