CHEMOSTRATIGRAPHY AND SEDIMENTOLOGY OF LOWER-MIDDLE PENNSYLVANIAN STRATA IN THE FOREST CITY BASIN, SOUTHERN IOWA

The fluvial to marine transition zone is defined in modern tidal-river systems by the segment of a river that is intruded by marine currents and saline waters, however, relatively few studies describe the stratigraphic record of this transition zone in the ancient realm. An ancient fluvial to marine transition zone is hypothesized to have been present in southern Iowa during the Early-Middle Pennsylvanian. During this time, accommodation on the North American mid-continent was linked to sea level rise, and consequently, the lower Cherokee Group in the Forest City Basin, southern Iowa, has been interpreted to locally contain both terrestrial and marginal marine deposits. This study employs the emerging analytical technique of handheld XRF analysis with the aim of recovering a record of paleosalinity from the lower Cherokee Group strata. This record is incorporated with a pre-existing palynostratigraphic framework to re-evaluate the Early–Middle Pennsylvanian regional depositional models and sediment dispersal patterns. Our preliminary results suggest that the Kilbourn, Kalo, and Floris Formations (Atokan-Lower Desmoinesian) of the lower Cherokee Group may be representative of a laterally extensive fluvial to marine transition zone as much as 100s of km from the coeval shoreline. This framework suggests a transition from dominantly freshwater conditions during the Early Pennsylvanian to dominantly brackish water conditions during the late Early-Middle Pennsylvanian, which is consistent with the long-term trend of eustatic sea level rise during this time. Strata in this study likely represent a relatively low-energy fluvial system confined to incised paleovalleys in a low accommodation setting. The funneling effect of the incised valleys likely permitted tidal influence to propagate far upstream and resulted in sedimentary structures and stratigraphic patterns that may be typical of the fluvial to marine transition zone.