CYCLOSTRATIGRAPHIC EVALUATION OF REPETITIVE SEDIMENTARY MICROFACIES FROM THE GREEN RIVER FORMATION VIA X-RAY FLUORESCENCE SCANNING: EVIDENCE FOR ANNUAL, EL NIÑO, AND SUNSPOT CYCLES?

The lacustrine Eocene Green River Formation is well-recognized for its extensive sub-millimeter lamination, which has been proposed to express annual, El Niño, and sunspot cycle variability. If validated, these paleoenvironmental signals could provide one of the richest available records of past warm climate behavior during the last major greenhouse climate, potentially spanning yearly to millennial scale resolution. Despite this promise, currently there is surprisingly little understanding of the actual processes behind the generation of these laminae. This is at least partially the result of the small spatial scales involved, usually 100 microns or less, which greatly inhibit traditional means of geochemical investigation. With continuous scanning capabilities down to 100 microns, X-ray fluorescence (XRF) core scanning can help to bridge this gap by providing elemental information at a spatial resolution and degree of continuity not previously possible. Using this powerful tool, 9.96 m of Green River Formation core have been scanned at a 5 mm resolution, and selected intervals have been scanned at a 100 micron resolution, in order to evaluate both large and small scale elemental variability within core material for 16 different elements (Mg, Al, Si, S, K, Ca, Ti, Fe, Cu, Zn, Ga, Pb, Rb, Sr, Zr, Mo). Preliminary results show pronounced antithetic oscillations between siliciclastic (Al, Si, K) and carbonate components (Ca, Sr) at multiple spatial scales, with intervals of highly-enriched redox sensitive elements (S, Mo, Cu, Zn) associated with some siliciclastic-dominated microfacies. Future work will use advanced spectral methods to allow the quantitative evaluation of hypothesized annual, El Niño, and sunspot variability.