16 



The Pleistocene section of the entire North Carolina coastal system rep- 

 resents a complex record of multiple cycles of coastal deposition and ero- 

 sion in response to numerous glacial-eustatic, sea-level cycles (Riggs, 

 Cleary, and Snyder, in press). During each glacial episode, fluvial chan- 

 nels severely dissected previously deposited coastal systems. The sub- 

 sequent sea-level transgression then produced a ravinement surface that 

 migrated landward and further eroded large portions of previously depos- 

 ited coastal sediments by inner shelf erosion. This process of older units 

 supplying sediment to the inner shelf of barrier islands was termed shore- 

 face, or inner shelf, bypassing by Swift (1976). The fluvial channels were 

 sequentially backfilled with fluvial, estuarine, and shelf sediments. Pre- 

 sent day sea level has produced a modem sequence of coastal sediments 

 that have been deposited unconformably over the eroded remnants of Pleis- 

 tocene sequences composed of different lithofacies. Niedoroda, Swift, 

 and Hopkins (1985) stated that this seaward thinning and fining veneer of 

 modem inner shelf sediments over the older Pleistocene lithofacies is 

 ephemeral and easily removed from the inner shelf during major storms. 



On a smaller scale, the Nags Head/Kitty Hawk and the Rodanthe/ 

 Buxton areas on the Outer Banks of North Carolina, although separated by 

 only 40 km, have distinctly different geological settings resulting in sig- 

 nificantly different inner shelf profiles (Pearson 1979) (Figure 6). At the 

 Nags Head/Kitty Hawk area, the inner shelf profiles contain two major 

 sediment units including a modem inner shelf sediment wedge, composed 

 primarily of reworked inner shelf sediments that thin in a seaward direc- 

 tion. These form a thin blanket over the in situ relict sediments that will 

 ultimately crop out on the inner shelf. Pearson (1979) stated that this mod- 

 em sediment wedge is periodically stripped away during extreme high- 

 energy periods; thus exposing, possibly eroding, and transporting the 

 relict units. By this mechanism, relict sediments are eroded and intro- 

 duced into the modem sediment regime. In addition, the relict sediments 

 underlying the thin, variable inner shelf sand sheet must also have a major 

 impact upon the shape of the entire inner shelf profile. 



In the Rodanthe/Buxton area, the inner shelf is controlled by Pleisto- 

 cene hard-bottom topographic features that act as headlands and intersect 

 the lower beach face at acute angles. These topographic features are be- 

 lieved to be a result of indurated Pleistocene stratigraphic units which out- 

 crop in the Rodanthe area (Pilkey et al. 1993). These features include 

 Wimble and Kinnakeet Shoals, permanent features up to 6 m in relief (Fig- 

 ure 7). 



According to Pilkey et al. (1993), these vastly different inner shelf fea- 

 tures have the following characteristics: 



a. They dramatically affect the cross section of the inner shelf and 

 beach profile. 



b. They create major changes in the orientation of the barrier island 

 (particularly at Rodanthe). 



Chapter 2 Inner Shelf Concepts 



