SPECTRAL ANALYSIS AND U-PB DATING OF THE EOCENE WILKINS PEAK MEMBER, GREEN RIVER FORMATION, WYOMING: SUPPORT FOR MILANKOVITCH CYCLICITY

The classic meter-scale sedimentary cycles of the Wilkins Peak Member of the Green River Formation in Wyoming have traditionally been interpreted as the record of an expanding and contracting lake forced by precession of the equinoxes (~20 ky) and modulated by eccentricity (~100 ky).  This interpretation has recently been called into question on the basis of Ar/Ar chronology of volcanic ashes, combined with cycle-counting, to calculate a mean cycle duration of ~10 ky (e.g., Pietras and Carroll, 2006, JSR v. 76, p. 1197).

Spectral analysis of oil yield records from four basinal cores shows significant peaks with the expected ratios for orbital origin (~400, ~100, ~40, ~20 ky). Sub-Milankovitch variability is also found, but only as a small part of the total variability.  Floating orbital age models are consistent with available Ar/Ar chronology for assumed eccentricity periods between 95 and 105 ky. Accordingly, it is not possible to eliminate orbital forcing with existing published chronology.

Orbital theory predicts two eccentricity periods of 95 and 125 ky, but the published Ar/Ar chronology is not precise enough to distinguish them. In addition, Ar/Ar dates for Green River ashes are complicated by diagenetic overgrowths on sanidine crystals, the presence of small crystals (requiring multiple grains per analysis), and possible bias between biotite and sanidine data.

U-Pb zircon geochronology has the potential to yield a precise chronology necessary for testing the orbital hypothesis in the Green River Formation because the U-Pb systematics are not affected by lake chemistry and/or fluid-related diagenesis and zircons can be dated with precision of ± ca 50 ky or better. Preliminary U-Pb zircon chronology from several ashes has yielded promising results. We will use the high-precision U-Pb chronology to quantify changes in sediment accumulation rates that will allow a direct spectral analysis of the dated records. Based on our preliminary results, combined spectral analysis of the four published proxy records and U- Pb dating of zircons support the orbital origin of the cyclicity although more consistent with the 125 ky than 95 ky period of eccentricity.