Testing the Milankovitch hypothesis: U-Pb geochronology and spectral analysis of Eocene lacustrine sequences, Green River Formation, Wyoming, USA

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Machlus, Malka۱; Ramezani, Jahandar۲; Bowring, Samuel۲; Hemming, Sidney۳
Holding Date 07 October 2008

The meter-scale sedimentary cycles of the Wilkins Peak Member of the Green River Formation in Wyoming (USA) have long been interpreted as a long-term record of an expanding and contracting intracontinental lake. The traditional hypothesis considers these cycles to reflect climatic change forced by precession of the equinoxes (~20 ky) and modulated by the eccentricity of Earth’s orbit (~100 ky). Recent spectral analysis of oil yield records that are used as a proxy for the sedimentary cycles is found to be permissive of the orbital forcing hypothesis but concludes that the available Ar/Ar chronology lacks the precision to allow independent testing (Machlus et al., EPSL, 2008); therefore orbital tuning is employed and independent testing is precluded.
U-Pb zircon geochronology by the ID-TIMS method has the potential of yielding a precise chronology necessary for independent testing of the orbital hypothesis in the Green River Formation. The U-Pb systematics in zircon are not affected by lake chemistry and/or fluid-driven diagenetic processes and zircons can be dated with precisions of ± ca 50 ky or better for many intercalated volcanic ash beds. Preliminary U-Pb dates from several ash beds yield suitable precision for testing orbital forcing. High-precision U-Pb chronology is used to constrain sediment accumulation rates which in turn allow direct spectral analysis of the dated records. Such a direct test will be the first independent testing of the Milankovitch hypothesis for any pre-Neogene record.
Our preliminary results, combining spectral analysis of published Fischer assays (expressed as oil-yields in units of gallons per ton of rock) with new geochronology support the orbital origin of the cyclicity but differ substantially from the classical identification of precessional cycles. Preliminary ages are consistent with an approximately 125 ky period of eccentricity and the 125 ky orbital age model of Machlus et al. (2008), implying the additional occurrence of roughly 20 precessional cycles in excess of the 77 previously identified cycles. We expect that the completed U-Pb chronology for this sequence will allow us both to characterize the cyclicity and to test the calculation of orbital parameters for the Early Eocene.