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
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Cassia Formation: Mid-Miocene record of large-scale Snake River–type volcanism 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 Thomas R. Knott1,†, Michael J. Branney1, Marc K. Reichow1, David R. Finn2, Robert S. Coe2, Michael Storey3, Dan Barfod4, and Michael McCurry5 1Department of Geology, University of Leicester, Leicester, LE1 7RH, UK 2Earth and Planetary Science Department, University of California, Santa Cruz, California 95064, USA 3Quaternary Dating Laboratory, Natural History Museum of Denmark, University of Copenhagen, 2100 Copenhagen, Denmark 4Scottish Universities Environmental Research Centre, East Kilbride, G75 0QF, UK 5Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA ABSTRACT relations, and rheomorphic transport indica- province remain little studied, and the overall tors in the successive dated ignimbrites. The volcanic stratigraphy is not resolved. Pioneer- The 1.95-km-thick Cassia Formation, de- subsidence is thought to have occurred in ing work by Bonnichsen et al. (2008) summa- fined in the Cassia Hills at the southern mar- response to incremental loading and modi- rized known rhyolitic volcanic units around the gin of the Snake River Plain, Idaho, consists fication of the crust by the mantle-derived central Snake River Plain, but an essential step of 12 refined and newly described rhyolitic basaltic magmas. During this time, the area toward understanding the magmatic and struc- members, each with distinctive field, geo- also underwent NW-trending faulting re- tural evolution of the province through time chemical, mineralogical, geochronological, lated to opening of the western Snake River will be to develop a stratigraphic and chrono- and paleomagnetic characteristics. It re- rift and E-W Basin and Range extension. logic framework of sufficient resolution to per- cords voluminous high-temperature, Snake The large eruptions probably had different mit the robust distinction and characterization River–type explosive eruptions between ca. source locations, all within the subsiding ba- of the individual eruptions. 11.3 Ma and ca. 8.1 Ma that emplaced in- sin. The proximal Miocene topography was This paper documents the 1.95-km-thick tensely welded rheomorphic ignimbrites and thus in marked contrast to the more elevated volcanic succession around the Cassia Hills on associated ash-fall layers. One ignimbrite present-day Yellowstone plateau. the southern flank of the central Snake River records the ca. 8.1 Ma Castleford Crossing Plain, Idaho (Fig. 1), where deep canyon inci- eruption, which was of supereruption mag- INTRODUCTION sion, together with a new 1.9-km-deep bore- nitude (~1900 km3). It covers 14,000 km2 and hole, reveal one of the most complete succes- exceeds 1.35 km thickness within a subsided, The Yellowstone–Snake River Plain volcanic sions of mid-Miocene Snake River volcanism in proximal caldera-like depocenter. Major- province, United States (Fig. 1), is the young- the region. The study builds on previous work and trace-element data define three succes- est and best-preserved silicic intraplate volcanic (Williams et al., 1990; Wright et al., 2002; Ellis sive temporal trends toward less-evolved province on Earth, with a protracted history of et al., 2010) and resolves earlier groupings and rhyolitic compositions, separated by abrupt voluminous explosive eruptions from the mid- miscorrelations of eruption units. We present returns to more-evolved compositions. Miocene to the present (e.g., Pierce and Mor- new stratigraphic definitions, and we character- These cycles are thought to reflect increas- gan, 1992; Bonnichsen et al., 2008). It produced ize 12 major rhyolitic explosive eruptions that ing mantle-derived basaltic intraplating and the largest known volume of low d18O deposits occurred between 11.3 Ma and ca. 8 Ma, during hybridization of a midcrustal region, coupled on Earth (e.g., Boroughs et al., 2005) and is a major ignimbrite flare-up within the province with shallower fractionation in upper-crustal the type locality of high-temperature “Snake- (Nash et al., 2006) broadly contemporaneous magma reservoirs. The onset of each new River–type” supereruptions, which generated with the opening of the western Snake River cycle is thought to record renewed intra- vast, intensely welded, and commonly lava- continental rift (Bonnichsen et al., 2008; Fig. 1). plating at an adjacent region of crust, pos- like rheomorphic ignimbrites, thick laminated Where possible, unit names are retained from sibly as the North American plate migrated ash-fall deposits, and uncommonly long block earlier accounts. New field descriptions, whole- westward over the Yellowstone hotspot. lavas (Branney et al., 2008). Of great interest is rock and mineral chemistry, geochronology, A regional NE-trending monocline, here the unusual physical volcanology of these large and paleomagnetic data are presented to char- termed the Cassia monocline, was formed eruptions, and the ways in which the magmas acterize and distinguish each eruption unit, and by synvolcanic deformation and subsidence formed and varied with time as the continen- the resultant stratigraphic framework is used to of the intracontinental Snake River basin. Its tal hotspot migrated eastward (Fig. 1; Leeman reveal repeated magmatic cycles in Snake River structural and topographic evolution is re- et al., 2008; Shervais et al., 2013). Some of the Plain magmas over time. Finally, the tectonic constructed using thickness variations, offlap eruptions are predicted to have been as large and magmatic evolution of the region during as the better-known eruptions from Yellow- the catastrophic eruptions is reconstructed from †trk2@ le .ac .uk stone (Ellis et al., 2012), yet large tracts of the structural and deposit thickness data. GSA Bulletin; July/August 2016; v. 128; no. 7/8; p. 1121–1146; doi: 10.1130/B31324.1; 19 figures; 1 table; Data Repository item 2016049; published online 10 February 2016. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 128, no. 7/8 1121 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/128/7-8/1121/3411955/1121.pdf by guest on 28 September 2021 Knott et al. ID MT orthoquartzite in the west, and more wide- Seattle spread Lower Permian sandstones and lime- CRB stone (Youngquist and Haegele, 1956; Mytton Kimberly 84 borehole et al., 1990; Williams et al., 1991). The cover is composed of intensely welded Snake River– wSRr Y–SRP Y Snake River type rhyolitic ignimbrites and associated ash- 42°N 30’ fall deposits (Branney et al., 2008), inferred to WA S n a k e R i v e r b a s N i n have originated from the central Snake River OR C UT WY Dry Gulch Plain (Pierce and Morgan, 1992; McCurry Salt Lake quarry Rock et al., 1996). 200 km City 5 km NV Cr Methods e ek Little Creek can Successions in canyon walls were subdivided 20′ y Shipper on Oakley into eruption units (deposit packages inferred to 27 Basin 93 record a single volcanic eruption) on the basis Steer Basin of intervening paleosols, sedimentary horizons, Magpie and contrasting paleomagnetic signatures that Basin Oakley indicate significant repose periods. Challenges Rogerson were presented by (1) the monotonous nature of 10′ the thick ignimbrite successions and (2) abun- Cassia k dant high-angle faults that repeat units along Trapper Cree canyon walls. Therefore, we targeted nonfaulted Shoshone Hills Hills reference sections (and one borehole) where Shoshone Basin numerous units are exposed in stratigraphic continuity. Links between these reference sec- Goose tions were then resolved by detailed mapping 42°N ID Creek to elucidate lateral lithological and thickness Jackpot NV UT basin variations. Lithostratigraphic units (members) are defined at type sections (coordinates in 1 114°W 40′ 30′ 20’ 10’ 114°W Table S1 ). More than 30 logs were described, and tops and bases of units were sampled (Table Figure 1. Map of the Cassia Hills in southern Idaho (C in inset) showing main canyons S1 [see footnote 1]) for petrographic, geochemi- and locations mentioned in the text. Gray—elevated terrain (main map); CRB—Columbia cal, radioisotopic, and paleomagnetic analysis. River basalts; Y-SRP—Yellowstone–Snake River Plain volcanic province showing NE mi- Fresh vitrophyres with negligible accidental gration of the Yellowstone hotspot track (white arrow); Y—Yellowstone; wSRr—western material were selected for whole-rock major- Snake River rift. State abbreviations in inset: ID—Idaho; MT—Montana; NV—Nevada; and trace-element X-ray fluorescence (XRF) UT—Utah; WY—Wyoming; WA—Washington; OR—Oregon. analysis. Mineral phases were characterized in thin section and by electron microprobe analy- ses. For consistency, all paleomagnetic samples Geological Setting 2008). This regional intra continental basin were drilled at the same sites as the geochemical crosses N-trending Basin and Range structures sampling, and were analyzed following methods Voluminous rhyolitic volcanism associ- and has undergone E-W extension (Miller outlined in Finn et al. (2015). Individual mem- ated with the Yellowstone hotspot has migrated et al., 1999). Several vast ignimbrite sheets are bers were dated variously by single-crystal ~600 km eastward from northern Nevada across exposed in massifs along the north and south 40Ar/39Ar dating of feldspars and U-Pb dating of southern Idaho, to the present-day Yellow stone flanks of the Snake River basin, and are thought zircons, and we present new data alongside pub- volcanic field (Leeman, 1982; Pierce and Mor- to derive from various rhyolitic eruptive cen- lished ages in Table 1. Analytical procedures, gan, 1992). It covers ~175,000 km2 and is com- ters concealed beneath Neogene basalt lavas data, sample locations, standards, and applied monly attributed to a mantle plume beneath within the Snake River basin (Bonnichsen et al., corrections are given in the GSA Data Reposi- the westerly migrating North American plate 2008).