The lack of penetration below the subbottojn reflector over unit II 

 appears to be due to the impenetrability of the unit's surface and not 

 to malfunction of the seismic reflection system or poor sea conditions. 

 Profiles of this area were made over a time period when sea conditions 

 varied and the profiles that were run continuously from the area under- 

 lain by unit II into units I and III showed good penetration in segments 

 underlain by the latter units. 



Reflection unit III extends from eastern Long Bay to Frying Pan 

 shoals, where it is deeply buried by shoal sands, through Onslow Bay to 

 as far north as New Topsail Inlet (Fig. 6). Internal secondary reflec- 

 tors in unit III dip southeast to south at 20 to 40 feet per mile (1:264 

 to 1:132). The pattern is distinctive and suggests that the deposit 

 consists of forset-type beds which were added in a progradational series 

 extending the deposit in a south and southeast direction across the 

 present inner shelf area. 



Figure 13 is a photo reproduction of a unit III seismic reflection 

 profile showing these distinctive beds. Profiles A, B, and C in Appendix 

 A provide a general illustration of the configuration and internal bed- 

 ding patterns of the unit as viewed on a shore-parallel course. Most 

 shore-normal profiles in Onslow Bay also show unit III; however, typical 

 characteristics are best shown by the longer profiles. 



The base of unit III appears to rest throughout the area on the prom- 

 inent green reflector. The dip of the green reflector is generally 

 southeast, while the more variable internal secondary reflectors in the 

 overlying unit III dip from east to southeast. The top of unit III, like 

 unit I, is beveled by an erosional surface (blue reflector) which trun- 

 cates the internal reflectors. This surface slopes very gently in an 

 east-southeast to southeast direction. Unit III is thickest (more than 

 61 meters or 200 feet) in the southeast part of the main grid area and 

 thins toward shore and to the north. Off Wrightsville Beach imit III is 

 little more than 6.1 to 15.2 meters (20 to 50 feet) thick. 



Filled channels, reaching a depth of more than 46 meters (150 feet) 

 in places, cut into units I and III. Fill in these channels often pro- 

 duces characteristic complex internal reflector patterns on seismic 

 reflection records typified by great variability in reflector length, 

 dip, and grouping. Figure 14 is a typical example of one of these 

 channels (see also App. A, profiles B and C) . These complex-bedded 

 channel-fill deposits are collectively called reflection unit IV. Their 

 location and configuration are shown in Figures 6, 7, and 8. 



Most of the large channels of unit IV are located in Onslow Bay. 

 Two large channels were detected in Long Bay but reflection coverage and 

 insufficient penetration provided only fragmentary information. 



Seismic reflection profiles are too sparse north of New Topsail Inlet 

 to define the reflection characteristics of subbottom strata in that area. 

 However, changes are indicated in seismic reflector patterns and sediment 



28 



