IMPLICATIONS FOR CLIMATE CHANGE AND SPELEOGENESIS IN SOUTHERN ILLINOIS BASED ON THE AGE OF REDEPOSITED GLACIAL SEDIMENT IN FOGELPOLE CAVE

Several second-order passages within Fogelpole Cave in southwestern Illinois were filled with a uniform to laminated, and in some places brecciated silt loam (glacial sediments). We trenched and sampled four of these passages and analyzed their organic carbon for concentrations of d¹³C and ¹⁴C to determine the timing and climatic implications of the deposits. The bedrock floor of two passages lies about 1.3 meters above the level of the stream bed in the main passage. The occurrence of the laminated sediment in elevated terraces adjacent to active channels, and the distribution of the sediment-filled passages indicate that the silt loam was rapidly deposited during flooding in a series of linked, subterranean slackwater lakes. The sediment filled the passages and probably the entire cave between 25,000 and 30,000 radiocarbon years before present (RCBP). d¹³C values for organic carbon within the sediment ranged from -15 to -27 ppt. The ¹⁴C ages of the organic carbon ranged from 3,000 to 36,000 RCBP with a gap between 18,000 and 27,000 RCBP. The lightest d¹³C values were found in the youngest samples that ranged from 3000 to 18,000 RCBP and are consistent with C3-type vegetation. The heavier d¹³C values (-15 to -19 ppt) were found in the oldest samples (27,000 to 36,000 RCBP) and fell into the range of C4-type vegetation. The gap in redeposited sediment between 18,000 and 27,000 RCBP is consistent with cold conditions, and possibly the development of permafrost which would reduce recharge to the cave system. The spatial relationship between the elevated floors of the second-order passages and the main passages were used to calculate the initiation of speleogenesis in this region. Using the age of the sediments as a uniform rate for down cutting, and given the present size of the cave, we calculated that Fogelpole Cave and probably 3 other large caves located to the north (inlcuding Illinois Caverns), began to form near the end of the Illinois Glacial Episode about 150,000 RCYBP. The presence of redeposited glacial sediment in the cave suggests a period of extensive flooding and concomitant erosion of surface deposits (sinkhole formation) during the end of the Mid-Wisconsin Interglacial Episode and the beginning of the Late Wisconsin Glacial Episode.