94 



Structure 



clastic sedimentary rocks of Oligocene age 

 are more common along the coast. The 

 general scarcity of pre-Miocene strata sug- 

 gests three explanations. (1) the strata were 

 not deposited in the offshore region because 

 it was then a topographically high area of 

 incompletely base-leveled metamorphic 

 rocks; or (2) they have been so deeply 

 buried by later strata that they have not 

 been exposed; or (3) a widespread later up- 

 lift caused them to be so completely eroded 

 away that few remnants have been found. 

 Probably each of these possible explanations 

 holds best in one part of the region or 

 another. The writer, however, believes that 

 the simplest and most general explanation 

 for this distribution of rocks is that during 

 Cretaceous and Paleocene times the conti- 

 nental borderland was a topographic high 

 which was being eroded to provide coarse 

 sediments to its margins and fine sediments 

 to distant areas. In Eocene time local 

 downwarping caused some of the area to re- 

 ceive rather than to provide coarse sedi- 

 ments. Afterward, in Oligocene time, 

 upwarping again reduced the areas of depo- 

 sition. Support for this explanation for pre- 

 Miocene strata is provided by the nature 

 and distribution of Miocene rocks. 



During most of Miocene time the region 

 must have been downwarped to account for 

 the unusually widespread distribution of 

 marine sediments, predominantly shales, of 

 this age. In several areas, Santa Catalina 

 Island, Fortymile Bank, and Thirtymile 

 Bank, the Miocene sedimentary rocks over- 

 lie metamorphic basement in such a way 

 that either no earlier strata was deposited or 

 it had been completely eroded away before 

 deposition of the Miocene rocks; at least, 

 none is now present. Along the present 

 coast and on the northern islands, however, 

 older sedimentary rocks are associated with 

 the Miocene rocks, indicating that in these 

 areas accumulation was more continuous. 

 Because of the widespread distribution of 

 the Miocene beds and their general uniform- 

 ity, commonly thin-bedded cherty shales, it 

 may be supposed that the continental border- 

 land had for the most part reduced to a low 

 gently sloping peneplain before the Miocene 



Epoch. This suggests a long pre-Miocene 

 period of quiessence. 



During Middle Miocene time extensive 

 volcanism, shown by sills, dikes, tuffs, and 

 ash beds, indicates a renewal of diastrophic 

 activity. A physiographic form of the 

 diastrophism was the faulting that is known 

 to have blocked out the Los Angeles and 

 Ventura Basins. Similar basins separated 

 by high areas which are now islands and 

 banks began to form in the offshore area, as 

 shown by the abundance atop the banks of 

 phosphorite, most of which contains only 

 Miocene Foraminifera. Since phosphorite 

 is an authigenic rock of very slow deposition, 

 its presence in a surface blanket deposit means 

 that inorganic detrital sediments could not 

 reach the bank tops to mask the phosphorite. 

 The absence of post-Miocene strata of inor- 

 ganic detrital sediments on most of the 

 banks and all the islands supports the belief 

 that blocking out of the continental border- 

 land began late in the Miocene. Recurrent 

 or additional block faulting continued, how- 

 ever, in some areas of the continental border- 

 land. It is shown by the thick blanket of 

 PHocene and Early Pleistocene shale atop 

 Coronado Bank off San Diego, now sepa- 

 rated from its source; by folded and beveled 

 Pliocene shale cropping out on the shelf in 

 San Pedro Bay and in the Ventura-Santa 

 Barbara region; and by Pliocene outcrops 

 on and near the scarps that form the sides 

 of Santa Catalina, San Pedro, and Santa 

 Monica Basins. Mid-Pleistocene folding 

 and uplift are well documented at Ventura 

 and at other places of the mainland and 

 islands where marine terraces occur 1000 

 feet or more above sea level, such as the 

 Palos Verdes Hills and San Clemente Island, 

 as well as in submerged areas where flat 

 erosional surfaces and gravel deposits occur 

 2000 feet or more below sea level (Fig. 81). 

 Local diastrophic movements are still occur- 

 ring, as shown by differences between 

 repeated geodetic surveys of the Newport- 

 Inglewood zone of the Los Angeles Basin 

 and of the Transverse Ranges near Cajon 

 Pass (Gilluly, 1949). The active seismicity 

 of the region is a result of such movements. 



On a larger scale than the block faulting 



