240 



Sediments 



being dependent on productivity, depth of 

 water, rate of deposition of other sediments, 

 and nature of interstitial water. The roles 

 of these factors in controlling the amount 

 and composition of organic matter will be 

 discussed in a later section. Radiocarbon 

 age measurements indicate that the organic 

 matter accumulates at an average rate of 

 about 1.5 mg/sq cm/yr at the basin floors. 

 Chemical precipitates are recognizable 

 chiefly in the areas of slow deposition, where 

 glauconite, phosphorite, and manganese ox- 

 ide are characteristically present. Either 

 these minerals do not form in the environ- 

 ment of the basin floors or they form such 

 tiny grains so diluted by other sediments that 

 they have neither been recognized nor sepa- 

 rated from the other sediments. Even in the 

 areas dominated by them, these authigenic 

 sediments accumulate at a nearly negligible 

 rate. Possibly more important in the basins, 

 but impossible to evaluate at present, is the 

 direct chemical formation of clay minerals 

 in sea water. Goldberg (1954) suggested this 

 as the origin of some of the clay minerals in 

 the deep-sea sediments, but it must be far 

 less important in the continental borderland 

 where the nearby land is a ready source of 

 clay minerals. 



Transportation 



Most of the sediments of the continental 

 borderland have been derived from land; 

 thus the methods by which they are trans- 

 ported from shore to final resting places are 

 of prime importance to an understanding of 

 them. There are only four possible zones of 

 transportation of sediments in the sea: over 

 the water, on the water, through the water, 

 and under the water. Transportation over 

 the water is of course the method for wind, 

 whereby fine sediments can be carried in 

 suspension far out to sea and coarser ones 

 moved in saltation must be left at the shore- 

 line. 



Several agents can transport sediment on 

 the water. Most important of these quanti- 

 tatively is the flotation of sediment-laden 

 fresh water atop the denser (higher salinity 



and lower temperature) sea water. After 

 heavy rains this "epithalassis" spreads its 

 brown color many miles offshore. The layer 

 is thin and sharp-bottomed, as indicated by 

 measurements of light transmission at vari- 

 ous depths and by the fact that boats tra- 

 versing it stir up the underlying clear water 

 and leave a semipermanent dark trail 

 through the discolored area. Secchi disk 

 readings in an area where 10 meters is nor- 

 mal may be reduced to 2 meters or less. 

 Excellent photographs of the discolored 

 areas are given by Bell (1942) and Natland 

 and Kuenen (1951). Eventual mixing with 

 the underlying sea water causes the fine- 

 grained sediments to become flocculated so 

 that clusters settle out of the layer and to the 

 bottom. Other agents of transportation at 

 the surface of the water are most eff'ective 

 for coarse sand, pebbles, and cobbles. AU 

 these are forms of rafting: kelp, sea lions, 

 and driftwood (Emery and Tschudy, 1941; 

 Emery, 1941Z>, \955b). The quantities of 

 sediment moved are small compared with 

 those carried by floating fresh water, but 

 rafting has the unique ability of explaining 

 the occasional presence of erratic pebbles 

 and cobbles in otherwise fine-grained sedi- 

 ments. Two pebbles with remnants of kelp 

 holdfasts still attached were dredged with 

 mud from a depth of 220 meters south of 

 Palos Verdes Hills, and other such erratic 

 pebbles are well known in Paleozoic and 

 other shales of the United States and else- 

 where. Sea lions carry as much as 5 kg of 

 pebbles and cobbles, some of which are up 

 to 10 cm in diameter. Rock identifiable as 

 to source almost invariably shows local deri- 

 vation and presumably therefore a short dis- 

 tance of transportation by the sea lions. 



Probably even more important is transpor- 

 tation through the water in a sort of diffusion 

 action. Sediment stirred up by the waves 

 or contributed by streams is coarsest and 

 most concentrated in shallow areas where 

 waves break and create high turbulence. 

 Marlette (1954) sampled the zone from shore 

 outward about 55 meters to a depth of 1.7 

 meters by wading out with empty quart 

 Mason jars and then opening and recapping 

 them at measured depths and distances from 



