EOCENE GREEN RIVER FORMATION: ORBITAL CYCLES VERSUS ARGON CHRONOLOGY

Smith et al. (2002) have challenged the traditional interpretation of the Green River Formation (Wilkins Peak Member) cyclicity as a product of orbital (precessional) forcing. In fact, the cyclostratigraphic positions of the dated tuffs all fall into Smith et al.'s error brackets with exception of the Firehole tuff, which appears to be misdated by some 300 ky. There remains, however, a residual discrepancy of about 7-8%. A time-series study of oil-yield values for the preceding Tipton Member, a comparatively uniform sapropelic marlstone of profundal origin, yields a classical spectrum for the full suite of expected orbital periods, with a powerful obliquity signal. A similar study of the Wilkins Peak Member is troubled by sporadic influx of extra sediment, and lacks the response to obliquity but records the full precession-eccentricity syndrome. The Roehler timescale is somewhat corrupted by excess number of lacustrine floodings, possibly attributable to occasional double-beats in the climatic response. For the extraordinary integrity of climatic response to orbital forcing we look to glaciation in the Laramide mountain range.