DETRITAL ZIRCON ANALYSIS OF THE GALICE FORMATION, KLAMATH MOUNTAINS, CA AND OR: UNDERSTANDING THE LATE JURASSIC NEVADAN OROGENY

Details of the Late Jurassic tectonic history of the Nevadan orogeny in the Klamath Mountains Province remain unresolved. Galice Formation provenance may help inform our understanding of the Nevadan orogeny because deposition of Galice strata in the Western Klamath terrane occurred during Nevadan deformation. We present detrital zircon U-Pb ages (n=4909) and ε_Hf values (n=864) from 18 metasandstone samples that span the ~275 km strike-length of the Galice Formation. All samples contain abundant near-depositional age detrital zircon, such that maximum depositional ages (MDAs) provide reasonable proxies for true depositional ages. Calculated MDAs limit the age of Galice deposition to ca. 160-151 Ma, consistent with biostratigraphy. A metasandstone sample collected from above the basal 100 m thick hemipelagic sequence has an MDA of ca. 160 Ma, requiring a rapid hemipelagic sedimentation rate of 50 cm/yr and suggesting that hemipelagic deposition represented distal Galice turbidites.

All 18 samples show different abundances of similar age modes. 16 samples have >40% pre-Mesozoic zircon, and 10 of those have >50%. Mesozoic age spectra include a large 170-155 Ma age mode in all samples, and a small 220-190 Ma age mode in nine samples. Selected Mesozoic grains from all samples show similar ε_Hf ranges of +16 to -15. Neither spatial (northern vs southern samples) nor age (samples with MDAs older than ca. 155 Ma vs younger) compilations reveal systematic provenance differences. Negative ε_Hf values in Mesozoic zircon and abundance of Precambrian detrital zircon document a North American continental signature throughout the Galice basin’s history. We suggest Galice provenance included Sierra Nevada pre-batholithic framework rocks, the Klamath-Sierran arc, and sources in the continental interior such as Jurassic eolianites, consistent with models that place the Galice Formation on the U.S. continental margin during Late Jurassic time. Overall Galice provenance does not clearly record the onset of orogenesis during nine million years of deposition, perhaps indicating that established sediment transport systems were not altered by Nevadan orogenesis, or that the beginning of the Nevadan orogeny coincided with the onset of Galice deposition at ca. 162 Ma.