214 



Sediments 



in the bank areas of abundant glauconite as 

 in the fine-grained sediments of Santa 

 Monica and San Pedro Basins where glau- 

 conite is absent except for rare grains that 

 are believed to have been reworked from 

 areas of shallow water. Most of the glau- 

 conite lacks the gross internal structure sug- 

 gestive of swelled flakes of biotite, and it 

 lies within even the very small inner cham- 

 bers of foraminiferal tests where biotite 

 must be excluded by its size. Possibly this 

 glauconite is derived from illite, the potas- 

 sium clay mineral that has a chemical com- 

 position and X-ray structure similar to that 

 of biotite and glauconite (Hendricks and 

 Ross, 1941). Variation in chemical and 

 molecular structure also suggests that several 

 diff'erent origins are possible (Burst, 1958). 

 The general appearance of many grains sug- 

 gests that the glauconite swells before it 

 hardens, permitting the inference that once 

 the enclosing test is broken away or dis- 

 solved the freed glauconite may expand into 

 a more or less shapeless grain. Completion 

 of a study of glauconite in the approxi- 

 mately 1000 available samples that contain 

 it should do much to uncover the origin or 

 origins of this interesting mineral. 



Glauconite is represented in ancient de- 

 posits of the region at several localities, 

 among the most interesting of which are 

 outcrops of the Pleistocene Lomita marl 

 around the margins of the Los Angeles Basin. 

 According to S. G. Wissler (personal com- 

 munication), foraminiferal zones several 

 thousand feet thick within the deep part of 

 the basin are represented by only a few tens 

 of feet in a glauconitic sand of Pliocene age 

 at Palos Verdes Hills, testifying to the rela- 

 tive slowness of deposition of glauconitic 

 sands. 



Sills of Basins 



A systematic study of the sediments cover- 

 ing the sills of basins has not yet been made; 

 however, sixteen cores or surface samples 

 have been collected at the sills of six of the 

 basins incidental to other work. These 

 samples show a wide range of types and 

 grain sizes of sediments (Table 13). At the 



sills of the nearshore Santa Barbara and 

 San Pedro Basins the sediments are only 

 slightly coarser than the average of the 

 normal muds that cover the basin floors. 

 One sample from another nearshore basin, 

 Santa Monica, is much coarser, 25 microns. 

 Two off"shore basins, Santa Catalina and San 

 Nicolas, have very much coarser sediments 

 at the sills than on the floors, but the sedi- 

 ment at the sill of another off'shore basin, 

 Santa Cruz, is only slightly coarser than 

 that on its floor. One sample in the sill 

 of Santa Catalina Basin is 125 microns 

 and one in San Nicolas Basin is 225 mi- 

 crons. Where coarse sediments are pres- 

 ent, they consist largely of Foraminiferal 

 tests, glauconite, and rock fragments; thus 

 they are more analogous to sediments of 

 bank tops than to those of basin floors. 

 Obviously, the sediments in sills and saddles 

 are so variable in both their horizontal and 

 vertical distribution that a closely spaced 

 sampling grid must be followed in order to 

 learn the true patterns of distribution. 



' Italics indicate number of sample analyses used for 

 obtaining average values. 



The presence of local areas of coarse- 

 grained sediments in basin sills suggests the 

 winnowing away of fine sediments or the 

 prevention of their deposition by stronger 

 currents than exist on the basin floors and 

 side slopes. It is unlikely that currents re- 

 quired for the known replenishment of basin 

 water are sufficiently fast to so control the 

 sediments. Instead, it seems more reason- 

 able to ascribe the currents to a periodic in- 

 ward and outward flow that must be pro- 



