of southern North Carolina are listed in Table 1. The formations are 

 shown in probable age relationships to the chronostratigraphic units of 

 Brown, Miller, and Swain. These relationships do not imply precise 

 equivalence; the implication is that the age of the part of the formation 

 which was accessible for study probably falls within the chronological 

 boundaries of the designated unit. 



4. Field and Laboratory Procedures . 



The field exploration phase of the ICONS program uses continuous 

 seismic reflection profiling supplemented by cores of the bottom sedi- 

 ment. Support data are obtained from NOS hydrographic smooth sheets, 

 engineering logs from boreholes, and from published literature. 



a. Data Collection Planning . Geophysical survey tracklines for the 

 study area are laid out in two basic patterns: grid and reconnaissance 

 lines. A grid pattern with variable line spacing depending on regional 

 geology, is used where a relatively detailed picture of sea floor and 

 subbottom geologic conditions is needed. Reconnaissance lines, which 

 are more widely spaced, are used for minimal coverage of the areas be- 

 tween grids. Reconnaissance lines reveal the general morphologic and 

 geologic aspects of the area and identify sea floor areas where more 

 detailed data collection may be advisable. Selection of individual core 

 sites is based on a continuous study of the seismic records as they be- 

 come available during the survey. 



b. Seismic Reflection Profiling . Seismic reflection profiling is a 

 technique widely used for delineating subbottom geologic structures and 

 bedding surfaces in sea floor sediments and rocks. Continuous reflections 

 are obtained by generating repetitive, high-energy sound pulses near the 

 water surface and recording "echoes" reflected from the sea floor-water 

 interface and subbottom interfaces between acoustically dissimilar mate- 

 rials. The compositional and physical properties (e.g., porosity, water 

 content, relative density), which commonly differentiate sediments and 

 rocks, also produce acoustic contrasts (indicated by dark lines on the 

 geophysical records). Thus, an acoustic profile is roughly comparable 



to a geologic cross section. 



Seismic reflection surveys of marine areas are made by towing variable 

 energy and frequency sound-generating sources and receiving instruments 

 behind a survey vessel which follows the predetermined survey tracklines. 

 The energy source used for this survey was a 50- to 200- joule sparker. 

 For continuous profiling, the sound source is fired at a rapid rate 

 (usually four pulses per second) and returning echo signals from sea 

 floor and subbottom interfaces are received by an array of towed hydro- 

 phones. Returning signals are amplified and fed to a recorder which 

 graphically plots the two-way signal traveltime. Assuming in this study 

 a constant velocity for sound in water at 1,463 meters (4,800 feet) per 

 second and for typical shelf sediments of 1,658 meters (5,440 feet) per 

 second, a vertical depth scale was constructed to fit the geophysical 

 record. Geographic position of the survey vessel is obtained by frequent 

 navigational fixes keyed to the record by an event marker. 



15 



