EVIDENCE FOR A REGIONALLY PERSISTENT CARBON ISOTOPIC CHEMOSTRATIGRAPHY IN THE NEOGENE HIGH PLAINS SUCCESSSION OF WESTERN KANSAS

Calcic paleosols occur with high stratigraphic frequency in Miocene, Pliocene, and Pleistocene fluvial strata of the High Plains Succession (HPS), and work by Fox and Koch (2003) demonstrated 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 second order polynomial regression: y = 0.11052x² – 1.7546x + 0.037283; where x = Age (Ma); y = δ¹³C of pedogenic carbonate; with R² = 0.75. 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 borehole drilled by USGS in the Cimarron National Grassland (CNG site; McMahon et al., 2003, USGS WRIR 03-4171). Our δ¹³C analyses of bulk organic carbon throughout the section are at 30 cm sample spacings, and pedogenic carbonates at spacings of 0.3-3 m. Results from lower parts of the CNG borehole show baseline bulk organic δ¹³C values ranging between -27 to -26‰ VPDB, and pedogenic carbonate δ¹³C values of about -7‰ VPDB. These results are compatible with 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 upper part of the CNG borehole shows bulk organic δ¹³C values up to -17‰ VPDB, while carbonates from this interval are still being analyzed. A fine-scale intricate chemostratigraphic structure from δ¹³C and δ¹⁸O data in the CNG profile offers prospects for developing high-resolution paleoclimatic correlations. Chronostratigraphic calibrations of the chemostratigraphy by ash-bed chronology are pending.