Paleomagnetic correlation and ages of basalt flow groups in coreholes at and near the Naval Reactors Facility, Idaho National Laboratory, Idaho

Abstract

Paleomagnetic inclination and polarity studies were conducted on subcore samples from eight coreholes located at and near the Naval Reactors Facility (NRF), Idaho National Laboratory (INL). These studies were used to characterize and to correlate successive stratigraphic basalt flow groups in each corehole to basalt flow groups with similar paleomagnetic inclinations in adjacent coreholes. Results were used to extend the subsurface geologic framework at the INL previously derived from paleomagnetic data for south INL coreholes. Geologic framework studies are used in conceptual and numerical models of groundwater flow and contaminant transport. Sample handling and demagnetization protocols are described, as well as the paleomagnetic data averaging process. Paleomagnetic inclination comparisons among NRF coreholes show comparable stratigraphic successions of mean inclination values over tens to hundreds of meters of depth. Corehole USGS 133 is more than 5 kilometers from the nearest NRF area corehole, and the mean inclination values of basalt flow groups in that corehole are somewhat less consistent than with NRF area basalt flow groups. Some basalt flow groups in USGS 133 are missing, additional basalt flow groups are present, or the basalt flow groups are at depths different from those of NRF area coreholes. Age experiments on young, low potassium olivine tholeiite basalts may yield inconclusive results; paleomagnetic and stratigraphic data were used to choose the most reasonable ages. Results of age experiments using conventional potassium argon and argon-40/argon-39 protocols indicate that the youngest and uppermost basalt flow group in the NRF area is 303 ± 30 ka and that the oldest and deepest basalt flow group analyzed is 884 ± 53 ka. A south to north line of cross-section drawn through the NRF coreholes shows corehole-to-corehole basalt flow group correlations derived from the paleomagnetic inclination data. From stratigraphic top to bottom, key results include the following: * The West of Advanced Test Reactor Complex (ATRC) flow group is the uppermost basalt flow group in the NRF area and correlates among seven continuously cored holes in this study under surficial sediments. The West of ATRC flow group is also found in coreholes near the ATRC, the Idaho Nuclear Technology and Engineering Center (INTEC), and in corehole USGS 129. * The ATRC Unknown Vent flow group correlates among seven continuously cored holes in this study underlying the West of ATRC flow group and a sedimentary interbed. Additional paleomagnetic inclination and stratigraphic data derived from the NRF coreholes changed the previously reported interpretation of the subsurface distribution of this basalt flow group. The ATRC Unknown Vent flow group also is found in coreholes near the ATRC and INTEC. * The Central Facilities Area (CFA) Buried Vent flow group correlates among all eight coreholes in the NRF area. It also is found in coreholes near the CFA and the Radioactive Waste Management Complex (RWMC) to the south. This basalt flow group is thickest near the CFA, which may indicate proximity to the vent. The State Butte flow group is found below the CFA Buried Vent flow group in the four northern NRF coreholes. It correlates to the State Butte surface vent located just northeast of the NRF. It is not found in coreholes south of the NRF. * The Atomic Energy Commission (AEC) Butte flow group is found in coreholes USGS 133, NRF 6P, and NRF 7P. It probably underlies coreholes NRF B18-1, NRF 89-05, and NRF 89-04, but those coreholes were not drilled deeply enough to penetrate the flow group. The AEC Butte flow group vent is exposed at the surface near the ATRC, and its flows are found in many coreholes near the ATRC and INTEC. The AEC Butte flow group abruptly pinches out against the Matuyama Chron reversed polarity flows of the East Matuyama Middle flow group between coreholes NRF 7P and NRF 15. * The East Matuyama Middle flow group correlates between coreholes NRF 15 and NRF 16 and may correlate to coreholes NPR Test/W-02 and ANL-OBS-A-001. * The North Late Matuyama flow group correlates among coreholes USGS 133, NRF 6P, NRF 7P, NRF 15, and NRF 16. It probably underlies coreholes NRF B18-1, NRF 89-05, and NRF 89-04, but those coreholes were not drilled deeply enough to penetrate the flow group. The vent that produced the North Late Matuyama flow group may be located in the general NRF area because it is thickest near corehole NRF 6P. * The Matuyama flow group is found in coreholes in the southern INL from south of the RWMC to corehole USGS 133 and may extend north to corehole NRF 15. The Matuyama flow group is thickest near the RWMC and thins to the north. * The Jaramillo (Matuyama) flow group is found in corehole NRF 15, which is the deepest NRF corehole, and shows that the basalt flow group is thick in the subsurface at NRF. This flow group is thickest between the RWMC and INTEC and thins towards the ATRC and NRF.