Neogene Fallout Tuffs from the Yellowstone Hotspot in the Columbia Plateau Region, Oregon, Washington and Idaho, USA

Abstract

Sedimentary sequences in the Columbia Plateau region of the Pacific Northwest ranging in age from 16–4 Ma contain fallout tuffs whose origins lie in volcanic centers of the Yellowstone hotspot in northwestern Nevada, eastern Oregon and the Snake River Plain in Idaho. Silicic volcanism began in the region contemporaneously with early eruptions of the Columbia River Basalt Group (CRBG), and the abundance of widespread fallout tuffs provides the opportunity to establish a tephrostratigrahic framework for the region. Sedimentary basins with volcaniclastic deposits also contain diverse assemblages of fauna and flora that were preserved during the Mid-Miocene Climatic Optimum, including Sucker Creek, Mascall, Latah, Virgin Valley and Trout Creek. Correlation of ashfall units establish that the lower Bully Creek Formation in eastern Oregon is contemporaneous with the Virgin Valley Formation, the Sucker Creek Formation, Oregon and Idaho, Trout Creek Formation, Oregon, and the Latah Formation in the Clearwater Embayment in Washington and Idaho. In addition, it can be established that the Trout Creek flora are younger than the Mascall and Latah flora. A tentative correlation of a fallout tuff from the Clarkia fossil beds, Idaho, with a pumice bed in the Bully Creek Formation places the remarkably well preserved Clarkia flora assemblage between the Mascall and Trout Creek flora. Large-volume supereruptions that originated between 11.8 and 10.1 Ma from the Bruneau-Jarbidge and Twin Falls volcanic centers of the Yellowstone hotspot in the central Snake River Plain deposited voluminous fallout tuffs in the Ellensberg Formation which forms sedimentary interbeds in the CRBG. These occurrences extend the known distribution of these fallout tuffs 500 km to the northwest of their source in the Snake River Plain. Heretofore, the distal products of these large eruptions had only been recognized to the east of their sources in the High Plains of Nebraska and Kansas.

Large, persistent rhyolitic magma reservoirs above Columbia River Basalt storage sites: The Dinner Creek Tuff Eruptive Center, eastern Oregon

“…Fallout tuffs either correlate with ignimbrites (cf. Nash and Perkins, 2012) or fall between ignimbrite eruptions. Minor amounts of mafi c magmas found in the Dinner Creek Tuff match CRBG magmas and thus provide direct evidence for CRBG fl ood basalt reservoirs beneath this large and long-lived rhyolitic eruptive center.…”

Field Mapping Investigation and Geochemical Analysis of Volcanic Units within the Dinner Creek Tuff Eruptive Center, Malheur County, Eastern Oregon

“…The Hunter Creek basalt is geochemically indistinguishable from the Birch Creek member of the Basalt of Malheur Gorge, and from the Grande Ronde Basalt (Evans and Binger, 1997). 40 Ar/ 39 Ar ages overlap with the Dinner Creek Tuff (15.7 -15.4 Ma), and the exact relationship between the two units is unclear (Nash and Perkins, 2012). In the Jonesboro Quadrangle, 10 km southeast of the study area, a lenticular segment of Hunter…”

Field-trip guide to a volcanic transect of the Pacific Northwest

“…Wanapum Basalt is >15.4 Ma (Nash and Perkins, 2012;Ladderud and others, 2015). Ages and volumes indicate that pre-Wanapum lavas erupted at an average rate of at least 0.2 cubic kilometers per year, but the true flux may be several times this, given the new age determinations (Jarboe and others, 2010).…”

“…The Roza Member consists of five separate lava flows, with a total volume of 1,300 km 3 . It has been dated at 14.98 ± 0.06 Ma (Barry and others, 2013;Reidel and others, 2013), but regional silicic ash tephrochronology does not support ages younger than 15.5 Ma for the Wanapum Basalt (Nash and Perkins, 2012;Ladderud and others, 2015).…”