Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)
Paper No. 40-2
Presentation Time: 8:00 AM-5:00 PM


GILLERMAN, Virginia S., Idaho Geological Survey, Boise State University, Boise, ID 83725-1535,

Mapping and reexamination of the “older” volcanic units exposed south of Hagerman is one phase of the Idaho Geological Survey’s STATEMAP project in the Twin Falls 30’ x 60’ quadrangle. The area includes Banbury Hot Springs, type locality for the Tertiary Banbury Basalt, which Malde and others (1963) mapped over a wide area of southwestern Idaho. Our work and the excellent mapping of Malde and Powers (1973) show much heterogeneity within the “type section.” The lower Banbury includes a field of hydrovolcanoes which appear to be localized along a northwest-trending structural zone which also influenced later graben development and formation of Pliocene Lake Idaho, geothermal activity, and canyon development. The inferred structure is suspiciously close to a projection (S49E) of the northern margin of the Western Snake River Plain.

Stratigraphically overlying rhyolite, the lower Banbury consists of altered basalt flows (Tbl) with a middle layer of basaltic pyroclastics (Tbls), overlain by sediments (Tbs) which thicken to the south, and upper basalt (Tbu) (Malde and Powers, 1973). The vent facies of Tbl includes at least two, exposed volcanic centers (Thousand and Blue Heart vents) which are characterized by tuff breccias with laterally extensive surge and tephra deposits, locally palagonitized. Spectacular, 10-meter high beds of tuff breccia contain blocks over a meter in size. Distal, phreatomagmatic tuffs with volcanic bombs, stream pebbles, and siltstone xenoliths, are overlain by a 1-meter bed of black spatter, which is overlain by an oxidized tuff with small glass bombs, overlain by 3 upward coarsening cycles of air fall tuff. The emergent hydrovolcano is unconformably overlain by a baked siltstone and the Tbu unit which consists of notably fresher, plagioclase-phyric, vesicular basalt flows. Elsewhere in the map area, “Tbu” is water-affected basalt (WAB) with a different magnetic polarity than the feldspar-rich flows. Pending geochronology and chemistry may improve age constraints and correlations. Even at its type locality, the Banbury is lithologically heterogeneous with lateral facies changes indicating basaltic volcanism within a lacustrine and fluvial setting prior to Lake Idaho and probably over a considerable time span. As noted by others, it is time to revise the nomenclature.

Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)
Session No. 40--Booth# 16
Products and Processes of Hydrovolcanism (Posters)
Boise Centre on the Grove: Flying Hawk and Falcon's Eyries
8:00 AM-5:00 PM, Wednesday, May 5, 2004

Geological Society of America Abstracts with Programs, Vol. 36, No. 4, p. 86

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