Middle to Late Miocene eruptive and climatic history in the Grande Ronde Valley, Northeast Oregon


The Minam River winds through the layered basalts of the Columbia River Basalt Group

WHAT: A study of enigmatic rocks and ancient soils associated with the huge eruptions of the Miocene.

WHEN: July 11-August 7

WHERE: Our field area is in northeastern Oregon near the Grande Ronde River. We will be in these locations: (1) orientation at Whitman College in Walla Walla Washington, (2) two weeks of camping along the Minam River in northeastern Oregon and field work in the nearby mountains and valleys, (3) return to Whitman for preliminary analysis and sample preparation, and (4) XRF, ICP, and other analysis at Washington State University in Pullman.

WHO: Six students, and Professors Kirsten Nicolaysen (Whitman College), Nick Bader (Whitman College), and additional help from Mark Ferns (USGS).

Project Overview and Goals

Northeastern Oregon marks the center of the largest continental eruption of lavas in the Cenozoic (Coffin & Eldholm, 1994). The importance of this Large Igneous Province (LIP) is difficult to overestimate. It completely reshaped the landscape in much of Oregon, Washington, and Idaho. Furthermore, it may also have significantly changed global climate (Sheldon, 2006); atmospheric loading by volcanic gases from bigger LIPs in India and Siberia are associated with the terminal Cretaceous and Permian mass extinctions respectively, the largest extinctions in the Phanerozoic (Wignall, 2001). Distinct changes in lava compositions reveal a protracted Miocene transition from the vast effusion of the Columbia River Basalt Group (CRBG) to the isolated, small volume eruptions of the Powder River Volcanic Field (PRVF) exposed near La Grande, OR. Contemporaneous with the volcanism, sedimentation and soil development hold clues both to the changing climate that affected the erupted lavas and to lengths of the pauses between eruptions. In collaboration with six motivated student researchers, we propose to (1) describe stratigraphic relationships within and between CRB and PRVF lavas and sedimentary interbeds, (2) investigate models for the genesis of the PRVF magmas using mineral and isotopic compositions, (3) augment our understanding of the timing of lava eruption and interflow pauses, and (4) elucidate regional Miocene climatic conditions.

Geologic Background

The lavas of Columbia River Basalt Group both obscure the underlying late Paleozoic-Jurassic accreted terranes (e.g., Schwartz et al., 2010 and references therein) and provide a defined surface later modified by folding, extensional faulting, and eruption (Figs. 1, 2). Although some CRBG lavas are compositionally as silicic as andesites, most of the 300+ flows are tholeiitic basalt and comprise a volume of ~224,000 cubic kilometers (Tolan et al., 2009 and references therein). In spite of complicating observations (Pierce and Morgan, 2009), mantle melts are thought to be largely responsible for the CRBG. Nonetheless clinopyroxene thermobarometry and isotopic compositions demonstrate that some CRBG lavas stalled in the crust and there assimilated partial melts of crustal material (e.g., Ramos et al., 2005; Caprarelli and Reidel, 2004). Although presumably the Yellowstone hotspot was under eastern Oregon when CRBG flows started effusing at Steens Mountain at approximately 16.6 Ma, the voluminous eruptions were finished by 14.7 Ma (Brueseke et al., 2007; Barry et al., 2010 and references therein). Synchronous with the waning eruptions of the CRBG, the Powder River Volcanic Field (PRVF) eruptions started as early as 14 Ma (Fig. 2; Bailey, 1990; Ferns et al., 2010). Throughout the ~12 million years of volcanism, PRVF lavas erupted lavas with truly diverse compositions, ranging from basalt to dacite and basanite to trachyandesite (Fig. 3; Bailey and Conrey, 1992; Ferns et al., 2010). In addition to being more siliceous and smaller in volume than most individual CRBG flows, PRVF lavas are calc-alkaline to alkaline in composition (i.e., higher in Na2O, K2O, and generally lower in MgO and FeO). These PRVF magma compositions cannot be explained by the same processes that produced the CRBG.

A second component of this project is ancient soils or paleosols. Paleosols are affected by processes occurring at the Earth’s surface. After controlling for diagenetic alteration, paleosols can therefore allow us to reconstruct paleoclimates (Sheldon and Tabor, 2009), paleoenvironments (Kraus, 1999) or paleotopography (Takeuchi, 2007) depending upon the variables held constant by the analysis.
Because most soils are in actively eroding landscapes, the global distribution of paleosols is sparse compared to the distribution of modern soils. However, widespread aggradation in the volcanically-active Cenozoic landscapes of eastern Oregon has preserved numerous paleosols. Especially well-studied examples include the late Eocene and Oligocene paleosols of the Clarno and John Day Formations (e.g., Retallack et al., 2000), which record the dramatic cooling at the Eocene-Oligocene transition. We will use the paleosols of the CRBG and the PRVF to examine Miocene climate and eruptive history.


View east across the upper Grande Ronde Valley from Mount Emily of the rocks of the Powder River Volcanic Field

Potential Student Projects:

  • Crustal history of alkaline magmas of the PRVF/EVF (1-2 students):  Some small buttes and ridges are interpreted as monogenetic eruptions of trachyandesite and andesite. Investigation of amphibole phenocrysts and pyroxene microphenocryst compositions could reveal crystallization depths and pre-eruptive volatile content.
  • Parental magmas of the PRVG/EVF (1 student): We’ll collect targeted rock samples for Sr, Nd, Pb isotopic analysis and petrogenetic modeling in order to understand the role of mantle melts and crustal contamination in the genesis of the lavas of the Powder River and Elgin Volcanic Fields.
  • Age relationships and post-emplacement extension of PRVF/EVF flows (1 student): The age relationships among the PRVF lavas are crucial to both aspects of the project. This project will focus on argon dating. With the appropriate home-institution adviser, the same student could drill paleomagnetic cores from a stratigraphic sequence of lavas to determine the paleoflow direction or the effect of regional extension on flow orientation.
  • Comparative clay mineralogy and microtexture of paleosols (1 student): This project has two components: a detailed field description and measurement of the Cricket Flats paleosol and a detailed mineralogical comparison of the Cricket Flats paleosol with an older CRBG paleosol using X-ray diffraction.
  • Geochemical analysis of weathering in two Middle Miocene paleosols (1 student): One student will use XRF and ICP-MS analysis of major and trace elements to compare two interesting paleosols developed on different volcanic units. This data will be used to generate weathering indices and reconstruct temperature and moisture profiles in conjunction with the paleosol mineralogy project.
  • Changes in weathering on CRBG flows after the Middle Miocene thermal maximum (1 student):  The interested student will use portable XRF to identify promising CRBG paleosols on Minam Grade Road, and collect samples for XRF and ICP-MS analysis at WSU Pullman. The resulting elemental abundance data will be used to reconstruct weathering profiles relative to the parent flow and to relate these soil processes to Middle Miocene paleoclimate.

Working Conditions

We will be outside for the main part of this project. For several projects we will be walking and sampling on steep slopes that form the sides of deep canyons. Students should have good depth perception, good physical condition, and be comfortable climbing rough ground off-trail. Temperatures may be in the 90’s during the day and the 40’s at night. Rain is unlikely but possible. Rattlesnakes are a possible danger. Our field accommodations will be primitive (tent-based for two weeks) and far from big cities, with vault toilets and no electricity or refrigeration.


You will need to bring camping gear for a two-week trip including a sleeping bag and tent. Have clothes for both heat and cold and possibly rain. Sturdy boots are essential, as are standard camping items such as hats, sunscreen, water bottles. You should bring a field notebook, writing utensils, and a hand lens. We’ll provide rock hammers, GPS, Brunton compasses, and other equipment. We’ll be staying in campus housing for part of the time but you will still need your sleeping bags as bedding is not provided. We’ll provide a more complete equipment list later on.

Recommended Courses/Prerequisites

Historical geology or sedimentology/stratigraphy; mineralogy and petrology. Courses in soils, geochemistry, and field courses are recommended but not required.



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  • Bailey, D.G., and Conrey, R.M., 1992, Common parent magma for Miocene to Holocene mafic volcanism in the northwestern United States. Geology 20, 1131-11-34.
  • Barry, T.L., Self, S., Kelley, S.P., Reidel, S., Hooper, P., & Widdowson, M., 2010, New 40Ar/39Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes. Lithos, 118(3-4), 213-222.
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    Pierce, K.L., and Morgan, L.A., 2009, Is the track of the Yellowstone hotstpot driven by a deep mantle plume? Review of volcanism, faulting and uplift in light of new data. J. Volcan. Geotherm. Res. 188(1-3), 1-25.
  • Retallack, G., & Bestland, E., 2000, Eocene and Oligocene paleosols of central Oregon (Special Papers. p. 192). The Geological Society of America.
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  • Schwartz, J.J., Snoke, A.W., Frost, C.D., Barnes, C.G., Gromet, L.P., Johnson, K., 2010, Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon. GSA Bulletin, 122, 517-536.
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