Locations, descriptions, densities, and magnetic susceptibilities of rock samples collected for analyses of gravity and aeromagnetic anomalies in Yellowstone National Park, Wyoming, Idaho, and Montana

Oliver, Howard; Christiansen, Robert

Centered beneath Yellowstone National Park in northwestern Wyoming and adjacent parts of Montana and Idaho (Fig. 1; USGS, 1972) is one of the world's largest active magmatic systems (Christiansen and Blank, 1972; Christiansen, 1982; 1984; in press). The well known hydrothermal activity of the national park is a direct expression of a magmatic system that has sustained activity of the Yellowstone Plateau volcanic field (Fig. 4) through latest Pliocene and Quaternary time, producing three cycles of voluminous rhyolitic eruptions that each climaxed with an exceedingly large ash-flow eruption and concomitant formation of a large collapse caldera. The first caldera-forming eruption (the Huckleberry Ridge Tuff) occurred at 2.0 Ma, the second (the Mesa Falls Tuff) at 1.3 Ma and the third (the Lava Creek Tuff) at 0.6 Ma; this third cycle formed the Yellowstone caldera that now occupies an area about 80 by 30 km across within the park (Fig. 1). Together the three voluminous ash-flow tuffs form a stratigraphic unit designated the Yellowstone Group. Rhyolitic lavas preceded and followed each caldera-forming eruption and partly filled each caldera. Basalts have erupted repeatedly surrounding the Yellowstone caldera and have partially flooded areas within the two older calderas. Stratigraphically beneath the rocks of the Yellowstone Plateau volcanic field and surrounding the plateau itself are rocks ranging in age from Precambrian to Eocene (Figs. 2, 3). The regional basement consists principally of high-grade metamorphic rocks of Archaean age. That basement is overlain by predominantly marine sedimentary rocks of Paleozoic age that aggregate nearly 1,000 m in thickness and by Mesozoic strata nearly 3,000 m thick (Fig. 2; Ruppel, 1972; Love and Keefer, 1975). The early Mesozoic strata include both marine and continental rocks; the Cretaceous section comprises more than 2,500 m of sandstones and shales that are mostly of marine origin but include a continental volcanic-bearing orogenic-basin fill sequence at the top. The region was strongly deformed during the latest Cretaceous and earliest Tertiary Laramide orogeny. Predominantly intermediate volcanic and associated shallow-intrusive rocks were emplaced after the Laramide during the Eocene to form the Absaroka volcanic field (Fig. 3; Smedes and Prostka, 1972). Most of the samples were collected in Yellowstone National Park, within the area of the gravity survey. Some samples of pre-Tertiary rocks, however, were collected just outside the park at Cinnabar Mountain (Fig. 1, numbers 11-26), where they are well exposed and easily accessible in a single measured section, and nearby at Yankee Jim Canyon (Fig. 1, number 27).

Locations, descriptions, densities, and magnetic susceptibilities of rock samples collected for analyses of gravity and aeromagnetic anomalies in Yellowstone National Park, Wyoming, Idaho, and Montana

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