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Department of Geology and Geophysics, The University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh, EH9 3JW, UK
Stratigraphic studies in the Wessex Basin, southern England, highlight the occurrence of a significant intra-Cretaceous unconformity which truncates progressively older stratigraphy in a westerly direction from Dorset into Devon. The erosion associated with the event can be traced at least to the west of Exmouth, where the Upper Greensand (U. Albian) sits upon beds of Permian age. A chronostratigraphic correlation diagram (Fig. 1), constructed from coastal outcrops between Hastings in the east and Budleigh Salterton in the west, and which avoids footwall highs, shows the convergent nature of this unconformity. The diagram suggests that the correlative conformity of this event lies between Worbarrow Bay and Swanage in the Valanginian Wealden Beds, below the prominent Quartz Grit bed, rather than in the Aptian as supposed by previous workers. Eastwards, the Wealden Beds thicken rapidly, with progressively more important marine incursions.
The same unconformity also appears to be traceable in the Normandy and Brittany regions of France, where the most complete stratigraphy is observed in the Pay de Bray region, northeast of Rouen. Westwards of this point the gap in stratigraphy widens, especially on the periphery of the Armorican Massif where Liassic, Triassic and Permian beds progressively sub-crop the unconformity in a relatively narrow band (less than 30 km wide).
Independent analysis of over 90 wells from the Celtic Sea, Western Approaches, English Channel and Brittany Basins reveals that an unconformity, which has a similar character to that observed from onshore, exists in the offshore basins also. The most complete stratigraphy can be found in
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