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

Abstract

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.

Geometry of Quaternary sediments along the north Norfolk coast, UK: a shallow seismic study

“…Firstly, the velocity log through the Chalk from the Somerton well (for location see Chroston & Sola, 1982) shows that lower horizons have a higher seismic velocity, and the low eastwards regional dip will result in higher velocities occurring in the west. Secondly, jointing can lead to seismic anisotropy if there is a dominant alignment (see e.g.…”

“…The possibility of faulting of the Chalk basement should be considered to explain the linearity of the southern boundary of the coastal zone (at least on a large scale) and the presence of the trough. In north Norfolk, the few deep boreholes and data from seismic surveys indicate a northerly deepening of the subMesozoic floor from the London-Brabant Massif towards the western part of the southern North Sea basin (Chroston & Sola, 1982;Chroston, 1985), and the increase in depth may be accomplished in part by east-west normal faults affecting the whole Chalk succession Although the dominant trends of the gravity and magnetic lineaments in eastern England are southeast or east-southeast (Lee, Pharoah & Green, 1991) there is some evidence of east-west faulting in this area with, for example, the limited published offshore data indicating such an orientation for the western end of the South Hewitt Fault (PESGB, 1992). Faulting in the coastal zone could have been exploited in the early stages of formation of the trough, in which case the marked increase in elevation to the south of the coastal zone may represent the remnants of a major fault scarp.…”

A seismic refraction line across Norfolk

“…Short seismic refraction lines providing estimates of the compressional wave velocities in the sub-Mesozoic floor have already been carried out (Bullard et al 1940;British Petroleum, personal communication;Chroston & Sola, 1982) and these reveal basal refractor velocities varying from 3.5 to about 6.0 km s~x (Fig. 1).…”

“…Precambrian) affinities have been found and the occurrence of similar rocks at Lexham and Hunstanton, northwest Norfolk, and in boreholes to the west has indicated their extension in a complex basement ridge to Leicestershire (Le Bas, 1972). Over much of the rest of East Anglia deep boreholes have revealed shales, slates, siltstones and mudstones, often steeply dipping and highly cleaved, of Lower Palaeozoic or Devonian age (Kent, 1968;Wills, 1978;Chroston & Sola, 1975, 1982. In northeast Norfolk boreholes show that these are overlapped by Carboniferous sediments (Chroston & Sola, 1982).…”

“…Over much of the rest of East Anglia deep boreholes have revealed shales, slates, siltstones and mudstones, often steeply dipping and highly cleaved, of Lower Palaeozoic or Devonian age (Kent, 1968;Wills, 1978;Chroston & Sola, 1975, 1982. In northeast Norfolk boreholes show that these are overlapped by Carboniferous sediments (Chroston & Sola, 1982). The depth to and nature of the igneous/metamorphic basement (hereafter referred to as 'basement') is, however, most uncertain.…”