PALEOMAGNETIC STRATIGRAPHY OF THE EASTERN SNAKE RIVER PLAIN IN THE SOUTHERN PART OF THE IDAHO NATIONAL LABORATORY, IDAHO

Facility operations at the Idaho National Laboratory (INL) contaminated the eastern Snake River Plain (ESRP) aquifer beneath the INL. U. S. Geological Survey researchers, in cooperation with the U.S. Department of Energy, are studying the subsurface geology at the INL to construct the geologic framework for conceptual and numerical models of groundwater movement and contaminant transport. Paleomagnetic inclination and polarity studies allow discrimination of the chemically nearly-identical ESRP olivine tholeiite basalt flow groups. Paleomagnetic data can be used with geochemical analyses, natural gamma logs, and radiometric dating of basalt flows to correlate basalt flow groups from surface to subsurface and among coreholes drilled at the INL.

Two stratigraphic cross-sections of the south-central part of the INL were constructed, across and sub-parallel to the trend of regional groundwater flow. Cross-section A-A’ runs from west to east from USGS 135 to ARA-COR-005, near the southern boundary of the INL. Cross-section B-B’ runs north to south from corehole USGS 134 near the Advanced Test Reactor Complex to corehole USGS 137 at the southern boundary of the INL. Stratigraphy of four partially-cored boreholes included in the cross-sections was interpreted from geophysical logs and paleomagnetic inclination stratigraphy of nearby coreholes.

Most ESRP basalt eruptions occurred on the Axial Volcanic Zone and the Arco-Big Southern Butte volcanic rift zone. Volcanoes erupted on these topographic highs, and basalt flowed into the lowlands. Correlation of basalt flow groups from surface to subsurface and between coreholes indicates that the subsidence in this region of the ESRP has changed through time. Before 542 ka, more subsidence occurred to the south. Since that time, Axial Volcanic Zone subsidence has continued, and subsidence or lack of basalt accumulation has occurred in the south-central area of the INL.