Rate of Deposition 



247 



ter (Fig. 204). In only a simple area can the 

 curves approximate straight lines, and pos- 

 sibly no large area can be that simple, 

 certainly not the continental borderland off 

 southern California. 



Rate of Deposition 



Methods 



Many geochemical computations require 

 information about the rate of deposition of 

 sediments. In order to satisfy this need, 

 several estimates have been made during the 

 past, mostly on the basis of supply. Revelle 

 and Shepard (1939) based their estimate on 

 supply for the entire region, obtaining a rate 

 of 0.13 mm/yr, or 60 mg/sq cm/yr in the 

 basins. Using the same basic data, Emery 

 and Rittenberg (1952) assumed a greater- 

 than-average rate for the nearshore Santa 

 Barbara Basin — 2 mm/yr, or 91 mg/sq cm/ 

 yr. Handin (1951; Luplow, 1950) estimated 

 that streams between Santa Barbara and San 

 Pedro discharge about 1,300,000 cu meters 

 of sand per year. If all eventually made its 

 way onto the floors of Santa Monica and 

 San Pedro Basins by turbidity currents, the 



rate of deposition of sand throughout the 

 2720 sq km area of flat floors of the two 

 basins (Table 5) would average about 0.48 

 mm/yr. Since the ratio of sand to shale in 

 the basins averages about 1:1, and since 

 compaction of mud to shale results in a 50 

 per cent decrease in thickness, the total rate 

 of deposition would be about 1.44 mm of 

 total sediment per year, or 9 1 mg of sediment 

 grains per square centimeter per year. 

 Lastly, the relative rates of deposition across 

 the floor of a basin can be determined by dif- 

 ferences from place to place in thickness of 

 sediment between subsurface sands; these 

 differences are detected by echo sounding. 

 By this stratigraphic method, deposition at 

 the east side of San Diego Trough was found 

 to occur more than twice as fast as on the 

 west side (Table 16). 



A different method is needed for obtaining 

 absolute rates of deposition in individual 

 basins or at different localities in the same 

 basin. The best of such methods available 

 is that of radiocarbon dating. Because of 

 the importance of proper dating, the Na- 

 tional Science Foundation provided funds 

 for collecting cores, determining radiocarbon 

 ages, and measuring necessary sediment 



Figure 204. Complex relation- 

 ship of grain size with contents 

 of calcium carbonate and or- 

 ganic matter in sediments of 

 continental borderland. In a 

 generalized way the curves 

 connect points at progressively 

 greater distance from shore. 

 The complexity results from 

 the interplay of supply and de- 

 mand for sediments in the var- 

 ious environments. 



60 



40- 



20 



I I I I — I — I — I 1 — 



bank tops-f 



island 



shelves 



mainland 



shelves 



beaches 



I I I I I — I — I 1 1 I I I I I — I — r 



continental slope-i 



nearshore basins 

 L. A. Basi 



1000 



100 10 



MEDIAN DIAMETER-MICRONS 



