A New Approach to Understanding Neogene Stratigraphic Architecture of the High Plains Succession in Western Kansas Using Carbon Isotope Chemostratigraphy
Greg A. Ludvigson1, Adel Haj1, David L. Fox2, Rolfe Mandel1, and P. Allen Macfarlane1. (1) Kansas Geological Survey, University of Kansas, 1930 Constant Avenue, Lawrence, KS 66047, phone: 785-864-2734, (2) Department of Geology & Geophysics, The University of Minnesota, Minneapolis, MN 55455 [email protected]
Traditional approaches to stratigraphic analysis of the Neogene High Plains Succession (HPS) in western Kansas (MioceneÐPliocene Ogallala Formation and undifferentiated younger units) are impeded by lack of widespread marker beds and the repetitive facies associations in fluvial deposits. Calcic paleosols are ubiquitous in the HPS and occur with high stratigraphic frequency. Stable isotopic investigations by Fox and Koch (2003) demonstrated prospects for dating these deposits using d13C values of pedogenic carbonates. They showed a systematic increase in d13C values over the Late MioceneÐPleistocene interval, a trend related to evolutionary innovation of the C4 grassland prairie biome. Their dataset from the last 10 Ma is used to formulate a stratigraphic-age model, correlating age of deposit with d13C of pedogenic carbonates using the exponential decay function:
y = 10.6690.51x -6.995
where x = Age (Ma); y = d13C of pedogenic carbonate; with R2 = 0.77
The correlation curve rises from a Late Miocene baseline value of -7ä up to Pleistocene values greater than 0ä VPDB. Recent works by Fox and others around Meade County, Kansas, developed highly resolved chemostratigraphic profiles in sections whose ages are constrained by land-mammal ages and volcanic-ash-bed chronology. They show discrete depositional sequences characterized by flat-line d13C profiles with characteristic d13C values for the Late Miocene (-7.1±0.91ä), Early Pliocene (-4.9±0.94ä), and Late Pliocene and Early Pleistocene (-2.6±1.12ä). Greater variability in the Late PlioceneÐEarly Pleistocene reflects the strong temporal gradient toward higher d13C values in the upper part of the section. We are investigating subsurface drillhole samples and well-dated exposed sections to evaluate whether results from Meade County can be extrapolated throughout the extent of the HPS in Kansas.
AAPG Search and Discover Article #90067©2007 AAPG Mid-Continent Section Meeting, Wichita, Kansas