54 



Physiography 



AREA IN SQUARE STATUTE MILES 



500 1000 



SANTA BARBARA 



SANTA MONICA- 



'S AN CLEMENTE 

 ■VELERO 



Figure 51. Hypsographic curves for basins. The slopes 

 are gentler for the shallower (nearer shore) basins, and 

 the deeper basins lack broad flat floors. From Emery 

 (1954 J. Fig. 2). 



were derived ultimately from the high Trans- 

 verse Ranges, the San Bernardino and San 

 Gabriel Mountains, with lesser amounts 

 from the smaller Santa Ynez, Santa Monica, 

 and Santa Ana Mountains. Support for this 

 belief is given by a cross section (Fig. 52), 

 which shows that the sediment is much 

 thicker in basins nearer the Transverse 

 Ranges, whether on the seaward or the land- 

 ward sides, than in basins farther away. So 

 much sediment has been deposited on the 

 seaward side that the Ventura, San Fer- 

 nando, Los Angeles, and San Gabriel Basins 

 on land and the San Diego Basin on the sea 

 floor have been filled to overflowing. Filling 

 of basins on the north side of the Transverse 

 Ranges has formed the Mojave Desert, a re- 

 gion of numerous small and very shallow 

 playa lake basins separated by low divides. 

 The presence of deep closed basins farther 

 seaward of the Transverse Ranges (such as 

 Santa Cruz, San Nicolas, East Cortes, and 



West Cortes Basins) and farther landward 

 (such as Death Valley and other basins of 

 the Basin and Range Province) means that 

 these distant basins have received detrital 

 sediments much more slowly than have 

 closer basins, provided all basins were of 

 about the same original size and date of 

 origin. 



A very rough estimate of the quantity of 

 sediment that has been deposited in the 

 basins can be obtained from the topography 

 and from seismic measurements of the depth 

 to basement. Evidence to be presented later 

 indicates that the basin topography was ini- 

 tiated mostly during Late Miocene times, so 

 the estimate of sediment volume will be con- 

 fined to post-Miocene strata. Well records 

 and seismic exploration show that the vol- 

 ume of post-Miocene sediment in the Los 

 Angeles Basin is about 1200 cubic miles and 

 that in Ventura Basin it is about 10,000 cubic 

 miles. Basins and other deep fairly flat areas 

 on the present sea floor total about 9000 

 square miles. If we assume that there is a 

 fill of 7000 feet above basement (based on 

 seismic surveys to be discussed later) and 

 that 60 per cent of this fill is post- Miocene 

 (the same as in Los Angeles Basin), the total 

 volume is about 7000 cubic miles. Thus, 

 about 18,000 cubic miles of sediment has 

 been deposited south of the Transverse 

 Ranges. This is nearly the same as the pres- 

 ent volume of water in the area of Chart I, 

 about 20,000 miles (30,000 square miles times 

 3500 feet, average depth). If we extend these 

 figures throughout the entire area of the con- 

 tinental borderland (south to Cedros Island), 

 the estimated total volume of sediment is 

 about 30,000 cubic miles (125,000 cu km) 

 and the volume of water about 50,000 cubic 

 miles (208,000 cu km). In comparison, the 

 present volume of the Transverse Ranges 

 (San Bernardino, San Gabriel, Santa Monica, 

 Santa Ynez Mountains) is only about 2400 

 cubic miles. 



The manner of filling as well as the amount 

 of filling can be inferred from the topogra- 

 phy (Fig. 53). Three main modes of filhng 

 appear to be worthy of consideration. If 

 sediment were deposited uniformly in aU 

 areas of the continental borderland, such as 



