Carbon isotopic chemostratigraphy of Neogene strata in the High Plains succession of Kansas
by Jon Jay Smith, Greg A. Ludvigson, P. Allen Macfarlane, R. Mandel, Adel Haj, Laura Murphy, and Arlo McKee
Geological Society of America, Abstracts with Programs, v. 40, no. 6, p. 257-258.
Presented at the 2008 Joint Annual Meeting, October, 2008, in Houston, TX.
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 δ13C 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 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. 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 δ13C analyses of bulk organic carbon from lower parts of the CNG borehole show baseline 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 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 show δ13C values up to +0.3‰ VPDB. In addition, δ13C 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 δ13C and δ18O 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.
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