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Sonic velocities from the Danian Chalk, the Upper Cretaceous Chalk, the Lower Cretaceous Greensand/Gault Clay, and the Triassic Mercia Mudstone were used to quantify apparent exhumation (i.e. amount of missing section) in the Celtic Sea/South-Western Approaches (southern Irish, south-western United Kingdom, and north-western French continental shelves). Analysis of all four of these stratigraphic units yield similar results. Since it is unlikely that sedimentological and/or diagenetic processes are responsible for similar amounts of overcompaction in the chalks, shaly sandstones and shales analysed, burial at depth greater than that currently observed is the most likely cause of overcompaction (anomalously high velocities). The use of lithologies other than shales in maximum burial-depth/exhumation studies is thus validated. The consistency of results from units of Triassic to Danian age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that the maximum Mesozoic-Cenozoic burial-depth in the Celtic Sea/South-Western Approaches was attained prior to Tertiary exhumation. The recognized inversion axes of the Brittany Basin, South-West Channel Basin and North Celtic Sea Basin are highs in apparent exhumation, however highs in apparent exhumation of approximately 1 km also occur in the uninverted St. Mary’s and Plymouth Bay Basins, and on the margins of the Armorican and Cornubian Platforms and the Pembroke Ridge. Tertiary exhumation was regional and not restricted to inversion axes, and thus had a thick-skinned origin. Heterogeneous lithospheric compression is the geodynamic model preferred to account for the regional exhumation of the Celtic Sea/South-Western Approaches. The identification of regional exhumation of approximately 1 km has important implications for modelling thermal maturation in the Celtic Sea/South-Western Approaches, with source rocks previously believed to be immature shown to be potentially mature for oil generation. When corrected for exhumation, the post-rift sequence in the Celtic Sea/South-Western Approaches area shows relatively little localization over the syn-rift Jurassic-Lower Cretaceous sequence, suggesting that during basin formation, the lower lithosphere, thermal re-equilibration of which controls post-rift subsidence, did not experience localized thinning on the scale of the syn-rift basins.

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-Porosity measurements of 22 Upper Cretaceous Chalk samples, and mean Chalk porosities derived from sonic logs in three wells, were used to quantify apparent exhumation (height above maximum burial-depth) in the onshore Cleveland Basin/East Midlands Shelf. Late Cretaceous/Tertiary exhumation of the East Midlands Shelf resulted in the removal of 1.2 km of section near the coast and more than 2 km of section inland, to the west. The southern margin of the Cleveland Basin was exhumed by 2 km, and exhumation increases northwards towards the recognized inversion axis running east-west along the basin's centre. The northwards increasing exhumation associated with the east-west trending inversion axis of the Cleveland Basin is superimposed upon the regional westward trend of increasing exhumation of eastern England. These two trends control the outcrop distribution of the Upper Cretaceous Chalk in the Cleveland Basin/East Midlands Shelf.