Abstract: Formation and Regional Significance of the Intra-Cretaceous Unconformity of Southern England and Northern France

Neil A. McMahon

Stratigraphic studies in the Wessex basin highlight the occurrence of a significant intra-Cretaceous unconformity which may have eroded important reservoirs and seals found in the nearby Wytch Farm oil province. A chronostratigraphic correlation diagram constructed from coastal outcrops in South England, and which avoids footwall highs, shows the convergent nature of this unconformity, which truncates progressively older stratigraphy in a westward direction. The diagram suggests that the correlative conformity of this event lies between Worbarrow Bay and Swanage on the Dorset coast, in the Barremian Wealden Beds, below the prominent Quartz Grit bed, rather than in the Aptian as supposed by previous workers. The same unconformity also appears to be traceable throughout the Celtic Sea b sins and in the northern France, where the most complete stratigraphy is observed northeast of Rouen. Westwards of this point the gap in stratigraphy widens, especially on the periphery of the American Massif. It is possible to explain the temporal and spatial dimensions of this event in two ways: either as a regional uplift in the North Atlantic associated with the emplacement of Aptian-Albian volcanics in the Western Approaches basin and/or as a consequence of a rejuvenation in buoyancy of the Amorican and Cornubian Massifs but not as a consequence of rift flank uplift. A knowledge of the subcrop and onlap patterns associated with this unconformity is likely to predict potential reservoirs in the underexplored Celtic Sea basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994