THE SEDIMENTARY ENVIRONMENT AT NEWPORT BAY, CALIFORNIA 



by 



Robert E. Stevenson 



Allan Hancock Foundation 



University of Southern California 



Newport Bay, Calitornia can be considered a relic estuary. The river 

 which carved the steep-cliffed channel in times past has not flowed into the 

 area for many decades, so that the present marsh deposits are strictly of 

 marine origin. Even when the combined flows of the Santa Ana River and 

 Santiago Creek entered the bay it is likely that the tidal currents were the 

 more important carriers of detrital material. This is because the climate of 

 southern California has been one of semi -aridity for many centuries and even 

 major streams flow only intermittently --following intense or prolonged rains. 

 Although Newport Bay has the physiographic features of an estuary, it is 

 probable that sedimentation throughout most of its history has been by marine 

 activity. 



The vertical and horizontal distribution of the marsh sediments graphic- 

 ally portray the depositional history of the bay. Surface samples are pre- 

 dominantly silt and clay with an average median diameter of 0.032 millimeter. 

 An increase in depth shows an increase in grain size until at 4 feet, 95 percent 

 is sand and the median diameter is 0.75 millimeter. This is the upper part 

 of the sand platform upon which the marsh deposits were lain. Today the dom- 

 inant material being deposited is of clay-size. Because no river enters the 

 area and it is well -protected from wind action by the high bluffs and wave ac- 

 tion by the spit and lagoon, erosion and deposition is primarily by tidal cur- 

 rents. The source of inaterial is mainly the marshes themselves, A slow 

 redistribution of the marshes is, therefore, taking place and the material 

 being deposited on the marshes at the points of tidal overflow is in the clay- 

 size range. The development of this depositional environment has resulted in 

 unusually well-sorted sediments in the clay-size fraction. 



The pH of the marsh sediments is low (4.6 ) and accounts for the minor 

 amounts of calcium carbonate and the fragile, crumbly nature of any shells in 

 the muds. In general, the pH decreases with distance from the marsh creeks, 

 with increase in organic content of the sediments, and with decrease in grain 

 size. 



Six floral communities are established on the marshes. The most exten- 

 sive is the Sueadetum which covers the flat surfaces of all the large marshes. 

 The Zosteretum and Distichlidetum are the narrowest and least extensive com- 

 munities as both are confined to the extremes of the marsh environment; the 

 former in the shallow channels and the latter where the upper marshes abut 

 the cliffs. The Spartinetum is extensive on portions of the lower marshes that 

 slope gently, but it may be narrow or even absent where the marsh slope is 



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