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South Carolina Coastal Erosion Study

Geologic Mapping

The USGS/South Carolina Sea Grant Consortium Coastal Erosion Study is centered within Long Bay, a large sediment-starved embayment bound to the north by Cape Fear and associated shoal complexes, and to the south by Santee Delta/Cape Romaine (Figure 1). South Carolina's Grand Strand, the focus of this study, is centered on a 100-km arcuate stretch of coastline within the apex of Long Bay. This region has few large inlet/fluvial systems and is characterized by a limited influx of modern sediment.

Geophysical surveys, designed to define the geologic framework within the Grand Strand region, were conducted in 1999 - 2003. The survey area extends from seaward of breaking waves (< 1 km) to ~10 km offshore, and covers approximately 700 square kilometers of the inner-shelf and lower shoreface (Figure 1). High-resolution sidescan-sonar and interferometric bathymetric sonar systems were utilized to map the surficial sediment distribution and regional bathymetry; seismic-reflection systems were used to define the underlying geologic structure. Surface grab samples, cores, video and photographs were collected throughout the region to ground-truth the geophysics.

The inner-shelf off South Carolina's Grand Strand has a low-relief, gently dipping seaward slope with depths ranging from 4 meters in the nearshore to 14 meters offshore (Figure 2).

Figure showing bathymetry offshore of Myrtle Beach, SC.
Figure 2. Bathymetric data collected offshore of Myrtle Beach,
SC 1999 - 2003. Depth ranges from 4 meters in the nearshore, to
14 meters offshore. Click on image to view larger version.

Bathymetric highs are present in several locations throughout the survey area; shoal complexes offshore of Waites Island and Murrells Inlet, a shore-oblique oblate feature trending NE-SW offshore of Myrtle Beach, and a ridge complex north of Winyah Bay, proximal to North Inlet. In general, these bathymetric highs correspond to regions on the inner-shelf that have some ( > 1 meter) accumulation of modern sediment.

The surficial character of the seafloor is somewhat variable within the Grand Strand region Figure 3). High-resolution sidescan-sonar systems were used to record the acoustic character of the seafloor. Variations in the strength of the return signal are represented as gray-scale values within an image; areas of low-backscatter (low-reflectance) are displayed as dark tones, areas of high-backscatter (high-reflectance) are displayed as light tones. Offshore of the Grand Strand, surficial grab samples and video show areas of low-backscatter to be characterized by fine-medium sand, silt, and mud.

Figure of sidescan-sonar data collected offshore of Myrtle Beach, SC.
Figure 3. Sidescan-sonar data collected offshore of Myrtle Beach,
SC, 1999 - 2003. High-backscatter is represented by light tones
within the image, low-backscatter is represented by dark tones. Click
on image to view larger version.

Conversely, areas of high-backscatter are shown to be characterized by coarse sand, shell hash, hardground (outcrop), and gravel. In general, fine-medium sand, low-backscatter areas correspond to relative bathymetric highs, while high-backscatter areas dominated by coarse-sand and outcropping older strata correspond to relative bathymetric lows or areas of constant relief.

Figure showing examples of seismic data collected with three different systems.
Figure 4. Sidescan-sonar data collected offshore of Myrtle Beach,
SC, 1999 - 2003. High-backscatter is represented by light tones
within the image, low-backscatter is represented by dark tones.
(Figure modified from Baldwin, 2002). Click on image to view larger version.

Boomer and chirp seismic-reflection systems were utilized to map underlying geologic structure offshore of the Grand Strand (Figure 4). Interpretation of the seismic records reveals several distinct units: Cretaceous/ Tertiary continental shelf strata influenced by regional tectonics, showing a gentle dip to the SE, with folding present in the northern portion of the survey area; a regional unconformity representing the latest marine transgression, or several regressive/transgressive cycles, marking the upper boundary to the underlying Cretaceous/Tertiary strata; paleochannels associated with old river systems which incised Cretaceous/Tertiary strata at low-stands of sea level; small paleochannels lying above the regional unconformity, most likely associated with local drainage of small tidal creeks/inlets and swales; and a patchy, discontinuous acoustically transparent modern sediment layer overlying the regional unconformity (Figure 5).

Figure showing seismic profile offshore of Murrels Inlet.
Figure 5. Chirp sub-bottom profile offshore of Murrells Inlet.
The seismic profile clearly displays underlying Cretaceous/Tertiary
strata (red), paleochannels (orange), and the regional unconformity
(blue). Also displayed is a shoal complex associated with Murrells
Inlet. Vertical scale is displayed as two-way travel time in
milliseconds and approximate depth in meters (assuming 1500
meters/second speed of sound). (Figure modified from Baldwin, 2002).
Click on image to view larger version.

Figure showing isopach map of modern sediment thickness within study area.
Figure 6. Map displaying modern sediment thickness within
study area. Sediment thickness ranges from 0.5 - > 6 meters.
Areas devoid of color represent sediment thickness less than 0.5 meters.
Chirp sub-bottom data were used to define the modern sediment thickness.
(Figure modified from Baldwin, 2002). Click on image to view larger version.

Regional tectonics, changes in sea-level through time, and modern oceanographic processes have shaped the character of the inner-shelf offshore of South Carolina's Grand Strand. Geophysical and sample data reveal the inner-shelf to be comprised of a patchy, discontinuous sand sheet, overlying a regional transgressive unconformity. Sediment thickness increases to the south (Figure 6). Large accumulations of sediment generally occur near inlet systems, and in some areas have been reworked by hydrodynamic processes to form shoal and ridge complexes. The shore-oblique sand body offshore of Myrtle Beach is an exception (Figure 7). Current research is focused on defining the origin of this feature. In areas devoid of modern sediment, or with only a thin veneer of sediment, Cretaceous/Tertiary units and paleochannel fill outcrop at the seafloor, primarily in large areas to the north and in troughs between sand ridges throughout the study area. Reworking of these older units by modern oceanographic processes generates a supply of sediment to this coastal region.

Figure showing oblate feature lying oblique to Myrtle Beach.
Figure 7. Map displaying location of bathymetric high offshore of Myrtle Beach.
This oblate feature lies oblique to the shore off of Myrtle Beach. This feature
is roughly 11 km long and 3 km wide. Current research is focused
on defining the origin of this feature. Click on image to view larger version.
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This page last modified on Monday, 14-Jan-2013 04:51:05 EST