Cyclostratigraphic Evaluation of Repetitive Sedimentary Microfacies From the Green River Formation, Utah: Evidence for Annual, El Niño, and Sunspot Cycles?
Organic-rich facies of the lacustrine Eocene Green River Formation are well recognized for their extensive sub-millimeter lamination, which has been proposed to express annual, El Niño Southern Oscillation (ENSO), and sunspot cycle variability during the last major greenhouse climate. However, the genetic significance of these laminae remains uncertain. X-ray fluorescence (XRF) scanning provides the opportunity to comprehensively evaluate the elemental composition of these deposits at multiple spatial scales down to the level of individual laminae (∼100 microns). In order to better characterize the Green River microfacies and evaluate paleoenvironmental influences on sedimentation, 9.96 m of core from the Uinta Basin has been scanned at 5 mm resolution (yielding Mg, Al, Si, S, K, Ca, Ti, Mn, Fe, Cu, Zn, Rb, Sr, Zr, Mo, Pb) and a 202 mm subset of the core has been scanned at 100 micron resolution (yielding Al, Si, S, K, Ca, Fe, Sr). The 5 mm XRF scan shows pronounced antithetic oscillations between lithogenic (K, Si, Al) elements and carbonate (Ca) at multiple spatial scales, including multi-meter variability consistent with precession. The 5 mm XRF scan also reveals punctuated intervals throughout the core where Si decouples from the lithogenic fraction, suggesting an alternate biogenic or authigenic Si source. High-resolution 100 micron XRF scans across an interval of strong Si decoupling reveal multiple distinct episodes of decoupling at the millimeter to centimeter scale. In order to evaluate the time scales associated with the Si variability, six different testable time scale constructions were developed. These temporal hypotheses utilize 40Ar/39Ar derived minimum, average, and maximum effective sedimentation rates combined with two independent models with varve and non-varve assumptions. Spectral analysis of these time series reveals interdecadal to centennial scale Si variability, precluding variability on the order of varves, ENSO, and sunspot cycles as the dominant driver of this authigenic or biogenic Si signal. Finally, intervals of decoupled Si variability, likely representing enhanced lake productivity, appear to coincide with a particular phase of the hypothesized precession cycle, revealing an interdecadal to centennial scale expression of Milankovitch-scale variability.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014