Reports: UR856245-UR8: Testing Models for the Formation of the Great Valley Basin using Detrital Zircon U/Pb Dates: An Early Cretaceous Forearc Basin or a Mid-Jurassic Pull-Apart Basin?

Diane Clemens-Knott, PhD, California State University, Fullerton

This ACS-PRF grant supports a field- and laboratory-based project focused on identifying the protoliths of metavolcanosedimentary pendants in the western Sierra Nevada batholith, and reconstructing their post-depositional tectonic histories. Specific goals of the study are to use detrital zircon “bar codes” to locate boundaries between metavolcanosedimentary pendants having unrelated protoliths. The overarching goal of the study is to assess whether any of these hypothesized terrane boundaries may represent structures that participated in the formation of the Late Jurassic (?) to Cretaceous forearc basin, in which California’s oil-rich Great Valley Group was deposited.

More than half of the planned fieldwork and sampling was completed between June 1st, 2016 and August 30th, 2017. The first analytical data was collected in April 2017 at the University of Arizona Laserchron facility. To date, five undergraduate students, one post-BA volunteer, and two graduate students have participated in research activities associated with this grant.

Substantial tasks completed during the grant’s first year include the following:

  1. Completion of one MSc thesis (Nancy Chen) and one undergraduate thesis (Rosa Murrieta).
    1. Chen’s MS thesis—initiated when this proposal was in review—completed one of the main objectives of this study. Specifically, she compared the detrital zircon barcode of sediments in the Golconda allochthon (north-central Nevada) to metasedimentary pendants in the western Sierra Nevada foothills in order to assess potential tectonic and provenance ties.
      1. Nancy is now employed as the geology instructor at Harvard-Westlake Prep School in southern California.
    2. Murrieta’s BS thesis compared the detrital zircon barcodes of three pendants in the Greenhorn Mountain sub-range of the southwestern Sierra Nevada Mountains. Each pendant turned out to preserve a different protolith, two of which were part of the Permian-Jurassic sedimentary blanket overlying the accreted ophiolite and western arc, and one of which appears to statistically correlate with the NeoProterozoic-Paleozoic North American passive margin.
      1. Rosa is now the laboratory manager at a local environmental geology firm.
    3. Completion of fieldwork for three new undergraduate theses (Rodrigo Avila, Christian Concha, Brian Magumcia).
      1. These students are continuing the characterization of metasedimentary pendants in the Sierra Nevada foothills.
      2. Much of their laboratory supervision is provided by senior Erin Boeschart, who took over the lab assistant position after Rosa Murrieta graduated.
    4. Securing permission to sample on the Tule River Indian Reservation.
      1. An optimistic goal of this project was to gain access to the reservation, the eastern portion of which appears not to have been described geologically prior to the early 1960’s.
      2. Permission was granted by the Tule River Tribal Council in February 2017. Conditions for access were approved by their legal counsel and environmental regulators.
      3. Sampling occurred (with a tribal guide) in March and June of 2017.
        1. post-BA student Zoe Kohler volunteered as a field assistant in June.
      4. Analysis of zircon samples at the University of Arizona’s Laserchron Lab.
        1. Four students (Chen, Murrieta with assistance from Duccini, Titular) worked to operate the instrument around the clock for 3.5 days.

Results from this first year of study are as follows:

  • Improved location of the eastern boundary of the accreted Ordovician-Carboniferous Kings-Kaweah ophiolite with its overlying blanket of PermoTriassic marine sediments.
  • Improved location of the western boundary of the NeoProterozoic-Paleozoic North American passive margin.
  • Identification of additional allochthonous metavolcanosedimentary pendants sandwiched between the accreted ophiolite and the North American passive margin.
  • In addition to new locations of the Jurassic marine sedimentary blanket (aka the Kings Sequence), we have identified a previously unknown package of Early Cretaceous blanket sediments extending along ~55 km of the foothills.
    • Most intriguing is a subhorizontal, ~124 Ma deposit, rich with detrital heavy minerals. I interpret this sample as a beach facies of the Cretaceous Great Valley Group that washed against the flanks of one of the Early Cretaceous stratovolcanos exposed in the Stokes Mountain region.
  • We applied multidimensional scaling analysis to pendant samples in order to compare/correlate to sediments from the Roberts Mountain and Golconda allochthons as well as to sediments of the North American passive margin.
    • We are currently experimenting with strategies to process the detrital zircon data to exclude grains affected by metamorphic disturbances.

Efforts during the second year of the grant (2017-18) will be directed at completing most of the remaining field work in the Sequoia National Forest and processing samples. I have requested time at the Arizona Laserchron Center this coming spring, and hope to take this current group of students to Tucson to analyze their own samples.

  • Based on this year's field work, I hypothesize that a terrane boundary actually crosses through one pendant, bisecting an area in which multiple pendants are in close proximity. If the detrital zircon work confirms this suspicion, I will add a structural mapping component to this project next summer.

One or more of these students will likely travel with me to Flagstaff, AZ in May 2018 to present their results at the joint Cordilleran-Rocky Mountain section meeting of the Geological Society of America. Two of the three current undergraduates will need to complete their senior thesis in Spring. I intend to recruit at least two more undergraduates to work on their required thesis research with me in 2018.