Testing Milankovitch Forcing of Eocene Lacustrine Sequences, Green River Formation, Wyoming, Usa, Using U-Pb Geochronology and Spectral Analysis
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 periodic climatic change forced by precession of the equinoxes (~20 ky) and modulated by the eccentricity of Earth’s orbit (~100 ky). Spectral analysis of oil yield records that are used as a proxy for cyclic changes in lake levels is found to be permissive of the orbital forcing hypothesis but cannot be tested using available Ar/Ar geochronology (Machlus et al., EPSL, 2008); therefore orbital tuning is employed and independent testing is precluded.
U-Pb zircon geochronology by the ID-TIMS method shows promise for providing a precise chronology necessary for independent testing of the orbital hypothesis in the Green River Formation. The U-Pb systematics are unaffected by fluid-driven diagenetic processes and can yield dates with precisions of ca. ± 50 ky or better for many intercalated volcanic ash beds; suitable 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.
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 roughly 20 additional precessional cycles in excess of the 77 that are known. We anticipate that a comprehensive 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.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009