discussed in this section and for future identification and correlation 

 of sediment bodies discussed in Section IV. 



II. SEISMIC REFLECTION RESULTS 



1. General . 



Seismic reflection records of the study area show acoustic reflectors 

 to depths ranging from a few meters to as much as 122 meters (400 feet) 

 below the shelf floor. Penetration of 30 meters (100 feet) or more was 

 obtained on most records. Selected line profiles reduced from the seis- 

 mic reflection records are contained in Appendix A. 



All reflectors on seismic reflection profiles are assumed to have 

 some geological significance. In this study reflecting horizons which 

 persist over a large area are called primary reflectors (Fig. 5); such 

 reflectors delineate a laterally extensive acoustic contrast between 

 bounding units related to one or more sediment properties. Between 

 primary reflectors there may be numerous localized reflectors called 

 secondary reflectors. Most secondary reflectors are associated with 

 internal bedding surfaces, local erosional discontinuities such as stream 

 channels, or relatively small sediment bodies of short, lateral extent. 

 The term reflection unit denotes extensive bodies of sediment or rock 

 which can be identified and traced on profiles by their bounding primary 

 reflectors and distinctive internal secondary reflector patterns. 



Most of the main survey area is underlain by four extensive reflec- 

 tion units (I, II, III, and IV) which are bounded by primary reflectors 

 and can be identified by distinctive internal reflector patterns. The 

 approximate areas of outcrop or shallow subcrop (generally less than 3 

 meters or 10 feet) are shown in Figure 6. No reflection units were de- 

 fined north of New Topsail Inlet (reconnaissance cirea in Fig. 6) because 

 of sparse data coverage. However, available cores and profiles indicate 

 that this area is probably closely underlain by more than one distinctive 

 sediment or rock unit. 



2. Primary Reflectors . 



The two most extensive primary reflectors in the study area are 

 called blue and green reflectors. The blue reflector marks the top of 

 reflection units I, II, III, and IV. The thickness of sediment above 

 the blue reflector, where it overlies units I, II, and III, is shown on 

 an isopach map in Figures 7 and 8. To show the location and configuration 

 of channels containing unit IV, the isopach base for areas underlain by 

 this unit was shifted from the blue reflector to the channel walls; thus, 

 the isopach interval includes unit IV. The thickness of sediment above 

 the blue reflector overlying unit IV generally does not exceed 3 meters; 

 however, in Frying Pan shoals off Cape Fear the shoal sands overlying 

 the blue reflector exceed 15.2 meters (50 feet) in places. 



The blue reflector marks an erosional unconformity because it trun- 

 cates internal reflectors in underlying units and it transgresses the 



