--> --> Geology of the western margin of the Cenozoic Western Snake River Plain basin near Ontario, Oregon

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Geology of the western margin of the Cenozoic Western Snake River Plain basin near Ontario, Oregon

Abstract

The sediments along the western margin of the Western Snake River Plain rest upon basalt and rhyolitic rocks erupted (>7 Ma) within the Oregon-Idaho graben. The basal sediments are a fluvial sequence of interbedded tuffaceous mudstone and conglomeritic sandstones bearing black chert-like pebbles. Exposure of fluvial sediments at Mitchell Butte is a 1000-ft section with conglomeritic sandstones 10 to 50 ft thick comprising ~200 ft of the section. This same section crops out south of Vale where it dips 5° west, and in Kiesal Estate #1 well (~ 6 miles west of the exposures), where it is described in cuttings from 150-3,160 ft depth. In the Ontario Ore-Ida well, from 4,170-7,990 ft depth are tuffaceous siltstones and fine-to-coarse sandstones with black-rock fragments believed to be a similar sequence. Sandstones make up ~ 30% of sediments in this interval. GeothermEx, Inc.(1981) interpreted an angular unconformity at 5,300 ft depth on the Ore-Ida dipmeter log, where upper strata generally dip 5-15° south and southwest, and lower strata dip 10-25° southeast and northeast. Fluvial sandstones and tuffaceous sediments below the unconformity appear to correlative with the enigmatic Miocene Payette and/or Chalk Hills Formations. The chert-like clasts are absent from these arkosic fluvial sandstones mapped in Idaho, Olivine basalt sills intrude the sedimentary section below 3,840-ft depth in the Kiesal well, and from 4,560-8,050 ft in the Ore-Ida well. Age of the sills is uncertain, but all basalt cuttings analyzed so far from the 4,620-9,690 ft interval from the Ore-Ida well are of Snake River olivine tholeite (SROT) geochemistry. Overlying the fluvial section are east-dipping lacustrine mudstones and interbedded fine-medium-grained turbidite sands, a section about 400 ft thick uplifted and exposed near the Malheur Butte intrusive. Sands make up ~12% of this section, the rest being mudstones and rare thin volcanic ashes. This section is similar to the subsurface pro-delta turbidite section beneath the plain described by Wood (1994, AAPG Bull 78, p,101), and the upper 4000 ft of the Ore-Ida well sediments. It is within this turbidite section that many of the shallow gas shows were noted during drilling in the years 1902-2010. These sediments are from the last major lake, ‘Lake Idaho’, and correlative with the Glenns Ferry Formation. Malheur Butte is a small (< 1.0 km) dacite (60-69% SiO2) intrusive stock into the sediments, 6 miles west of Ontario. The dacite gives 3 surprising Pleistocene whole-rock 40Ar/39Ar dates of 2.0±0.2, 1.1±2.6, 1.5±0.4 Ma (Hooper et al., 2002, GSA data repository 2002014). Both andesite and dacite occur in the Butte, and trace element analysis suggested to KR Lees (1994, PhD diss. The Open University) that the rocks are from mixed magma sources. These studies show that a variety shallow igneous bodies need to be considered in interpretation of the subsurface geology in this part of the Western Snake River Plain basin.