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Marine Geologic Mapping of Nearshore Region off Myrtle Beach, South Carolina

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sidescan-sonar mosaic—offshore South Carolina
Sidescan-sonar mosaic of the study area: High backscatter is represented by light tones, low backscatter by dark tones.
[larger version 54KB]
The geologic framework beneath and seaward of the littoral zone significantly influences coastlines with limited sand supplies such as much of the Atlantic margin of the United States. This antecedent geology can strongly influence modern beach behavior. Aboard the M/V Atlantic Surveyor from October 30 to November 12, 1999, Bill Schwab, Jane Denny, Bill Danforth, Ken Parolski, Tom O'Brien, Jenna Hill (Woods Hole Field Center), and Shawn Dadisman (St. Pete Field Center) began a program designed to map the nearshore region off northern South Carolina. A 100-105 kHz high-resolution sidescan-sonar system, a 234 kHz interferometric (direct phase comparison) sidescan-sonar system, a 300-3000 Hz Geopulse boomer, a 27 kHz swept FM (CHIRP) subbottom profiler, and a 50-200 kHz fathometer were used to map the sea floor. The study area extends ~65 km along the inner shelf of northern South Carolina from the ~7-m isobath to about 10 km offshore (water depths <14 m).

The goals of the investigation are to determine the regional scale sand resource availability needed for planned beach nourishment programs, to investigate the roles that the inner-shelf morphology and geologic framework play in the evolution of this coastal region, and to provide baseline geologic maps for use in proposed biologic habitat studies. The use of a Submetrix interferometric sidescan-sonar system enabled, for the first time, collection of swath bathymetric data on a regional scale in extremely shallow water. These data were edited and gridded at a 10-m cell size. An interpolated surface generated from the gridded data was used in conjunction with the sidescan-sonar imagery to construct perspective imagery in real time.

digital sidescan-sonar mosaic draped over bathymetry
3-D Sidescan-Sonar Plot: Digital sidescan-sonar mosaic draped over bathymetry and swath bathymetry of the northernmost segment of the study area. Vertical exaggeration 245X; sun illumination 45°; sun altitude 30°.
[larger version 54KB]
Although formal interpretation of the data requires intensive analysis, preliminary interpretations indicate that outcrops of Cretaceous and younger sedimentary strata and a limited, mobile, modern sediment deposit dominate the inner shelf. Geomorphic expression of outcrop strata (an erosional surface) follows the variability in the sequence and internal geometry of the eroding strata. In the northern segment of the study area, hardgrounds are generally Cretaceous in age, exhibit higher relief due to differential deflation of the inner shelf, and produce a relatively lower shelf gradient. In the central section of the study area, the hardgrounds are Tertiary in age, are expressed as relatively low-relief erosional surfaces, and produce a relatively steeper inner-shelf gradient. In the southern segment of the surveyed area, the ancestral Pee Dee River system, channel fill, and estuarine and fluvial deposits dominate the inner-shelf stratigraphy and morphology. This area has a more continuous modern sediment veneer overlying a well-defined ravinement surface.

Coastal erosion rates reportedly increase from the northern part of the study area to the south. We speculate that this variation in regional erosion rates might be related to differential erosion of the local substrate, i.e., the Cretaceous strata are more resistant than the Tertiary strata, resulting in the relatively lower shelf gradient and thus, lower coastal erosion rates. The shoreline in the southern segment of the study area can be characterized as a series of low-relief bay-mouth barriers. It is relatively sediment rich (the sediment source is thought to originate from erosion of the submerged headlands and outcrops to the north) and has the accommodation space for the development of a more traditional "Atlantic" barriered coast with higher rates of shoreline change.

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