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 (type-Morrowan). Pitkin cements reflect extensive, meteoric, phreatic diagenesis prior to karstiflcation, 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 warmer, wetter tropics during periods of high-latitude cooling. Basal Cane Hill strata consist of high-energy, shoreface, boulder-cobble conglomerate containing reworked Pitkin clasts. The conglomerate contains an unequivocal rockyshore community consisting of encrusting bryozoans and corals, Trypanites , acrothoracican barnacle borings, and, possibly, the earliest occurrence of Gastrochaenolites . 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.