1330h
U12A-0005
Milankovitch Cyclicity in the Eocene Green River Formation of Colorado and Wyoming
* Machlus, M
machlus@ldeo.columbia.edu
Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9W, Palisades, NY 10964-8000
United States
Olsen, P E
polsen@ldeo.columbia.edu
Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9W, Palisades, NY 10964-8000
United States
Christie-Blick, N
ncb@ldeo.columbia.edu
Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9W, Palisades, NY 10964-8000
United States
Hemming, S R
sidney@ldeo.columbia.edu
Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9W, Palisades, NY 10964-8000
United States
The Eocene Green River Formation is a classic example of cyclic lacustrine sediments. Following Bradley (1929, U.S.G.S. Prof.
Paper 158-E), many descriptive studies suggested precession and eccentricity as the probable climatic forcing to produce the
cyclic pattern. Here we report spectral analysis results that confirm this hypothesis. Furthermore, we have identified the
presence of a surprisingly large amplitude obliquity cycle, the long-period eccentricity cycle (400 k.y.) and the long period
modulators of obliquity.
Spectral analyses of data from Colorado were undertaken on an outcrop section and core data using two different proxies for
lake depth. In a section measured in the west Piceance Creek basin, three lithologies (ranks) were used as a proxy for
relative water depth, from relatively shallow to deep water: laminated marlstones; microlaminated, light-colored oil-shales;
and microlaminated black oil shales. A multi-tapered spectrum of the 190-m-thick record in the depth domain shows
significant peaks at periods of 2.1, 3.4, 12 and 39 m. These are interpreted as the precession, obliquity and eccentricity
cycles. The precession cycle confirms Bradley's independent estimate of 2.4 m per 20 k.y. cycle, based on varve counts at the
same location. A high-amplitude, continuous 3.4 m (obliquity) cycle exists in the evolutive spectrum of this record. A
second spectral analysis of an oil-shale-yield record was made on a 530 m core near the basin depocenter. This record
includes the time-equivalent of the outcrop section, spans a longer interval of time, and has a higher sedimentation rate.
Peaks are found at 5, 10, 25 and 79 m. Again, the probable obliquity peak, at 10 m, is continuous along the record. Initial
tuning of this record to a 39.9 k.y. cosine wave improves the resolution of the precession, short and long eccentricity
cycles.
Spectral analysis of oil shale yield and sonic velocity data of cores from the Green River basin, Wyoming, gives similar
results. Spectral peaks at 6, 13, 31 and 122 m appear mainly in the Tipton and the Wilkins Peak members. The correlation
between oil shale yield, lithology and relative water depth was examined in the upper part of the Wilkins Peak Member and the
Lower part of the Laney Member. The succession from microlaminated black oil shale to laminated micrite corresponds with
documented lateral changes in facies from deep to shallow environments, thus confirming the use of these facies as relative
water-depth proxies. Furthermore, the upsection record of oil shale yields correlates with these facies, with higher yields
corresponding to deeper water facies. This correlation supports the use of the oil shale yield record as a proxy for
short-term lake-level changes, and therefore a proxy for climate.
The spectral analysis results from both basins show the importance of the obliquity cycle in these continental records. This
cycle cannot be identified by cycle-counting, and therefore was not previously recognized. Earlier published attempts at
spectral analysis of short records from the Piceance Creek and Uinta basins misinterpreted the observed cycles. This is the
first time both the obliquity cycle and the long-term eccentricity cycle have been identified in the Green River and Piceance
Creek basins.
4239 Limnology
9350 North America
9604 Cenozoic
U
2001 AGU Fall Meeting
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