Late Atokan siliciclastic cave-fill deposits within preAbsarokan paleokarst in the Burlington-Keokuk limestone, Dade county, Missouri
Two siliciclastic-filled paleocaverns within the Mississippian (Osagean) Burlington- Keokuk Limestone, which were formed in pre-Desmoinesian (Middle Pennsylvanian) time, are described from exposures in Dade County, Missouri. Petrographic and sedimentological analyses suggest that these cave-fills were derived from terrestrial sources. The age of the cave-fills were determined to be Late Atokan (Lower Pennsylvanian) based on their spore and pollen assemblage. One of the caves examined is filled primarily with coarse to fine-grained sand. Sedimentary structures include plane-beds, cross-stratification, scours, flaser bedding, softsediment deformed layers, flame structures, and normal-graded bedding. Cave-roof collapse and associated boudinage, as well as slack water shales, separate thick sandstone layers. This paleocavern has been interpreted to have been filled by fluviokarst processes with sediment entering primarily through sinking streams. The cave-fill in the other paleocave examined is primarily mudrock, shale, and sandy siltstone with some roof-collapse breccia. These rocks contain abundant organic matter, including wood fragments and charcoal, as well as abundant pyrite and secondary sulfate minerals. This paleocave has been interpreted to have been filled by soil and residuum entering through sinkhole collapses beneath marshes or swamps and by some in-flowing, low-energy water currents. The recharge points of a cave can be inferred by the type of sediments that it is filled with and recharge points dictate cave geometry. These recharge points can be seen on subsurface maps and an area of possible paleocaves can be drawn between these points. Sandstone cave-fill and fractured limestone host-rock around shale-filled caves are potential hydrocarbon reservoirs in the subsurface of Kansas and northern Oklahoma.