b. Resuspension and settling of inner shelf sediment. 



c. Seaward transport of terrigenous sediment in freshwater plumes. 



By taking 15-cm cores, Gagan, Chivas, and Herczog (1990) show that on 

 a shelf-wide scale, in the -20- to -40-m water depth, sediment was eroded 

 to a depth of 6.9 cm, and in water depth less than -20 m, sediment was 

 eroded to a depth of 5.1 cm. Particles finer than medium sand were 

 eroded and transported out of the mid shelf. 



Gagan, Chivas, and Herczog (1990) found that at least 10-30 percent of 

 inner shelf storm sediment is composed of mid-shelf mud, thus indicating 

 the landward movement of fine material. In summary, Gagan, Chivas, and 

 Herczog (1990) support other findings that significant storms are capable 

 of sporadic but efficient cross-shelf transport of suspended sediment. 



Wright et al. (1991) and others (Swift et al. 1983; Niedoroda, Swift, 

 and Hopkins 1985; Niedoroda, Swift, and Thorne 1989) concur with 

 Gagan, Chivas, and Herczog (1990) that the inner shelf is dominated by 

 storm flows, which produce a fining sequence of grain size in an offshore 

 direction, and storm beds including hummocky cross-stratification and 

 storm-graded bedding. 



Wright et al. (1991), using a digital sonar altimeter, also documented 

 bed-level changes of 15 cm at 8 m due to a 'Northeaster' storm. This in- 

 crease is inferred to be a result of offshore migration of sediment lobes 

 possessing abrupt leading edges, which migrate well seaward of the -8-m 

 depth contour. These lobes are indicative of energetic cross-shelf advec- 

 tion, as opposed to gradual diffusion. 



Wright et al. (1991) documented the response of the bed primarily as a 

 result of hydraulic roughness during different weather conditions. Bed re- 

 sponse during fair-weather conditions was characterized by pronounced 

 wave-induced ripples, low sediment mobility, and high apparent hydraulic 

 roughness heights (up to 1 cm). During post-hurricane fair-weather condi- 

 tions, the bed was mantled with redeposited fine sediment and exhibited 

 subtle ripples surmounting irregular ridges and depressions. This mor- 

 phology yielded the lowest hydraulic roughness of all four cases. 



During storm-dominated conditions (wave heights and periods of 3-6 m 

 and 10-20 sec, respectively, and near-bottom wind-driven mean currents 

 of 0.5 m/s) while there were no ripples, a highly mobile plane bed was pre- 

 sent. However, strong wave agitation and a thick wave boundary layer re- 

 sulted in an effective hydraulic roughness moderately larger than that of 

 the ripple-dominated normal fair-weather case. Skin friction and total bed 

 stresses during the storm exceed those of fair-weather conditions by more 

 than an order of magnitude. 



62 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



