MAPPING OF MIOCENE-PLIOCENE LAKE IDAHO AND PAYETTE SEDIMENTARY DEPOSITS NORTH OF THE WESTERN SNAKE RIVER PLAIN

New STATEMAP-supported mapping and U-Pb zircon dating help delineate sediments on the north side of the Western Snake River Plain in terms of the relatively well-studied Miocene-Pliocene formations on the south side of the plain and allow subdivision of these sedimentary deposits into their respective formations for the first time. Basin formation included a complex interplay between sedimentation and emplacement of the Columbia River Basalt Group (~16.9-16.4 Ma), newly identified rhyolitic, dacitic, and andesitic lavas and ignimbrites (≥16.4 Ma), Weiser volcanics (~15.1-12.4 Ma), and other localized flows. Sedimentary deposition occurred as 1) fluvial and lacustrine beds of the Payette Formation between and immediately above these volcanic flows; and 2) ancient Lake Idaho deposits consisting of tuffaceous lacustrine clay-siltstone with associated subaqueous channels and Gilbert deltas deposited 10-6 Ma (Chalk Hills Formation) and 5-2 Ma (Glenns Ferry Formation). In addition to pollen and fish biozones, we also have five new U-Pb TIMS dates that aid in our correlation of these formations. Above the mixed rhyolite and andesite lavas is a more laterally restricted rhyolite (rhyolite of Indian Creek) with a date of 16.394 ± 0.008 Ma. Payette Formation sediments overlying this rhyolite contain a 15-20 ft thick partially welded tuff dated at 15.881 ± 0.012 Ma. This tuff is overlain by fluvial-lacustrine sediments containing pumice lapilli beds from which zircon ages of 9.0057 ± 0.0082 Ma, 9.041 ± 0.014 Ma, and 7.776 ± 0.011 Ma establish their correlation to the Chalk Hills Formation on the south side of the plain. Although the Payette Formation commonly has steeper dips (~15-25°), a clear boundary with the similarly tuffaceous Chalk Hills Formation has not yet been identified. The youngest sediments are fluvial-deltaic sand with oolite beds, beach gravels, coarse sands, and fish fossils near the base which we regard as the Glenns Ferry Formation. The newly discovered Willow Creek hydrocarbon field lies 16 km west of these outcrop areas, from which 11,100 MCF/day natural gas and 380 bbls/day condensate are produced mostly from sands below ~3,400 ft deep. This ongoing project contributes to understanding the stratigraphy, groundwater resource, and these young hydrocarbon occurrences.