| Abstract: | Non-marine strata of Early Cretaceous age ('Wealden facies') are found at outcrop in the type localities of the Wessex Basins, southern England (and adjacent subsurface) and in extensive and thick successions filling the North Celtic Sea Basin. Sedimentology, paleontology, petrology and geochemistry have traditionally been used as evidence in determining the climatic, tectonic and sea level controls on Wealden facies, sedimentary processes and stratigraphy. Analysis of seismic data, through seismic facies and sequence stratigraphic analysis, allows direct comparison of the Wealden in these basins and new interpretations to be made of the tectonic and depositional influences. In the north-eastern end of the North Celtic Sea Basin, tectonic controls on seismic facies can be demonstrated and are related to coarse-grained fan-delta horizons documented in core. In the south-western North Celtic Sea Basin, adjacent to the Cretaceous proto-Atlantic, tectonic controls are less apparent, and changes in relative sea level were probably more important in controlling the preservation of stratigraphic sequences. Where the non-marine Lower Cretaceous succession is imaged clearly, the stratigraphic similarity between parts of the North Celtic Sea and the Weald and Channel Basins suggests a wider control by relative sea-level fluctuations. However, important variations in seismic facies within the basins indicate localized, dominant tectonic control. The recognition of a very distinctive 'lower' Wealden seismic facies, observed as undulatory (?channelized) and downlapping reflections, is contrasted with either the parallel or synsedimentary fault-dominated 'upper' Wealden facies. These seismic characteristics reflect the previously recognized climatic/tectonic change from Hastings to Weald Clay Group environments. The debate on tectonics versus eustacy is further complicated: palaeoclimate appears to be a third process responsible for stratigraphic variation. |
