NO. 1 EMERY AND HULSEMANN: SUBMARINE CANYONS 61 



SUMMARY AND CONCLUSIONS 



In many ways submarine canyons are intermediate between shelves 

 and basin floors. Their axial slopes are intermediate in steepness; thus 

 the canyons not only dissect the basin slopes but their heads extend 

 landward of the shelf break. Where the heads of the canyons are very 

 close to shore they may serve as local sites for upwelling in response to 

 the action of wind in driving surface water toward the open sea. This 

 upwelling, however, appears to be weak and probably discontinuous. It 

 does not establish a very unique ecological environment, but the minor 

 differences in the waters of canyons or basins which do exist may pos- 

 sibly be significant for some animals. 



Canyons which cross much of the width of shelves and of basin 

 slopes receive sediments in at least three different ways. Most important 

 quantitatively is grain-by-grain deposition of silts and clays carried m 

 suspension from the mouths of streams and from the turbulent shore 

 zone. When deposited, this sediment forms a homogeneous blanket of 

 green mud on the steep walls of the canyons as well as on the basin 

 slopes and floors farther seaward. The steepness of the canyon walls, 

 possibly aided by animal activities, allows the sediment to move down- 

 slope to the canyon axes. This movement not only exposes rock outcrops 

 on the sides of the canyons but also produces interbeds of the green 

 mud with coarser sediment on the canyon floors. Whether the mud 

 moves downslope slowly and continuously or rapidly and intermittently 

 is unknown. The outer parts of the canyons, the channels on the basin 

 floors, also receive the grain-by-grain deposits, but because of the gentle 

 slopes of the sub-sea aprons there probably is little mass movement of 

 this sediment. 



Second most important, but probably of greatest interest, is the depo- 

 sition of sand and fine gravels which move down coast along beaches 

 and atop the inner part of the shelves, under the influence of longshore 

 currents. These currents are partly the inshore portions of the general 

 southern California eddy but mostly they are produced by the diagonal 

 approach to shore of the dominant waves from the northwest (Emery, 

 1960a). Where canyons extend close in to shore, they serve as traps for 

 this moving sediment. The sediment may accumulate slowly until it 

 finally moves out en mass, causing a sudden deepening of the water of 

 the canyon head (Shepard, 1951a, and other papers). The moving mass 

 may become transformed into a turbidity current which carries sand into 

 deep water (Shepard, 1951b), building up sub-sea fans or aprons at 

 the mouths of the canyons (Gorsline and Emery, 1959; Emery, 1960b). 

 These sands have the same general grain size as the nearshore sediments 



