GEOPHYSICAL INVESTIGATION OF THE CONTINENTAL MARGIN BETWEEN CAPE HENRY, VIRGINIA, AND JACKSONVILLE, FLORIDA

Abstract

Forty seismic-refraction and reflection profiles on the eastern continental shelf and adjacent deep-water areas of the Atlantic Ocean, from 29°39′ to 36°30′ N. Lat. and 73°30′ to 81°10′ W. Long, trace the transition from deep-oceanic to continental-type structures. The transitional area divides naturally into three parts: the continental shelf, the Blake Plateau, and the adjoining deep-water area. The results on the continental shelf are correlated with adjacent continental geology. The deepest horizon traced along the shelf is interpreted as granitic basement, which has compressional velocities of 5.82–6.1 km/sec. At the southern extremity it is at a dept of 6 km, shoals to 0.86 km near Cape Fear, and deepens north of Cape Hatteras to more than 3 km. North of Charleston, South Carolina, there is excellent depth correlation with granitic basement in coastal wells; to the south all deep wells are inland. Age correlations are based on well data near the coast, which indicate to us that most of the observed section is Cretaceous. On the Blake Plateau, several layers (1.83–4.5 km/sec.) are interpreted as sedimentary. A 5.5-km/sec. layer is found only south of a line from 30°30′N., 78°W. to Cape Canaveral. Velocities higher than 5.5 km/sec, have been measured on six profiles on the Blake Plateau. The 5.5-km/sec. layer and a 6.2-km/sec. layer appear to form a positive feature to the south of the above-mentioned line. Higher velocities, 8.0 km/sec, and 7.28 and 7.3 km/sec., which are probably not the same horizon, are found at markedly different depths. Possibly these represent the M layer and ultrabasic material, depending on relations not now known. The deep-water area is a continental slope and rise modified by the Blake Plateau and by a ridge trending southeastward from Cape Fear and deepening from about the 1500-fathom contour to more than 2000 fathoms (3657 m). The ridge is underlain by thick low-velocity layers (1.83–2.96 km/sec.), interpreted as sediments, and higher-velocity layers which form a distinct linear structure having the same general trend as the ridge. At its northwestern end this trend terminates against a thick lower-velocity section interpreted as a sediment-filled trough. South of the ridge profiles are similar to those of the ocean basins. Excellent seismic-refraction evidence of faulting indicates subsidence of the ridge relative to its surroundings. A hypothesis interprets the ridge as a former chain of islands and reefs on a structural trend colinear with the Cape Fear Arch. The structural pattern formed by the Piedmont crystallines and the Peninsular Arch of Florida and the Cape Fear Arch and the ridge resembles that formed by the Japanese archipelago, the Ryukyus, and the Bonin ridge. The pattern of the trough of the Blake Plateau and the deep-water area is somewhat similar to such modern features as Exuma Sound and the Tongue of the Ocean in the Bahamas. Such comparisons should not be regarded as strictly homologous but are suggested as possibly reflecting similarities in deep-lying tectonic activity. Similarly it would seem fruitful to consider similaritiesof deep structure beneath isolated seamounts, chains of seamounts, submerged ridges, island arcs, and mature mountain ranges.