Paleokarst, paleosol, and rocky-shore deposits at the Mississippian-Pennsylvanian unconformity, northwestern Arkansas

The Mississippian-Pennsylvanian unconformity in northwestern Arkansas contains pedogenically altered regolith preserved in low-lying areas on paleokarst on the Chesterian Pitkin Limestone. The regolith is succeeded by transgressive rocky-shore deposits of the Cane Hill Member, Hale Formation (typeMorrowan). Pitkin cements reflect extensive, meteoric, phreatic diagenesis prior to karstification, and paleokarst features include large, dissolution-fragmented Pitkin lithoclasts with numerous solution pipes on a low-relief surface. The overlying, pedogenically altered regolith contains calcretized lithoclasts in clay matrix, suggesting a shift from ever-wet conditions to more seasonal rainfall prior to the Pennsylvanian transgression. That shift may represent local climatic change or short-term fluctuations in mid-Carboniferous tropical climate. The latter interpretation may support global climate models that predict wanner, wetter tropics during periods of high-latitude cooling. Basal Cane Hill strat a consist of highenergy, shoreface, boulder-cobble conglomerate containing reworked Pitkin clasts. The conglomerate contains an unequivocal rockyshore community consisting of encrusting biyozoans and corals, Trypanites, acrothoracican barnacle borings, and, possibly, the earliest occurrence of GastrochaenoUtes. Preservation of poorly indurated paleosol beneath high-energy transgressive deposits suggests very rapid transgression consistent with a glacio-eustatic mechanism. Truncated Pitkin strata were removed by karstification and subaerial erosion during the hiatus and not by erosion during transgression.