Pre-orogenic volcanics (Group B sites)

Abstract

The sites covered in this chapter are listed below, and arranged in broad stratigraphic—tectonic groups from west Cornwall to east Devon. The majority are examples of extrusive and intrusive basaltic rocks covering the late Devonian—early Carboniferous period of maximum volcanic activity in south-west England. The actual intrusion age of the large sills is not known, although many in western Cornwall appear to be pre-main-phase deformation, that is, prelate Devonian (Taylor and Wilson, 1975). Other bodies showing evidence of high-level intrusion, soft-sediment deformation and spatially associated lavas, have an age corresponding with the enclosing sediments. In central south-west England, where faunal evidence of depositional age is good, lavas interbedded with dated sediments are typically Frasnian (for example, Pentire Point pillow lavas), Famennian (for example, Chipley pillow lavas) or Dinantian (for example, Tintagel Volcanic Formation). Lavas within the poorly fossiliferous Mylor Slate and Gramscatho Formations of western and southern Cornwall are probably late Devonian (Turner et al., 1979; Wilkinson and Knight, 1989).