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April 12, 2019

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Brian Webb Coauthored an Article on the Littlefield Rhyolite in the Geological Society of America journal Geosphere

Brian Webb (Oregon) was the lead author on a research article entitled "The Littlefield Rhyolite and associated mafic lavas: Bimodal volcanism of the Columbia River magmatic province, with constraints on age and storage sites of Grande Ronde Basalt magmas" that was published in the Geological Society of America (GSA) journal Geosphere in volume 15, Issue number 1, on pages 60-84, on December 20, 2018.

His coauthors were Martin J. Streck, William C. McIntosh, and Mark L. Ferns.

Brian Webb recently joined Geosyntec's water resources team as a Senior Staff Geologist in Portland, Oregon.

Geosphere is an open access, online-only journal that targets an international audience and publishes research results from all fields of the geosciences. It particularly seeks papers that take advantage of the electronic format.

The Geological Society of America is a global professional society with a membership of more than 24,000 individuals in 115 countries. GSA provides access to elements that are essential to the professional growth of earth scientists at all levels of expertise and from all sectors: academic, government, business, and industry. The Society unites thousands of earth scientists from every corner of the globe in a common purpose to study the mysteries of our planet (and beyond) and share scientific findings.


We present data that distinguishes the long-known Littlefield Rhyolite of eastern Oregon (northwestern United States) into two distinct, voluminous, Snake River–type, high-temperature rhyolite lava packages that erupted in short sequence over <100 k.y., with minimum volumes of 100 and 150 km3 respectively, contemporaneous with flood basalt volcanism of the Grande Ronde Basalt phase of the Columbia River Basalt Group. Contemporaneity of rhyolites with flood basalts is exceptionally demonstrated within the Malheur Gorge by intercalated mafic units belonging to the Grande Ronde Basalt that are stratigraphically constrained by underlying and overlying Little­field Rhyolite flows, and the underlying Dinner Creek Tuff (unit 1). Our new ages of 16.11 Ma and 16.02 Ma for the lower and upper Littlefield Rhyolite, respectively, provide a narrow age constraint on the controversial lower age of Grande Ronde Basalt volcanism. Petrological data on local, intercalated Fe‑rich andesitic (icelanditic) lavas provide further evidence for coeval existence of rhyolitic and mafic magmas, and additionally provide location evidence for storage sites of Grande Ronde Basalt magmas. Based on these data in addition to similar data on the nearby Dinner Creek Tuff rhyolite center, as well as the locations of other rhyolite centers that fall within the same period of intense rhyolite volcanism of ca. 16.1 Ma, we infer that Grande Ronde Basalt crustal magma reservoirs were widespread in this area of eastern Oregon. We further infer that the main eruptions of stored flood basalt magmas followed the magmas' lateral transport from these reservoirs to the well-known dike swarms located at the periphery of the rhyolite distribution area where local eruptions of rhyolites are notably absent. Our study highlights the interplay of mafic and crustally derived rhyolite magmas, with implications for other continental flood basalt provinces that are less well preserved than the Columbia River Basalt province.

More Information

Learn more about the article: The Littlefield Rhyolite and associated mafic lavas.
For consultation regarding mafic lavas, contact Brian Webb at This email address is being protected from spambots. You need JavaScript enabled to view it..