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Recognition and Measurement of Basin Inversion |
Department of Geology and Geophysics, University of Adelaide, SA 5005, Australia
Sonic velocities in the Upper and Middle Chalk, the Bunter Sandstone and the Bunter Shale were used independently to quantify apparent exhumation (amount of missing section) in the UK Southern North Sea (SNS). Apparent exhumation results derived from these units are statistically similar. The consistency of results from chalks, shales, and sandstones in the UK SNS, and in other areas (Inner Moray Firth and Celtic Sea/South-Western Approaches), suggests that, at a formation and regional scale, burial-depth is the primary control on velocity (and porosity) in these units, and validates the use of lithologies other than shale in maximum burial-depth determination. The consistency of apparent exhumation results from units of Early Triassic to Late Cretaceous age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that maximum Mesozoic-Cenozoic burial-depth was attained prior to Tertiary exhumation.
The analysis of sonic velocities, apatite fission tracks, vitrinite reflectance, normalized drilling rate and clay mineral diagenesis all suggest that there was a regional component of approximately 1 km of exhumation during Tertiary times, which affected structurally uninverted areas in and around the UK. The widely-recognized inversion-related exhumation is superimposed on regional Tertiary exhumation. Regional Tertiary exhumation need not have been contemporaneous with structural inversion. Further refinement beyond Tertiary age is not possible from the sonic velocity data. Regional exhumation, however, is most likely to be associated with regional Palaeocene or Oligocene/Miocene unconformities.
The evidence for regional exhumation implies that there was a pre-exhumational phase of burial (during which the eroded sedimentary rocks were deposited). Exhumation and prior burial associated with the regional Tertiary event need to be incorporated in aspects of basin analysis such as sediment decompaction and diagenetic and maturation modelling, and can, for example, account for the maturity of the Carboniferous of the onshore East Midlands Shelf, and the Lias exposed on the north coast of Somerset. The regional, Tertiary tectonic uplift associated with exhumation must have had a thick-skinned origin, and, in areas where there is no evidence for significant Tertiary igneous activity, a decoupled, two-layer model of lithospheric compression is invoked to account for the subsidence/uplift patterns of both inverted and uninverted areas.
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