Island Shelves 



209 



seaward increase in the percentage of cal- 

 cium carbonate and in the number of fora- 

 miniferal tests per gram of sediment. This 

 dilution supports grain size in indicating a 

 seaward decrease in rate of deposition of 

 present-day detrital sediment. The highly 

 organic sediments on the shelf north of the 

 east end of the island are likened by Wood- 

 ring, Bramlette, and Kew (1946) to the Pleis- 

 tocene Lomita marl that crops out north and 

 east of the Palos Verdes Hills. General com- 

 position and depth of accumulation are 

 similar. Calcareous algae in the Lomita 

 marl also have their living counterparts in 

 shallow water near the isthmus of the island. 

 Two other studies of island shelves war- 

 rant attention because of the dissimilarity of 

 sediment patterns around them, even though 

 both are small and consist dominantly of 

 volcanic rocks. These are Anacapa Island 

 (Fig. 182) and Santa Barbara Island (Fig. 

 183), studied by Scholl (1959) and Grady 

 (in preparation), respectively. The maps of 

 median diameters of whole sediments are 

 strikingly different, with Anacapa Island 

 being fringed by medium-to-coarse sand 

 grading outward to very fine sand, and Santa 

 Barbara being fringed by fine sand that 

 grades outward to medium-to-coarse sand be- 

 fore again becoming fine-grained beyond the 



119'30 21 



Figure 182. Sediments of shelf around Anacapa Island 

 based on 100 samples. Maps show diameter, heavy min- 

 erals in insoluble residues, calcium carbonate (gasometric 

 analyses), and organic matter (1.7 x organic carbon). 

 Dashed line is 100-meter contour. Adapted from Scholl 

 (1959). 



% CALCIUM CARBONATE 



Figure 183. Sediments of shelf around Santa Barbara 

 Island based on about 90 samples. Maps show median 

 diameter, Trask sorting coefficient, calcium carbonate 

 (gasometric analyses), and organic matter (17 x nitrogen). 

 Dashed line is 200-meter contour. Adapted from Grady 

 (in preparation). 



shelf-break. The difference in the two pat- 

 terns is largely due to faster erosion of Ana- 

 capa Island which provides coarser detrital 

 sand for the area immediately adjacent to 

 the island. Rapid modification of the sea 

 cliffs was especially evident following a 

 storm during the winter of 1957. In addi- 

 tion, the shelf south of Anacapa Island is so 

 narrow that there is insufficient space for 

 development of the ring of coarse shell sand 

 found on the outer half of the wider shelf 

 off Santa Barbara Island. Also, the strait 

 between Anacapa and Santa Cruz Islands is 

 the site of strong currents which prevent the 

 deposition of sediments other than relatively 

 coarse sands — chiefly debris of mollusk 

 shells and calcareous red algae. Because of 

 the strait, the area of coarsest sediment ex- 

 tends from the fringing area of the sea cliffs 

 more or less due westward in line with the 

 island. The organic content of the sedi- 

 ments around Anacapa Island is about dou- 

 ble that around Santa Barbara Island, in spite 

 of the fact that the total sediment of the en- 



