Palaeotemperatures, thermal modelling and depth of burial studies in northern and eastern England

Abstract

SUMMARY It is widely agreed that Mesozoic rocks formerly were present over much of northern England, well beyond the erosional limits of their present-day outcrops to the east and west of the Lake District and the Pennines. However, the extent, age, lithology, sequence and thickness of these vanished rocks have proved to be a controversial subject. The principal methods employed to determine the burial and thermal history of the region, and to estimate the thickness and nature of the eroded rocks, are (1) extrapolation of stratigraphical information from the preserved strata, (2) variations in rock physical properties with depth in boreholes and (3) depth conversion of palaeotemperature measurements and estimates from apatite fission track analysis, diagenetic mineral assemblages and alteration of organic matter. The exploration for oil and gas in the region and adjacent offshore areas provided a spur to such investigations. All the methods used are, to a greater or lesser extent, dependent on unverifiable assumptions. The results obtained have been somewhat contradictory, with estimates of the thickness of eroded Mesozoic overburden in the Northern Pennines, the Lake District and the Cleveland Hills ranging from a few hundred metres to more than 3000 m. Early modelling studies of the palaeotemperature data, derived from apatite fission track analysis on samples from the Lake District and some adjacent parts of the Northern Pennines, have suggested that the minimum thickness of Mesozoic rocks over much of this area was in excess of 3000 m. However, a more recent investigation which took into account the locally high heat flows in this area, and made use of more refined methods of converting palaeotemperatures to palaeodepths, indicates an eroded overburden thickness of only c . 1500 m, a value consistent with regional stratigraphical information and some sonic log studies. The depth conversion of some palaeotemperature data from the East Midlands Shelf and the Cleveland Basin, where heat flow is somewhat lower than in the Lake District, is reassessed. Several modelling options have been considered. The palaeotemperature data are consistent with the removal of c. 2500 m of post-Middle Jurassic overburden from the central parts of the inverted Cleveland Basin. Similar conclusions have been reached from several sonic log studies. Regional stratigraphical trends suggest that it is unlikely that all of these eroded rocks were of Mesozoic (Upper Jurassic and Cretaceous) age, and it seems more probable that up to c. 1000 m were of Early Cenozoic age. Modelling of palaeotemperature data, combined with regional stratigraphical information and sonic log compaction studies, suggest that the maximum thickness of Chalk on the East Midlands Shelf, before erosion, was c. 700 m. The modelling is unable to discriminate between alternative explanations for the inferred thicknesses of missing strata from the East Midlands Shelf (Cleethorpes and Biscathorpe boreholes): either (1) the missing rocks were all Late Cretaceous Chalk, or (2) the missing rocks were mainly Chalk, partially eroded in end-Cretaceous/Early Cenozoic times, and overlain by a few tens of metres of Early Cenozoic strata.