588 KAPLAN AND RITTENBERG [CHAP. 23 



animal and plant exoskeletons such as calcareous foraminiferal tests, siliceous 

 diatom fnistules and coprolitos from benthonic animals living on the bottom 

 of well aerated basins. The contribution by siliceous tests and faecal pellets to a 

 sediment can generally best be determined visually, since chemical analysis is 

 misleading owing to the masking effect of detrital material. Carbonates can 

 easily be determined chemically. 



The second category comprises organic compounds, largely unorganized, of 

 complex structure. A large number of analyses have been made to determine 

 the absolute and relative contents of organic matter in sediments, but little is 

 yet known of the actual constituents. 



h. Recent detrital 



This class undoubtedly represents the most important type of sediment being 

 deposited in present day basins. Its origin is weathered and eroded terrigenous 

 material with an overall composition of alumino-silicates. It is brought to the 

 ocean by rmi-oflF, streams and rivers, and by wave erosion of cliffs. Such sedi- 

 ments are distributed in suspension by currents which allows for the settling 

 out of the heavy and larger particles near shore and a concentration of finer 

 particles in the deep water. As water movement in basins is generally at a 

 minimum, the winnowing out effect of fine-sediment removal does not occur and 

 fine particles of clay and silt can accumulate. Recent detrital sediments, as 

 well as other types, may be redistributed by slumping or sliding down the wall 

 of a steep-sided basin, or by turbidity currents which can disperse sediments 

 over very large distances and introduce coarse-grained materials on an other- 

 wise fine-grained bottom (Gorsline and Emery, 1959). 



B. Structure 



It is known that many ancient basin sediments have well preserved lamina- 

 tions (Bramlette, 1946; Baldwin, 1959). In present day basins this phenomenon 

 is not as well represented. One of the best examjDles is that occurring in Malo 

 Jezero (Seibold, 1958), a brackish-water lagoon connected to the Adriatic. 

 Here annual cycles of carbonate and quartz deposits alternate, forming bed 

 thicknesses of 0.25 mm; larger beds also occur at regular intervals. Grippenberg 

 (1939) reports varves in Baltic sediments that were evidently formed in the 

 late glacial period and contain little organic matter. Layering in fjords is not 

 common, perhaps because rainfall and run-off are usually high and the sedi- 

 ments are coarser than normal basin deposits. Strom (1939) found that of 32 

 fjords, two had irregular lamination and only one had regular varves. The 

 u])per sediment in the Cariaco Trench contains numerous thin, discontinuous 

 horizontal laminae, distinguishable by lighter and darker shades of coloring 

 as well as by slight differences in grain size of the sediments (Athearn, 1959; 

 Heezen et al., 1959). Seasonal variations, which might cause differences in the 

 type and amount of sediment being deposited, may best explain these laminae. 



Of the southern California basins, laminations have been observed only in the 

 Santa Barbara Basin (Hiilsemann and Emery, 1961). which is also the most 



