Mid-Miocene record of large-scale Snake River–type explosive volcanism and associated subsidence on the Yellowstone hotspot track: The Cassia Formation of Idaho, USA

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 covers 14,000 km 2 and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Majorand 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.

Rheomorphic ignimbrites of the Rogerson Formation, central Snake River plain, USA: record of mid-Miocene rhyolitic explosive eruptions and associated crustal subsidence along the Yellowstone hotspot track

“…Similar occurrences of abundant blocky vitric lapilli at horizons within thick massive ignimbrites have been described in Snake River-type ignimbrites elsewhere (e.g. Big Bluff Member, Knott et al 2016). …”

“…The succession around Rogerson, Idaho, has been resolved and is now divided into 11 clearly defined eruption-units. The work builds upon recent studies further east, in the Cassia Hills (Knott et al 2016), at Rogerson Graben (Andrews and Branney 2005;Andrews et al 2008), and to the west, at Brown's Bench escarpment ; Fig. 1).…”

“…XRF and electron microprobe analytic procedures, including standard data, are detailed in Knott et al (2016). Separated sanidine feldspars were dated by 40 Ar/…”

“…1) and we use new structural, kinematic and thickness-variation data to reconstruct the timing of major, magma-induced continental subsidence of the 100-km-wide Snake River Basin during this phase of intense magmatism. A complementary account of a rhyolitic volcanic succession further east, in the Cassia Hills, is given in Knott et al (2016).…”

“…Although the ignimbrites in the central Snake River Plain are unusually intensely welded and develop flow-folding and upper flow breccias, they can readily be distinguished from true lavas by several features, key of which are (a) the widespread absence of basal autobreccias, which are ubiquitous in rhyolitic lavas (Bonnichsen and Kauffman 1987;Henry and Wolff 1992;; and (b) the tendency of rheomorphic ignimbrites to thin gradually as they mantle topographic slopes, and as they approach their tapering distal terminations (e.g. Branney and Kokelaar 2002;Knott et al 2016), in contrast to the more abrupt, lobate, steep and breccia-fringed terminations of true blocky lavas (Bonnichsen and Kauffman 1987).…”