THE LITHOSTRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE PELLA FORMATION (MISSISSIPPIAN) IN KEOKUK AND WAPELLO COUNTIES, SOUTHEASTERN IOWA

The Pella Formation is the uppermost Mississippian rock formation in Iowa, cropping out mainly in the southeastern part of the state. It typically consists of a relatively thin (< 2m) basal limestone that is overlain by thicker (up to 7.5m), highly fossiliferous marl. The Pella Formation rests unconformably on the St. Louis Formation, the upper part of which is nonmarine limestone and plant-bearing sandstone. The top of the Pella Formation is a major erosional unconformity corresponding to the Mississippian-Pennsylvanian systemic boundary. Carbonate petrography of the lower part of the Pella Formation enabled the identification of distinct brachiopodal and oolitic lithofacies that are locally correlatable and correlatable less precisely over greater distances. Bioclasts increase in diversity up-section and quartz sand decreases in abundance up-section, suggesting that at least the lower part of the formation is a deepening-upward deposit in which restricted marine and then normal marine conditions became established pursuant to transgression of the underlying unconformity surface. The unconformity between the upper Pella Formation and overlying Pennsylvanian rocks is rarely exposed in outcrops because it is bracketed by relatively nonresistant beds that are eroded and covered by vegetation. The unconformity is well preserved in a core from Wapello County. Uppermost Pella beds in the core are deeply weathered and rubbly, interpreted as a coarse soil residue.