Regional Tertiary Exhumation in and around the United Kingdom

Abstract

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 I 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.

Signature of cryptic sedimentary basin inversion revealed by shale compaction data in the Irish Sea, western British Isles

“…This decrease is largely irreversible upon subsequent exhumation, and thus compaction-driven porosity reduction is an effective and widely used measure of former burial depths in exhumed sedimentary basins [Corcoran and Doré, 2005;Japsen et al, 2007]. Likewise, sonic velocity increases with compaction and consequently former burial depths in many exhumed basins have been quantified using anomalously high sonic velocities [Hillis, 1991[Hillis, , 1995Heasler and Kharitonova, 1996;Japsen, 1998Japsen, , 2000Japsen et al, 2002;Corcoran and Mecklenburgh, 2005]. Exhumation studies using sonic velocity data generally compare the anomalously high velocities measured in an overcompacted formation with a lithologically analogous, normally compacted velocity-depth baseline [Japsen et al, 2007].…”

“…The best known of these basins is the East Irish Sea Basin (EISB), the most northeasterly depocenter of the NE-SW trending Irish-Celtic Sea Basin system, a $700 km long chain of exhumed MesozoicCenozoic extensional basins [Tappin et al, 1994;Jackson et al, 1995]. Exhumation studies along this corridor have consistently highlighted the principal role of Cenozoic inversion in the uplift and erosion of this region [Tucker and Arter, 1987;van Hoorn, 1987;Hillis, 1991Hillis, , 1995Bulnes and McClay, 1998;Williams et al, 2005], and consequently the absence of clear inversion structures in the EISB [e.g., Knipe et al, 1993] is puzzling.…”

Magnitude of Cenozoic erosion from mean sonic transit time, offshore Taiwan

“…These techniques resolve only apparent erosion, which is strongly related to post-erosion burial and can be easily masked. Post-erosion burial correction was also discussed and applied to apparent erosion in previous studies (Hillis, 1993;1995;Japsen, 1998;Katz, Pheifar & Schunk, 1988;Menpes & Hillis, 1995). In contrast to the single-well approach, mean sonic transit times of some selected stratigraphic units from all wells in a wide region are cross-plotted with midpoint depths of the unit.…”

Evidence for kilometre-scale Neogene exhumation driven by compressional deformation in the Irish Sea basin system

“…Compaction-driven porosity reduction is an effective and widely used measure of former burial depths in sedimentary basins (Hillis 1991(Hillis , 1995Japsen 2000;Ware & Turner 2002). The term compaction refers to the three-dimensional reduction in sediment volume which occurs as a result of mechanical and thermochemical processes during burial (Giles et al 1998;Corcoran & Doré 2005).…”

“…In recent years, considerable attention has been paid to the late Mesozoic-Cenozoic exhumation history of the British Isles (Lewis et al 1992;Brodie & White 1994;Cope 1994;Hillis 1995;Japsen 1997;White & Lovell 1997;Rowley & White 1998;Green et al 2002;Ware & Turner 2002;Holford et al 2005b). A great deal of this discussion has been presented in terms of Palaeogene uplift, which has been assumed to represent the dominant exhumation episode.…”