ERUPTIVE SOURCES OF SILICIC FALLOUT TUFFS IN THE GREEN RIVER FORMATION, WYOMING

The Eocene (53.5 to 48.5 Ma) Green River Formation of Wyoming preserves many layers of ash-fall tuff whose eruptive 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 so that correlations with ignimbrite sources could be attempted. We have 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, with a basin-wide average of about 6 cm. X-ray diffraction analysis reveals that the tuffs' glassy matrices have been extensively altered to illite, calcite, clinoptilolite, analcime, albite, and K-feldspar; the alteration was controlled by the temporal variations in salinity and alkalinity of the lake waters. 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 (or “calc-alkaline”) rhyolites or dacites and erupted from subduction zone volcanoes. Moreover, tuff beds deposited farther east in the Greater Green River Basin have sanidine, plagioclase, and biotite compositions similar to those in 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 A-type 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 definitively the eruptive source of most 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.