202 



Sediments 



COARSE 

 FRACTION 



ROCK FRAGMENT 

 BIOTITE 



OUARTZ-FELDSPAR 

 GLAUCONITE 



% CALCIUM CARBONATE 



50 



119°40' 



Figure 178. Sediments of main- 

 land shelf off Santa Barbara. 

 Contours of percentage calcium 

 carbonate and of organic mat- 

 ter (1.7 X organic carbon) are 

 based on samples indicated by 

 small circles. Maps of grain 

 size of total sediment and of 

 composition of coarse fractions 

 are based on approximately 

 twice as many samples. Dashed 

 line is 100-meter contour. 

 Compare with benthic faunal 

 distribution of Figure 144. 



hills, abundance of calcareous organisms, 

 recent explosive vulcanism, and presence of 

 lag materials. Although these factors can 

 be determined for the present ocean floor in 

 well-known areas, they usually cannot be 

 estimated for ancient strata. 



As a further step toward reaching an un- 

 derstanding of the distribution of sediments 

 and its causes, Emery (1952^?) proposed that 

 samples be classified according to the dom- 

 inant cause of deposition and that sediments 

 of similar origin be mapped as units. The 

 major kind of sediment found on continental 

 shelves is usually the present-day detrital 

 material contributed to the ocean mostly by 

 streams and sea cliff's. Having a similar 

 origin but a longer and different history are 

 relict sediments, material once deposited on 

 the sea floor in equilibrium with then-exist- 

 ing waves and currents but now above or 

 below that equilibrium position, owing to 

 relative change of sea level. A third kind 

 of sediment is residual inorganic debris left 

 from in situ weathering of rocks cropping 

 out on the sea floor; the relative roles of ex- 

 isting submarine and former subaerial 

 weathering cannot usually be determined. 

 Sediments of organic origin, a fourth type, 

 consist of calcareous foraminiferal tests and 

 shell fragments, siliceous skeletal debris, and 

 organic matter. Fifth is chemical precipi- 

 tates, or authigenic sediments, chiefly 

 glauconite, phosphorite, and manganese 

 oxide. In some regions other types of sedi- 



ments occur in amounts large enough to be 

 worthy of consideration. Among them are 

 rafted sediments. Since ice is the chief mass 

 rafting agent, these sediments are largely re- 

 stricted to high latitudes and probably never 

 occurred in southern California even during 

 the Pleistocene Epoch. Another unrepre- 

 sented kind of sediment is volcanic ash and 

 cinders, which in Japan's Kagoshima Bay, 

 for example, covers many hundreds of 

 square kilometers. The five most important 

 kinds of sediment in southern California 

 (present-day detrital, relict, residual, organic, 

 and authigenic) occur in varying proportions 

 on the mainland shelves, island shelves, and 

 bank tops. Each of these three areas will 

 be described separately. In the well-known 

 areas of the southern California mainland 

 shelf the sediment distribution maps clearly 

 reveal the reason for the apparently erratic 

 distribution of grain sizes (Fig. 180). The 

 areas of anomalous grain size correspond 

 for the most part to areas of sediment de- 

 rived from organic, authigenic, relict, and 

 residual origins. They occur where the na- 

 ture of bottom topography is such that 

 modern detrital sediments cannot be de- 

 posited to bury older kinds of sediment. 



Present-day detrital sediments are the 

 most important in terms of area of the 

 mainland shelf of southern California. The 

 approximate percentage of area covered 

 by them in the four best-known parts 

 of the mainland shelf are as follows: off 



