|
|
 |
|||||||||||||||||
|
|||||||||||||||||
 |
|||||||||||||||||
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
,1,11 Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA
The deposits of Eocene Lake Gosiute that constitute the Green River Formation of Wyoming contain numerous tuff beds that represent isochronous, correlatable stratigraphic markers. Tuff beds selected for 40Ar/39Ar analysis occur within laminated mudstone, are matrix supported, and lack evidence of reworking. These tuffs contain 2%–15% euhedral phenocrysts of quartz, plagioclase, sanidine, biotite, and minor amphibole, pyroxene, and zircon, encased in a matrix of altered glassy ash. Air abrasion and handpicking under refractive- index oils were required to obtain clean, unaltered phenocrysts of sanidine. 40Ar/39Ar age determinations from single-crystal and <1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination. Laser-fusion experiments on phenocrysts from the Rife, Firehole, C Bed, Grey, Main, Sixth, and Analcite tuff beds from the Tipton, Wilkins Peak, and Laney Members yielded weighted-mean ages (±2
analytical uncertainties) of 51.25 ± 0.31 Ma, 50.70 ± 0.14 Ma, 50.56 ± 0.26 Ma, 50.39 ± 0.13 Ma, 49.96 ± 0.08 Ma, 49.70 ± 0.10 Ma, and 48.94 ± 0.12 Ma, respectively. Ages for sanidine and biotite from the Main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation. Moreover, the 40Ar/39Ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the Absaroka and Challis volcanic fields.
Our 40Ar/39Ar-based age model indicates that sediment accumulated three times more rapidly (327 ± 86 µm/yr) during the evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 ± 34 µm/yr) and Laney (104 ± 18 µm/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for <1-mm-thick laminae and precessional forcing of 1–3-m-thick depositional cycles in these units. However, previously described cycles in the Wilkins Peak Member have average durations that are significantly shorter than the 19–23 k.y. precessional modes.
The Green River Formation encompasses an 
5 m.y. period between ca. 53.5 and 48.5 Ma, spanning magnetic chrons 24n through 21r. The Green River Formation was therefore deposited during the warmest period of the Cenozoic corresponding to the early Eocene climatic optimum as defined by the global marine O isotope record. Deposition of bedded evaporites (trona) of the Wilkins Peak Member began at ca. 51 Ma, or 2 m.y. after the onset of the highest inferred temperatures of the climatic optimum. The Bridgerian–Wasatchian faunal turnover occurred subsequently during Wilkins Peak time, at ca. 50.6 Ma. Thus, our 40Ar/39Ar ages strongly suggest that Wilkins Peak evaporite deposition and the turnover from Wasatchian to Bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions.
Key Words: Green River Formation • sedimentation rates • lacustrine • 40Ar/39Ar, • Wasatchian-Bridgerian • Early Eocene Climate Optimum
This article has been cited by other articles:
|
|
 |
|
  J. P. Craddock, D. H. Malone, J. Magloughlin, A. L. Cook, M. E. Rieser, and J. R. Doyle Dynamics of the emplacement of the Heart Mountain allochthon at White Mountain: Constraints from calcite twinning strains, anisotropy of magnetic susceptibility, and thermodynamic calculations Geological Society of America Bulletin, May 1, 2009; 121(5-6): 919 - 938. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. E. Chew Paleoecology of the early Eocene Willwood mammal fauna from the central Bighorn Basin, Wyoming Paleobiology, January 1, 2009; 35(1): 13 - 31. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. R. Carroll, A. C. Doebbert, A. L. Booth, C. P. Chamberlain, M. K. Rhodes-Carson, M. E. Smith, C. M. Johnson, and B. L. Beard Capture of high-altitude precipitation by a low-altitude Eocene lake, western U.S. Geology, October 1, 2008; 36(10): 791 - 794. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Giorgis, W. McClelland, A. Fayon, B. S. Singer, and B. Tikoff Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho Geological Society of America Bulletin, September 1, 2008; 120(9-10): 1119 - 1133. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. E. Smith, B. S. Singer, A. R. Carroll, and J. H. Fournelle Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques American Mineralogist, May 1, 2008; 93(5-6): 784 - 795. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. R. Meyers Resolving Milankovitchian controversies: The Triassic Latemar Limestone and the Eocene Green River Formation Geology, April 1, 2008; 36(4): 319 - 322. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. E. Smith, A. R. Carroll, and B. S. Singer Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States Geological Society of America Bulletin, January 1, 2008; 120(1-2): 54 - 84. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. M. Hora, B. S. Singer, and G. Worner Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcan Parinacota, Chile Geological Society of America Bulletin, March 1, 2007; 119(3-4): 343 - 362. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B.R. Jordan, H. Sigurdsson, S. Carey, S. Lundin, R.D. Rogers, B. Singer, and M. Barquero-Molina Petrogenesis of Central American Tertiary ignimbrites and associated Caribbean Sea tephra Geological Society of America Special Papers, January 1, 2007; 428(0): 151 - 179. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. T. Pietras and A. R. Carroll High-Resolution Stratigraphy of an Underfilled Lake Basin: Wilkins Peak Member, Eocene Green River Formation, Wyoming, U.S.A. Journal of Sedimentary Research, November 1, 2006; 76(11): 1197 - 1214. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. E. Smith, B. S. Singer, A. R. Carroll, and J. H. Fournelle High-resolution calibration of Eocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation Geology, May 1, 2006; 34(5): 393 - 396. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Secord, P. D. Gingerich, M. E. Smith, W. C. Clyde, P. Wilf, and B. S. Singer Geochronology and Mammalian Biostratigraphy of Middle and Upper Paleocene Continental Strata, Bighorn Basin, Wyoming Am J Sci, April 1, 2006; 306(4): 211 - 245. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. R. Carroll, L. M. Chetel, and M. E. Smith Feast to famine: Sediment supply control on Laramide basin fill Geology, March 1, 2006; 34(3): 197 - 200. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. Wilf, C. C. Labandeira, K. R. Johnson, and N. R. Cuneo Richness of plant-insect associations in Eocene Patagonia: A legacy for South American biodiversity PNAS, June 21, 2005; 102(25): 8944 - 8948. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Machlus, S. R. Hemming, P. E. Olsen, and N. Christie-Blick Eocene calibration of geomagnetic polarity time scale reevaluated: Evidence from the Green River Formation of Wyoming Geology, February 1, 2004; 32(2): 137 - 140. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. C. Clyde, W. S. Bartels, G. F. Gunnell, and J.-P. Zonneveld 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming: Discussion Geological Society of America Bulletin, January 1, 2004; 116(1-2): 251 - 252. [Full Text] [PDF]
|
|
|
|
|
|
M. E. Smith, B. S. Singer, and A. R. Carroll Reply Geological Society of America Bulletin, January 1, 2004; 116(1-2): 253 - 256. [Full Text] [PDF]
|
|
|
|
|
|
J.T. Pietras, A.R. Carroll, B.S. Singer, and M.E. Smith 10 k.y. depositional cyclicity in the early Eocene: Stratigraphic and 40Ar/39Ar evidence from the lacustrine Green River Formation Geology, July 1, 2003; 31(7): 593 - 596. [Abstract] [Full Text] [PDF]
|
|
| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
