STRUCTURE OF THE LOWER CONTINENTAL RISE HILLS OF THE WESTERN NORTH ATLANTIC

Data acquired during a recent seismic reflection survey of portions of the western North Atlantic have provided new information on the movement and deposition of sediments on the ocean floor. The area surveyed included portions of the lower continental rise hills which form the transition between the continental rise and the abyssal floor east of Cape Hatteras. The lower rise hills are ridges having a general east‐west orientation and an average length of 20 miles. Individual hills have 25 to 50 fathoms relief and are two to three miles wide. Each of the hills has a relatively short, steep out‐crop slope facing seaward and a shallow dip slope. Shallow subbottom reflectors dip opposite to the regional slope; deep reflectors are structurally independent of the upper reflecting layers. In profile, the ridges appear to be imbricate blocks with a maximum thickness of 100 fathoms. Ridges become progressively smaller seaward and are eventually onlapped by the horizontal beds of the abyssal floor. No fault structure is visible. The imbricate structure of individual hills, dip reversals between the continental rise and lower rise hills, dip reversals between the lower rise hills and the abyssal floor, and the apparent recent denudation of sediment cover from the continental rise indicate that the lower rise hills are gravitational glide blocks. Published literature on the engineering properties of marine sediments illustrates progressive gravitational gliding on slopes as low as 1:100.