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Regional Studies |
1 Arthur D. Little, Berkeley Square House, Berkeley Square, London W1X 6EY, UK
2 Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
The development of the Wessex Basin, Celtic Sea Basins and the Western Approaches Trough was initiated during the break-up of Pangea with active subsidence continuing throughout the Mesozoic until the basins were inverted in the Tertiary. The Mesozoic stratigraphy in these basins is characterized by a number of unconformities. In the Early Cretaceous two major erosional events can be identified, one in the Berriasian and the second in the Aptian. These unconformities have a significant bearing on both the Cretaceous stratigraphy and the hydrocarbon potential in the region.
Stratigraphic data from onshore exposures in the Wessex Basin, from over 100 boreholes, and seismic data document the evolution and erosion pattern of both unconformities. The Aptian unconformity has been well documented by previous authors, and may be related to uplift associated with commencement of sea-floor spreading in the Bay of Biscay. However, an older unconformity can be identified as Berriasian in age on the basis of tracing its correlative conformity using vertical and lateral facies changes and a rapid lateral increase in the thickness of the Wealden Group. This older unconformity is herein interpreted to have been formed by a latest Jurassic-earliest Cretaceous uplift event and can be recognized in all basins surrounding the Cornubian Platform. The areal extent of the unconformity implies that the interpreted uplift was of regional significance, centred on the Cornubian Platform, but unlike the younger Aptian event was unrelated to any specific intra-basinal extensional event or the Apto-Albian sea-floor spreading in the nearby Bay of Biscay. Its effects dominated Early Cretaceous sedimentation, the results of which are demonstrated in subsidence patterns within these basins.
The thermal maturity of the Lias (Early Jurassic), the main hydrocarbon source rock in the area, has also been affected by the latest Jurassic-earliest Cretaceous uplift, as well as Tertiary uplift and inversion. The latest Jurassic-earliest Cretaceous event resulted in the erosion of much of the Jurassic succession in the basins closest to the Cornubian Platform, and in many areas uplifted the Lias from the top of the oil window. Only on the flanks of the uplift, where high sedimentation rates occurred during the deposition of the Purbeck and Wealden Beds, were source rocks buried to depths sufficient to generate hydrocarbons. Only by understanding the tectonic evolution of the latest Jurassic-earliest Cretaceous uplift can accurate play risk modelling and prediction of the hydrocarbon fairway take place.
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