Abstract

A 900-m-thick section of tuffaceous sedimentary rock, vitric fallout tuff, and ash-flow tuff is well exposed along Trapper Creek in south-central Idaho. This section provides nearly continuous exposure through the fill of the Goose Creek basin, a major north-trending Miocene extensional basin located along the southern margin of the Snake River Plain volcanic province (SRPVP). Some 51 separate units of vitric fallout tuff are recognized in the Trapper Creek section. Petrographic and chemical characteristics of these vitric tuffs indicate that most are from SRPVP sources. New 40Ar/39Ar laser-fusion dating, along with prior isotopic age determinations, show that the Trapper Creek tuffs span the period ca. 13.9 – 8.6 Ma. Chemical correlation indicates that fallout tuffs in the central part of the Trapper Creek section (12.5 – 10.0 Ma) are from sources in the Bruneau-Jarbidge volcanic field of the SRPVP centered ≈100 km west of Trapper Creek. Underlying fallout tuffs may have had sources in the Owyhee-Humboldt field of the SRPVP centered ≈200 km west of Trapper Creek, while overlying fallout tuffs, interlayered with several ash-flow tuffs, had a relatively proximal source, possibly in the proposed Twin Falls volcanic field centered ≈60 km north of Trapper Creek.

The Trapper Creek tuffs provide insight into the characteristics of explosive silicic volcanism within the SRPVP during middle – late Miocene time. From ca. 13.9 to ca. 9.5 Ma, major eruptions (those depositing ≥1.5 m of fallout tuff) were frequent (about one event per 200 k.y.); their products display a trend toward the eruption of progressively less evolved, higher temperature silicic magma after 12.5 Ma. This trend to higher temperature eruptions, termed the Cougar Point “flare-up,” culminated in the eruption of high-temperature (≈1000°C), plagioclase-rich magma during the period 10.5 – 9.5 Ma. In contrast to these eruptions, later (<7.0 Ma) major silicic eruptions within the SRPVP were characterized by the lower temperature (≈850°C) of the erupted magma and by the longer intervals (about one event per ≈500 – 600 k.y.) between eruptions. Variations in the character of SRPVP explosive silicic eruptions may reflect changes in the structure, composition, or state of stress in the crust beneath the eastward propagating SRPVP, or, perhaps, changes in the Yellowstone hot-spot plume that may drive the SRPVP volcanism.

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