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This ACS-PRF grant supports a field- and laboratory-based project focused on characterizing a recently discovered sedimentary and volcanic formation preserved in a metamorphic pendant in the western Sierra Nevada batholith. Specific goals of the study are to confirm whether an independent estimate of the formation's Early Cretaceous depositional age is correct as well as to assess where the ancient river sediments were originally derived from (i.e., the sedimentary provenance).  The overarching goal of the study is to assess whether these rivers could have contributed sediment to California's oil-rich Great Valley Sequence.

Substantial tasks completed during the first year, 2009-10, include the following:

a)   completion of field work and sampling for one undergraduate thesis (Kevin Tomita) and one masters thesis (Mike Martin);

b)   adaptation of University of Arizona mineral separation protocol for use at CSUF by Mike Martin and undergraduate student Natalie Hollis;

c)    attendance (by Mike Martin and PI) at a 1-day short course on detrital zircon analysis and interpretation taught by the Laserchron Lab co-Director, Dr. George Gehrels (5/10);

d)   analysis of 6 detrital zircon samples from the Goldstein Peak formation at the University of Arizona's Laserchron Lab (Mike Martin);

e)   microprobe analysis of Kings Sequence rocks at the California Institute of Technology's Analytical Facility, as part of a second undergraduate thesis (Crystal Castellanos); and

f)     presentation of preliminary results at the GSA Cordilleran section meeting (5/10) by Castellanos and Martin, and by the PI at a meeting of the San Joaquin Geological Society in Bakersfield, CA (6/10) and at a GSA Penrose Conference convened in the eastern Sierra Nevada (8/10).

In addition, a recently recruited freshman geology major, Natalie Hollis, was hired to assist in the mineral separation process. This short-term employment enabled her to pay tuition for the Physical Geology lab she took during CSUF's 2010 January Intersession, putting her on-track for completion of the geology major in 4 years. I have rehired Natalie this fall to assist in the mineral separation process and potentially to complete her own undergraduate thesis as part of this ACS-PRF-funded study.

Preliminary results of this study are as follows:

a)   Mike Martin's detrital zircon data confirm that the depositional age of the Goldstein Peak (GP) formation is indeed Early Cretaceous (139-135 Ma). As such, the GP formation is the oldest recognized nonmarine deposit in the Sierra Nevada arc-Great Valley forearc system.

b)   The majority of GP zircons have ages consistent with local derivation from the Jurassic Sierra Nevada arc.

c)    A minority of the detrital zircons (dz) date back almost to 3 Ga. We are in the process of assessing whether the neighboring metamorphic pendants could have provided any of these older zircon populations, before considering sources further east of the Sierran Nevada arc.

Unfortunately, many of the likely sources of detrital zircon within the westernmost Cordillera have not yet had their dz populations characterized. This absence presents a significant challenge to completing the provenance analysis of the GP fluvial sediments. Luckily, many other researchers are starting to work on this issue, and the community is trying to coordinate efforts in order to avoid duplication of efforts. My group's contribution to this effort will be to characterize the detrital zircon population within the central Jura-Triassic Kings Sequence, which forms a 200-km-long belt along the western Sierra Nevada, just inboard of the GP formation.

The Kings Sequence is a particularly complex package of rocks: multiply deformed and multiply metamorphosed, the protoliths are hypothesized to have been deposited in an submarine fan complex draining the Jurassic and Triassic Sierran arcs. The fan system abutted a submarine fault system along which ophiolitic material was uplifted and shed into the distal fan sediments. Undergraduate student, Crystal Castellanos, is currently completing her thesis on the petrography and geochemistry of the Kings Sequence exposed within the Lake Kaweah pendant. One somewhat surprising (though not unreasonable) result of her study is the identification of a block of mantle peridotite within the Lake Kaweah pendant. Crystal received a stipend for her work through the NSF-Louise Stokes Minority Access Program (grant HRD-0331537 to CSUF), but her use of Caltech's electron microprobe to identify particularly fine-grained minerals was funded by this ACS-PRF grant. Crystal's petrographic descriptions of the Kings Sequence will help us target potentially zircon-rich horizons to sample for detrital zircon analysis.

We must consider the Kings Sequence as a potential source for detrital zircons because the GP formation is deposited on top of the Kings Sequence, forming a depositional unconformity. Two students will begin this effort in fall: new graduate student Chris Buchan will analyze zircons from the Kings Sequence within the Lake Kaweah pendant as well as in pendants located due east of the field area (the Sequoia Park pendants) and undergraduate Kevin Tomita will analyze Kings Sequence zircons from one of the Tule River pendants, located approximately 65 km south of the Lake Kaweah pendant. In this latter field area, the pervasively deformed Kings Sequence forms an angular unconformity with weakly metamorphosed and undeformed sediments. Whether these undeformed sediments are coeval with the Goldstein Peak formation or are slightly older (Å170-140 Ma) or younger (Å135-125 Ma) in age is the subject of Kevin Tomita's undergraduate thesis. We are scheduled to analyze these Kings Sequence samples at the Laserchron facility in February 2011.