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SummaryHigh-resolution seismic stratigraphy interpreted from CHIRP and boomer seismic reflection profiles provide a detailed description of the shallow geologic framework within South Carolina's Long Bay. This framework includes: Cretaceous/Tertiary continental shelf strata; a regionally defined marine transgressive unconformity that serves as the upper boundary to underlying Cretaceous/Tertiary strata; widespread paleochannel incision and channel fill of varying age; and a patchy and discontinuous lens of modern sediment (Pleistocene and younger) blanketing the shoreface and inner continental shelf. Lithified late Cretaceous and Paleocene sedimentary strata provide the substrate for mobile sediment on the inner continental shelf. Monoclinal dip and folding of these strata indicate uplift to the north, and regional north – south compression. Layers within these strata exhibit considerable variation in erosional resistance. Fluctuating sea level and modern hydrodynamics have caused the strata to differentially erode, yielding an identifiable upper boundary. The elevation of the unconformity has been mapped across the area and is interpreted to represent an erosional surface formed during the last marine transgression. Variations in the seismic signatures generated by truncation of these strata, both at the seafloor and on the margins of channel incisions, indicate that individual horizons within these strata have undergone differential erosion. Paleochannel incisions are common features within this framework, and two general types have been identified. Type I paleochannels are large channels incised into underlying Cretaceous/Tertiary strata. These incisions have been truncated by a recent transgression, and are likely the result of large fluvial systems cutting down into subaerially exposed continental shelf strata during prior low stands in sea level. Differential erosion exerts a primary control over the positioning and long-term reoccupation of these features. Channel fill appears to be largely lithified and generates a variety of seismic signatures indicating infill patterns and relative age. Type I incisions increase in size and number towards the southern portion of the study area, south of the inferred onshore Cretaceous/Tertiary contact. Type II paleochannels are observed within sediment accumulations above the regional transgressive unconformity. These incisions are much smaller and have little cross shelf continuity. Type II paleochannels are interpreted to represent previous locations of local drainage systems, such as tidal creeks, swashes or small inlets. Where multiple channels exist in an area, relative ages can be assigned through vertical stratigraphic position. Modern surficial sediment exists primarily as a patchy and discontinuous sand sheet overlying the transgressive unconformity. Surficial sediment cover increases towards the southern portion of the study area, south of the inferred Cretaceous/Tertiary contact. Throughout much of the northern portion of the area sediment accumulation is below the resolution of seismic reflection systems utilized here. In general, significant accumulations of surficial sediment occur in proximity to tidal inlet systems. The large shore oblique sand body offshore North Myrtle Beach is the exception to this trend. Sediment comprising the shoals associated with tidal inlet systems has been reworked by modern hydrodynamic processes into shoreface attached and detached ridges. Where sediment cover thins, both in broad expanses throughout the northern portion of the study area and in the swales between sand ridges, underlying Cretaceous/Tertiary strata and paleochannel fill are exposed, at the seafloor. Seismic interpretations are supported by backscatter patterns in side-scan sonar imagery, swath bathymetry, and groundtruth data.
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