Origin and Evolution of Illinois' Longest Caves: An Integrated Approach to Interpreting the Geologic and Paleoclimatic Records
Geologists at the Illinois State Geological Survey (ISGS) are taking a multidisciplinary approach in their investigations of the origin and evolution of long, branchwork type caves and their deposits in southwestern Illinois. Fogelpole Cave and Illinois Caverns are Illinois' longest caves. Both caves have active streams flowing through them. The caves also contain abundant fluvial sediments, flowstone, speleothems (stalagmites and stalactites), and breakdown, thus showing evidence of past climate change, large floods, and major earthquakes. By systematically mapping and dating these deposits, the geologists are beginning to see correlations among groups of deposits and their relationships to specific historic and prehistoric climatic and seismic events. From these results, the geologists are developing a conceptual model of the timing and mode of cave initiation, development, and deposition of sediments and speleothems within the caves and their relationship to paleoclimate conditions.
The results obtained to date suggest that the large caves in southwestern Illinois were initiated by glacial melting between 140,000 and 170,000 years ago (sometime near the end of the Illinois Glacial Episode and the beginning of the Sangamon Interglacial Episode). Cold, glacial meltwaters probably began infiltrating into vertical fractures and flowing along horizontal bedding planes within the calcite rich St. Louis Limestone. The present cave ceilings mark the top of the water table during that period. The continuous flow of water through these developing crevices and conduits result in additional dissolution of rock and downcutting of the caves that continue today (incision rates appear to have ranged from 0.032 to 0.048 cm/yr). Remnants of flowstone near the cave ceilings and stalagmites on benches recorded the time of exposure of these cave levels, as well as timing of wet and dry periods in Illinois. Side passages filled with fine grained sediment are evidence of a major flood or series of floods that nearly filled the caves about 42,500 years ago. This event was also recorded in the stalagmites that have been sampled. Finally, the presence of many small, white stalagmites have been dated; these are thought to have initiated at two distinct times, about 90 and 190 years ago, which correlate with two major earthquakes in the region, one that occurred in 1917 and another in 1811 1812, the latter being generated by the New Madrid Seismic Zone. These results indicate that caves contain a wealth of paleoclimatic and possibly seismic information that the ISGS geologists are only beginning to uncover.