56 



Physiography 



Figure 53. Cross sections of basins showing sediment fill 

 consisting of: A, organic debris and chemical precipitates 

 from near the water surface; B, detrital sediments diffused 

 from mainland by volume suspension; C, detrital sedi- 

 ments contributed by turbidity currents from near 

 mainland. 



would be approximately true for organic de- 

 bris and some chemical precipitates, we 

 would expect the basin floors to become flat 

 eventually, except for marginal slopes con- 

 sisting of sediment that would have slid down 

 the flanks of islands and banks. Secondly, 

 clastic sediments may diffuse outward from 

 the mainland shore, float outward with fresh 

 water atop sea water, or be blown out to sea 

 by wind. When deposited, these sediments 

 should be progressively finer-grained and 

 thinner with distance from shore. The cross 

 sections of the basins would be similar to 

 that produced by deposition of organic de- 

 bris, except for a gradual slope away from 

 shore. A third form of deposition is that of 

 turbidity currents which may travel along 

 the sea floor outward from the mainland, 

 perhaps largely from the mouths of subma- 

 rine canyons, depositing their sediment as 

 extensions of the canyon mouth fans. Such 

 sediments would form a steeper slope across 

 the basin floors than would deposition from 

 the surface or from suspension throughout 

 the whole water column. 



Examination of the contours of the basins 

 shows that the floors of the outer basins have 

 a more or less symmetrical cross section 

 which fits either deposition of organic debris 



or deposition from general suspension (the 

 first two forms of deposition). This conclu- 

 sion is supported by the known high content 

 of planktonic tests and debris in these ba- 

 sins. The floors of some of the nearshore 

 basins, however, are quite different. The 

 flat bottoms of the Santa Monica and San 

 Pedro Basins have a gentle seaward slope 

 (0.3°) (not obvious from the 300-foot con- 

 tours of Chart I). Sediment appears to form 

 a southward slope into the San Diego Trough 

 (0.4°) and westward into Santa Catalina 

 Basin (0.4°) as though turbidity currents 

 from the north (perhaps also partly from San 

 Pedro Basin) divide, some flowing southward 

 and some westward (Fig. 54). The bottom 

 of the western end of Santa Catalina Basin 

 is slightly steeper, as though only a small 

 amount of sediment has been able to reach 

 that far. The San Diego Trough also has a 

 steep slope (2.0°) which extends southward 

 out of the basin and into part of the border- 

 ing San Clemente Basin. It is therefore 

 probable that sediment in these nearshore 

 basins has an additional form of deposition 

 to that in the offshore basins. If turbidity 

 currents are in reality the chief source of 

 sediments in the nearshore basins, the islands 

 and banks form effective barriers preventing 



Figure 54. Source of bulk of sediment in marine basins. 

 Solid lines show axes of canyons, dashed lines show 

 probable routes followed by turbidity currents, and S indi- 

 cates that most of the fill is from suspension in the water 

 column or from near the water surface. 



