Orbital forcing of continental Eocene climate: Detailed stratigraphy and argon-40/argon-39 dating of the Green River Formation in Wyoming.

摘要

The Eocene Green River Formation is a classic example of cyclic lacustrine sediments. This is the first study to quantify this cyclicity, in the Green River Formation of Wyoming, using radiometric dating, detailed stratigraphy, and proxies for paleo - lake-level.; Tuffs 6 and 3 of the Wilkins Peak Member were dated by the single crystal 40Ar/39Ar method. Ages of these tuffs, combined with existing stratigraphy and magneto-stratigraphy show that the 46.8 Ma calibration-age of the Global Polarity Time Scale is too old by 1.5 m.y. I show that previously reported precision and accuracy associated with dated tuffs of the Green River Formation are underestimated. Therefore, a long cyclic record, preferably longer than 3 m.y., is required for rigorous testing of the orbital forcing hypothesis for this formation.; Spectral analysis and radiometric dating suggest that the cyclical record of the Wilkins Peak Member is orbitally forced. Analyses were undertaken on published Fischer assay (oil yield) records of four cores. Two methods of tuning were employed on the basinal cores, yielding similar orbital age models that are in agreement with the preferred radiometric age model. The long-period eccentricity is the dominant cyclicity, accompanied by short eccentricity, precession and a weak obliquity. The existence of a weak half-precession cycle suggests a possible equatorial influence of ENSO-like climate processes on the mid-latitudes of the early Eocene.; A new high-resolution stratigraphy, based on numerous tuffs, was developed for the lower Laney Member. This stratigraphy is substantially different from the published one, requiring reassessments of previous interpretations. Lateral distribution of water-depth sensitive facies between consecutive tuffs suggests that the oil shale was deposited in a deep, stratified lake with a relatively stable lake level. These contemporaneous facies associations cannot fit into the lake classification of A. R. Carroll and K. M. Bohacs that is partly based on the previous stratigraphy. I suggest that this two-dimensional lake classification lacks the dimension of climate variability that is typical of orbitally forced strata.