No abstract

Summary On the basis of the measured seismic velocities, the layering found at 25 seismic stations in the area of the Western Approaches of the English Channel is divided into four classes which are respectively correlated with semi-consolidated Cretaceo-Tertiary sediments, the New Red system, the Palaeozoic system, and a metamorphic basement. The last of these appears to form a long, deep trough. The Palaeozoic floor is depressed in a trough of somewhat variable depth, bounded on the north by an outcrop of the metamorphic basement which is possibly a westward extension of the upthrust Lizard-Start metamorphic belt. Contour maps of the layering are produced.

The seamounts of the deep oceans are strongly magnetized and are made up of basic volcanic rocks with or without a capping of limestone. Some seamounts of the continental margins differ from these by being weakly magnetized. Three such seamounts are described. Magnetic surveys over them strongly suggest that they do not have cores of basaltic rocks. There is good evidence that pellet limestones are exposed in the flanks of one of them (Vigo Seamount). These rocks resemble pellet limestones of Jurassic age which occur in Portugal. The results of dredging and photography on the top of Vigo Seamount and on Galicia Bank suggest that detrital limestones, porcellanous limestones, and soft foraminiferal limestones, of Cretaceous to Eocene age, are exposed there. The facies of these calcareous rocks resembles that of the Mesozoic limestones of Portugal and southern France. The remainder of the dredged rocks are believed to be ice-rafted erratics from farther north in Europe or America. The results of seismic refraction work on Galicia Bank are consistent with a structure in which 0.4 km of uncemented limestones overlie 4.2 km of hard limestones that rest on a crystalline basement. The mode of origin tentatively suggested for the seamounts involves the formation of limestones during Mesozoic time on a gradually subsiding platform of continental rocks. As there is no substantial gravity anomaly, subsidence must have been accompanied by density changes or by removal of the lower part of the continental crust near the present position of the continental margin.

A series of seismic refraction profiles has been shot between Kenya and the Seychelles Bank and in the neighbourhood of the Bank itself. Thick sediments have been observed for 300 to 400 km from the African coast. Near Kenya, great thicknesses of material of about 4.8 km/s velocity match closely the 9 to 10 km of Karroo beds expected on the coast at Lamu. The Mohorovicic discontinuity has been traced from 100 km off the African coast to the Seychelles Bank. West of the Bank the mantle is unusually shallow, rising to only 8.5 km below the surface, and the 6.8 km/s crustal layer unusually thin or absent. The absence of a gravity anomaly associated with this very shallow mantle raises a problem which has yet to be resolved.

Summary Recent geological information obtained by core sampling in the English Channel off the South Devon coast is set forth. This information has been combined with the results obtained from a seismic survey along a line southwards from Plymouth. The evidence from these two sources points to the existence of a trough filled with New Red Sandstone deposits which extends from within a few miles of the English coastline to a point south of the middle of the Channel. The thickness of these deposits reaches approximately 3000 feet. The line of reefs formed of metamorphic rocks extending from Bolt Tail to the west of the Eddystone projects through the New Red Sandstone formations, which appear to be swamping an irregular topography. Southwards from the Eddystone the breccias and sandstones of the Permian and Trias are followed by Keuper Marl and a small thickness of Lower Jurassic formations. These in turn are covered unconformably by a few hundred feet of Chalk.