Newport Submarine Canyon, California: An example of the effects of shifting loci of sand supply upon canyon position

Abstract

Newport Submarine Canyon, located off Newport Beach, California, is typical of the small shelf canyons of the California continental borderland. The canyon has formed at a point where the shelf is narrow and sand moved by waves in longshore drift is concentrated at a convergence zone. This situation has apparently existed for much of the Pleistocene although the loci of sand concentration have shifted in response to changes in position of stream input, or local coastal configuration due either to sea level changes or as a result of coastal erosion or deposition. The present topographic expression of the submarine canyon represents its most recent position, but one that is now in process of abandonment as a result of shifts in loci of sand input. The Santa Ana River is the principal local sand source. Changes in the location of the river mouth during the past century have shifted the point of longshore drift convergence and sand concentration approximately one kilometer northwest of the present canyon head. The canyon head is now receiving only fine organic-rich sediments that are building an apparently stable deposit that will ultimately fill the head. The sand deposition necessary for canyon development is now being distributed as a lobe over the shelf north of the canyon and this lobe is just beginning to extend over the local shelf edge. Thus, the locale illustrates the process of canyon origin, development and ultimate abandonment. Evidence from local bathymetry and sedimentary distributions on the shelf and slope suggest that the cycle also occurred in Holocene and Late Pleistocene time and that the canyon has shifted laterally several times in response to the changing point of input of sand to the canyon system. Newport Canyon is therefore an exception that proves the rule for the current theories regarding small coastal submarine canyon development by mass movements of sand as described by Dill, Chamberlain and others over the past five years.