2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 6
Presentation Time: 8:00 AM-6:00 PM

Cyclic Sedimentation in the Mississippian Pride Shale: Quantitative Paleoenvironmental Analysis of Tidal Rhythmites Using X-Ray Fluorescence Scanning and Advanced Spectral Methods

LARKINS, Kristin1, MEYERS, Stephen1 and BARTEK, Louis2, (1)Geological Sciences, University of North Carolina - Chapel Hill, Mitchell Hall, 104 South Road, CB 3315, Chapel Hill, NC 27599, (2)Geological Sciences, Univ of North Carolina, Chapel Hill, NC 27599, lkristin@email.unc.edu

Tidal rhythmites offer a unique opportunity to better constrain the orbital dynamics of the Earth-Sun-Moon system (e.g., lunar recession rate, changes in Earth's precessional constant), and provide high-resolution paleoclimate archives with exceptional time control. The Mississippian Pride Shale of West Virginia is an unusually thick tidal rhythmite deposit (~60 m), representing hundreds to thousands of years of nearly continuous sedimentation. Previous analyses of the Pride Shale, using visual observations of layer thicknesses, identified a hierarchy of tidal cycles (semidiurnal to the 18.6-year nodal cycle) and an annual monsoonal climate cycle. In this study, we develop a new methodology for the investigation of ancient tidal rhythmites, using high-resolution X-ray fluorescence (XRF) scanning and advanced spectral methods. Importantly, the XRF-scanning technique provides the opportunity to deconvolve multiple paleoenvironmental signals (detrital flux, biogenic flux, redox state) preserved within these deposits, and thus allows a more complete analysis of their mechanism of formation. Assessment of the individual paleoenvironmental proxy records using advanced spectral methods (time-frequency analysis, coherency analysis, etc.) permits evaluation of their linkage to tidal and climatic forcing agents. This coupled XRF-scanning and spectral approach provides an opportunity to extract important paleoclimate information from tidal rhythmites. For this study, samples of the Pride Shale have been collected using a variety of approaches, including the extraction of continuous sections from outcrop using a concrete saw. Analyses of the Pride Shale samples suggest robust detrital (e.g., titanium), biogenic (e.g., silicon/titanium), and redox proxies (e.g., total sulfur), with variable sensitivity to the individual forcing mechanisms.