The geology of the western approaches of the English Channel. II. Geological interpretation aided by boomer and sparker records

Abstract

A geological map of about 1700 square miles of the sea-bed south of the Cornish coast between Bolt Head and Fowey was prepared by coring and dredging for rock samples; a boomer-sparker survey was then selected in relation to the supposed geology to fix more precisely the geological boundaries and to ascertain the geological structure of the rocks underlying the sea-bed, in this case to a depth of about 400 ft. The acoustic apparatus is described; the records are interpreted according to mathematical analysis and the probable limits of inaccuracy assessed. 91 rock samples are described lithologically and the microfossils identified in all productive cases to give the stratigraphical ages. In some samples radiometric determinations have been made of the ages of the metamorphism of ‘gneisses’ and slates. All contacts are unconformable between the following major divisions: Metamorphic Complex, ?Devonian, New Red Sandstone, Upper Cretaceous, Eocene; the base of the Lias is unknown but these rocks form an inlier and are unconformably surrounded by the Upper Cretaceous. Pre-Santonian, Santonian, Campanian, Lower and Upper Maestrichtian are recognized; Danian appears conformable to the Maestrichtian but is included in the Tertiary although it is unconformably succeeded by Eocene; there is presumptive evidence of ? Oligocene in one cored sample. True-scale sections drawn along the course of the ship using corrected apparent dips and applying different velocities to different rock-types gave the following approximate thicknesses: New Red Sandstone, 3100 ft.; Lower Jurassic, greater than 640 ft. (base not seen); Upper Cretaceous, 1225 ft., comprising Pre-Santonian 75 ft., Santonian 175 ft., Campanian 375 ft., Maestrichtian 600 ft.; Danian, 375 ft.; Eocene, greater than 430 ft. (top not seen). In this particular combined experiment the boomer-sparker equipment has proved invaluable in correcting geological boundaries, in determining geological structure which with present coring methods at sea is almost impossible to detect, and in providing reasonably reliable figures of the thickness of major stratigraphical divisions. These two geological and geophysical investigations are mutually complementary and, with much greater energy output than was used in 1960, the promise is great for the determination of geological structure many thousands of feet below the sea-bed.