Carbon Isotopic Chemostratigraphy of Neogene Strata In the High Plains Succession of Kansas

Calcic paleosols occur with high stratigraphic frequency in Miocene–Pleistocene fluvial strata of the High Plains Succession (HPS). Fox and Koch (2003) showed a systematic long-term increase in d¹³C values of HPS pedogenic carbonates since the Late Miocene and demonstrated the potential for dating HPS deposits in Kansas using isotope chemostratigraphy. Their dataset is used to formulate a stratigraphic age model correlating age of deposit with d¹³C of pedogenic carbonates with the second order polynomial regression: y = 0.11052x² – 1.7546x + 0.037283; where x = Age (Ma); y = d¹³C of pedogenic carbonate; with R² = 0.75. To test this age model, we are analyzing the isotopic composition of bulk sediments and carbonate nodules from three USGS-drilled boreholes in southwestern Kansas; one located in the Cimarron National Grassland (CNG) in Morton County and the others (CAL-121 and CAL-122) in Finney County (McMahon et al., 2003, USGS WRIR 03-4171). Initial d¹³C analyses of bulk organic carbon from lower parts of the CNG borehole show baseline values ranging between -27 to -26‰ VPDB, and pedogenic carbonate d¹³C values of about -7‰ VPDB. These results are compatible with Neogene paleofloras dominated by C3 biomass, and suggest correlation with Late Miocene portions of the Ogallala Formation. The upper part of the CNG borehole shows bulk organic d¹³C values up to -17‰ VPDB, while carbonates show d¹³C values up to +0.3‰ VPDB. In addition, d¹³C values from pedogenic carbonates show at least three discrete positive isotope excursions within Pliocene age deposits. Isotopic analyses of CAL-121 and CAL-122 boreholes are on-going. A fine-scale intricate d¹³C and d¹⁸O chemostratigraphy of the HPS offers prospects for developing high-resolution paleoclimatic correlations. Chronostratigraphic calibrations using ash-bed chronology and optically stimulated luminescence (OSL) dating are pending.