surface. Core 19, collected well within the confines of the shoal in 

 15.5 meters (51 feet) of water, contains, 2,7 meters (9 feet) of sand 

 overlying the mud surface. Core 20 (Fig. 32), however, was obtained from 

 the extreme edge of the shoal in water 18.3 meters deep. Cored sediments 

 represent the feather edge of shoal sand as it migrates across a mud sub- 

 strate. Included in the relatively thin (1 meter) surface sand are com- 

 ponents derived from the mud substrate (Fig. 32). At about 0,5 meter 

 (1.6 feet) below the core surface is a peat clast approximately 3 centi- 

 meters in diameter. Between 0.61 and 0.73 meter (2.0 and 2.4 feet) are 

 four large pelecypod valves (oysters, clams). Both components (peat, 

 shells) are characteristic of the underlying mud and show that as the 

 shoal sand shifted southward, certain components of the underlying mud 

 unit were deposited in the shoal. Sediments below the surface sand down 

 to about 2.4 meters are dark-gray, fine sandy, clayey silt with few or 

 no shells. From 2.4 meters to the bottom of the core are alternating 

 silty clays and clayey sandy silts which contained a large white oyster 

 shell at 2.5 meters (8.2 feet). 



In summary, shoals are composed of medium-grained, well -sorted quartz 

 sand that is megascopically massive and locally bioturbated. Mollusk 

 shells (both whole and fragmented) are common. The sand rests on a nearly 

 flat interface with the underlying, typically fine grained sediments. 

 This underlying mud unit is a relict deposit from a low-energy environment 

 (lagoon, estuary). The upper surface of the mud unit was, and is, being 

 eroded by current scour. As shoals migrate over this surface, constitu- 

 ents of the mud are incorporated into the base of the shoal sand. 



V. DISCUSSION 



1. Origin and History of Sediments . 



Shallow-subsurface sediments of the northern Delmarva inner shelf 

 range from Pliocene age to modern and represent the entire sequence of 

 marginal marine depositional environments from fluvial to inner shelf. 

 All sediments are representative of lateral facies existing today in 

 the mid-Atlantic Coastal Plain; there is no sedimentary or structural 

 evidence of ancestral glacial or tropical reef environments. 



Sediments consist almost entirely of terrigenous sand and silt. Clay 

 is present in significant quantity in only one lithologic type. Nonter- 

 rigenous materials include biogenic carbonates (principally mollusks, 

 echonoids, and foraminifera) and reworked authigenic grains, such as 

 glauconite. The terrigenous fraction is dominated by quartz and feldspar. 

 Mica and various high-density silicate minerals compose a few percent. 

 Composition of the heavy mineral fraction indicates derivation from the 

 adjacent coastline and ultimate derivation from the complex metamorphic 

 rocks of the Piedmont province. Nieheisel (1973) showed that sediments 

 from the New Jersey shelf are not transported south beyond the Delaware 

 Bay entrance and that sediments from the northern Delaware shelf are 

 transported north into the entrance. Mineral suites indicate derivation 

 of shelf sediment from a combination of land sources (Piedmont, Coastal 

 Plain, Pleistocene terraces). 



72 



