DEVELOPING A CARBON ISOTOPE CHEMOSTRATIGRAPHIC APPROACH TO UNDERSTANDING NEOGENE STRATIGRAPHIC ARCHITECTURE OF THE 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 units) have not been fruitful. The lack of widespread marker beds, and the occurrence of repetitive facies associations in these fluvial deposits have hampered efforts to develop reliable stratigraphic correlations. Calcic paleosols are ubiquitous in the HPS, and occur with high stratigraphic frequency. Stable isotopic investigations by Fox and Koch (2003, Geology 31:809-812) in Meade Co, KS demonstrated the prospect for dating Neogene deposits using the δ¹³C values of pedogenic carbonates. They (ibid.) showed a systematic long-term increase in δ¹³C values over the Late Miocene-Pleistocene interval, a trend related to evolutionary innovation of the C₄ grassland prairie biome. Their (ibid.) dataset over the last 10 Ma has been used to formulate a stratigraphic age model correlating age of deposit with δ¹³C of pedogenic carbonates using 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 correlation curve rises from a Late Miocene baseline δ¹³C value of -7‰ VPDB up to Pleistocene values greater than 0‰ VPDB. Moreover, more recent works by Fox and others in and around Meade Co., KS have developed highly-resolved chemostratigraphic profiles in sections whose ages are constrained by land mammal ages and volcanic ash bed chronology. They show that discrete depositional sequences are characterized by flat-line δ¹³C profiles with characteristic δ¹³C values for the Late Miocene (-7.1±0.91‰), Early Pliocene (-4.9±0.94‰), and Late Pliocene to Early Pleistocene (-2.6±1.12‰). The somewhat greater variability in the Late Pliocene and Early Pleistocene primarily reflects the strong temporal gradient toward higher δ¹³C values in the upper part of the section. We are carrying out investigations on subsurface drillhole samples and well-dated exposed sections in western Kansas to evaluate whether results from Meade County can be extrapolated throughout the extent of the HPS in Kansas.