218 



Sediments 



similar to sediments on basin slopes. Many 

 of the more numerous cores along the can- 

 yon axes, however, contain sand layers, 

 some of which are so thick and hard that 

 core barrels have been bent on striking bot- 

 tom. The sand layers in canyons that reach 

 close to shore have textural and composi- 

 tional similarities to beach sands and sedi- 

 ments of the nearshore parts of the shelf, as 

 recognized long ago by Ritter (1902). 

 Coarse sediments on the floors of canyons 

 having heads farther from shore are more 

 like the sediments of the outer part of the 

 shelf, with common shell fragments and 

 glauconite. The sequence of fine- and 

 coarse-grained sediments in the canyon axes 

 is probably a result of intermittent deposi- 

 tion of turbidity currents produced by mass 

 movements at the head of the canyons, as 

 discussed later. Included in the sediments 

 of both kinds of canyons, but more abundant 

 in canyons heading far from shore, are 

 pebbles and cobbles similar to strata that 

 constitute the canyon walls. Some pieces 

 of rock were probably carried away by 

 small mass movements of sediment, but 

 others were probably dislodged by burrow- 

 ing activities of organisms (Limbaugh and 

 Shepard, 1957). 



Basins and Troughs 



A. Areal variation of grain size and per- 

 centage of calcium carbonate. More than 

 600 samples from about 250 stations in the 

 various basins and troughs have been ana- 

 lyzed in the laboratory. These samples show 

 that the bulk of the sediment consists of 

 detrital materials and calcium carbonate of 

 foraminiferal tests with only minor amounts 

 of authigenic minerals and organic matter; 

 thus grain size is controlled by the detrital 

 and calcium carbonate fractions. Within 

 individual basins there is some variation in 

 grain size that is related to proximity to the 

 basin side slopes (Fig. 187 top) and a greater 

 variation that results from changes at depth. 

 Owing to these variations and possibly also 

 to alteration of the samples during prepara- 

 tion for analysis, the regional pattern of 

 grain size is not easily recognized. A basin- 



by-basin summary (Table 14) reveals no 

 clear-cut trend of grain size, but when the 

 basins are grouped into five series at pro- 

 gressively greater distances from shore and 

 compared with the continental slope and 

 deep-sea floor west of San Nicolas Island, 

 the average median diameters may be seen 

 to present the following general changes by 

 group: (1) 14.0, (2) 6.5, (3) 4.0, (4) 4.0, (5) 

 4.6, (6) 7.8, and (7) 1.8 microns. Thus the 

 grain sizes of whole samples decrease from 

 the Los Angeles Basin to the middle basins, 

 increase to the continental slope, and de- 

 crease again on the deep-sea floor. This is 

 exactly the change that we would expect 

 from an observed progressive increase in 

 percentage of coarse foraminiferal tests of 

 sediments in an offshore direction. We 

 should also expect that removal of calcium 

 carbonate by acid treatment of the samples 

 would reveal that the insoluble residue does 

 not increase in grain size near the continen- 

 tal slope. The corresponding average median 

 diameters of insoluble residues are (1) none, 

 (2) 8.1, (3) 9.4, (5) 6.7, (6) 6.7, and (7) 2.9 

 microns, a series that indicates a more uni- 

 form decrease of diameter with distance 

 from shore than that exhibited by analyses 

 of the whole samples. Comparison of the 

 two sets of diameters (for whole samples 

 and for insoluble residues), however, shows 

 that the insoluble residues for most of the 

 areas are coarser than the whole samples. 

 Whether this is true, whether the acid treat- 

 ment enlarged the grains (although floccula- 

 tion did not occur), or whether the number 

 of samples is too small to be statistically 

 valid (121 insoluble residues versus 608 

 whole samples) is not clear. 



Another way of investigating the effects 

 of calcium carbonate on grain size of whole 

 samples is illustrated by Figure 188, a plot 

 of median diameter against percentage of 

 calcium carbonate. With a continuous sea- 

 ward increase in percentage of calcium 

 carbonate, the median diameters of whole 

 samples decrease from the Los Angeles 

 Basin through the nearshore basins (Groups 

 2 and 3 of Table 14) to minimum diameters 

 in the offshore basins followed by an in- 

 crease in diameter in the outermost basins 



