tops of units with different geological ages . None of the structural 

 and erosional features in underlying units affect the configuration of 

 the blue reflector surface which is relatively level and featureless. 

 Most of the development of the blue surface probably occurred during 

 marine transgressions and regressions associated with worldwide Plio- 

 Pleistocene sea level fluctuations. The slope and surface morphology of 

 the inner shelf floor are largely controlled by the blue reflector sur- 

 face which has little or no sediment cover in most places. 



Figures 9 and 10 show contours on the surface of a second mapped 

 primary reflector, the green veflectov, which lies well below the blue 

 reflector. This reflector is the strongest and most persistent found on 

 the profiles. The reflections indicate that the green reflector is a 

 relatively smooth surface dipping in an east-southeast to southeast direc- 

 tion. A projected terrestrial outcrop line trends north-south along the 

 coast of southern Onslow Bay and extends southward into eastern Long Bay. 

 It is believed from evidence, which will be discussed later, that the 

 green reflector marks the top of the Eocene-age rocks in this region. 



3. Reflection Units . 



Reflection unit I, which underlies the southwest part of the study 

 area, extends offshore to the seaward survey limits and probably westward, 

 beyond study limits, into the South Carolina shelf region. Profiles 21, 

 22, and 23 (App. A) and Figure 11 show reflection features of unit I seen 

 in shore-normal profiles. The distinctive internal reflectors suggest 

 forset-type bedding and growth of the deposit by southward progradation. 

 The upper surface lies very close to the shelf floor and there appear to 

 be extensive areas of outcrop. The top of the unit has been beveled by 

 erosion truncating the distinctive internal reflectors and forming an 

 even surface dipping very gently southward. This surface essentially 

 determines the gradient of the inner shelf floor within the area where 

 unit I comprises the uppermost subbottom unit as shown in Figure 6. 



At the base of unit I a strong reflector (profiles E and 21, App. A) 

 extends only a short distance seaward before being lost due to the rela- 

 tively shallow penetration of seismic records in the area. Where the 

 base reflector is visible, the thickness of unit I ranges from nearly 

 to 37 meters (120 feet) and continually increases southeastward. The 

 thickness is probably much more than 37 meters in most of the study area. 



The dip of the base reflector where it can be seen is toward the 

 southeast. The dip of the internal forset-type reflectors within the 

 unit is predominantly toward the south at about 20 feet per mile (1:264). 



Unit II appears to horizontally separate units I and III; however, it 

 may actually overlie one of these units. The top of unit II produces a 

 strong reflection (blue reflector) marking its slightly uneven surface. 

 Coherent reflectors below this surface are rare (Fig. 12; App. A profiles 

 D, E, and 20); thus, the thickness of the unit, with its bedding charac- 

 teristics and relationship to other units, is obscure. 



23 



