Chemostratigraphic framework of a fluvial-tidal transition zone in the Middle Pennsylvanian Cherokee Group of the Forest City basin, Iowa, USA

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ABSTRACT The Cherokee Group, Forest City basin, Iowa, preserves a Lower–Middle Pennsylvanian fluvial-tidal transition zone. We used a combination of detailed core descriptions and whole-rock geochemical data obtained with a portable X-ray fluorescence spectrometer to interpret depositional facies, identify distinct geochemical zones for correlation, and interpret element-mineral associations. In the lowermost unit, the Kilbourn Formation, two geochemical zones record dominantly tidally influenced environments. In the overlying Kalo Formation, three geochemical zones are also dominated by tidally influenced environments. Up section in the Floris Formation, four geochemical zones were identified; this unit is dominated by terrestrial fluvial deposition with occasional tidal influence. The lowermost Swede Hollow Formation has one geochemical zone that records the first major fully marine incursion in the study area. These geochemical zones illustrate subtly fluctuating depositional environments within an ancient fluvial-tidal transition zone experiencing an overall relative fall in sea level while eustatic sea level was rising, likely driven by variations in climate and tectonics. Periodic increases in rainfall may have forced increased stream power in fluvial systems that overwhelmed any tidal influence and pushed marine conditions farther to the southwest throughout the deposition of the Kilbourn, Kalo, and Floris Formations. Furthermore, the final assembly of Pangea during deposition of the Floris Formation resulted in higher topographic regions that provided increased sediment flux to the basin via transcontinental fluvial systems. Eventually, eustatic controls outpaced climatic and tectonic influence, resulting in the first major marine transgression at the base of the Swede Hollow Formation.

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