Scientific Drilling

Abstract

HOTSPOT is an international collaborative effort to understand the volcanic history of the Snake River Plain (SRP). The SRP overlies a thermal anomaly, the Yellowstone-Snake River hotspot, that is thought to represent a deep-seated mantle plume under North America. The primary goal of this project is to document the volcanic and stratigraphic history of the SRP, which represents the surface expression of this hotspot, and to understand how it affected the evolution of continental crust and mantle. An additional goal is to evaluate the geothermal potential of southern Idaho.Project HOTSPOT has completed three drill holes.(1) The Kimama site is located along the central volcanic axis of the SRP; our goal here was to sample a long-term record of basaltic volcanism in the wake of the SRP hotspot.(2) The Kimberly site is located near the margin of the plain; our goal here was to sample a record of high-temperature rhyolite volcanism associated with the underlying plume. This site was chosen to form a nominally continuous record of volcanism when paired with the Kimama site. (3) The Mountain Home site is located in the western plain; our goal here was to sample the Pliocene-Pleistocene transition in lake sediments at this site and to sample older basalts that underlie the sediments.We report here on our initial results for each site, and on some of the geophysical logging studies carried out as part of this project.

Understanding volcanic facies in the subsurface: a combined core, wireline logging and image log data set from the PTA2 and KMA1 boreholes, Big Island, Hawai`i

“…Sonic, resistivity, spectral gamma, dipmeter, magnetic susceptibility, temperature, total magnetic field and televiewer tool logging were planned for both wells in order to allow detailed volcanic facies assessment and to test correlations between the two boreholes. Neutron and density tools have also been demonstrated to give important insights into volcanic facies (e.g., Planke, 1994;Shervais et al, 2013); however, due to the active radioactive sources in both tools, it was not possible to include these logs in the logging suite on Hawaii due to operational restrictions. In the end two logging trips were undertaken, the first in February 2016 and the second in June 2016, and only the open hole section of PTA2 was logged due to major complications associated with the final stages of drilling within KMA1 and subsequent efforts to resolve the resulting stuck pipe problem.…”

“…Due to the general incompatibility of these elements during early fractional crystallization of basaltic melts, their abundance and therefore related API gamma responses increase with fractional crystallization and differentiation. Gamma ray responses can therefore be a good proxy for the degree of differentiation in volcanic rock suites (Stefansson et al, 2000;Delpino and Bermúdez, 2009), with higher gamma responses indicating more evolved compositions. Alteration and weathering processes are also known to affect gamma responses in basaltic rocks largely due to the water solubility and therefore mobility of K during alteration (e.g., Planke, 1994).…”