Sub-bottom reflection measurements on the continental shelf, Bermuda banks, West Indies arc, and in the West Atlantic basins

During a recent cruise of R.V. Vema, a short survey was made with the sub-bottom depth recorder in the vicinity of the Hudson Canyon between the 40-fathom and the 400-fathom contours. The results show that on this part of the continental shelf the upper layers are approximately horizontal, whereas the deeper ones dip seaward at a slightly greater angle than the bottom. Several reflecting horizons were observed at depths as great as 400 feet below the bottom, and many were traced for distances of the order of 10 to 20 miles. Where the canyon cuts through the sediments of the shelf, several of the sub-bottom horizons outcrop on the canyon walls, sometimes correlating with benches or knees. Some of the horizons at the outcrops are practically horizontal, in particular those on the northeast wall; others, notably the deep ones on the southwest wall, are sloping, tending to follow the contour of the canyon wall. Sediment cores were taken in a few locations, providing some information about the reflecting horizons (reflectors). On Challenger Bank and on Plantagenet Bank near Bermuda, no sub-bottom reflectors were found. On the West Indies Arc near Puerto Rico and the Virgin Islands, shallow reflectors were seen in some areas. During a run from New York to Bermuda to Puerto Rico, sub-bottom reflectors were seen on the echo-sounder fathogram at least 40 per cent of the time. The best results were obtained on the topographic rises, particularly on the outer rise north of the Puerto Rico Trench.