Abstract

Recent surveys and sampling of the V-shaped canyon that cuts into parts of the broad troughs separating the Bahama Banks have given a greatly improved picture of this gigantic valley and the processes operating to shape it. The canyon has two major branches, one following Northwest Providence Channel and the other the Tongue of the Ocean, which join 15 mi north of New Providence Island, and continue seaward as a submarine canyon with walls almost 3 mi high. These, so far as we know, are the world's highest canyon walls (either submarine or subaerial), and the canyon length, including the branch in Northwest Providence Channel, is at least 150 mi, exceeded only by two submarine canyons in the Bering Sea.

Bottom photographs from the outer portions of Northwest Branch and Tongue Branch show wall rock, rounded cobbles, and boulders along their axes, as well as ripple-marked sand to indicate the importance of currents moving along the canyon floor. Further evidence that erosion has at least kept the valleys open as the Bahama Banks grew comes from the winding courses and the numerous tributaries that descend the walls from the shallow Banks, particularly on the south side of Northwest Branch. The possibility that limestone solution has been important comes from the finding of more depressions along Northwest Branch than in other submarine canyons of the world, and the discovery of caverns along the walls by observers during deep dives into Tongue Branch in the Alvin and Aluminaut. It seems to us highly probable that the modern canyons are due primarily to submarine erosion, partly re-excavating old filled troughs.

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