136 



Water 



Since correlation within and between 

 basins is based on biozones, the correlations 

 follow ecological lines rather than time lines, 

 making uncertain the contemporaneity of 

 events in the two basins. Even within a single 

 basin the differential movements of separate 

 blocks provided differences in nature of sedi- 

 ment, depth, and conditions of deposition. 

 Nevertheless, along very general lines both 

 the Ventura and the Los Angeles basins ap- 

 pear to have begun to subside during the 

 Miocene from the largely shallow water and 

 nonmarine environment of Oligocene and 

 earlier times. The water depth appears to 

 have reached its maximum of more than 

 4000 feet (1200 meters) in each basin during 

 the Early or Middle Pliocene and, although 

 subsidence continued, the rate of sedimen- 

 tation increased so that both basins shoaled 

 until the surface of both rose above sea level 

 in about the Middle Pleistocene Epoch, there- 

 after receiving nonmarine sediments. Dur- 

 ing the times of greatest depth, analogy with 

 modern basin biozones suggests that the tem- 

 perature was lower than about 2.5 ° C. After- 

 ward, as the water depth decreased, the tem- 

 perature of accumulation increased to the 

 present temperature or even higher for some 

 Late Pleistocene and Recent sediments 

 deposited in shallow lagoons. 



The sediments deposited when the Ventura 

 and Los Angeles Basins were physiographic 

 depressions between Middle Miocene and 

 Middle Pleistocene times (White, 1952) 

 yield a partial confirmation of the conclu- 

 sions reached from faunal studies. Neither 

 evaporites nor fresh-water sedimentary rocks 

 have been reported for this time span from 

 the central areas of these basins, but of course 

 such sediments could have been deposited 

 along the shores and subsequently removed 

 by erosion. 



Inferences about paleoceanography can 

 also be reached on the basis of paleogeog- 

 raphy. As discussed in the section on lith- 

 ology, the distribution and nature of Tertiary 

 sedimentary rocks are such that the seas of 

 the Eocene, Oligocene, and Early to Middle 

 Miocene Epochs are believed to have been 

 broad and quite shallow. Existing areas of 

 broad shallow water are generally character- 

 ized by seasonal extremes of temperature 



resulting from the facts that the currents are 

 sluggish and that the losses and gains of 

 heat through the surface must be absorbed 

 in only a short column of water with no op- 

 portunity for mixing with underlying water 

 as in the deep ocean. As a result the sea 

 off southern California during the early 

 Tertiary Epochs must have been warmer 

 during the summer and colder during the 

 winter than the surface of the deep ocean at 

 the same times. During Late Miocene and 

 Pliocene Epochs the sea floor became broken 

 into blocks, many of which sank to become 

 areas of deep water. Owing to the increased 

 depth, the water temperatures should have 

 been more uniform, with less difference be- 

 tween summer and winter. Summer tem- 

 peratures are probably more important than 

 winter ones for organisms because this is 

 usually the time of maximum growth and 

 reproduction. If the surface temperatures 

 of the deep ocean during the entire Tertiary 

 were similar to those at present, the area of 

 the continental borderland should have 

 been populated during early Tertiary by 

 mollusks and other fossil-forming animals 

 which now live only in water warmer than 

 that presently in the area, whereas during 

 later Tertiary the animals should have been 

 similar in their temperature requirements to 

 those now living there. These inferences on 

 topographic control of water temperature 

 during the Tertiary appear to support con- 

 clusions reached from the fossil record by 

 Durham (1950, 1954) and others. 



During the Pleistocene stages of glacially 

 lowered sea level, nearly the entire length of 

 the Santa Rosa-Cortes Ridge must have 

 been exposed, as shown by the distribution 

 atop it of basalt cobbles from Santa Rosa 

 Island and by the presence of a series of 

 submerged marine terraces. When exposed, 

 the ridge prevented the westward flow of 

 surface water that now is entrained by the 

 California Current. In the absence of such 

 flow, upwelling due to entrainment was not 

 possible, although some upwelling due to 

 wind may have existed. With the conse- 

 quent almost certain reduction in upwelling 

 during stages of lowered sea level, the water 

 temperature must have been higher relative 

 to that of the deep ocean than at stages of 



