216 



Sediments 



NUMBERS 



OiOtOtOk 



I I I I 



Figure 186. Sediments from 

 Palos Verdes Slope, north side 

 of San Pedro Basin (profile D 

 of Fig. 43) (Emery and Terry, 

 1956, Fig. 6). Top, grain-size 

 composition of surface samples 

 and of ones at a core depth of 

 54-60 inches (137-152 cm), or 

 an in situ depth of 108-120 

 inches (274-305 cm). Middle, 

 relationship of water content 

 of sediments to Atterberg 

 limits. Note that the sediments 

 have such a high relative water 

 content that they must be clas- 

 sified as hquids, neglecting 

 thixotropic effects. Bottom, 

 positions of samples on the 

 profile of slope. 



these samples are probably more typical of 

 shelves than of continental slopes, they will 

 not be further described. Cores from the 

 lower parts of this slope also contain glau- 

 conite but are fine-grained, about 9 microns, 

 and contain an average of 22 per cent of 

 calcium carbonate. The deepest core con- 

 tains less calcium carbonate than the others, 

 owing probably to the dissolving action of 

 deep-sea water. The second area of investi- 

 gation is west of Northeast Bank where two 

 long cores have been taken chiefly for study 

 of organic constituents. Because this area 

 is about 150 km farther from the mainland 

 shore than the area studied by Dill, the rate 

 of deposition of detrital sediments is less, 

 causing the calcium carbonate to be more 

 than double that in the first area, 47 per 

 cent. On removal of all calcium carbonate 

 by acid treatment, the average median di- 

 ameter decreased from 8 to 7 microns, 

 showing that the foraminiferal tests, which 



make up most of the carbonate, are coarser 

 than the detrital sediment from shore. By 

 strict definition (Revelle, 1944, p. 16) this 

 sediment and that of some of the outer 

 basins should be termed Globigerina ooze 

 because calcium carbonate as foraminiferal 

 tests constitutes more than 30 per cent of 

 the sample. However, the gradational na- 

 ture of the sediment from the more typical 

 terrigenous green muds closer to shore sug- 

 gests that the term calcareous green mud is 

 more appropriate for this particular study. 

 A unique kind of sediment, barite con- 

 cretions, was found on the steep basin slope 

 of San Clemente Island in a single dredging 

 made in 1938 (Revelle and Emery, 1951). 

 Enclosed Foraminifera show that the con- 

 cretions were formed in Pliocene sediments 

 or more probably in Recent sediments con- 

 taining many reworked Pliocene Foramini- 

 fera. Barite occurs in amounts of up to 77 

 per cent and consists mostly of small radial 



