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

Michael E Perkins,Victoria C Smith,Barbara P Nash

doi:10.1371/journal.pone.0044205

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.

Highlights

Commencing at about 16.6 Ma, the initial phase of the Yellowstone hotspot was dominated by the voluminous basaltic volcanism of the Columbia River Basalt Group (CRBG), whereas in the subsequent evolution of the hotspot along the Snake River Plain in Idaho to its present location at Yellowstone National Park, rhyolitic volcanism has been volumetrically dominant over basaltic
The identification of widespread fallout tuffs, together with published 40Ar/39Ar ages, allows us to establish the synchroneity of a number of these fossil assemblages as well as to provide a tephrostratigraphic framework for disparate basins across a wide region of active volcanism
These localities contains an abundance of fallout tuffs, but in this paper we focus on major fallout tuffs with regional distribution

Summary

Introduction

Commencing at about 16.6 Ma, the initial phase of the Yellowstone hotspot was dominated by the voluminous basaltic volcanism of the Columbia River Basalt Group (CRBG), whereas in the subsequent evolution of the hotspot along the Snake River Plain in Idaho to its present location at Yellowstone National Park, rhyolitic volcanism has been volumetrically dominant over basaltic. In this paper we describe the occurrences of widespread hotspot source rhyolitic tuffs in sedimentary sequences associated in time with the CRBG, those that occur in sedimentary interbeds within the CRBG, as well as a few younger occurrences in the region. The earliest dated silicic eruptions occurred in northwestern Nevada and southern Oregon in the High Rock volcanic center between 16.5 and 15.4 Ma [4,5,6,7], the McDermitt caldera complex (16.45– 14.6 Ma) [8], the Lake Owyhee volcanic fields of northern Nevada and southeastern Oregon (15.9–14.7 Ma) [9,10] the Oregon High Plains (15.6–15.3 Ma) [11] and the Oregon-Idaho graben (15.3– 10.5 Ma) [12]. By 15.2 Ma the hotspot had encountered the craton as the North American Precambrian lithosphere overrode the deep-seated thermal anomaly, and subsequent eruptions involved magmas derived from the interaction of mantle basalt and Precambrian crust that produced a more focussed and narrow track leading to the development of the Snake River Plain and the modern day Yellowstone Volcanic Plateau

Methods

Results

Conclusion