210 



Sediments 



tire shelf at Anacapa is slightly coarser than 

 that off Santa Barbara Island. Perhaps this 

 higher content is due to the greater produc- 

 tion of diatoms in surface waters near Ana- 

 capa Island owing to its position within the 

 chief area of upwelling off the coast. 



The mineralogy of the detrital sediments 

 at Anacapa Island is unique in its over- 

 whelming abundance of feldspar, an average 

 quartz-to-feldspar ratio of 0.09 : 1. Augite 

 is the flood heavy mineral, as on the main- 

 land shelf and beaches immediately to the 

 east. Glaucophane is present, but not abun- 

 dant, owing to reworking from land and sea 

 floor outcrops of the San Onofre breccia. 

 Sediments of both shelves, as on other island 

 shelves, contain much glauconite. 



Bank Tops 



A few samples were collected more than 

 20 years ago from the tops of Thirtymile, 

 Fortymile, and Sixtymile Banks, but they are 

 sufficient only to show that the sediments of 

 these banks are typical of those on other 

 banks that were later studied more thor- 

 oughly. A more general study was made for 

 Coronado Bank off San Diego by Emery, 

 Butcher, Gould, and Shepard (1952) on the 

 basis of about 250 samples collected in 1943- 

 1944. Its sediments, shown in Figures 175, 

 179, and 180, are highly calcareous and con- 

 tain abundant glauconite and phosphorite, 

 minor amounts of weathered residual or 

 relict grains, and little organic matter. They 

 form a thin blanket atop thick Pleistocene 

 strata which cap the bank. Inorganic grains 

 are similar to those present on the nearby 

 mainland shelf, but in all other respects the 

 sediments of the bank are utterly unlike 

 those of the shelf — an indication of the effect 

 on sediments of the exclusion of present-day 

 detrital materials. 



A survey of sediments atop a completely 

 isolated bank in the region was made in 1950 

 by Holzman (1952) for Cortes and Tanner 

 Banks. The tops of these banks contain out- 

 crops of rock, mostly basalt, probably larger 

 in terms of percentage area than outcrops of 

 rock on island and mainland shelves. Sedi- 

 ments are coarse, especially for the shal- 



lowest parts of the banks (Fig. 184). Cal- 

 careous organic debris comprises more than 

 80 per cent of the sediment at the top and 

 lesser amounts at greater depth. In this re- 

 spect the sediment is closely similar to that 

 of the isolated Ranger Bank off Mexico 

 (Emery, 1948). On both bank areas the cal- 

 careous debris is coarser than most of the de- 

 trital inorganic sediment. At Cortes and 

 Tanner Banks the total sediment has an av- 

 erage median diameter of 310 microns in 

 contrast to only 220 microns for inorganic 

 sediment. Organic calcareous and siliceous 

 material averages 47 per cent foraminiferal 

 tests, 34 per cent broken gastropod and 

 pelecypod shells, 7 per cent bryozoans, 7 per 

 cent echinoids — mostly spines — and 5 per 

 cent sponge spicules. Comprising most of 

 the inorganic sediments are quartz and feld- 

 spar having an average ratio of 0.17: 1, far 

 lower than on the mainland beaches and 

 shelves, doubtlessly because of the basaltic 

 composition of most of the underlying rocks. 

 In line with this source, the flood heavy min- 

 eral is augite which comprises about 45 per 

 cent of the total crop. Most of the remain- 

 der of the inorganic sediment is of authi- 

 genic origin, with glauconite averaging 16 

 per cent and cellophane 5 per cent. Be- 

 cause of the coarse grain size of the total 

 sediment the average content of organic mat- 

 ter is low, less than 0.8 per cent on the bank 

 top. This low organic matter and high cal- 

 cium carbonate yields a gray color for that 

 sediment in contrast to an olive color in sur- 

 rounding deeper ones. In summary, the 

 sediments of Cortes and Tanner Banks are 

 dominantly residual, relict, and authigenic. 

 Little or no present-day detrital sediments 

 are present, owing to the absence of a nearby 

 land source. Thus, the bank top sediment is 

 greatly different in origin from that of the 

 mainland and island shelves where most of 

 it is present-day detrital. 



The Santa Rosa-Cortes Ridge is a feature 

 that is neither true island shelf nor true bank 

 top. Because it is elongate at right angles 

 to shore, the outer part can be expected to 

 receive little island-contributed debris, so its 

 sediment should resemble that of bank tops. 

 A study of the ridge by Uchupi (1954) 



