66 



Lithology 



bulk of the rocks are of Miocene age, but 

 Jurassic, Cretaceous, Eocene, Oligocene, 

 Pliocene, and Pleistocene are also locally 

 present. One of the most interesting of the 

 sedimentary rocks from the point of view of 

 origin is the San Onofre breccia, originally 

 described by Woodford (1925). Character- 

 istically, it consists of small-to-large, angular- 

 to-subround pieces of glaucophane schist 

 and quartzite in a clayey to sandy matrix. 

 It crops out at San Onofre (northwest of 

 Oceanside), Palos Verdes Hills, Point Dume. 

 Anacapa Island, and Santa Cruz Island, and 

 it is recognized in well cuttings in other 

 areas. In addition, occasional loose pieces 

 of quartzite and schist occur atop San 

 Nicolas and Santa Barbara Islands, on which 

 there are no outcrops of the breccia or of 

 the metamorphic source rock; possibly these 

 scattered fragments are remnants of a 

 former thick bed now removed by erosion. 

 On the sea floor San Onofre breccia has 

 been found in place in Santa Monica Bay 

 (Terry, Keesling, and Uchupi, 1956, pp. 102, 

 160), on Lasuen Seamount (Fowler, 1958), 

 and olf Anacapa Island (Scholl, 1959). 



Igneous rocks are represented by both 

 extrusive and intrusive types. Andesite and 

 basalt are about equally abundant, with 

 andesite probably somewhat concentrated 

 in the landward half of the borderland and 

 basalt in the seaward half. These extrusive 

 or fine-grained intrusive rocks are con- 

 sidered to be in place for only about 35 

 samples at Point Dume, the shelves or slopes 

 of Santa Barbara, San Clemente, Anacapa, 

 and Santa Cruz Islands, Cortes and Tanner 

 Banks, San Juan Seamount, and on several 

 small hills in the San Diego Trough. Trans- 

 ported specimens are much more common, 

 probably occurring in about half of all the 

 rock samples (Emery and Shepard, 1945; 

 Holzman, 1952). This wide distribution is 

 a result of both resistance to abrasion and 

 the probable presence of many lava flows 

 and thin sills interbedded in Miocene sedi- 

 mentary strata of the sea floor, as along much 

 of the mainland and on most of the islands 

 (Smith, 1898; Woodring, Bramlette, and 

 Kew, 1946; Shelton, 1954). In fact, one 

 island, Santa Barbara, is probably the north 



slope of a Miocene volcano, as indicated by 

 lava bombs, thick coarse agglomerate, and 

 steep dips near its southern end and on the 

 small adjacent Sutil Island. Only San 

 Nicolas Island has but minor amounts of 

 igneous rock; there it is in the form of small 

 diabase dikes (Norris, 195 1). Coarse-grained 

 intrusive rocks, such as granite and grano- 

 diorite, were found only as rounded and 

 transported pieces. Nearly all these speci- 

 mens came from samples of the mainland 

 shelf between Hueneme and Newport, from 

 which they had probably been carried by 

 streams from outcrops in the Santa Monica 

 and San Gabriel Mountains (Bailey and 

 Jahns, 1954). They are also present as rare 

 single pebbles associated with other rocks 

 in a few scattered samples farther from 

 shore; these are almost certainly examples 

 of rafting, probably by kelp. 



Metamorphic rocks, schists, gneisses, and 

 quartzites mostly containing glaucophane 

 were sampled in place on Sixtymile, Forty- 

 mile, and Thirtymile Banks. Transported 

 pieces are abundant on these banks and also 

 around Santa Catalina Island, in Coronado 

 Canyon, in La Jolla Canyon, and at the 

 center of Santa Monica Bay (Emery and 

 Shepard, 1945). At Santa Catalina Island 

 and possibly Sixtymile Bank these trans- 

 ported pieces have been derived from out- 

 crops of metamorphic rocks on the island or 

 sea floor, but in the other areas they prob- 

 ably represent the Miocene San Onofre 

 breccia or reworked fragments of it. The 

 lithology of these metamorphic rocks strongly 

 suggests that they are part of the great 

 Jurassic-Cretaceous Franciscan complex 

 which in southern California is known as 

 the Catalina facies (Woodford, 1924; Bailey, 

 1941; Woodford, Schoellhamer, Vedder, and 

 Yerkes, 1954; Irwin, 1957). Schist in place 

 on a bank about 20 miles north of Sixtymile 

 Bank is similar to the possibly pre-Cambrian 

 Pelona schist of the San Gabriel Mountains. 

 Cobbles of the Triassic-Jurassic Black 

 Mountain metaigneous series found in many 

 dredgings near La Jolla and San Diego are 

 doubtlessly reworked from the nearby Eocene 

 conglomerates. A few pieces of transported 

 slate, probably Triassic (?) Santa Monica 



