Continental Slope 



57 



the turbidity currents that start near the 

 mainland from reaching the offshore basins. 

 Turbidity currents from island and bank 

 sources are probably too small to exert much 

 effect on the topography of the offshore ba- 

 sins. The net result is that all three modes 

 of deposition are important in nearshore 

 basins, whereas only the first two are impor- 

 tant in offshore basins and one of these is 

 even less important than in nearshore basins. 

 These relationships may easily account for 

 the fact that the rate of deposition is faster 

 in nearshore than in offshore basins. 



Continental Slope 



A length of about 200 miles of continental 

 slope lies off southern California (Chart I) 

 at a distance of 50 to 160 miles beyond the 

 shoreline. The general trend is N25°W, 

 about the same as its continuation farther 

 north and south. Five gaps are present, none 

 of which is related to known submarine can- 

 yons; therefore, they may be of tectonic 

 origin. The northernmost, west of San Mi- 

 guel Island, appears in fact to represent a 

 slight landward offset of the continental 

 slope. 



Off southern California the average width 

 of the continental slope is about 10 miles and 

 the depth at its base is about 12,000 feet. 

 Its profile shown by contours is nearly 

 straight, but where more detailed soundings 

 are available (Fig. 55) small irregularities are 

 shown. At its base the continental slope 

 locally descends into shallow depressions 

 that have the appearance of remnants of a 

 nearly filled deep-sea trench (Fig. 56). The 

 straightness of profile and the absence of an 

 apron at the base show that its physiographic 

 age, according to Dietz' (1952) classification, 

 is initial. To the south, off Cedros Island, 

 Mexico, and farther south the depression is 

 deeper and much longer (Heacock and Wor- 

 zel, 1955; Fisher and Shor, 1956). Farther 

 north, off San Francisco, the basal depression 

 has been replaced by an apron, and the 

 physiographic age is late youth. Thus the 

 base of the continental slope off southern 

 California is intermediate in character be- 



tween that to the south and that to the north. 



Trenches are much more highly developed 

 along island arcs of the western Pacific 

 Ocean, where the supply of sediment is ex- 

 tremely small. They may have been formed 

 by downbuckling of the crust on compres- 

 sional downfolding (Vening Meinesz, 1954; 

 Hess, 1948) or by compressional faulting 

 (Emery, 1950fl). Recently, however, Worzel 

 and Ewing (1954) have shown that the large 

 negative gravity anomahes associated with a 

 trench in the West Indies are due to great 

 thickness of sediment, rather than to local 

 thickening of the crust by downfolding or 

 ramp faulting. This may also prove to be 

 true off southern California when suitable 

 measurements are made. Where the rate of 

 deposition of sediments in the trenches is 

 slow, as off island arcs, the trenches are 

 very deep, but where sedimentation is rapid 

 the trenches may become filled faster than 

 they are formed. If the rate of trench for- 

 mation is about equal off northern Califor- 

 nia, southern California, and Mexico, the 

 presence of an apron at the base of the con- 

 tinental slope in northern California indi- 

 cates that sediments are being deposited 

 rapidly, whereas the presence of a trench at 

 the base as off Mexico indicates that sedi- 

 ments are being deposited slowly. The pres- 

 ence of neither a thick apron nor a deep 

 trench off southern California means that 

 sediments are being deposited about as fast 

 as the trench is being formed by folding or 

 faulting. Fast sedimentation off northern 

 California is promoted by the high coast and 

 perennial streams, sedimentation off south- 

 ern California is limited by trapping of most 

 of the sediment in the offshore marine ba- 

 sins, and very slow sedimentation off Mexico 

 is caused by the barrier provided by the 

 peninsula of Baja California as pointed out 

 by Menard (1955). 



Hypotheses for the origin of the continen- 

 tal slope have included warped peneplain, 

 steep normal step faults, gentle normal fault, 

 delta-Hke foreset beds, and prograding of 

 sediments over a subsiding continental mar- 

 gin (Shepard, 1948, p. 191; Kuenen, 1950, 

 p. 160). The finding of bedrock at several 

 points on and near the top of the continental 



