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Marine Geologic Mapping of Nearshore Region off Myrtle Beach, South Carolina
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Sidescan-sonar mosaic
of the study area: High backscatter is represented by light tones,
low backscatter by dark tones.
[larger version 54KB]
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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.
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]
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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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December 1999
in this issue:
cover story:
Myrtle Beach Mapping
Long Island Sound
Moloka'i Coral Reefs
North Carolina Outer Banks
Southern California Benthic Habitat
Cape Cod Community College
Nat'l Environmental Careers Conference
Coral Reef Task Force
Chesapeake Bay
Atlantic Offshore Aggregates
DOE Gas Hydrates
Cooper Co-Chief of Antarctic ODP Cruise
Knauss Fellows Tour Reston
Woods Hole Visitors
Seminar Series
Staff News
December Publications List
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U. S. Department of the Interior | U.S. Geological Survey
