Project Hotspot: Subsurface Stratigraphy and Petrologic Evolution of Snake River Plain Basalts from Kimama Core

A key to understanding the volcanic and magmatic history of the Snake River Plain is identifying stratigraphic and geochemical trends through time. Project Hotspot, the Snake River Scientific Drilling Project, seeks to understand the long-term volcanic and sedimentary history of the Snake River volcanic province and its potential as a geothermal resource through the detailed logging of the Kimama drill core. One of three new, deep drill holes in the Snake River Plain (SRP), the Kimama drill hole (1912 m depth) is located along the volcanic axis of the central SRP. As one of the deepest coreholes on the Snake River Plain, Kimama core provides the opportunity to identify and evaluate these trends to better understand the sequence of magma genesis and volcanism. Detailed lithologic logging has identified at least 550 basalt flows (0.1-50 m thick) grouped into at least 30 flow groups, or individual magmatic episodes, that range in thickness from 13 m to 170 m thick (most 20-100 m thick). Basalt 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 and indicate hiatuses in volcanic activity. 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. Previous studies have shown that Snake River Plain basalt flow groups typically progress from less-evolved, more primitive basalts at depth to more evolved basalts upsection. These inter-flow group fractionation cycles are visible in preliminary plots of element concentrations by depth from the Kimama core. However, basalt flows at depths of 318.7 m and 526.6 m show higher values of Fe2O3 wt. % as well as higher incompatible element concentrations outside of expected ranges, indicating more evolved compositions similar to those at Craters of the Moon, Idaho. These data suggest the presence of multiple magma sources at varying depths that fed simultaneous eruptions. Additionally, they have broader implications for the source of mafic volcanism on the SRP and the overall evolution of Yellowstone Hotspot volcanism through time.