Sediments 



Sediments contain within them the evi- 

 dence of their origin. They are the end 

 products of the interplay of water move- 

 ments, Hfe processes, and physiographic con- 

 trols. Thus they are best considered only 

 after their sources, agents, and processes are 

 at least partly known. Since physiography 

 is the easiest measured and best understood 

 of the controls, the sediments have been ar- 

 ranged in a sequence of physiographic 

 groupings. One of the most important 

 properties of sediment is its role as host to 

 petroleum; consequently, some of the threads 

 of evidence for the origin, migration, and 

 concentration of petroleum can be drawn 

 together in a discussion of sediments. 



General Composition and Minor 

 Features 



Marshes 



The sedimentary characteristics of marshes 

 in southern California as elsewhere are 

 closely controlled by two factors, topography 

 and vegetation. Topography is the primary 

 factor, dividing the area into narrow wind- 

 ing tidal channels and broad, relatively 

 smooth, intertidal flats. The flats are dis- 

 tinctive in that their surfaces are covered by 

 patterns of vegetation (Fig. 154) as described 

 earlier. Marshes are world-wide in distribu- 

 tion, and enough is known about them to 



recognize that they closely resemble each 

 other in most respects. A thorough study 

 from the point of view of topography, water, 

 flora, fauna, and sediments was made at 

 Newport Bay by Stevenson (1954), and most 

 of the following remarks are based on this 

 study and particularly on one unit of the 

 marsh for which about 160 sediment sam- 

 ples were analyzed. Another very complete 

 study of marshes in several lagoons of Baja 

 California by Stewart (1956) shows them to 

 be similar in general to those of Newport 

 Bay. In discussing the sediments of New- 

 port Bay, it is important to realize that until 

 about 1825 the bay was open to the ocean 

 and contained a large population of oysters 

 whose broken shells established the founda- 

 tion for extensive sand bars within the bay. 

 In 1825 a bar began to close the bay mouth, 

 and finer sediments started to accumulate 

 within the bay atop the sand bars. The fine 

 sediments form the present marsh surface 

 and are less than 1 meter thick (Table 12). 

 The coarsest sediment in the bay is on the 

 floors of the deep tidal channels which have 

 cut into the former sand bars. This sand 

 averages about 320 microns in diameter. 

 Along the banks of the channels are narrow 

 beaches of fine sand and coarse silt existing 

 in equilibrium with the currents and waves. 

 Small tidal creeks that drain the water from 

 the marsh surface during ebbing tide are also 

 understandably coarser than the marsh sur- 



180 



