CENOZOIC BASALTIC VOLCANISM IN JACKSON HOLE, WYOMING (U.S.A.) COEVAL WITH DEPOSITION OF THE TEEWINOT FORMATION: PHYSICAL VOLCANOLOGY OF THE FLAT CREEK BASALT

In northwestern Wyoming (U.S.A.), monogenetic and polygenetic eruptions distinct from the Yellowstone Plateau volcanic field occurred from ~9 Ma to ~480 ka, in two different episodes. The youngest episode, from ~4.8 Ma to ~480 ka, is focused in the upper Wind River Basin. The oldest episode, from ~9 to 8 Ma, is characterized by basaltic andesite to rhyolite eruptions of the Jackson Hole volcanics (JHV). The most voluminous exposures of the JHV are centered on the town of Jackson, but lavas also crop out to the west at Teton Pass. The Flat Creek Basalt (FCB), crops out along the eastern edge of Jackson Hole and may be part of the JHV suite. The FCB was originally mapped by Love (2001; Trb) and described as a package of “sills and flows” that intrude the ~16 - 6 Ma Teewinot Formation (e.g., ~1800-m-thick package of sedimentary strata and air-fall tuffs deposited in a fault-formed lake) and overlie Paleozoic rocks. Thus, the FCB represents the first known instance of basaltic eruptions in the Jackson Hole region and northwest Wyoming, prior to the development of the Yellowstone Plateau volcanic field. New mapping of the FCB has identified at least one dike; we suggest that the package was locally erupted, where lavas flowed westward into the Teewinot basin. We find no evidence of sills. Lavas are rubbly pāhoehoe to a’a and individual lobes are on average, 2-3-m-thick and the thickest measured lobe is ~6-m-thick. The thickest section of lava is ~35-m-thick and composed of ~16 lobes, which alternate between blocky/massive, vesiculated, lava interiors and rubbly basal/upper breccias. Some of the massive interiors have vesiculated tops (1-m-thick), but most lack upper vesicular zones, even in inflated lobes. The breccias are ~3-m-thick, monolithologic, and are characterized by subangular to subrounded, up to boulder-sized, clasts. FCB breccias (and many massive interiors) are also affected by post-emplacement alteration via abundant Ca and Si mineralization. This has made geochemical analyses of the FCB challenging. However, three FCB samples plot as basalts on the total alkalis vs. silica diagram and also on a Nb/Y vs Zr/wt. TiO2 plot. 40Ar/39Ar geochronology and more bulk rock geochemical work is in progress on newly sampled fresher material to better constrain the timing and petrogenesis of this basalt package.