Tuff of Wooden Shoe Butte (lower member).

The Wooden Shoe Butte tuff is an extensive rhyolitic (72% SiO2) outflow ignimbrite, 100 m thick. Geothermometry indicates very high magmatic temperatures, and high emplacement temperatures are indicated by intense welding, widespread rheomorphism and a predominance of ‘lava-like’ lithofacies. The ignimbrite lacks fiamme, and granulometric analysis of rare non-welded parts show much better sorting than is typical for ignimbrites from other provinces with few lithic and pumice lapilli. Rather, it contains abundant small chips of dense black obsidian. A basal ashfall deposit contains similar obsidian chips, and its lower 4 m are fused to vitrophyre. It is well sorted and composed of megascopic glass shards, crystals and obsidian chips, but pumice and lithics are scarce.

Eruption mechanisms responsible for the Wooden Shoe Butte Tuff are enigmatic. The fall deposits are atypical of Plinian deposits, yet coarser (even distally) than is likely of phreatoplinian deposits. The ignimbrite was emplaced in the Miocene  as is part of the Yellowstone hotspot track.

The recognition of a highly unusual assemblage of vast rhyolitic lava-like rheomorphic ignimbrites, extensive, large volume rhyolite lavas (<200 km3), and widely dispersed laminated rhyolitic ashfall deposits with ash pellets and unusually large shards, has given rise to the notion of ‘Snake River-type volcanism’ that contrasts with conventional Plinian-ignimbrite volcanism elsewhere. These are the features of the single eruption unit of the Wooden Shoe Tuff whick is > 30 km3 in volume.

See next image for details of the tuff unit.