STRATIGRAPHIC, CHEMOSTRATIGRAPHIC, AND GEOCHEMICAL INVESTIGATIONS OF THE FIRST INTACT DRILL CORE OF THE NEOGENE HIGH PLAINS SUCCESSION IN WESTERN KANSAS

Stratigraphic investigation of the terrestrial High Plains Succession (HPS) in western Kansas (Neogene Ogallala Formation and overlying, undifferentiated units) has been deficient due to the lack of outcrops, regional marker beds, and its unconsolidated nature. These obstructions are significant because they have limited the reliability of stratigraphic correlations in what is the most important regional aquifer in the United States for agricultural production. Due to the highly variable lithologic nature of the HPS, paleoclimatic and geochemical analysis provide non-traditional opportunities for stratigraphic analysis. With state-of-the-art rotary-vibratory and hydraulic piston coring technology, we have recovered the first intact drill core from the HPS in western Kansas for such analysis. Initial δ¹³C results from 0.3 m to 38.1 m depth below the land surface at the HP1A site show bulk organic δ¹³C values from -27 to -12‰ VPDB. Numerous investigations of the HPS have demonstrated a systematic temporal increase in δ¹³C values over the late Miocene-Pleistocene interval and relate this trend to the evolution of C₄ grasses. Our results from 38.1 to 20 m depth interval are indicative of a stable C₃ biomass; and, by analogy to Fox and Koch (2003; Geology 31: 809-812), this suggests correlation with late Miocene portions of the Ogallala Formation. Above the 20 m depth interval the δ¹³C record becomes increasingly variable with systematic excursions of up to 7 permil, indicating episodic transition to increasing C₄ biomass, simliar with other bulk organic δ¹³C studies of the HPS. The fine-scale δ¹³C structure of HP1A offers prospects for high-resolution paleoclimatic and depositional interpretations that may prove useful for future regional correlations. Isotopic data from the remainder of the section is pending, as well as volcanic ash-bed chronology, paleomagnetics, isotopic analysis from pedogenic carbonates, optically stimulated luminescence (OSL) dates, particle size analysis, and whole-rock geochemistry by ICP-AES to undertake weathering indices.