Seismic profiles from shoreface shoals also clearly show the shoals 

 to be underlain by a relatively flat reflecting surface. A basal reflec- 

 tor is traceable beneath the shoreface shoal off northern Assateague to 

 where it intersects or "crops out" on the sea floor, thereby indicating 

 a change in surface sediment character in that region. Profiles from 

 the shoreface shoal complex just offshore of Ocean City also show no 

 acoustic evidence of internal bedding or structure that might indicate 

 mode or direction of growth. The shoals composing the shoal complex 

 are all underlain by a flat reflector that can be projected to intersect 

 the sea floor. 



IV. CHARACTERISTICS AND DISTRIBUTION OF INNER SHELF SEDIMENTS 



1 . Background . 



The availability in recent years of a low-cost coring technique 

 (as opposed to continuous borings) for obtaining information on shallow- 

 subsurface lithostratigraphy and the spatial distribution of sand bodies 

 has provided much insight, and on occasion, has led to revision of previ- 

 ous concepts on shelf Quaternary history (Swift, et al . , 1971; Duane, et 

 al., 1972; Stahl , Kozan, and Swift, 1974; Field, 1974; Sheridan, Dill, 

 and Kraft, 1974; Field and Duane, 1976). Vibratory cores collected for 

 this study are no exception; they have yielded much information on three- 

 dimensional sediment relationships, and when compiled with shallow seismic 

 reflection data, they provide a strong basis for understanding the Quater- 

 nary evolution of the shallow Delmarva shelf. 



There are few published studies on sediments of the study area. 

 Milliman (1972) discusses mineralogy of Atlantic shelf surface sediments, 

 including the Delmarva inner shelf, but few samples were obtained during 

 this investigation. Most of the detailed studies of this region have 

 been conducted on the Delaware inner shelf (Moody, 1964; Neiheisel , 1973; 

 Sheridan, Dill, and Kraft, 1974). By analyzing the heavy mineral content 

 of samples from Delaware Bay and the New Jersey and Delaware shelves, 

 Neiheisel (1973) concluded that sediment mixing does not occur across 

 the bay entrance. Based on the frequency abundance of nonopaque heavy 

 minerals, particularly sillimanite, he identified four mineralogic prov- 

 inces: (a) fluvial Piedmont, in the Upper Bay; (b) glacially derived 

 continental shelf source, on the southern New Jersey inner shelf (silli- 

 manite content <4 percent) ; (c) a mixed Delaware Bay and glacially 

 derived shelf source in the center of Delaware Bay (sillimanite content 

 4 to 8 percent) ; and (d) a mixed fluvial Piedmont and Coastal Plain source 

 in the west-central Bay and on the Delmarva Atlantic shelf (sillimanite 

 content >8 percent). Neiheisel further concluded that sediment has moved 

 north and into the bay mouth from the Delaware shelf. 



Based on feldspar to feldspar plus quartz ratios (10 percent to 25 

 percent) , Milliman (1972) classed the Delmarva inner shelf sands as sub- 

 arkosic to arkosic. He also reported several anomalies for heavy mineral 

 grains along the central section (Maryland shelf) of the study area. In 

 this region garnet ranges from 31 to 45 percent, but decreases to between 



54 



