PRECISE 40AR/39AR LASER FUSION GEOCHRONOLOGY OF THE GREEN RIVER FORMATION, WYOMING

This study focuses on ⁴⁰Ar/³⁹Ar dating of several distal tuffs in the Green River Formation (GRF), Greater Green River (GGR) basin, Wyoming that were derived from coeval volcanism in the Absaroka province. Thus far, laser fusion experiments have been performed on the Grey tuff (Wilkins Peak Member, ~3m above the "D" arkose bed), Main tuff (Wilkins Peak Member), and Analcite tuff (Laney Member). From each tuff, between 15 and 32 individual age determinations from single crystals or multigrain aliquots of sanidine and biotite grains facilitated the exclusion of minor xenocrystic contamination, yielding weighted mean ages* (± 2s) of: 50.86 ± 0.20 Ma (Grey), 50.00 ± 0.13 Ma (Main), 49.02 ± 0.15 Ma (Analcite). These ages constrain deposition of the upper half of the GRF to a period of 1.84 m.y., yielding net sediment accumulation rates from 1.5 mm/yr in the evaporative Wilkins Peak Member, to 0.7 mm/yr in the saline Laney Member.

Our new age determinations coupled with published results permit correlation of GGR basin magneto- and bio-stratigraphy with the global GPTS and marine O-isotope record. Specifically, Wing et al. (1991) showed that the Luman/Niland contact in the GGR basin coincides with the Lysitian/Lostcabinian North American Land Mammal Age boundary at 53.12 ± 0.32 Ma* in the Bighorn basin. When considered with our new ages, the GRF spans ~5 m.y. from ca. 53.5 Ma to 48.5 Ma and is thus 4 to 5 m.y. (~10%) older than previously thought on the basis of imprecise K-Ar and ⁴⁰Ar/³⁹Ar ages. Contrary to the magnetostratigraphy of Clyde et al. (2001) that shows two reversed zones within lacustrine GRF and equivalent alluvial strata, the GRF encompasses chrons 24n through 21r*. Accordingly, the Bridgerian/Wasatchian boundary in the GGR basin, stratigraphically between the middle Wilkins Peak and Tipton Members, is older than ca. 50.5 Ma and younger than ca. 52.5 Ma. From a global perspective, these new ⁴⁰Ar/³⁹Ar ages indicate that the GRF was deposited during the early Eocene climatic optimum defined by marine O-isotope records, thereby providing a unique high-resolution continental record of this period that will be further exploited to distinguish orbital from regional influences on sedimentation.

*Ages reported (or adjusted) relative to 28.34 Ma Taylor Creek sanidine standard.