Ocean Basin Structure as Revealed by Seismic Profiler Studies

Abstract

A new technique for making continuous reflection profiles in the oceans has been developed in which reflection shots are recorded in a manner analogous to that employed in standard echo sounding equipment. This provides automatic correlation and integration of arrivals and makes possible rapid examination and analysis of large volumes of data. During recent months, several thousands of miles of track, mostly in ocean basin areas, have been studied with the Seismic Profiler. The deepest reflector seen in the Profiler records is the upper surface of a layer with seismic velocity 4.5–5.5 km/sec. This reflecting surface is very uneven. In large areas of the ocean basins, it is almost entirely devoid of sedimentary cover, except in pockets, and its roughness is accurately expressed by the sea floor topography. In other areas, principally in the abyssal plains and on the continental rises, the relief in this layer is no different, although it is obscured by up to a few kilometers of flat lying sediments. A prominent reflector within the sedimentary column is seen throughout the latter areas, usually at a depth of about 400–500 m below the sea floor. Its smooth, flat surface which is comparable to the present sea floor in abyssal plains, indicates that turbidity current deposition has long been the dominant process in deep sea sedimentation. The universal roughness of the top of the 4.5–5.5 km/sec layer and the absence of reflections from beneath it are taken as evidence that the layer is part of the original oceanic crust, possibly a less dense differentiate of the crustal material. This opposes the view held by some that the layer consists of older sediments, possibly covered by lava flows.