228 



Sediments 



Figure 195. Sand-shale ratio of sediments in present basins and in the now-filled Los Angeles and Ventura Basins. 

 In order to make the two sets of data comparable, the thickness of "shale" in existing basins was taken as half the 

 thickness of the uncompacted green muds. Note the restriction of high sand-shale ratios to nearshore basins and their 

 close relationship to submarine canyons. Compiled in part from Slosson (1957), Gorsline (1958), and Baldwin (1959). 



The accumulated data verify earlier con- 

 clusions that organic matter is present in the 

 basins adjacent to the mainland in percent- 

 ages equal to or shghtly greater than per- 

 centages in shales of the now filled Los 

 Angeles Basin. Farther off'shore, in Santa 

 Catahna, Santa Cruz, San Nicolas, and Tan- 

 ner Basins (Table 14), organic matter rises 

 to the highest percentages found in the en- 

 tire continental borderland. Still farther 

 seaward organic matter decreases in the 

 outermost basins, and this decrease is con- 

 tinued by the very low percentages found in 

 sediments on the continental slope and the 



deep-sea floor beyond. The peak concentra- 

 tion of organic matter at intermediate dis- 

 tance from the mainland is attributed to a 

 very rapid rate of deposition of detrital sedi- 

 ments near shore, resulting in the dilution or 

 masking of organic matter there, and to a 

 very slow rate of deposition far from shore, 

 resulting in extensive oxidation of organic 

 matter before it can become protected by 

 burial. Thus the highest concentration oc- 

 curs under optimum conditions of neither 

 too rapid nor too slow a rate of deposition 

 of detrital sediments. 



Viewed in vertical section, the organic 



