592 KAPLAN AND KITTENBERG [CHAP. 23 



Reducing conditions on the bottom preserve the organic matter since de- 

 composition by anaerobic bacteria is generally not as efficient or as complete 

 as decomposition by aerobes. Since clays have fewer pore spaces, fine-grained 

 sediments ^\■ill in general promote reducing conditions by preventing water 

 circulation and so help preserve organic matter. The binding of organic material 

 by clays may decrease its availability to bacteria. 



The above discussion is concerned with the areal distribution of the compo- 

 nents relative to the total dry weight of the sediment. It is possible to consider 

 absolute distribution if the stratigraphy is well known. Thus, for the Black Sea, 

 Strakhov (see Smirnow, 1958) found that by considering a unit area and a 

 column of sediment equivalent to a constant time lapse (2500 years), the total 

 amounts of insoluble residues, calcium carbonate and organic matter were 

 greatest inshore and decreased toward the center. 



The highest organic content reported for recent sediments is for a fjord, 

 23.4% (Strom, 1939). In the recent Black Sea sediments the highest content of 

 organic matter is about 7.7% (Gaspers, 1957, p. 823), which is considerably 

 lower than the 25% reported for the residual Euxinian deposits. In the Baltic 

 Sea and Lake Maracaibo the maximum is about 8.5%. The highest content 

 measured in the southern California basins is about 11.5%. These values are 

 much greater than the 1-3% of the deep-sea sediments. All data available show 

 a sharp decrease in the organic content with depth below the top 5-10 cm, 

 indicating that diagenetic processes occur there most rapidly. 



d. Composition of detrital material 



The insoluble residue of the detrital material in basins is composed largely 

 of silt (16[jt,-4[x) and clay ( < 4fx). It has been found in southern California basins 

 that the mineral composition of the silt is similar to that of the sand on the 

 beaches and on the continental shelf. Here the dominant minerals are quartz 

 and feldspar (ratio 2.2:1), the plagioclase to orthoclase ratio in the feldspar 

 being 4.5:1 (Emery, 1960). 



The clay minerals show an enrichment of illite and montmoriUonite over 

 kaoUnite. The greater the distance from the shore the higher the illite-kaolinite 

 ratio in the southern Cahfornia basins (Emery, 1960). MontmoriUonite appears 

 to be an intermediate in the diagenetic alteration of land-derived kaolinite to 

 illite. 



e. Color 



The color of basin sediments varies considerably depending on their state of 

 oxidation and reduction. Highly oxidized sediments are generally brown; 

 highly reduced sediments vary from gray to green to black. Strom (1939) 

 emphasized that muds covered by H2S-containing waters are invariably black, 

 but this is not always the case. Green muds occur in Walvis Bay (Copenhagen, 

 1934), Kaoe Bay (Kuenen, 1943) and in the Cariaco Trench (Athearn, 1959; 

 Heezen et al., 1959) — all stagnant environments. Lake Maracaibo, on the other 



