IMPORTANCE OF XRF CHEMOSTRATIGRAPHY IN CHARACTERIZING SEDIMENTARY MUDROCK SUCCESSIONS AND ITS LIMITATIONS

The availability of rapid acquisition x-ray fluorescence (XRF) spectrometry has revolutionized characterization of sedimentary rock successions, especially mudrocks. These rocks, which, because of their small grain size commonly reveal little or no diagnostic features from visual description, have, in the past, rarely been described in detail. Key advantages of XRF include the ability, using major elements, to define mineralogical facies and, using trace elements, to characterize bottom water redox conditions that can be used to define settings conducive to organic matter preservation, at virtually any scale of resolution. These data are especially critical in characterization of subsurface mudrocks, as they provide a basis for calibrating borehole geophysical logs. However, XRF data cannot alone define such critical sedimentological parameters as facies, depositional environments, diagenesis, water depth, or organic matter abundance. Because many mudrock successions contain quartz and/or calcite derived from both landward and basinal sources that reflect very different conditions of sediment production and deposition, as well as diagenesis, these minerals must be more specifically characterized by conventional analysis. Our studies of the Jurassic Haynesville Fm show that it contains calcite derived from both landward and basinal sources as well as point-sourced detrital quartz. The Barnett (Mississippian) and Woodford (Late Devonian) formations contain both landward-derived and basin-derived quartz. Despite being dominated by planktic and benthic calcite, the Eagle Ford Fm also contains abundant diagenetic calcite. Although XRF chemostratigraphy is crucial for high resolution definition of facies and redox, it must be coupled with conventional (e.g., megascopic and microscopic) description to be useful in correlations, identification of facies architecture (i.e., sequence stratigraphy), and defining the distribution of paleo-environments.