EXCEPTIONALLY WELL-PRESERVED SUB-ABSAROKA PALEOKARST AND LOWER PENNSYLVANIAN FILL: A WINDOW INTO EARLY PENNSYLVANIAN PALEOENVIRONMENTS

A newly discovered Lower Pennsylvanian cave fill deposit in northern Illinois is remarkable for a diverse macrofossil and pollen assemblage, including the remains of possibly the earliest North American conifer; the superb preservation of the macrofossil remains as charcoal; and the size and condition of the paleokarst and associated fills, which are essentially unconsolidated. The paleokarst is exposed in a 310 m section along the wall of a quarry in Kendall County, Illinois. Well data suggests that the paleokarst extends at least 1 km to the south. The preserved features are all roofed by limestone, so we consider them to be part of a cave system, rather than sinkholes. The approximately one-dozen exposed features range in size from a meter or less in size up to approximately 10 m high and 18 m across. The host rocks for the paleokarst are Late Ordovician limestones of the Dunleith Formation, Galena Group. Karstification probably took place in the late Mississippian. The cave fill itself is highly variable, both within and among caves. The material is very poorly lithified, disaggregating very easily when wet. Many of the smaller caves are filled solely with fine grained laminated clays. Others have distinct units comprised of roof collapse limestone breccias, sandstone, siltstone, clays, and coals. The macrofossils are preserved as charcoal as a result of wildfire and reveal detailed morphologic and anatomical features including stomata with overarching papillae on needle-shaped leaves. The pollen record, along with the lithostratigraphy, supports a correlation with the lower part of the Tradewater Formation (Lower Pennsylvanian). Leaves, rare leafy shoots, and pollen represent one of the earliest occurrences of conifers in the fossil record and indicate the spread of vegetation into extrabasinal habitats. The site is important for our understanding of environmental conditions in the early Pennsylvanian, in particular, the nature of xeric and upland floras during this interval. It is also significant for our understanding of pre-early Pennsylvanian (sub-Absaroka sequence) erosion and the earliest stages of Absaroka deposition on the northern edge of the Illinois basin.