68 



Lithology 



closely corresponds to that of Reed's (1933, 

 p. 28) metamorphic Southern Franciscan 

 and granitic Anacapia Provinces. For sim- 

 plicity, however, both types of basement are 

 shown on the geological map as "basement," 

 with no distinction made between them. 



Pre-Miocene rocks consist of Jurassic (?) 

 green chert and chloritic altered sandstone 

 on several deep banks west of Tanner Basin. 

 Because no schist was found associated, this 

 area should probably be considered one of 

 sedimentary rocks — possibly the oldest of 

 the entire continental borderland. Fos- 

 siliferous Cretaceous (Chico) shale and sand- 

 stone are present nearshore at La JoUa and 

 Point Loma. Eocene shale, sandstone, and 

 conglomerate also occur nearshore at La 

 Jolla. Shale, sandstone, and limestone of 

 Eocene age surround San Nicolas Island, 

 which consists exclusively of rocks of that 

 age. 



Miocene rocks bulk largest by far of all 

 the Tertiary strata, occurring on nearly all 

 topographic highs of the region. They con- 

 sist of Early, Middle, and Late Miocene 

 shale, chert, and limestone. The enclosed 

 fauna is dominantly Luisian and Mohnian. 

 Volcanic rocks on the sea floor are also 

 probably mostly of Middle Miocene age, 

 just as they are on the islands and adjacent 

 mainland. Because of the insufficient sample 

 density throughout most of the borderland, 

 no distinction is made on the geological map 

 for diff"erent parts of the Miocene series. 



Pliocene strata are represented chiefly by 

 mudstones, some of which are so soft as to 

 be confused with some Recent muds. On 

 foraminiferal evidence Pliocene mudstones 

 have been recognized in two samples on the 

 northeast slope of San Clemente Island, in 

 one from near the sill of San Nicolas Basin, 

 in one from the west slope of the Santa Rosa- 

 Cortes Ridge, and in five samples from the 

 northeast slope of the San Pedro and Santa 

 Monica Basins (Emery and Shepard, 1945; 

 Moore, 1954a) and in two from Lasuen Sea- 

 mount (Fowler, 1958). Eleven samples from 

 Coronado Bank ofl" San Diego contain 

 Foraminifera which range from Late PHocene 

 to Recent. On the basis of the fauna and 

 of lithologic and physiographic evidence. 



Emery, Butcher, Gould, and Shepard (1952) 

 considered that the bank is overlain by 

 strata of Late Pliocene to Late Pleistocene 

 age. Large areas of Repetto (Pliocene) shale 

 also occur on the shelf of San Pedro Bay and 

 along the coast between Ventura and Santa 

 Barbara, mostly adjoining coastal outcrops. 

 In addition, Foraminifera of Pliocene age 

 occur in phosphorite which cements to- 

 gether older nodules atop several banks 

 (Dietz, Emery, and Shepard, 1942). Un- 

 consolidated iron-stained sands found at 

 numerous localities on the mainland shelf 

 are believed to be of Late Pleistocene age — 

 they will be discussed in a later section 

 on sediments. Finally, basins and their 

 lower slopes are floored by thick muddy sedi- 

 ments which probably range in age from 

 Pliocene at depth to Present at the surface 

 (as in the now-filled Los Angeles Basin). 



Phosphorite 



Description and Composition 



One of the most important kinds of rock 

 present on the sea floor off southern Cal- 

 ifornia is authigenic phosphorite. Its com- 

 position and distribution provide useful in- 

 formation on the paleogeographic develop- 

 ment of the region. About 330 samples 

 containing phosphorite have been obtained 

 from the continental borderland (Fig. 63). 

 These samples are from bank tops, ridges, 

 deep hills, some basin slopes, and parts of 

 the mainland shelf that are shallower than 

 their surroundings. All these areas are 

 characterized by a very slow rate of deposi- 

 tion of modern detrital sediments. More 

 than 95 per cent of the samples are from 

 depths between 100 and 1000 feet. Most 

 shallower areas border the mainland shore 

 where either detrital sediment is deposited 

 so rapidly as to dilute or bury all authigenic 

 materials or where vigorous wave action is 

 actively eroding bedrock. In many areas 

 deeper than about 1000 feet a mantle of fine 

 sediment also dilutes or buries authigenic 

 materials; however, the tops of deep hills 

 are free of such sediment, and phosphorite 

 may be found on them, although few have 



