182 



face, averaging about 90 microns. Border- 

 ing the tidal channels of some large marshes 

 in other regions are low natural levees built 

 of silts deposited by the flooding tide, but 

 levees are only poorly developed at Newport 

 Bay and on all other marshes of southern 

 and Baja California. Sediments on the 

 marsh surface have median diameters rang- 

 ing between 100 and 1.6 microns, with an 

 average of 10 microns. Actually, 97 per 

 cent of the area is covered by sediments finer 

 than 16 microns (Fig. 155). Sediments hav- 

 ing median diameters in the clay size (finer 

 than 4 microns) occupy about one-fifth of 

 the area and occur mostly along the edges 

 of the marsh where they are covered by 

 nearly every high tide and thus are the places 

 of most active deposition. Median diame- 

 ters in this area mostly range between 1.6 

 and 1.9 microns. Sediments present a grad- 

 ual coarsening inward from the margins of 

 the marsh so that the coarsest (86 microns) 

 occurs near the center. This reversal of the 

 usual trend toward finer grain size near the 

 center of a marsh is probably the result of 

 absence of natural levees. When the marsh 

 becomes older and levees develop, the finest 



^___ Sediments 



sediment may be expected to accumulate in 

 the shallow basin behind the levees instead 

 of largely being carried away by ebbing tides 

 as at present. 



The best sorted sediments of the bay are 

 in the tidal channels, where the sorting co- 

 efficient averages about 1.5. The Trask sort- 

 ing coefficient is 



So 



/coarse quartile 

 fine quartile. 



Atop the marsh surface the sands near the 

 center average about 3.7. The silts (4 to 62 

 microns) are also poorly sorted, averaging 

 5.4, with the worst sorting (7.0) for coarse 

 silts. Clays are relatively well sorted, aver- 

 aging 3.35. Because of their deposition in 

 quiet water, the marsh sediments are not as 

 well sorted as are those from many open-sea 

 environments, as indicated by the gentle 

 slope of the cumulative curves of their grain- 

 size distribution (Fig. 156). 



Organic matter derived almost entirely 

 from the vegetation living atop the marshes 

 is the most concentrated of any common 

 sediment of southern California but not 

 enough to be called peat. Contours of or- 



SEDIMENT TYPE 



Figure 155. General sediment types and percentage organic matter of marsh at Newport Bay. Adapted from Steven- 

 son (1954) and Stevenson and Emery (1958, Figs. 31, 36). 



