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The Santee River of North Carolina and South Carolina emptied into the sea 75 km west of its modern mouth when the shoreline was 15 to 21 m above present sea level in early Pleistocene time. For a short time, the river deposited a fluviomarine delta lobe (volume 5 cu km) that covered 400 sq km near Summerville, South Carolina, 35 km northwest of Charleston. The Summerville lobe was abandoned before the late Pleistocene, and the sea has not covered the area since then. The original wave-constructed ridge-and-swale topography is still visible; drill holes have revealed the subsurface lithofacies relationships. In the modern Santee delta and the chenier plain of Louisiana, similar topography and patterns of lithofacies reflect alternating dominance of flood-plain deposition nd shoreface redistribution.
Paleontology, paleomagnetic stratigraphy, and sediment mineralogy contribute to the age determination of the Summerville lobe. On the basis of fossil pollen and invertebrates identified by U.S. Geological Survey paleontologists, the Summerville lobe deposits are tentatively believed to be equivalent in age to the Waccamaw Formation (late Pliocene and early Pleistocene) of northern South Carolina. Surficial heavy- and light-mineral suites are more mature, and thus older, than paleontologically dated late Pleistocene shoreline deposits nearby. Less weathered mineral suites below the water table in the Summerville lobe reflect the Piedmont (Santee River) source of the sediment. Preliminary paleomagnetic data are compatible with this age determination.
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