86 



on the outer shelf between Santa Monica and Redondo Canyons. 

 This is illustrated in Figure 27 in which average values of 

 calcium carbonate percentages in each of a series of depth 

 intervals are plotted with increasing depth. The great 

 increase of the average percentage in the 201-300 foot 

 depth range is due to the large amounts of calcium carbonate 

 occurring in the sediments on the central shelf projection. 



The high concentration of shells on the outer shelf is 

 due to (1) ecological conditions which are more favorable to 

 the growth of shelled organisms, (2) winnowing of sediment 

 finer than the shells, (3) the low supply of terriginous 

 sediments in this area, or (4) nondeposition in this area. 

 As pointed out elsewhere in portions of this report, this 

 region is probably an area of nondeposition or very slow 

 accumulation of detrital material. The main cause for the 

 increase in the percentages of calcium carbonate then is due 

 to winnowing or nondeposition of fine material. Since the 

 shells are relatively coarse they can accumulate in abundance 

 and thus constitute a high proportion of the sediment in this 

 area. 



The high values of calcium carbonate on the shelf south 

 of Redondo Canyon result from the red sand which contains a 

 large number of shell fragments. This sediment is possibly 

 a submarine outcrop of the Palos Verdes sand, an Upper 

 Pleistocene age terrace deposit which contains a great 

 quantity of shell debris. 



The general offshore increase in calcium carbonate in 

 the other parts of the bay is due to the diminishing supply 



