Some geophysical aspects of the deep geology of eastern England

SUMMARY A review of the geophysical data available in the East Midlands, in particular gravity and magnetic data, confirms the influence of major basement structures on the formation of Carboniferous ‘blocks’ and ‘basins’. Two profiles were selected over two negative gravity anomalies. The first – over the Widmerpool Gulf, containing a proven thick Carboniferous sedimentary sequence; the second – in the Newark area, over an anomaly of uncertain geological origin. Modelling of the Widmerpool Gulf, with good geological control, provides a suitable fit to the observed gravity field. The uncertain origin and lack of geological control prevents a definitive gravity interpretation of the Newark anomaly either as a sedimentary basin or as an igneous intrusion. Both models provide a reasonable correlation with the observed profile. A compilation of available seismic refraction lines provide an assessment of basement topography, but there may still be ambiguities in the interpretation in some areas where massive Dinantian limestone is present. Interpreted velocities increase northwards from the Suffolk/Essex region, where Devonian and Ordovician rocks have velocities less than 5 km s −1 , to Norfolk, where Precambrian and Cambrian rocks whose velocities always exceed this value. Recently acquired seismic data from East Anglia suggest that there may be anisotropic variations in basement velocities depending on the directions of the refraction lines relative to the regional cleavage/foliation trend (WNW-ESE), the maximum velocity values occurring on lines parallel to this direction.