Revealing deep structural influences on the Upper Cretaceous Chalk of East Anglia (UK) through inter-regional geophysical log correlations

Abstract

New borehole geophysical log interpretations between Wiltshire and north Norfolk show detailed lateral changes in the spatial relationships of Chalk Group marker beds. They show how marker beds in the Turonian and Coniacian Chalk Group in East Anglia pass laterally into their correlatives further west, and reveal unusual lateral thickness changes affecting stratigraphical intervals in the East Anglian succession. Newly enhanced regional gravity and magnetic data indicate that these thickness changes are probably related to WNW to ESE trending structural lineaments in the Palaeozoic basement rocks of the buried Anglo-Brabant Massif.The later part of the Mid Turonian and early part of the Late Turonian succession across East Anglia is greatly thickened, and shows almost no lateral variability. These relatively soft, smooth-textured chalks equate with thinner, hard, nodular beds formed in both shallow marine and deeper basinal settings elsewhere in southern England. Since it seems unlikely that there was greater sediment accommodation space across East Anglia at this time compared to basinal areas, this thickening may reflect a localised coccoliths productivity pulse, or perhaps a sheltered palaeogeographical position that protected the area from sediment-winnowing marine currents.A residual gravity low across north Norfolk, previously interpreted as a granite pluton, may instead represent two elongated (fault-bounded) sedimentary basins.