58 



Additional differences between the two sites include the transition 

 zone, which is between the -2.0- and -5.0-m water depths at Sapelo Island, 

 and between the -9.3 and -18.7-m depths at Port Hueneme. Offshore 

 facies are characterized by the presence of palimpsest sediments, defined 

 as reworked sediments of the continental shelf, and occur seaward of 

 -5.0 m at the Sapelo Island site, and seaward of -18.7 m at the Port Hue- 

 neme site. In addition, storm units (parallel laminated to burrowed beds, 

 separated by erosional contacts) are more clearly developed at the Port 

 Hueneme site sequence. 



In a study of Topsail Island, North Carolina, Schwartz, Hobson, and 

 Musialowski (1981) collected data supporting the subdivision of the inner 

 shelf into upper, middle, and lower inner shelf zones. These zones corre- 

 spond to the inner shelf, transition zone, and offshore facies attributed to 

 the Sapelo Island, Georgia, coast site by Howard and Reineck (1981). 

 Each zone is related to a particular set of nearshore processes and result- 

 ing stratigraphical characteristics. The upper inner shelf is dominated by 

 surf conditions (including longshore currents) and maximum wave shoal- 

 ing effects just prior to breaking. The approximate water depth range of 

 the upper inner shelf is estimated to be between 0.0 m and -2.0 m based 

 on sedimentary structures, sediment grain size characteristics, and 

 changes in profile shape). Stratigraphically, the upper inner shelf is char- 

 acterized by subhorizontal laminae and very low-angle, thinly laminated 

 units, and by local occurrences of inverse textural grading. 



The middle inner 5/ie// (approximate water depth from -2.0 to -4.0 m), 

 is dominated by relatively strong shoaling effects and coastal currents that 

 produce significant downward scour and sediment transport during storm 

 events. This facies is dominated by subhorizontal laminae, trough cross- 

 bedding, low-angle foreset laminae, and minor bioturbation structures. 

 The lower inner shelf, (water depth from -4.0 m to -6.5 m), is slightly to 

 moderately affected by fair-weather waves, is stratigraphically dominated 

 by subhorizontal to low-angle laminar bedding, small-scale trough or rip- 

 ple bedding, and has moderate to locally abundant bioturbation. Nor- 

 mally, graded beds, although sometimes poorly defined, occur throughout 

 the inner shelf. 



Storm-related stratigraphy 



Numerous authors have identified storms as controlling sedimentation 

 and stratigraphy of the inner shelf (Appendix B, "Significant (Storm) 

 Event References"). Smith and Hopkins (1972) state that erosion of the 

 continental shelf by severe storms ranges from a few millimeters to centi- 

 meters; sediment is transported off the continental shelf into deeper 

 water. Smith and Hopkins (1972) suggest that deposits are layered, and 

 perhaps graded by storms as sands are covered by silt that settles out in 

 suspension after the storms. 



Chapter 4 Sedimentary Features/Stratigraphy of the Inner Shelf 



