74 



whereas silt and clay which are mainly carried in suspension 

 should increase in an offshore direction. 



There are certain known factors related to the composition 

 of the sediment which will disturb this theoretical gradation, <. 

 These ares (1) the occurrence of authigenic minerals having 

 sizes not related to depositional mechanics of the host sedi- 

 ments, (2) shells and their fragments occurring in a sediment 

 which are formed by local organic processes and bear no 

 relationship to the transported detrital components where they 

 occur, and (3) relic accumulations which were formed during 

 a previous depositional period, usually that of a lowered sea 

 level, and have not been covered or removed during subsequent 

 periods of sedimentation. 



The distribution of relic sediments, authigenic minerals, 

 rocks, and organic remains in the sediments of Santa Monica 

 Bay often causes a general offshore increase, or a local in- 

 crease in grain size as compared to the surrounding material. 

 The relationship of these factors to the general pattern of 

 sedimentation of the shelf and slopes is shown in Figure 23, 

 which is a graph showing the variation of median diameters 

 with depth. The diagram consists of the arithmetic mean, 

 median, and highest and lowest values of median diameters of 

 the deposits in each of a series of depth ranges. It should 

 be noted that the axis of this diagram indicating size is 

 logarithmic. The deeper depth intervals were made greater 

 than those at shallow depths to include a comparable number 

 of samples because of the decreasing proportion of samples 

 with depth as noted earlier. The arithmetic mean is the 



