Estimates of the eroded overburden and the Permian–Quaternay subsidence history of the area west of Orkney

D. J. Evans
Scottish Journal of Geology, 33, 169-181, 1 November 1997, https://doi.org/10.1144/sjg33020169

Synopsis

Due partly to the perceived lack of preservation of strata younger than Devonian in age, the area onshore mainland Scotland and that offshore occupied by the West Orkney Basin, has long been regarded as emergent or close to sea level since end-Devonian times. Boreholes drilled within the West Orkney Basin have revealed much of the assumed Devonian fill of the West Orkney Basin to be Permo-Triassic in age, questioning this long held view. The post-Triassic subsidence history of the basin remains unknown. Well 202/19-1 penetrated Triassic mudstones, from which sonic and density data may provide evidence of an eroded overburden and thereby information regarding the subsidence or burial history of the West Orkney Basin. The availability of just one borehole does not permit the establishment of a porosity-depth relationship (shale burial curve) for shales/mudstones within the basin. However, available data when plotted against published shale depth–density and depth–velocity curves, provide eroded overburden estimates of between 1.3 and 1.6 km for the basin. Additionally the deposition of 250 m of post-erosional Late Tertiary/Quaternary sediments indicates that the eroded overburden was 1.85 km prior to reburial, which it is suggested, represents a reasonable estimate of the total amount of eroded late and post-Triassic overburden. Absolute uplift of 1.75–2.25 km from end-Cretaceous to Late Tertiary times is then calculated for the basin in general. Implications arise for palaeogeographic reconstructions and the timing of Devonian age source rocks entering the oil window. In addition to hydrocarbon generation during Late Devonian–Carboniferous times, further phases would be likely during Permian–Cretaceous burial. The eroded overburden and absolute uplift values suggest the submergence beneath Cretaceous seas of areas long considered at or close to sea level since end-Devonian times. This includes large areas of the Scottish Highlands.

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