Composite Origin of Submarine Canyons

Abstract

The investigation of submarine canyons is still in its infancy, and complete reversal of ideas may come from further studies. However, the information which has become available in recent years offers an explanation which seems to eliminate the necessity of calling upon the inadequately supported hypotheses of submarine erosion or huge sea-level changes. The new explanation includes: subaerial excavation of portions of the canyons at various times during the past when the margins were elevated above the ocean level; building of deltas on the outer slopes during this excavation; submergence of the canyons, with some accompanying fill but with preservation of the canyons through slides and turbidity currents acting along the old canyon axes; reshaping of the old deltas, with some enlargement of shallow trenches characteristic of steep delta fronts; marine deposition on the old land surfaces into which the canyons were cut but concurrent maintenance of the canyons by slides, so that the canyon walls grew higher as the submergence continued; and, finally, the reshaping of the canyon heads by the Pleistocene sea-level changes of a few hundred feet. This composite sequence is supported in part by the detailed soundings obtained recently off the San Diego area, where delta-like features have been found out beyond the rock-walled canyons. These supposed deltas are trenched by marginally rirnmed valleys similar to those of the Swiss lakes. The hypothesis is also supported by new evidence showing the adequacy of submarine mass movements in keeping open the canyon heads. Finally, the evidence for large-scale submergence of continental margins and of oceanic islands is becoming continually more complete. On the other hand, the new reasons offered for supporting submarine origin of the canyons are shown to be based on misunderstanding of the character of the canyons. The advocates of submarine erosion continue their failure to observe what is actually taking place on steep aqueous slopes where sediment is available for the erosion they infer.