Geology, geomorphology, and tectonics of the Congaree River Valley, South Carolina

The Pliocene to recent Congaree River Valley is a major feature of central South Carolina. Several observations hint that the valley architecture is tectonically influenced. New geologic mapping has recognized 14 surfaces in the Congaree River Valley terrace complex and numerous features within the floodplain. The geomorphic complexity of the floodplain and lower terraces illustrates the difficulty with generalizing terrace surfaces and indicates that river migration is more of a gradual process than an abrupt, dramatic cut-and-fill process.; Macromorphologic analysis of the modern channel indicates five distinct reaches. Reach 1 is a straight channel influenced by shallow bedrock. Reach 2 is an incised, symmetrically meandering reach influenced by the Augusta Fault. Reach 3 is an irregularly meandering channel dominated by autocyclic processes. Reach 4 is an extremely sinuous channel influenced by the Bates Mill Creek Fault. Reach 5 is a sinuous channel influenced by both resistant southern bluffs and recent deposition to the north.; Several measurements indicate that the position of the modern channel is shifted 80-90% towards the southern margin of the floodplain. Abandoned channel orientations indicate that net late Pleistocene to recent river migration is directed ∼225°.; Structure contour maps indicate regional horizons striking 054-065° (parallel to regional structural trends) and dipping 0.13-0.30° southeast. Regional drainages oriented ∼150-160° (perpendicular to these trends) appear most geomorphically stable. Several regional elements, including terraces in the upper Congaree River Valley, record progressive rotation towards this regional drainage direction. Terrace geometries in the lower valley trend oblique to this direction.; The Congaree River is attempting to rotate from a Pliocene bearing of ∼110° to the modern downdip orientation of ∼150-160°. This rotation is successful in the upper valley but progressively hindered downstream by incision into increasingly thick and resistant regional strata. This has resulted in apparent southward migration of the terrace complex. These regional trends are complicated by at least two discreet faults. Floodplain geometries indicate that at least some of this deformation is geologically recent and ongoing. This subtle, up-to-the-northwest tectonic tilting is interpreted to reflect post-middle Miocene denudational isostatic rebound of the Appalachians and not Cape Fear Arch uplift.