--> Abstract: Heise Volcanics Caldera Boundaries and Basaltic Fill of Paleoriver Channels: A Preliminary Geologic Strip Map of the Ririe Reservoir Area, ID, by B. Engleman, T. Atkinson, D. Doherty, V. Thornton, R. Tracy, D. Moore, and G. Embree; #90181 (2013)

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Heise Volcanics Caldera Boundaries and Basaltic Fill of Paleoriver Channels: A Preliminary Geologic Strip Map of the Ririe Reservoir Area, ID

B. Engleman1, T. Atkinson1, D. Doherty2, V. Thornton1, R. Tracy1, D. Moore1, and G. Embree1
1Brigham Young University – Idaho
2Renaissance Alaska & Stellar Oil & Gas

The Ririe Reservoir area lies near the southern margin of the Snake River Plain in southeastern Idaho and is part of the Yellowstone Snake River Plain (YSRP) magmatic province. We report detailed geologic field mapping of the area, and compositional and paleomagnetic data of two basaltic units (Qbb and Qbw). Bedrock geology in the area records multiple episodes of local rhyolitic volcanism from the Heise calderas (~6.5 – 4.5 Ma), including the formation of a rhyolitic caldera; multiple episodes of local basaltic volcanism; and the Huckleberry Ridge Tuff (Qyh), from the Yellowstone I caldera to the northeast (2.1 Ma). We map the Heise rhyolites as three units: Blacktail Tuff (Thb, ~6.5 Ma), intermediate tuffs and local air-fall deposits (Thi), and the Kilgore Tuff (Thk, ~4.5 Ma). We identify a caldera boundary indicated by the vertical contact between Thb and Mzs in the Meadow Creek Drainage and near the Blacktail boat ramp. Basaltic volcanism includes diktytaxitic units (Qbb, and Qbbt) that lie in the same stratigraphic position (below Qyh) and are petrographically indistinguishable. An additional basalt unit (Qbw) with plagioclase megacrysts (up to ~4cm) lies above Qyh. Where basaltic units are thick they record the location of paleoriver channels, which are adjacent to present canyons. Bedrock volcanic units are underlain and juxtaposed next to Mesozoic sedimentary rocks, and are locally covered by alluvium, colluvium, loess, and landslide deposits. Compositional data for Qbb and Qbw record tholeiitic / ferroan and Nb-rich. The megacrystic basalt plots with the extensivelyfractionated YSRP magma series. The diktytaxitic basalt plots with the normal basaltic YSRP magma series. The megacrystic basalt records a normal polarity; whereas the diktytaxitic basalt is reversed.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013