TESTING A CARBON ISOTOPIC STRATIGRAPHIC AGE MODEL FOR THE NEOGENE HIGH PLAINS SUCCESSSION IN WESTERN KANSAS

Traditional stratigraphic approaches to analysis of the Neogene High Plains Succession (HPS) in western Kansas (Miocene-Pliocene Ogallala Formation and undifferentiated younger Pleistocene units) have not been fruitful. Repetitive facies associations and a lack of widespread marker beds have hampered development of stratigraphic correlations. Calcic paleosols occur with high stratigraphic frequency in the HPS, and work by Fox and Koch (2003) demonstrated the potential for dating HPS deposits in Kansas using the δ¹³C values of pedogenic carbonates and organic matter. Their work showed a systematic long-term increase in δ¹³C values over the Late Miocene-Pleistocene interval, a trend related to the evolutionary innovation of the C₄ grassland prairie biome. Their dataset over the last 10 Ma are used to formulate a stratigraphic age model correlating age of deposit with δ¹³C of pedogenic carbonates with the exponential decay function: y = 10.669^0.51x -6.995; where x = Age (Ma); y = δ¹³C of pedogenic carbonate; with R² = 0.77. The pedogenic carbonate correlation curve rises from a Late Miocene baseline value of -7‰ VPDB up to Pleistocene values greater than 0‰ VPDB. A test of this model is being carried out on a research core drilled by USGS in the Cimarron National Grassland (CNG site; McMahon et al., 2003, USGS WRIR 03-4171). δ¹³C analyses of bulk organic carbon throughout the section are at 30 cm sample spacings, and pedogenic carbonates at spacings of 1-3 m. Initial results from the 37 m to 60 m depth levels below the land surface at the CNG site show bulk organic δ¹³C values from -27 to -26‰ VPDB, and pedogenic carbonate δ¹³C values of about -7‰ VPDB. These results are indicative of Neogene paleofloras dominated by C₃ biomass, and by analogy to Fox and Koch (2003) suggest correlation with Late Miocene portions of the Ogallala Formation. The fine-scale chemostratigraphic structure of the profile offers prospects for high-resolution paleoclimatic/paleobotanic correlations. Chronostratigraphic calibration of the chemostratigraphy by volcanic ash-bed chronology is pending.