ORGANIC SEDIMENTATION IN MARGINAL FLOODPLAIN ENVIRONMENTS: A WORKING STRATIGRAPHIC MODEL FOR QUATERNARY GROUNDWATER RIMSWAMP DEPOSITS, CONGAREE NATIONAL PARK

Recessed embayments along the northern margin of the Congaree River valley in Congaree National Park (CNP), SC, contain a forested wetland ecosystem that is largely distinct from the adjacent floodplain. These areas, termed groundwater rimswamps (GRS), are underlain by thick accumulations (10+’) of organic-rich sediments constantly saturated by groundwater exfiltration from the adjacent bluff. Theoretical considerations and reconnaissance data, including LOI and oriented microtome sections, suggest that accumulation of organic matter in these areas is highly sensitive to allocyclically-influenced changes in the regional groundwater table. Climatic and eustatic signals are relatively strong in these areas because these deposits are not subject to reworking or complicated by competing changes in sediment supply and channel dynamics.

GRS deposits in CNP can be divided into three intervals. The basal interval, dating to approximately 21 ka at 10’, consists of a series of Late Pleistocene terraces containing ox-bow lake, point bar, and proximal floodplain deposits, and capped by a significant, inorganic-rich, non-erosional unconformity. The dominant, central interval consists of organic-rich sediments deposited in a true GRS environment. Oriented microtome sections indicate a wide range of constituents including charcoal, fungal spores, pollen, macrophyte tissue, cell fillings, fecal pellets, and humified matrix. Systematic changes in these parameters indicate a number of paleoenvironmental changes. LOI data confirm wt% organic matter contents ranging from 20 to 80%. The basal age of this interval is > 3. 5 ka, consistent with a 4-6 ka rise in the regional groundwater table. The uppermost interval (20 cm), which reflects environmental conditions during historic and modern times, consists of inorganic-rich sediments. These sediments likely represent an oxidized weathering residue related to a drop in the groundwater table caused by the 1930 emplacement of the Saluda dam, but could also represent colluvium related to erosion caused by historical land use patterns. Further research on these systems is relevant to a refined (re)assessment of the ecology, function, and/or stratigraphy of floodplain margins as well as management of analogous systems, both modern and ancient.