300-Million-Year-Old Soils and Their Implications on Predicting Ancient Climate Conditions

Monica M. TurnerÐWilliams and Wan Yang. Department of Geology, Wichita State University, Wichita, KS 67260 [email protected]

Alternating beds of nonmarine and marine siliciclastic and carbonate rocks of the Late Pennsylvanian age (300 million years ago) are exposed in southeast Kansas and northeast Oklahoma. They form the Oread Cyclothem and were deposited during shoreline transgression and regression when Kansas and Oklahoma were located in the equatorial climatic belt. Climate was expected to be warm and humid year round. Paleosols developed upon floodplain shale and sandstone. Close to the channel area, Protosols are thick (150 cm) and Calcisols are thin (50 cm) with cm-scale slickensides. In contrast, the floodplain Protosol is thin (50 cm), and thick (80Ð200 cm) Vertisols, rich in calcitic nodules, replace Calcisols. Commonly, 3Ð4 paleosols stack upward from thin (50-cm) Protosol to thick (80Ð200 cm) Calcisol or Vertisol. Changes of type, thickness, and number of stacked soils away from river channels indicate increasing soil maturity and exposure following episodic deposition. Lateral and vertical variability were influenced by topography and parent rock from the high, siltstone-rich levee area of active sedimentation to the low, mudstone-rich floodplain. We interpreted a semi-arid to subhumid paleoclimate with strong seasonal precipitation based on the regional occurrence of Calcisols and Vertisols and their modern analogs. Paleosols are good paleoclimatic indicators because soil formation is influenced by climate, but the variability caused by local bedrock type, topography, and hydrography complicates climate interpretation.