Abstract
The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River−type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (∼1900 km 3 ). It correlates regionally and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.
Highlights
The Yellowstone–Snake River Plain volcanic province, United States (Fig. 1), is the youngest and best-preserved silicic intraplate volcanic province on Earth, with a protracted history of voluminous explosive eruptions from the midMiocene to the present (e.g., Pierce and Morgan, 1992; Bonnichsen et al, 2008)
We present new stratigraphic definitions, and we characterize 12 major rhyolitic explosive eruptions that occurred between 11.3 mean age (Ma) and ca. 8 Ma, during a major ignimbrite flare-up within the province (Nash et al, 2006) broadly contemporaneous with the opening of the western Snake River continental rift (Bonnichsen et al, 2008; Fig. 1)
Fresh vitrophyres with negligible accidental material were selected for whole-rock majorand trace-element X-ray fluorescence (XRF) analysis
Summary
The Yellowstone–Snake River Plain volcanic province, United States (Fig. 1), is the youngest and best-preserved silicic intraplate volcanic province on Earth, with a protracted history of voluminous explosive eruptions from the midMiocene to the present (e.g., Pierce and Morgan, 1992; Bonnichsen et al, 2008) It produced the largest known volume of low d18O deposits on Earth (e.g., Boroughs et al, 2005) and is the type locality of high-temperature “SnakeRiver–type” supereruptions, which generated vast, intensely welded, and commonly lavalike rheomorphic ignimbrites, thick laminated ash-fall deposits, and uncommonly long block lavas (Branney et al, 2008). Voluminous rhyolitic volcanism associated with the Yellowstone hotspot has migrated ~600 km eastward from northern Nevada across southern Idaho, to the present-day Yellowstone volcanic field (Leeman, 1982; Pierce and Morgan, 1992). The cover is composed of intensely welded Snake River– type rhyolitic ignimbrites and associated ashfall deposits (Branney et al, 2008), inferred to have originated from the central Snake River Plain (Pierce and Morgan, 1992; McCurry et al, 1996)
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