The Provenance of Eocene Tuff Beds in the Fossil Butte Member of the Green River Formation, Wyoming: Relation to the Absaroka and Challis Volcanic Fields
M. R. Chandler1, E. H. Christiansen1, B. J. Kowallis1, M. Dorais1, A. Aase2, and B. Singer3
1Brigham Young University
2Fossil Butte National Monument – National Parks Service
3University of Wisconsin-Madison
The Eocene Green River Formation of Wyoming preserves many layers of ash-fall tuff whose sources are unknown. 40Ar/39Ar dating of sanidine, X-ray fluorescence analysis of bulk rocks, and electron microprobe analysis of phenocrysts were used to characterize the ash. We focused on 21 beds deposited in ancient Fossil Lake and 6 in Lake Gosiute. Ash beds range in thickness from paper-thin to 22 cm. X-ray diffraction analysis reveals that the tuffs' glassy matrices have been extensively altered; the alteration was controlled by the temporal variations in salinity and alkalinity of the lake. Alteration is extensive and complicates identifying primary magmatic characteristics. However, immobile element systematics, phenocryst assemblages, and mineral compositions indicate that most of the tuffs were magnesian rhyolites or dacites and erupted from subduction zone volcanoes. Moreover, tuff beds deposited in the Greater Green River Basin have sanidine, plagioclase, and biotite compositions similar to tuffs from Fossil Basin and are interpreted to have the same eruptive sources. Based on age and proximity, the Absaroka and Challis volcanic fields are the likely sources of the tephra. Although distinctive Atype rhyolites erupted from the Challis volcanic field, they were preceded by magnesian ignimbrites similar to those in the Absaroka volcanic field. Thus, it is difficult to determine the eruptive source of the tephras. However, two samples, one from Fossil Basin and one from the Greater Green River Basin, have biotite and pyroxene compositions with A-type chemical affinities; consequently, we conclude that they were erupted from volcanoes within the Challis volcanic field.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas