Fallout tuffs of Trapper Creek, Idaho; a record of Miocene explosivevolcanism in the Snake River plain volcanic province

Abstract

A 900-m-thick section of tuffaceous sedimentary rock, vitric fallouttuff, and ash-flow tuff is well exposed along Trapper Creek insouth-central Idaho. This section provides nearly continuous exposurethrough the fill of the Goose Creek basin, a major north-trendingMiocene extensional basin located along the southern margin of theSnake River Plain volcanic province (SRPVP). Some 51 separate units ofvitric fallout tuff are recognized in the Trapper Creek section.Petrographic and chemical characteristics of these vitric tuffsindicate that most are from SRPVP sources. New (super 40) Ar/ (super39) Ar laser-fusion dating, along with prior isotopic agedeterminations, show that the Trapper Creek tuffs span the period ca.13.9-8.6 Ma. Chemical correlation indicates that fallout tuffs in thecentral part of the Trapper Creek section (12.5-10.0 Ma) are fromsources in the Bruneau-Jarbidge volcanic field of the SRPVP centerednearly equal 100 km west of Trapper Creek. Underlying fallout tuffsmay have had sources in the Owyhee-Humboldt field of the SRPVPcentered nearly equal 200 km west of Trapper Creek, while overlyingfallout tuffs, interlayered with several ash-flow tuffs, had arelatively proximal source, possibly in the proposed Twin Fallsvolcanic field centered nearly equal 60 km north of Trapper Creek. TheTrapper Creek tuffs provide insight into the characteristics ofexplosive silicic volcanism within the SRPVP during middle-lateMiocene time. From ca. 13.9 to ca. 9.5 Ma, major eruptions (thosedepositing > or =1.5 m of fallout tuff) were frequent (about oneevent per 200 k.y.); their products display a trend toward theeruption of progressively less evolved, higher temperature silicicmagma after 12.5 Ma. This trend to higher temperature eruptions,termed the Cougar Point ""flare-up,"" culminated in theeruption of high-temperature ( nearly equal 1000 degrees C),plagioclase-rich magma during the period 10.5-9.5 Ma. In contrast tothese eruptions, later (<7.0 Ma) major silicic eruptions within theSRPVP were characterized by the lower temperature ( nearly equal 850degrees C) of the erupted magma and by the longer intervals (about oneevent per nearly equal 500-600 k.y.) between eruptions. Variations inthe character of SRPVP explosive silicic eruptions may reflect changesin the structure, composition, or state of stress in the crust beneaththe eastward propagating SRPVP, or, perhaps, changes in theYellowstone hot-spot plume that may drive the SRPVP volcanism.

51 samples from this reference