74 



Lithology 



at that time in place of tlie thin laminar beds 

 of phosphorite that occur within many Mid- 

 dle Miocene shales. Deposition probably 

 ceased or was negligible during latest Mio- 

 cene and early Pliocene time, and then it was 

 renewed in latest Pliocene time and con- 

 tinued through part of the Pleistocene. The 

 scarcity of phosphorite on the walls of sub- 

 marine canyons and at the topmost peak of 

 Fortymile Bank suggests that deposition 

 again ceased in Late Pleistocene to Recent 

 time. 



Paleogeography 



When stratigraphic information for a re- 

 gion is fairly complete, one of the best 

 methods of summing and testing it is the 

 construction of paleogeographic maps, pro- 

 vided we keep in mind the large number of 

 uncertainties inherent in such maps. Be- 

 cause of the rarity of shoreline deposits, the 

 ancient shorelines can almost never be traced 

 with exactness; locally, however, nearness to 

 an ancient shoreline may be shown by the 

 presence of thick and very coarse marine 

 detrital sediments, especially if supplemented 

 by the inclusion of remains of shallow-water 

 animals. For any given time unit the gen- 

 eral distribution of marine and continental 

 deposits may be the chief criteria used for 

 paleogeography. Because continental de- 

 posits are usually thin or not preserved at 

 all, to a large extent paleogeographic maps 

 depend on the negative evidence of absence 

 of marine strata. The first paleogeographic 

 map of California was drawn by Clark 

 (1921), largely on the basis of presence or 

 absence of strata. It is obvious that the ab- 

 sence may also result from deep erosion at 

 some time after marine deposits have been 

 formed. In addition, it cannot be assumed 

 that all marine areas are characterized by 

 deposition, for some, such as the tops of 

 banks, receive very minor quantities of de- 

 trital sediment. 



In some circumstances the presence of a 

 former land area may be assumed to account 

 for the presence of detrital sediments having 

 a lithology or texture such that no other 



known land area could have been the source; 

 such is the chief basis for Woodford's (1925) 

 Catalinia. Among the remaining uncertain- 

 ties of paleogeographic maps resulting from 

 incomplete information are the questions of 

 changes in land and sea areas within the 

 particular time interval of the map and the 

 known distortion of land and sea areas dur- 

 ing postdepositional times of folding and 

 faulting (Reed, 1933, p. 296). 



Following Clark's and Woodford's lead, 

 Loel and Corey (1932) constructed a paleo- 

 geographic map of Vaqueros time, and then 

 Reed (1933) made maps for many intervals 

 between Cretaceous and Pliocene times. Of 

 necessity these maps were based entirely on 

 data from outcrops on the mainland and 

 islands. When stratigraphic data from sam- 

 ples of the sea floor were presented by Dietz, 

 Emery, and Shepard (1942) and Emery and 

 Shepard (1945), some modification of the 

 maps was needed. In succeeding years ad- 

 ditional changes have been required by new 

 lithologic, paleontologic, and structural data 

 from land and sea floor areas. Many of 

 those new data have been incorporated by 

 Corey (1954) in maps that also considered 

 facies distribution of sediments for seven 

 units of the Tertiary Period. These maps 

 were supplemented by Clements (1955) for 

 the Early and Late Pleistocene Epochs on the 

 basis of topography alone. Still more data 

 have become available from the sea floor 

 since these maps were published, requiring 

 additional changes. The present maps (Fig. 

 66) are shown as highly generalized and are 

 subject to further corrections, especially 

 when more consideration is given to faunal 

 facies. Their main value is perhaps in show- 

 ing that the sea floor off" southern California 

 has undergone great changes during the dis- 

 tant as well as the recent past. 



Eocene and Oligocene seas appear to have 

 been restricted by the presence of broad and 

 probably low land areas. Beginning in Early 

 Miocene time the seas advanced until by 

 Middle and Late Miocene they extended far 

 inland of the present shoreUne. Extensive 

 block faulting began in the Ohgocene, prob- 

 ably reached a maximum rate in Late Mio- 

 cene, and continued into the Pliocene Epoch. 



