GEOPHYSICAL EVIDENCE FOR BASEMENT STRUCTURE IN SUFFOLK, EAST-ANGLIA

Boreholes through the Quaternary, Tertiary and Mesozoic sequences of E ast Anglia provide general evidence that the Palaeozoic basement in th e northern part of the UK segment of the London-Brabant Massif occurs at depths of less than 150 m to more than 1000 m below sea level. The widely scattered nature of the borehole evidence gives an impression t hat the basement surface is planar. However, in the northern part of S uffolk, Bouguer gravity anomaly data indicate an elongate basement rid ge which also seems to be related to the presence of a concealed Chalk high separating basins of Crag (Plio-Pleistocene) deposits. An invest igation of this gravity feature using transient electromagnetic (TEM) soundings confirmed that the gravity high is due to a ridge rising to 140 m below sea level, about 100 m above the surrounding basement. The ridge is believed to be associated with NE- to NNE-trending faults, w hich also exerted some form of overall structural control on the depos ition of the Crag. The feature appears to be comparable with structure s affecting Eocene and older rocks reported in the southern North Sea. Part of the success of the TEM method in the area is due to the prese nce of a very low resistivity zone in the power part of the Chalk whic h contrasts strongly with the underlying resistive basement. Modelling of the geophysical data indicates that the zone is almost certainly d ue to the presence of saline formation water in the Chalk. The TEM met hod therefore has potential applications to the hydrogeological invest igation of the Chalk, the major groundwater aquifer in the UK.