208 



bris bypasses, leaving behind on the shelf 

 only sediments much different from those off 

 Texas. Eventually, when the basins become 

 filled, sediments can accumulate in broad 

 off-lapping blankets over the shelf and the 

 basin fillings. The thin layer of coarse sedi- 

 ment will then be found atop bedrock of the 

 shelf and below a blanket of fine sediment. 

 Possibly similar sediments exist in the geo- 

 logical record, but if the ancient shelves had 

 been very narrow, owing to the limitation on 

 erosion imposed by a constant sea level, the 

 unique shelf sediments would have been de- 

 posited only in a narrow belt around the 

 former shore. This restriction in area would 

 certainly have contributed to failure to find 

 and recognize ancient shelf sediments like 

 the existing ones. 



Island Shelves 



Sediments of the island shelves have been 

 studied less thoroughly than have those of 

 the mainland shelf. Nevertheless, specific 

 studies have been made for four island 

 shelves: Santa CataUna (Shepard and Wrath, 

 1937; McGlasson, 1957), San Nicolas (Nor- 

 ris, 1951), Anacapa (SchoU, 1959), and Santa 

 Barbara (Grady, in preparation). The type 

 and distribution of their sediments can be 

 presented in the same ways as for the main- 

 land shelves, but only two methods will be 

 used (similar to those of Figures 175 and 180 

 for the mainland shelves) for comparison 

 with various other details of the sediments. 



The earliest study of island shelf sedi- 

 ments, made by Shepard and Wrath (1937), 

 was based on about 45 scattered samples 

 plus many chart notations. The latter, how- 

 ever, failed to satisfactorily distinguish be- 

 tween organic and inorganic sands. A later 

 study of McGlasson (1957) was based on 92 

 samples arranged mostly in 16 pofiles. 

 Neither set of samples is very satisfactory 

 for a real description, so Figure 181 is a 

 composite of both. Here, organic sediment 

 clearly is more abundant than along the 

 mainland; moreover, the detrital sediment 

 also contains fairly large percentages of fora- 

 miniferal tests, fragments of gastropods and 

 pelecypods, and bryozoans. Although not 



Sediments 



Figure 181. Sediments of shelf around Santa Catalina 

 Island based on 137 samples. Maps show sediment types, 

 percentage of calcium carbonate (acid-soluble material), 

 and number of foraminiferal tests per gram of total sedi- 

 ment. Dashed line shows shelf-break 60 meters. Adapted 

 from McGlasson (1957). 



separated and plotted, the authigenic min- 

 eral glauconite forms an appreciable compo- 

 nent, particularly on the outer parts of the 

 shelf. When organic and authigenic sedi- 

 ments and minor amounts of residual sedi- 

 ments are ignored, the remaining present-day 

 detrital sediments present a general seaward 

 decrease in grain size. In these sediments 

 the ratio of quartz to feldspar is about 1.1: 1 , 

 about the same as on the mainland shelves. 

 Progressively greater dilution of detrital sedi- 

 ment by organic remains is indicated by a 



