45737-B8
Detailed Provenance Analysis of the Hornbrook Formation, Oregon and California: Testing Paleogeographic Models of the Cretaceous Cordillera in the Western United States
Current results from provenance analysis of the Hornbrook Formation indicate that the previous interpretation of a Klamath Mountains source needs to be revised to include a significant, younger, non-Klamath arc source for Hornbrook strata. Detrital zircon ages reveal a Late Cretaceous age peak (ca. 80-120 Ma) that requires a non-Klamath source and matches well with the timing of the major Late Cretaceous magmatic pulse in the Sierra Nevada. Trace element geochemistry indicates an arc signature with strong enrichment in LREEs and negative Eu enomalies throughout the stratigraphy. Nd isotopic signatures and geochemistry also suggest a dissected arc source, with decreasing volcanic input throughout the section. The overall trend to more evolved (negative) εNd values in the Hornbrook Formation also corresponds to a similar εNd trend in the GVG that reflects dissection of the arc and migration of source region within the Sierra Nevadan arc to the east through time. Although more data are needed, preliminary results suggest that the Blue Gulch mudstone, the thickest and youngest member of the Hornbrook Formation, has a more mixed provenance than the older strata. The Blue Gulch Mudstone member reverses many of the trends that are consistent throughout the other four members of the Hornbrook Formation: the detrital zircon age signature includes fewer Upper Cretaceous grains and more Early Cretaceous and Jurassic grains, and the geochemical signature is less evolved and has more volcanic input. These preliminary results suggest that as the Hornbrook basin deepened, the distal turbidites of the Blue Gulch Mudstone member incorporated both the dissected Sierran arc source and Klamath sources.
Thus, the integrated provenance results from the Hornbrook Formation describe a source dominated by a Late Cretaceous magmatic arc that was increasingly dissected with time; the Klamath Mountains may have been an additional source, but could not have been the sole source region for the Hornbrook Formation. Moreover, the similarity between the Hornbrook Formation and the GVG in detrital zircon age signatures and the trends up-section in both trace element geochemistry and εNd values suggest that the Hornbrook Formation represents the northern continuation of the Great Valley Group, and the primary source of Hornbrook sediment was in the northern Sierra Nevada arc. These results contradict previous interpretations of Hornbrook provenance and raise the question of how these once-connected basins became separated since Cretaceous time. Continuing research will focus on substantiating possible trends in geochemical data and strengthening correlation with other basins to better guide paleogeographic reconstructions.
During this first year of the project, I completed two summer field seasons and one full academic year of research in northern California and Oregon with undergraduate students. This PRF Grant has supported the research of four undergraduate students (two male, two female), two of whom completed Honors Theses in April 2008, a third will complete a Senior Thesis in April 2009, and the fourth will complete a Junior Directed Studies Project in April 2009. The grant paid three of the students a summer stipend (the fourth secured a Trinity University Summer Fellowship), all field expenses, 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 Las Vegas in March 2008.
The impact of this research on the students is significant. Students thought of themselves as “scientists” and even “geologists” for the first time during the course of this research, and worked well both independently and in collaboration. One student is now starting graduate school at the University of Oklahoma, after quality discussions about his research prior to and during the Cordilleran Section meeting with his new research advisor. Two other students are applying to graduate schools this spring and plan to attend beginning Fall 2009. All of these 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. The grant funds my research program through my final years before tenure and allows me to complete meaningful research with students fully involved. Results are significant and will lead to multiple publications. These results also raise important new questions about paleogeography and Cordilleran tectonics, and will guide my future research in these areas.
