242 



Sediments 



grains are flakes of biotite, thus pointing out 

 the route by which the abundant biotite in 

 nearshore basins makes its way there. Con- 

 ceivably this suspended sediment can be car- 

 ried by currents throughout the area of the 

 nearshore basin before its settUng velocity 

 will allow it to reach bottom. 



Lastly, much sediment is transported sea- 

 ward at the bottom in the form of turbidity 

 currents. Evidence presented in other sec- 

 tions shows that the submarine canyons 

 which reach close inshore intercept sands 

 that travel along the beaches; the canyons 

 thus divide the mainland beaches into a 

 series of semi-independent sections (Fig. 28). 

 Finer sediments that move longshore atop 

 the inner half of the continental shelf are also 

 intercepted. High sand-shale ratios in the 

 bottom sediments of nearshore basins reveal 

 the resting place of these sandy sediments 

 by their concentric pattern around the 

 mouths of the submarine canyons that de- 

 bouch at the sides of the basins (Figs. 194, 

 195). In these areas the sands are thicker 

 than elsewhere and the topography has the 

 shape and slope suggestive of alluvial fans, 

 subsea fans. Similar deposits of Pliocene 

 age are known in the now-filled Los Angeles 

 and Ventura Basins (Natland and Kuenen, 

 1951; Slosson, 1958; Conrey, 1959; Baldwin, 

 1959), and these basins commonly have 

 graded bedding as do the sea floor deposits. 

 They also have deep-water current ripples 

 which have not yet been detected on their 

 sea floor counterparts. Although turbidity 

 currents have not been seen in action off" this 

 coast, these pieces of circumstantial evidence 

 are very persuasive of sediment transporta- 

 tion in the form of turbidity currents. Both 

 sedimentary and physiographic evidence in- 

 dicates that turbidity current deposits occur 

 in every basin off the coast but that they are 

 far more important in the nearshore Santa 

 Monica and San Pedro Basins and the San 

 Diego Trough than elsewhere. The high 

 sand-shale ratios in Santa Monica and San 

 Pedro Basins show that the volume of sedi- 

 ment deposited in them by turbidity currents 

 is about equal to that contributed by all 

 other means, chiefly diff'usion through the 

 water column. Not until the two basins be- 



come filled to their sills can turbidity cur- 

 rents move completely through them and 

 reach the next basins farther seaward (Fig. 

 54). This is the situation in the now-filled 

 San Diego Trough which is spilling turbidity 

 current sediments into the adjacent San 

 Clemen te Basin (Fig. 201). 



Deposition 



The composition and grain size of accum- 

 ulated sediment are not a simple function of 

 derivation, transportation, and deposition. 

 Instead, there is a complex interplay of 

 sources, one dominant here and another 

 there. Transportation may be by several 

 agents, each separated by shorter or longer 

 periods of rest followed by physical or chem- 

 ical reworking. We can cite examples of 

 complexity in sediments of many kinds of 

 areas, but it is especially well documented 

 for the sea floor off southern California. 



Most detrital sediments reach the ocean 

 from land in a wide spectrum of grain sizes. 

 In general, the beaches act as a sort of filter 

 that allows the finer sizes to continue sea- 

 ward and retains the coarser sizes until they 

 too are worn fine enough to be transported 

 seaward in suspension, or, in a few special 

 cases, such as mass movements and turbidity 

 currents in submarine canyons, they may be 

 carried seaward en mass as sands. At the 

 same time that detrital sediments are being 

 transported seaward from land, even less 

 well-sorted sediments are extracted from 

 solution in the water. The coarsest of these 

 are mollusk shells grown on the bottom; the 

 most abundant are foraminiferal tests grown 

 throughout the overlying water column, and 

 probably the finest is organic matter formed 

 by phytoplankton living near the water sur- 

 face. All these organic materials experience 

 changes parallel to but diff'erent from those 

 undergone by detrital sediments. These 

 changes consist of chemical alteration by 

 herbivores, carnivores, and scavengers, and 

 by simple contact with sea water. Mechani- 

 cal comminution also occurs through organic 

 activities and water motions. Again, the 

 sediments undergo a filtering action by water 



