ABSTRACT

The Yellowstone Volcanic Group contains deposits of three major cycles of rhyolitic ignimbrite eruptive activity: first cycle 2.0 Ma Huckleberry Ridge Tuff, second cycle 1.3 Ma Mesa Falls Tuff, and 0.6 Ma Lava Creek Tuff. The Mesa Falls Tuff was studied in three well exposed localities of the Island Park area, Idaho, all within 35 km of the inferred vent region Two localities (Thurmon Ridge and the Henrys Fork Canyon) consist of welded pyroclastic flows which were ponded within an older (first cycle) caldera These welded tuffs comprise a multiple-flow, simple cooling unit 85-150 meters thick. The other locality, near Ashton, Idaho, exposes thin (25 m) non-welded tuffs consisting of basal airfall deposits overlain by pyroclastic flows and related ground surge deposits which surmounted the older caldera rim. The Mesa Falls Tuff was produced by very explosive activity that generated an eruption column tens of kilometers high; such eruptions are known as "Plinian". Eruption and emplacement of the Mesa Falls Tuff generated large volumes of fine vitric material (Pearlette-S ash) which was dispersed by prevailing winds over much of the central-western United States Granulometric, component and petrographic analyses of the Mesa Falls Tuff show that all deposits in the Island Park area are crystal enriched. This indicates the widely dispersed Mesa Falls airfall ash was produced both from the early eruption column and from subsequent winnowing (elutriation) of fine vitric material during column collapse and emplacement of pyroclastic flows.