A RADIOISOTOPIC AND ASTROCHRONOLOGIC CROSS-CALIBRATION OF THE WILKINS PEAK MEMBER OF THE GREEN RIVER FORMATION, WYOMING, USA

The Eocene Green River Formation has long been recognized as preserving apparently cyclicly deposited lacustrine sediments with abundant intercalated volcanic ashes, an ideal place to test a new approach to astronomical calibration of strata. In order to test the assumptions of astronomical calibration with geochronology, it is necessary that the precision of radioisotopic ages be comparable to the period of the cycle being tested, and that there are abundant control points. U-Pb CA-TIMS (chemical abrasion- thermal ionization mass spectrometry) zircon dates for seven volcanic ashes from the Green River Formation with precisions of ±11,000 to 52,000 years from the Wilkins Peak Member in Wyoming (USA), spanning approximately 2 million years are used to construct a new age model.

Tuning-based orbital age models deviate from the new radioisotopic constraints. There are a number of possible explanations including changes in orbital periods, gaps in the sedimentary record, and varying rates of sediment accumulation. In this paper we present a general model for testing astronomical calibration that establishes a strategy to test for varying sediment accumulation rate and/or depositional hiatuses. Our method compares three age-depth models: the simplest model assumes constant accumulation rate between pairs of dated volcanic ashes and the other two models vary sediment accumulation rates between the dated horizons.

Comparisons between varying sediment accumulation rate models and constant sediment accumulation rate allow us to identify certain intervals where further dating of available ashes could distinguish among the three models. Once dated, some tuning options can be discarded and the remaining tuning options will be limited. Repeating the process will allow insight into the importance of orbital forcing and may be used to support a specific tuning. In the best case scenario, it may even be possible to identify changes in the periods of orbital parameters themselves. We propose that the iterative approach proposed here should become the standard for establishing a rigorous orbital calibration of the stratigraphic record where sufficient ashes exist.