Deep boreholes, seismic refraction lines and the interpretation of gravity anomalies in Norfolk

Deep boreholes and seismic refraction data are reviewed and used to constrain the interpretation of gravity anomalies in Norfolk. The sub-Carboniferous rocks range from ? Precambrian to Devonian in age, with the Palaeozoic sediments showing shales, mudstones and siltstones, often steeply dipping and cleaved. Seismic refraction lines show that compressional wave velocities in the sub-Mesozoic floor range from 3.5 to 6.0 km s −1 . High velocities (5.6–6.0 km s −1 ) are found in N and NW Norfolk, low velocities (3.5–4.5 km s −1 ) in W Suffolk and Cambridgeshire, and intermediate velocities in the rest of the area. A broad gravity low is found in W Norfolk, and two interpretive models are considered: either a thick sequence of Palaeozoic sediments, or granitic intrusions in the metamorphic basement. The latter is preferred.