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

It is widely agreed that Mesozoic rocks formerly were present over much of northern England, well beyond the erosional limits of their present-day out crops 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 t o determine the burial and thermal history of the region, and to estimate t he thickness and nature of the eroded rocks, are (1) extrapolation of strat igraphical information from the preserved strata, (2) variations in rock ph ysical properties with depth in boreholes and (3) depth conversion of palae otemperature measurements and estimates from apatite fission track analysis , diagenetic mineral assemblages and alteration of organic matter. The expl oration 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 le sser extent, dependent on unverifiable assumptions. The results obtained ha ve been somewhat contradictory, with estimates of the thickness of eroded M esozoic overburden in the Northern Pennines, the Lake District and the Clev eland 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 f lows in this area. and made use of more refined methods of converting palae otemperatures to palaeodepths, indicates an eroded overburden thickness of only c. 1500 m, a value consistent with regional stratigraphical informatio n 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 th e Lake District, is reassessed. Several modelling options have been conside red. 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 th at all of these eroded rocks were of Mesozoic (Upper Jurassic and Cretaceou s) age, and it seems more probable that up to c. 1000 m were of Early Cenoz oic age. Modelling of palaeotemperature data, combined with regional strati graphical information and sonic log compaction studies, suggest that the ma ximum thickness of Chalk on the East Midlands Shelf, before erosion, was c. 700 m. The modelling is unable to discriminate between alternative explana tions for the inferred thicknesses of missing strata from the East Midlands Shelf (Cleethorpes and Biscathorpe boreholes): either (1) the missing rock s were all Late Cretaceous Chalk, or (2! the missing rocks were mainly Chal k, partially eroded in end-Cretaceous/Early Cenozoic times, and overlain by a few tens of metres of Early Cenozoic strata.