PROJECT HOTSPOT: INSIGHT INTO THE SUBSURFACE STRATIGRAPHY AND PETROLOGIC EVOLUTION OF THE SNAKE RIVER PLAIN

Project Hotspot seeks to understand the long-term volcanic history of the Snake River Plain, with particular focus on ubiquitous basalts exposed in two new drill holes in the central Snake River Plain (SRP). The Kimama drill hole (1912 m depth) is located along the volcanic axis of the central SRP. Core from this drill hole comprises basalt flows with minor sediment interbeds; no rhyolite was encountered, in contrast to expectations. The Kimberly drill hole is located on the southern margin of the Twin Falls eruptive complex (projected depth 1500 m). The Kimberly drill hole penetrates basalt that overlies welded rhyolite ash-flows with up to 20% phenocrysts, which in turn overlies basalt with minor sediment.

Preliminary stratigraphic sections for Kimama are based on gamma and neutron logs as well as lithologic observations. The gamma logs document the depth and thickness of sedimentary interbeds, whereas the neutron logs respond to the contrast between massive flow interiors and more porous flow tops. These data show the presence of at least 340 basalt flows (0.3-26 m thick), grouped into at least 20 flow groups, 30 m to 300 m thick (most 60-130 m thick). Flows are designated based upon morphological characteristics, including the presence of rubbly, highly fractured flow tops, massive to vesicular flow interiors, and rubbly flow bases; flow groups are commonly separated by sedimentary interbeds, which range in thickness from 3 m to 52 m. The compilation of well log data shows an apparent agreement between lithologic and geophysical stratigraphy, with observed basalt flow group breaks and sediment interbeds at least roughly mirrored by spikes and dips in natural gamma and neutron log signals. These observations demonstrate the usefulness of wireline geophysical data to determine basalt flow group boundaries.

Initial geochemical analyses of basalts in core from Kimama show a progression from primitive SRP basalts (MgO 8-9%, and Fe₂O₃ 12-14%) at depth to more evolved ferrobasalts (MgO 7-8.5% and Fe₂O₃ 15-18%) upsection. One flow at 535 m depth has a composition similar to but slightly less evolved than lavas at Craters of the Moon. These data have broader implications for the evolution and duration of basaltic volcanism in the central SRP as well as the sequence of volcanism associated with caldera formation along the track of the Yellowstone Hotspot.