REFINED STRATIGRAPHY OF THE WEISER EMBAYMENT, WEST-CENTRAL IDAHO

Recent and ongoing mapping by the Idaho Geological Survey (and partially funded by the USGS StateMap program) uses whole-rock XRF data, paleomagnetic data, and U-Pb zircon dating to refine the stratigraphy of the Weiser embayment in west-central Idaho. In addition, this work sheds light on the stratigraphy of the northeast flank of the recently active New Plymouth oil and gas field. Basement rocks of the embayment includes Permian to Jurassic accreted terrane metasedimentary and metavolcanic rocks, Mesozoic and Cenozoic intrusive complexes, and Eocene volcanics. Tertiary volcanism in the basin is divided into four intervals. Oldest are the lower formations of the Columbia River Basalt Group, the Steens Basalt and the Imnaha Basalt erupted from 16.9-16.0 Ma. The second interval is the Grande Ronde Basalt of the Columbia River Basalt Group from 16.0-15.6 Ma. The third interval is the basalt and basaltic-andesite of the Weiser volcanics from 15.1 to 12.75 Ma. The fourth and final volcanic interval is the post-Weiser volcanics which includes flows of rhyolites, andesites, basaltic andesites, and basalts that extend from 12.75 Ma into the Pliocene. Many of these younger flows had previously been assigned to the Weiser volcanics.

During all phases of volcanism sedimentary interbeds of granitic and tuffaceous material were deposited with stratigraphic thickness increasing significantly after the Grande Ronde Basalt. The majority of the sedimentary deposits reside southwest of the volcanics as the basin deepens towards the western Snake River Plain. A U-Pb TIMS zircon date of 9.0057 ± 0.0082 Ma was obtained from a pumice-rich zone in one of the lowest exposed sedimentary intervals north of Emmett. Recent and historic wells drilled in the Little Willow Creek and Big Willow Creek drainages of the New Plymouth oil and gas field have tapped into economically viable hydrocarbon reserves in these fluvial and lacustrine deposits. Recent producing wells mark the first production of natural gas and liquid condensates in Idaho’s history. Regional volcanic stratigraphy and geochemistry applied to well cuttings and geophysical well data could prove valuable in understanding basin stratigraphy at depth (particularly in the post-Weiser section) and assist in the search for additional reserves below the uppermost volcanic units.