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Pennsylvanian paleokarst and cave fills from northern Illinois, USA: A window into late Carboniferous environments and landscapes

¹ University of Illinois at Chicago, Earth and Environmental Sciences, 845 W. Taylor St., Chicago, Illinois, 60607, USA
² Royal Holloway University of London, Department of Earth Sciences, Egham, Surrey TW20 0EX, UK
³ Field Museum of Natural History, Geology Department, 1400 S. Lake Shore Drive, Chicago, Illinois, 60605, USA
⁴ University of Kentucky, Kentucky Geological Survey, 228 Mining and Minerals Building, Lexington, Kentucky, 40506-0107, USA
⁵ StrataPower, 1023 Homestead Dr., Yorkville, Illinois, 60560, USA
⁶ plotnick{at}uic.edu

A new fault-associated paleokarst and cave fill has been discovered in north-central Illinois, emplaced in Ordovician limestones. The paleokarst preserves many original solution features, such as oriented grooves, pendants, and half tubes. Many of the ancient cave passages have rounded bottoms and flat roofs. Together these suggest that the original elliptical, phreatic cave passages grew upward by paragenesis, in which the floor of the cave is protected from dissolution by the presence of sediment, while the ceiling of the cave grows upward by dissolution. The fill is dated as Moscovian (Middle Pennsylvanian) based on palynological data and can be correlated with the Tradewater Formation. The fills are composed of a fining-upward sequence of relatively unindurated clastic sediments that contain well-preserved plant fossils, most notably voltzialean conifer and cordaite remains, representative of vegetation living in well-drained areas. Many of the macrofossils are fragmentary but charcoalified and, along with the megaspores, are uncompressed and preserve exceptional morphological and anatomical data. The presence of abundant charcoal in the fills, as well as diagnostic polycyclic aromatic hydrocarbons, indicates significant wildfire activity in this area during this interval.