15 deg (to be described in detail in the next section). Hummocky cross- 

 stratification laminae are probably the most persistent physical sedimen- 

 tary structure in this facies, with small-scale oscillation- ripple laminae 

 second in abundance. Cross-bedding, pebbles, and heavy-mineral laminae 

 are not present. This facies contains stratigraphic structures of both the 

 offshore and nearshore zones. The cross-shore transition between bio- 

 genic and physical structures indicates fluctuation of wave energy. The 

 onshore limit of this area is most likely normal wave base, while the off- 

 shore limit is storm wave base. No shells or shell fragments were present 

 in this facies (Howard and Reineck 1981). 



In the offshore facies at Port Hueneme (> -18-m water depth), the pri- 

 mary texture is sandy silt and bioturbation is the dominant sedimentary 

 structure (Howard and Reineck 1981). Energy decreases with increasing 

 water depth, which results in increasing amounts of biogenic activity and 

 a fining of grain size in an offshore direction. Biogenic processes affect 

 up to 90-100 percent of this facies due to the following: 



a. Slow rates of sedimentation. 



b. Brief storm events. 



c. Long periods of relative quiescence. 



Remnant parallel laminae are the only physical sedimentary structures pre- 

 sent. Shells and shell fragments are abundant. Direct or indirect effects 

 of storms are rare. 



In comparing the stratigraphy of the inner shelf off Port Hueneme, Cali- 

 fornia, and Sapelo Island, Georgia, Howard and Reineck (1981) found sev- 

 eral differences in the sedimentary sequences resulting from different 

 wave characteristics (as the tidal range for the two areas is similar). A 

 major difference between sedimentary sequences at the two sites was the 

 water depth at which facies boundaries occur. At the Port Hueneme, Cali- 

 fornia, site, the foreshore-inner shelf boundary is distinct as the parallel 

 laminated sand of the foreshore facies is replaced by large-scale cross- 

 bedding, and small-scale ripple laminae of the inner shelf facies. At the 

 Sapelo Island, Georgia site, a distinction between the foreshore/inner shelf 

 boundary could not be made because the parallel laminated sand of the 

 foreshore facies continues as the dominant sedimentary structure well into 

 the upper inner shelf facies. 



Thickness of the inner shelf facies was also different between the two 

 sites. At Sapelo Island, the inner shelf is 250 m wide and 2 m thick. The 

 upper inner shelf is characterized by parallel laminated sand, and the 

 lower inner shelf is characterized by small-scale ripple laminae. In con- 

 trast, the Port Hueneme inner shelf is 300 m wide and 9 m thick. Large- 

 scale cross- bedding as well as parallel laminated sand and small-scale 

 ripple laminae occur on this inner shelf. 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



57 



