Basins and Troughs 



231 



ment is that of the variable composition of 

 the source materials; for example, stream- 

 borne sediments in an area of weathering 

 Tertiary marine shales may contain much 

 illite. Another factor is that of differential 

 flocculation and deposition of clay minerals 

 (van Andel and Postma, 1954), whereby illite 

 may be precipitated closer to shore than 

 montmorillonite. Rate of deposition with 

 respect to rate of diagenesis can also control 

 the ratio of illite to montmorillonite. An- 

 other general limitation is the low degree of 

 precision in determining the composition of 

 complex mixtures of fine-grained clay min- 

 erals, even though all available methods are 

 used, such as X-ray diffraction, differen- 

 tial thermal analysis, chemical and base- 

 exchange analyses, and optical and electron 

 microscopy. These various factors clearly 

 require caution in the interpretation of en- 

 vironments of deposition for ancient shales 

 on the basis of their clay mineralogy. 



The only comprehensive study of the clay 

 minerals on the sea floor off southern Cah- 

 fornia was made by Dietz (1941) and re- 

 ported also in part by Grim, Dietz, and 

 Bradley (1949). Samples were analyzed from 

 the Colorado River and Mission Bay (near 

 San Diego), from cores of the mainland 

 shelf off Encinitas and Oceanside, from cores 

 of some slopes and the bottoms of Santa 

 Barbara, San Pedro, Santa Monica, San 

 Diego, Santa Cruz, Santa Catalina, East 

 Cortes, and Tanner Basins, and from cores 

 of the continental slope and the deep-sea 

 floor. In general, kaolinite, montmorillonite, 

 and illite were found to be about equally 

 abundant in the Colorado River and Mission 

 Bay. Samples from the mainland shelf 

 showed a relative loss of kaolinite. In the 

 basins there appeared to be a further sea- 

 ward decrease in kaolinite accompanied by 

 an increase in illite, so that in offshore basins 

 kaolinite averaged about 20 per cent, mont- 

 morillonite 30 per cent, and ilUte 40 per cent. 

 Presumably, this change in ratio means that 

 illite is being formed in the basin sediments 

 at the expense of kaolinite. The seaward 

 decrease of the kaolinite-to-illite ratio can 

 be an effect of the rate of deposition which 

 is slower in the offshore basins than in near- 



shore ones, thus providing more time for 

 diagenesis during burial of the sediments; 

 the decrease may also be influenced by the 

 higher percentage of calcium carbonate in 

 offshore than in nearshore basin sediments, 

 giving rise to lesser stability of kaolinite off- 

 shore. A slight seaward decrease of the 

 montmorillonite-to-iUite ratio indicates that 

 montmorillonite is also being lost by 

 diagenesis. 



In 1955 X-ray analyses of core samples 

 from Santa Barbara, Santa Monica, and 

 Santa Catalina Basins were made by the 

 Petroleum and Oil-shale Experiment Station, 

 U. S. Bureau of Mines (H. M. Thorne, per- 

 sonal communication of November 10, 1955). 

 The results showed that illite is dominant but 

 that kaolinite, montmorillonite, and chlorite 

 are present in minor amounts, probably less 

 than 10 per cent each. Again no change of 

 relative proportions of the clay minerals with 

 depth, to as much as 360 cm, could be de- 

 tected. Two more analyses of clays from 

 Santa Barbara Basin by Grim (Orr and 

 Emery, 1956fl) yielded the following results: 

 montmorillonite 55 per cent, illite 30 per 

 cent, and chlorite 15 per cent. The dissimi- 

 larity between the three sets of clay mineral 

 analyses must be due to differences in tech- 

 niques of analysis. 



In addition to their content of clay min- 

 erals, the clay-sized sediments in the basins 

 contain much fine-grained calcium carbo- 

 nate, as discussed in an earlier section. The 

 X-ray analyses by the U. S. Bureau of Mines 

 indicate that it is in the form of calcite. 

 These analyses also show the presence of 

 fine-grained feldspar, quartz, dolomite, and 

 pyrite in order of decreasing abundance. 

 More detailed X-ray diffraction determina- 

 tions of quartz, plagioclase, and orthoclase 

 were made by E. D. Goldberg aided by D. S. 

 Gorsline (personal communication). Analy- 

 sis for quartz was an extension of studies 

 made by Rex and Goldberg (1958), in which 

 the quartz content of deep-sea sediments was 

 determined with a reproducibility of 3 per 

 cent. Analysis for feldspar is less certain 

 because of dependence of the diffraction pat- 

 tern on the type and history of the feldspar, 

 as pointed out by Smith and Yoder (1956). 



