9-4 Extending the Record of Jurassic Strata in the Western Sierran Foothills, CA: Detrital Zircon Analysis of the Colfax Formation

Session: Sedimentary Systems and Provenance in the Western Cordillera (Posters)

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Abstract:

Understanding the development of the Western Sierra Nevada Metamorphic Province is essential for resolving Triassic-Jurassic subduction beneath western North America. We focus on characterizing the detrital zircon provenance of the Colfax Formation, which was deposited along the continental margin during the Middle to Late Jurassic. Our analysis permits comparison with potentially correlative Galice and Mariposa Formations, as well as with potential sources of zircon grains > 300 Ma within the arc framework and interior regions. We present U-Th-Pb data from six Colfax Formation samples (n=621). Maximum depositional ages (MDAs; YSP method) span about 15 Ma, from  172 ± 2 Ma to 156 ± 2 Ma.  All samples include abundant grains > 300 Ma (49%-76%) and a significant Middle-Late Jurassic age mode. Samples show intersample variability in the presence and size of Permo-Triassic and Early Jurassic age modes, as well as older Paleozoic and Precambrian age distributions. Multi-dimensional scaling (MDS) analysis reveals no correlation between statistical similarity and sample location. 

A compilation of all Colfax samples shows that 70% (n=435) of grains are >300 Ma, with a multimodal signature characteristic of western North America, whereas 30% (n=186) are Permo-Triassic and Jurassic grains (<300 Ma), with age modes at 254, 196, 169, and 157 Ma.  Th/U ratios for grains <300 Ma are mostly between 0.1-1, typical of zircon from intermediate to felsic igneous sources. Lu-Hf analyses of 143 selected grains largely record a juvenile signature, but 170-160 Ma grains span epsilon-Hf values that range from +14.1 to -12.1. The Colfax Formation MDAs of ca. 172-156 Ma are older than and overlap with MDAs of ca. 159-150 Ma of the Mariposa Formation (WSNMP) and the Galice Formation (Klamath Mountains). Although the Colfax Formation samples have substantially lower yield of zircon grains, all three formations have similar detrital zircon age distributions, suggesting similar sediment sources. MDS analysis of the Colfax, Mariposa, and Galice Formations, along with potential sediment sources, reveals the greatest similarity between the Colfax and Mariposa Formations, consistent with shared sediment sources. Thus, our results indicate an extended duration (>20 million years) of Colfax-Mariposa-Galice sedimentation along the continental margin, recording consistent provenance from a Jurassic continental arc and older sediment sources within western North America.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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