seaward sediment transport mechanisms responsible for the formation of 

 these structures. Field and Roy (1984) indicate that the most plausible 

 mechanism is the seaward transport of sediment during storm-induced 

 downwelling currents. 



Cacchione et al. (1984), in a study associated with the Coastal Ocean 

 Dynamics Experiment, have identified three types of sedimentary features 

 of the Central California inner shelf up to 2 km from the coast in -65 m of 

 water. These included: 



a. Rocky outcrops. 



b. Elongate depressions of low relief on the inner shelf slightly oblique 

 or normal to the general trend of the isobaths. These depressions 

 contain ripples (heights of 0.40 m; wavelengths of 1.7 m) believed 

 to be formed by large-amplitude, long-period winter surface waves. 



c. Smooth areas of no perceptible relief, but covered with well-defined 

 wave ripples (heights of 0.02-0.05 m, wavelengths of 0.20-0.30 m). 



The proposed generation mechanism of these features is storm-generated 

 bottom currents associated with strong, storm-driven downwelling flows 

 during late fall and winter, steered by underwater rock ledges which scour 

 the surficial fine-grained sediment and expose the coarser-sand substrate 

 in the depressions (Cacchione et al. 1984). 



Small-scale sedimentary features 



Bed form classification. Harms et al. (1975) presented a classification 

 of bed forms in which bed form formation is a function of energy (depend- 

 ent upon the energy source and water depth), and grain size, where a 

 larger grain size effectively reduces the amount of energy affecting the 

 bed (Table 4). The hierarchy of bed form formation by increasing energy 

 includes ripples, megaripples, and sand waves. Within the ripple classifi- 

 cation, a gradation exists from short-crested (0- to 20-cm wavelength), to 

 medium-crested (20- to 40-cm wavelength), to large-crested (40- to 60-cm 

 wavelength) ripples (Reineck and Singh 1986). Within the megaripple 

 classification, a gradation exists from two-dimensional (straight-crested) 

 megaripples, to three-dimensional or lunate (sinuous-crested) megarip- 

 ples, to flat (plane) beds (Figures 11 and 12). 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



45 



