RIMSWAMP COAL-FORMING ENVIRONMENTS IN FLOODPLAINS OF THE SOUTHEASTERN UNITED STATES

Although coals formed in floodplain settings are well represented in the geologic record, little research has been undertaken on the petrologic characteristics of one of the most common peat-forming settings in the modern floodplains of the southeastern US, that is, those formed at floodplain edges within arc-shaped recessed embayments in the bluffs caused by past meandering of the river. These wetlands can become enriched in phytogenic organics due to: (1) lack of detrital inorganic input from the river, (2) persistent flow of ground water into them from the bluffs, and (3) slight doming of the peat surface. Preliminary results from one of these sites in the Congaree River floodplain (Congaree National Park, SC) reveals a peat deposit that is thicker and older than any similar deposit yet encountered in the Southeast. As a consequence, this study focuses on petrographic and palynologic characteristics of this site and compares these findings with similar sites in other floodplains of the Southeast.

Four distinct petrologic zones were defined. The basal zone is dominated by biogenic and detrital silica, fine-grained alginite-sporinite rich organics, and tissue fragments indicative of ponded conditions. Zone 2 contains components indicating emergent aquatic vegetation, and Zone 3 is characterized by an increase in arboreals. These two middle zones are marked by a dominance of matrix, but the matrix is more organic-rich and contains indicators of bioturbation. Abundant resins occur here as well. Zone 4 is characterized by enrichment in pre-vitrinites and pre-cutinites, but also contains significant pre-fusinites and pre-funginites. This zone represents a slightly domed, forested wetland now largely controlled by fluctuations in groundwater flow from the bluffs. These results are being used to develop models that may be useful in predicting occurrence and potential petrographic composition of rimswamp coal beds.