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DISCUSSION

[ Introduction | Methods | Discussion & Conclusion ]


Generalized geologic column for the South Carolina coast
The Quaternary stratigraphy within the study area can be divided into two sedimentary units: (1) late Pleistocene with at least three stratigraphic sequences and (2) Holocene with only a transgressive portion of a sequence. The late Pleistocene unit overlies a highly variable Tertiary framework comprised of Oligocene to Pliocene aged materials. Late Pleistocene inlets and streams have incised to and into the Tertiary Marks Head and Ashley Formations. The top of the Tertiary is an erosional unconformity as identified in seismic profiles. Pliocene to Pleistocene formations reported from the coastal plain of South Carolina around Charleston Harbor are absent on the inner shelf, but may be found below 10 m water depth. Pleistocene units are discontinuous beneath the islands and thin offshore. This suggests that if deposits from these formations were present and/or discontinuous on the inner shelf they were stripped away by subsequent erosion. The late Pleistocene unit ranges from 3 to 30 m in thickness with an average of 12 m thickness and is composed of relatively thin, stacked, discontinuous, overlapping bedded sequences. The regional distribution of Pleistocene sediments is dependent on the structure and antecedent topography of the Tertiary units. The distinguishing seismic character of this unit includes the numerous erosion channels formed during at least three regressions. Regional drainage patterns for coastal South Carolina reportedly were established during this period of time (240 to 200 ka BP). The exposed late Pleistocene deposits were reworked and/or removed as sea level rose after the last glacial episode creating a ravinement surface with shell lag deposits that separates the late Pleistocene unit from the Holocene transgressive deposits. Thickness of the Holocene transgressive unit ranges from 3 to 5 m inshore and averages approximately 4 m. Offshore the Holocene thins to a feather edge in approximately 7 to 10 m of water, coincident with the attachment of shoreface attached ridges. Barrier-island stratigraphy indicates that the Holocene transgression has been punctuated by eustatic fluctuations.

Figures
Folly Island Kiawah Island Isle of Palms
Depth to Base of Pleistocene Depth to Base of Pleistocene Depth to Base of Pleistocene
Thickness of Quaternary Sediments Thickness of Quaternary Sediments Thickness of Quaternary Sediments
Location of Tertiary Channels Location of Tertiary Channels Location of Tertiary Channels
Seismic Profile:
[ high res | low res ]		 Seismic Profile:
[ high res | low res ]

CONCLUSION

The modern configuration of the central South Carolina coastline is defined by a variety of sand-dominated depositional environments, primarily barrier islands and ebb-tidal deltas. The barrier islands have evolved in response to Holocene sea-level rise and have been previously described as regressive and accretionary (i.e., Kiawah Island, SC) with a basal contact on the late Pleistocene unconformity. The Quaternary sediments prism thins seaward, from approximately 6 meters immediately offshore of the barrier islands, to negligable thickness in about 16 meters water depth. The Pleistocene erosional surface dips seaward, from approximately 7 meters below sea level at the shoreline to about 15 meters at about 16 meters water depth. The current bathymetry appears to mimic the attitude of the Pleistocene erosional surface in this area. The bathymetry as well as the Pleistocene erosional surface is influenced by the presence of older incised channels running normal to the modern shoreline. Finally, seismic profiles collected across the region indicate that inlets associated with the barrier islands have remained in the same general position, migrating within a single incised valley throughout the late Quaternary.

[ Introduction | Methods | Discussion & Conclusion ]
U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
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Updated: 20 November, 1998 @ 04:42 PM (THF)