Basins and Troughs 



219 



Figure 1 87. Areal variation of 

 texture in Santa Monica and 

 San Pedro Basins. From Gor- 

 sline and Emery (1959, Fig. 2). 

 Top, median diameters of sur- 

 face sediments (based on about 

 260 samples), showing general 

 decrease of grain size away 

 from shore, across continental 

 and island shelves, down basin 

 slopes, and toward center of 

 basins. Data from coarsest 

 sediment omitted in interest of 

 simplification. Middle, sand- 

 shale ratio computed on the 

 basis of green mud eventually 

 becoming compacted to half 

 its present thickness. Note 

 close relationship of high ra- 

 tios to mouths of submarine 

 canyons. Bottom, average 

 thickness of sand layers in 

 centimeters, showing seaward 

 thinning. 



and the continental slope. These data in- 

 dicate that the high calcium carbonate from 

 foraminiferal tests causes an increase in the 

 median diameter of the sediments in the 

 basins farthest offshore (Group 5) and on 

 the continental slope, but the effects on 

 basins closer to shore appear to be negligible 

 owing to a smaller percentage of calcium 

 carbonate and detrital grains. Histograms 

 of sediments separated by decantation sup- 

 port the conclusions reached above (Fig. 

 189) and show in addition that grains of 

 calcium carbonate in the sediments of near- 



shore basins have nearly the same size dis- 

 tribution as do the detrital grains. This, of 

 course, suggests that much of the calcium 

 carbonate is itself detrital, reworked pos- 

 sibly from areas of shallower water. The 

 alternative possibility, that the foraminiferal 

 tests have been partly dissolved and the 

 calcium redeposited, is refuted by the ab- 

 sence of solution even on delicate tests of 

 hyaline forms and by the fact that there is 

 no marked increase in percentage of fine 

 fraction of calcium carbonate at depth in 

 the cores, even though as much as 20,000 



