Inner Shelf Geologic Framework Importance 



Coastlines characterized by limited sand supplies, such as much of the 

 U.S. Atlantic margin, are significantly influenced by the geologic frame- 

 work occurring underneath and in front of the inner shelf (Figure 3). Pas- 

 sive margin coastlines, in particular, are significantly influenced by the 

 geologic framework occurring underneath and in front of the inner shelf. 

 This underlying geological framework can act as a subaqueous headland 

 or hard ground that dictates the shape of the inner shelf profile and con- 

 trols beach dynamics and the composition of the sediment. 



The Atlantic coast of North America is an example of a coast affected 

 by its geological framework. The advance of glaciers during the Pleisto- 

 cene Epoch (characterized by continental glaciations at North America 

 from approximately 2 million years to 10,000 years before present (ybp) 

 (Evernden et al. 1964, Pratt and Schlee 1969) extended as far south on the 

 Atlantic coast as northern New Jersey. North of the moraine terminus, gla- 

 cial moraines composed of till (mixture of clay, silt, sand, gravel and boul- 

 ders) underlie much of the land, islands (i.e. Long Island, Nantucket, and 

 Martha's Vineyard), and offshore banks (i.e. Georges and Nova Scotian 

 Banks). Coastal erosion of some of these features provides a variety of 

 materials to the continental shelf. Conversely, south of the glacial mo- 

 raine (Mid- Atlantic coast south of New Jersey), sediments are dominated 

 by riverine sediments of piedmont streams that intersect the coastal plain 

 strata. 



Along the North Carolina coast, Pilkey et al. (1993) discuss that there 

 exist three categories of underlying geologic framework which influence 

 the inner shelf profile shape: 



a. Subaerial headlands, which are composed of semi-indurated to 

 indurated Pleistocene Epoch or older deposits incised by a wave-cut 

 platform with a perched sand beach on the platform. 



b. Submarine headlands, composed of semi-indurated to indurated 

 Pleistocene Epoch or older units, which form the platform upon 

 which the modern barrier island is perched and either crop out on 

 the eroding inner shelf or occur on the inner shelf as 

 paleotopographic highs in front of the modem inner shelf. 



c. Nonheadland-transgressive inner shelf, commonly composed of 

 Holocene Epoch (the Epoch from approximately 10,000 ybp to the 

 present, which follows the continental glaciations of the Pleistocene 

 Epoch) peat and mud deposits that extend from the modern 

 estuaries, under the modern barrier islands, to crop out in the surf 

 zone and inner shelf. 



Chapter 2 Inner Shelf Concepts 



15 



