VOLCANIC STRATIGRAPHY AND AGE MODEL OF THE KIMAMA DEEP CORE HOLE (PROJECT HOTSPOT), CENTRAL SNAKE RIVER PLAIN, IDAHO

The Snake River Plain is the world’s best example of a mantle hotspot track in continental crust, with a record of bimodal volcanism extending from over 12 Ma to the present. Project Hotspot, the Snake River Scientific Drilling Project, recovered almost 2 km of continuous core from the Kimama drill site, located in central Idaho on the Axial Volcanic Zone of the Snake River Plain. Within 1912 m of core, we identify a total of 462 basalt flow units, representing 155 basalt flows, seventy-one basalt flow groups, twenty-seven super groups, and four compositional basalt types. We use volcanic facies observations, geochemical data, stratigraphic relationships, sedimentary interbeds, borehole geophysical logs, ⁴⁰Ar/³⁹Ar dating, and measurements of paleosecular variation in the magneto-stratigraphy to produce a comprehensive record of volcanic output from 6.5 Ma through 700 ka.

Sediment deposits between basalt flows represent lulls in regional volcanic activity of up to thousands of years. Gamma ray logs document the depth and thickness of sedimentary interbeds, and also highlight the occurrence of high-K₂O basalt lavas. Neutron logs document individual flow units through the contrast between massive flow interiors and more porous flow tops.

We dated six basalt lava flows using ⁴⁰Ar/³⁹Ar incremental heating by CO₂ infrared laser. Flows sampled at 320 m, 454 m, 1155 m, 1184 m, 1284 m, and 1489 m provide ages of 1.54 ± 0.15 Ma, 1.62 ± 0.15 Ma, 3.74 ± 0.13 Ma, 4.18 ± 0.58 Ma, 4.39 ± 0.30 Ma, and 5.05 ± 0.81 Ma, respectively. We measured paleomagnetic inclination in over 1200 samples collected at 2 m depth intervals. From these, we identified twenty-one magnetic reversals and correlated them to dated paleomagnetic Chrons and Subchrons using the radiometric ages. A linear fit to ages determined from ⁴⁰Ar/³⁹Ar dates and paleomagnetic time scale extrapolates to a bottom hole age of 6.4 Ma and defines a mean igneous accumulation rate of 335 m/Ma, slightly below the 346 m/Ma average accumulation rate of basalt lavas in the eastern SRP. Evidence from Kimama core stratigraphy and paleomagnetic and radiometric age data demonstrate the relative continuity of mafic volcanism on the central Snake River Plain over the last 6.4 Ma and the probable steady-state tectonic subsidence of the region.