RECONNAISSANCE MAPPING OF THE EOCENE THUNDER MOUNTAIN CALDERA COMPLEX, CENTRAL IDAHO

Reconnaissance geologic mapping in 2012-2015, combined with earlier published maps by the Idaho Geological Survey (Stewart et al., 2012) and USGS (Fisher et al., 1992) indicates that at least four of the ash-flow deposits associated with the Eocene Challis Volcanic Group near Thunder Mountain were produced by large-volume eruptive events. The oldest is the dime and quarter tuff sequence, followed by buff rhyolite, lower Sunnyside, and finally upper Sunnyside tuffs. Various workers have pointed out that while the Thunder Mountain area clearly contains collapse features, the affected area is extremely large, seemingly too large to be a caldera. We term the area a “caldera complex”, and propose a sequence of events to produce the extremely large and structurally complex volcanic field we see at present. A regional map presents the outlines of a postulated series of linked calderas that may have produced the caldera complex, based primarily on outcrop patterns and, in the case of the upper Sunnyside tuff (Sunnyside II caldera), the presence of collapse-related mega-breccia. None of the four pyroclastic deposits contain significant sediments and each presumably underwent relatively rapid collapse. The presence of at least one thick (200 m?) sequence of sediments above the dime and quarter tuff indicates that formation of its caldera was followed by a protracted depositional event prior to emplacement of the buff rhyolite. Following the four major eruptions, each forming its respective caldera, there was a fifth collapse event to form the Big Creek graben. The Big Creek graben is continuous with the Pistol Creek dike swarm to the SW, and the two features share the NNE trend characteristic of Challis-aged extension. However, the structural block into which dikes of the Pistol Creek swarm intruded is composed of dime and quarter tuff and Cretaceous and Tertiary intrusives rather than the Sunnyside tuffs of the Big Creek graben itself, and may be a continuation of the graben that was down-dropped less and therefore exposes a deeper structural section.