‘Snake River (SR)-type’ volcanism at the Yellowstone hotspot track: distinctive products from unusual, high-temperature silicic super-eruptions

Abstract

A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) largevolume, lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarsegrained ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre; (3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association between the rhyolitic rocks and numerous, coalescing lowprofile basalt lava shields; and (8) widespread evidence of emplacement in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6-8), and involved H 2 O-poor, low-δ 18 O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900-1,050°C). SR-type volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme, and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos Texas, EtendekaParaná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea Dacite of Australia.

A review of volcanic ash aggregation

“…Stratigraphic context of accretionary pellets (AP) in tephra fallout layers AP-bearing fall layers derived from eruption clouds or co-PDC clouds ('co-PDC deposit clouds') are common in the geological record and recent past (e.g., Self, 1983;Hayakawa, 1990;Schumacher and Schmincke, 1991;De Rita et al, 2002;Branney et al, 2007;Brown et al, 2010;Ritchie et al, 2002). Typically these deposits contain AP1 aggregates or, less commonly, AP2 aggregates (Table 1).…”

“…Most occurrences are of AP2 aggregates (e.g., McPhie, 1986;Schumacher and Schmincke, 1991;Ui et al, 1992;Sohn, 1996;Baer et al, 1997;Colella and Hiscott, 1997;De Rita et al, 2002;Palladino et al, 2001;Scolamacchia et al, 2005;Branney et al, 2007;Brown et al, 2007;Andrews et al, 2008;Brown et al, 2010;Ellis and Branney, 2010). AP2 aggregates are commonly found within the matrices of PDC deposits or clastsupported in discontinuous layers or lenses within bedded PDC deposit (Fig.…”

Extensive soft-sediment deformation and peperite formation at the base of a rhyolite lava: Owyhee Mountains, SW Idaho, USA

“…Rhyolite magmas typically extrude comparatively smallvolume lava domes of high viscosity, yet some rhyolitic volcanic fields produce voluminous lava flows, often hundreds of metres thick (Bonnichsen 1982;Henry et al 1990;Bonnichsen and Kauffman 1987;Branney et al 2008). Large volume silicic volcanism producing extensive, high temperature silicic lava flows and lava-like ignimbrites is characterised as BSR-type^, or BSnake River-type^volcanism by Branney et al (2008) and is typical of the bimodal Columbia River-Yellowstone Volcanic Province in the NW of the USA.…”

“…Large volume silicic volcanism producing extensive, high temperature silicic lava flows and lava-like ignimbrites is characterised as BSR-type^, or BSnake River-type^volcanism by Branney et al (2008) and is typical of the bimodal Columbia River-Yellowstone Volcanic Province in the NW of the USA. Thick rhyolite lava flows (>50 m) can impose huge amounts of downward force and heat on to the underlying substrate.…”

Volcanism of the Palaeoproterozoic Bushveld Large Igneous Province: The Rooiberg Group, Kaapvaal Craton, South Africa

“…Younger examples include the Snake River Plain (Bonnichsen and Kauffman, 1987;Manley, 1996;Branney et al, 2008), Trans-Pecos Texas (Henry et al, 1989(Henry et al, , 1990, Yellowstone National Park (Christiansen and Hildreth, 1989), and the Etendeka Igneous Province in Namibia (Milner, 1986;Milner et al, 1992).…”