STRATIGRAPHIC CHARACTERIZATION OF THE MOUNTAIN HOME BORE HOLE, WESTERN SNAKE RIVER PLAIN

From 2010 to 2012 Project HOTSPOT completed three drill holes in the Snake River Plain to depths of ~2 km each. The three drill sites (Kimama, Kimberly and Mountain Home) were strategically chosen to sample a nearly continuous chronologic record of emplacement and deposition of the northeast migration of the Yellowstone Hotspot relative to the North American Plate. The objective of the drilling project was to investigate geothermal potential in three distinct regimes of the SRP. We characterize the flow-unit scale stratigraphy of whole-rock core from the western-most drill hole (Mountain Home drill site) and identify and describe outcrop analogs. The methods used for this characterization include identification of volcanic facies observations, stratigraphic and textural relationships, and sedimentary and volcaniclastic marker horizons. We correlate the lithologic logs acquired at Mountain Home to the borehole geophysical data in an effort to identify signatures that represent fine-scale variations in stratigraphy, composition and/or alteration.

Flow boundaries are identified by key flow-units. Major flow units include vesicular oxidized flow tops, massive flow interiors, and rubbly flow bases. Periods of non-emplacement are marked by sedimentary deposition. Hyaloclastites indicate rapid quenching and proximal water sources or sub-aerial emplacement. Massive, nearly black units indicate fractionated remnant melt with a very high abundance of iron. Previous chemical analysis of Western Snake River Plain basalts identified anomalously high abundances of iron, when compared with basalts of the central and eastern SRP. The oldest basalts are highly fractured, and the flow units are more difficult to discern. Fractures and vesicles are filled with calcareous and zeolitic alterations, indicating a history of hydrothermal fluid-rock interactions.