The complex Mesozoic development of the western Sierran foothills is not well understood due to significant deformation, magmatism, erosion, and limited age data from metasedimentary rocks. We focus on determining the depositional age and characterizing sediment sources for the metasedimentary Colfax Formation of the western Sierran foothills. Determining whether these strata formed during Middle or Late Jurassic time is essential to understanding how these rocks compare with strata of similar age within the foothills in order to build a greater understanding of the region's geologic history.
We will complete U-Pb age dating and Hf analysis of detrital zircon grains isolated from sandstones in the Colfax Formation. The ages of the youngest zircon grains in the Colfax Formation provide the oldest age limit of deposition, and the span of zircon ages present reflects the ages of sediment sources for the Colfax Formation. In addition, Hafnium data helps characterize the magmatic origin of the zircon grains.
We completed mineral separation processes to isolate zircon grains from five of our eight Colfax Formation metasandstone samples and analyzed them at the Arizona LaserChron Center. Preliminary results indicate the presence of Precambrian detrital zircon grains based on their purple color when viewed under a microscope, but further analysis of our data is necessary to determine absolute detrital zircon ages.
This research allows geologists to better understand the geologic history of the western Sierran foothills, thereby providing researchers a stronger foundation from which to understand more recent geological processes in the region.