PRELIMINARY STRATIGRAPHIC INTEREPRETATION FROM BOREHOLE STRAINMETER DRLLING, YELLOWSTONE NATIONAL PARK

Between 2007 and 2008, the Plate Boundary Observatory (PBO) drilled seven boreholes (~200 feet to 800 feet deep) in Yellowstone National Park for the NSF Earthscope project. Five of the seven boreholes contained strainmeters, downhole seismometers, and tiltmeters located in non-hydrothermal areas. During 2013, PBO drilled an additional borehole to replace one damaged by lightning in 2011. Data collected during drilling included cuttings, stratigraphy within the hole, temperature, water samples from selected depths, and geophysical logs. Examination of cuttings assisted the selection of suitable horizons for grain mounts, thin sections, and geochemical analyses. Geochemical analyses of cuttings (x-ray fluorescence, XRF; inductively coupled plasma mass spectrometry, ICP-MS) determined major element and trace element values for selected horizons.

A preliminary stratigraphic interpretation of the geochemical analyses generally showed a correlation between borehole cuttings and previously mapped geology. Six of the boreholes encountered rhyolite lavas and explosive tuffs: the Canyon flow, the Gardner River flow, the Gibbon River flow, the Hayden flow, the Nez Perce flow, the West Thumb flow, the Tuff of Bluff Point, the Lava Creek Tuff. The borehole near the Panther Creek vent drilled through a sequence of Sheepeater Cliff basalts outside the Yellowstone caldera.

Data from the drilling of the borehole strainmeters improved the understanding of Yellowstone’s hydrothermal system: The boreholes encountered fluid flow at permeable boundaries between rhyolite lavas and fractures within the welded Lava Creek Tuff. Additionally, water flow rates during drilling support the interpretation that fractured rhyolite flows and tuffs are good aquifers.