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Unconsolidated Bottom Materials 



Sediments are classified on the basis of a three-component 

 system of gravel j, sand, and silt content, and are subdivided 

 further on the basis of color o Although the sediments represent 

 a gradual progression from sand nearshore to silts offshore, 

 this progressive change is disrupted by areas of coarse sedi- 

 ments These coarse deposits are composed of relic sediments, 

 organic debris, and authigenic minerals » 



The highest percentages of sand are in the nearshore 

 regions, at the head of Santa Monica Canyon, and in some iso- 

 lated patches on the outer portions of the shelf. Sand is the 

 main sediment component in the bay. The greatest percentages 

 of silt are along the basin slope and in the submarine canyons. 

 Clay rarely exceeds 20 per cent of the sediment except in parts 

 of the submarine canyons and in the Santa Monica Basin. 



All deposits have fine-grained sediment associated with 

 them, indicating that fine material is deposited to some extent 

 in all parts of the bay. Intermittent removal of fine-grained 

 sediment may be the most important factor in preserving the relic 

 deposits. 



An examination of the sand fractions of the sediment 

 resulted in the establishment of six distinct types j (1) fine 

 quartz-feldspar ssmd, (2) rock fragment sand, (3) glauconite 

 sand, (4) phosphorite-glauconite-shell sand, (5) shell sand, 

 and (6) red sand. 



Fine quartz-feldspar sand covers most of the shelf, and is 

 the terriginous detrital material being deposited in the bay at 

 the present time. 



