from: ILIC3: The Third International Limnogeology Congress Abstract Volume, Tuscon, Arizona 29 March - 2 April, 2003, p. 171.]

Milankovitch Cyclicity in the Eocene Green River Formation of Colorado and Wyoming

Machlus, Malka, Olsen, Paul E., Christie-Blick, Nicholas, and Hemming, Sidney R.,

Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9W, Palisades, NY 10964

The Eocene Green River Formation is a classic example of cyclic lacustrine sediments. Numerous authors have suggested that the cyclicity is due to climatic forcing at precession (~20 ky) and eccentricity (~100 ky) timescales. Spectral analysis results are consistent with hypothesized orbital forcing, but they suggest that the dominant cyclicity in the Green River and Piceance Creek basins relates to long-period eccentricity (~400 ky) rather than shorter-period variations in eccentricity, obliquity or precession. The 400 ky cycles may also be modulated by the 2.4 m.y. eccentricity cycle.

Spectral analyses were undertaken mainly on Fischer assay records (≥oil shale yields≤), which were used as a proxy for lake-level change on the basis of documented correlation with depositional facies. In the Green River basin, lateral changes in facies from deep to shallow environments correlate with a decrease in oil shale yield. In the Piceance Creek basin, shallow water facies correlate with minima in yields. Spectra obtained from Fischer assays are similar to those derived from sonic logs, and from relative water depth (depth rank) records.

Spectra were obtained from a series of cores in a transect across the Piceance Creek basin (1). A high-amplitude cycle is interpreted in all cores, with a period of about 75m at the basin center. ⁴⁰Ar/³⁹Ar ages for the "wavy" and "curly" tuffs, which bracket one cycle (the Mahogany oil shale zone), and previously published estimates of sedimentation rate based on varve counts (2) suggest that the observed periodicity corresponds with the long-eccentricity (400 ky) cycle. Eight such cycles are present, two above and five below the oil-rich Mahogany zone. Those cycles are themselves modulated, with minima located within the lower and upper parts of the Green River Formation.

Cycles are about 50 m thick in the upper part of the Wilkins Peak Member of the eastern Green River basin and 100 m thick in the trona-rich lower part. These cycles are also attributed to long-period eccentricity, with four present between the Firehole tuff and tuff #6, in agreement with a ⁴⁰Ar/³⁹Ar age of 49.3±0.2 for tuff #6 and a published age of 50.7±0.1 for the Firehole tuff (3). Tuning that record to a 404 ky cosine results in a spectrum with significant peaks at periods of about 20, 40, 90 and 115 ky. The long-period eccentricity cycles appear to be modulated by a 2 m.y. cycle, with a minimum in the middle of the Wilkins Peak Member.

(1) Dyni, 1998, USGS open file report 98-483 (2) Bradley, 1929, USGS prof. paper 158 (3) Smith et al., in press, GSA Bulletin