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During this second year of my ACS-PRF Grant, I focused research on the little-studied Ochoco basin in central Oregon. In recent reconstructions of Cretaceous Cordilleran paleogeography, the Ochoco basin represents the eastern continuation of the Hornbrook Formation (the focus of my research in Year 1). My students and I completed conglomerate clast counts, sandstone petrography, trace element geochemistry, and detrital zircon geochronology on Ochoco basin strata to detail the sedimentary provenance through time, thus permitting comparison with the Hornbrook Formation provenance. As in the Hornbrook Formation provenance completed in Year 1, our provenance analysis of the Ochoco basin revealed that the basin’s source must have included the northern Sierran batholith, in addition to the Blue Mountains. Detrital zircon age spectra from the Ochoco basin include a prominent 98 Ma age peak in many of the samples, as well as Early and Middle Jurassic ages. Trace element geochemistry indicates an arc source, with decreasing volcanic input. To date, the provenance results from the Ochoco basin are very similar to the Hornbrook Formation results, suggesting that the two basins were indeed connected during the Late Cretaceous. Thus, the integrated provenance results from the Hornbrook Formation and the Ochoco basin describe a source dominated by a Late Cretaceous magmatic arc that was increasingly dissected with time; the Klamath Mountains and the Blue Mountains likely were additional sources for the two systems, respectively, but could not have been the sole source regions. These results contradict previous interpretations of Hornbrook and Ochoco provenance and raise the question of how these once-connected basins became separated since Cretaceous time. This possible connection between the Hornbrook Formation and Ochoco basin needs additional geochemical work to corroborate, including neodymium analysis. Moreover, further research may demonstrate whether a combined Hornbrook-Ochoco basin was also connected with the Great Valley Group to the south, forming a single, complex forearc basin system west of an active volcanic arc during the Late Cretaceous. Therefore, continuing research in the region will focus on substantiating possible trends in geochemical data and strengthening correlation between basins to better guide paleogeographic reconstructions.

In related research, a second undergraduate student and I undertook provenance analysis of the Payne Cliffs Formation, a poorly-dated unit in southwestern Oregon, to better understand if and how Payne Cliffs deposition relates to older, Late Cretaceous deposition. Detrital zircon geochronology from the Payne Cliffs Formation reveals an Eocene depositional age, and the age spectra include both distal and proximal sources, much like sediment in the coeval Tyee Formation on the Oregon Coast. This research project helped determine how the Payne Cliffs Formation fits in to the overall Cordilleran history of this region.

During this second year of the project, I completed one full academic year of research in Oregon with two undergraduate students, followed by a summer of compiling and writing results. This PRF Grant supported the research of two undergraduate students (one male, one female) during the academic year (note that the 2009 summer stipends for these students were included in Year 1), one of whom completed  an Honors Thesis in April 2009, and the other completed a Junior Directed Studies Project in April 2009.  In addition to supporting students’ summer field expenses and stipends (under Year 1), the grant paid travel expenses to the LaserChron Center at the University of Arizona to collect detrital zircon U-Pb age data, all analytical expenses, and travel expenses to attend and present results at the Cordilleran Section meeting in Kelowna, British Columbia in May 2009. 

The impact of this research on the students is significant. One student is now starting graduate school at New Mexico State University, working on tectonics research in Alaska that will include provenance analysis. The other student supported in Year 2 is currently a senior working on an Honors Thesis with me that focuses on the provenance of Eocene strata in southeastern Alaska (funded through separate sources). All of my research students indicated that their research experiences have confirmed and intensified their interest in pursuing an advanced degree and meaningful employment in the geosciences. The impact of this research on my own career has also been substantial. My research results raise important new questions about paleogeography and Cordilleran tectonics, and will guide my future research in these areas. Results to date formed the basis of a newly-funded NSF CAREER grant that will support my continuing research in these areas for the next five years.