New seismic refraction evidence on the origin of the Bouguer anomaly low near Hunstanton, Norfolk

SUMMARY Possible geological explanations for the negative Bouguer gravity anomaly between Hunstanton and King’s Lynn in north-west Norfolk include a thickening of the Mesozoic succession, a concealed basin filled with Carboniferous sediments, or an acid igneous intrusion into the sub-Mesozoic basement. In order to constrain the interpretation, a seismic refraction experiment was carried out to determine the configuration and seismic velocities of the basement. Five reversed seismic lines were shot between the Hunstanton and Wiggenhall boreholes and were interpreted using both the plane-layer and ‘plus-minus’ methods. The results indicate that the sub-Mesozoic basement surface at the centre of the anomaly lies at a depth of approximately 650 m and has a consistently low apparent dip of 0.5 degrees towards the north. P-wave velocities in the basement range from 5.1 km s −1 to 5.8 km s −1 . Consequently a low-density granitic intrusion seems the most likely explanation of the anomaly.