Abstract

Boreholes through the Quaternary, Tertiary and Mesozoic sequences of East Anglia provide general evidence that the Palaeozoic basement in the northern part of the UK segment of the London-Brabant Massif occurs at depths of less than 150 m to more than l000 m below sea level. The widely scattered nature of the borehole evidence gives an impression that the basement surface is planar. However, in the northern part of Suffolk, Bouguer gravity anomaly data indicate an elongate basement ridge which also seems to be related to the presence of a concealed Chalk high separating basins of Crag (Plio-Pleistocene) deposits. An investigation 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 l00 m above the surrounding basement. The ridge is believed to be associated with NE- to NNE-trending faults, which also exerted some form of overall structural control on the deposition of the Crag. The feature appears to be comparable with structures 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 presence of a very low resistivity zone in the power part of the Chalk which contrasts strongly with the underlying resistive basement. Modelling of the geophysical data indicates that the zone is almost certainly due to the presence of saline formation water in the Chalk. The TEM method therefore has potential applications to the hydrogeological investigation of the Chalk, the major groundwater aquifer in the UK.

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