300-5 Characterizing Sediment Sources of Jurassic Strata in the Western Sierran Foothills, CA: Colfax Formation Conglomerate Composition and Detrital Zircon Analysis

Session: Reconstructing Earth Surface Processes in Orogenic Systems (Posters)

Poster Booth No.: 138

Presenting Author:

Lucas Craig

Authors:

Craig, Lucas¹, Shaffer, Cate², Sikes, Wiley³, Surpless, Kathleen DeGraaff⁴, Quigley, Nathan⁵, Botha, Brandon Joaquim Louis⁶, Orme, Devon A.⁷

(1) Earth and Environmental Geosciences, Trinity University, San Antonio, TX, USA, (2) Earth and Environmental Geosciences, Trinity University, San Antonio, Texas, USA, (3) Earth and Environmental Geosciences, Trinity University, San Antonio, TX, USA, (4) Earth and Environmental Geosciences, Trinity University, San Antonio, TX, USA, (5) Montana State University, Bozeman, MT, USA, (6) Montana State University, Bozeman, MT, USA, (7) Montana State University, Bozeman, MT, USA,

Abstract:

Understanding the Jurassic development of the Western Sierra Nevada Metamorphic Belt (WSNMB) is essential for interpreting more recent tectonics in the central North American Cordillera, but the number and polarity of Jurassic subduction zones in the WSNMB remains uncertain. Models include east-directed subduction of the Farallon plate beneath North America, west-directed subduction of the North American plate beneath an island archipelago, or east- and west-directed subduction of the Mezcalero plate beneath North America and an island archipelago. We characterize sediment sources of the Jurassic Colfax Formation within the northern WSNMB to evaluate proposed subduction models. We completed U-Pb age dating (n=506) and Lu-Hf isotopic analysis (n=143) of five detrital zircon samples, as well as six conglomerate clast counts. Colfax conglomerates are both clast- and matrix-supported with sandy or muddy matrices and clast sizes that range from pebble to boulder. Clast composition varies, but all six conglomerates include chert and volcanic clasts. Maximum Depositional Ages (MDAs) calculated from detrital zircon age spectra range from 172–156 Ma, with the 156 Ma as the most robust MDA. Four of five samples include few Jurassic zircon, so their older MDAs may be too conservative. Of the 506 Colfax detrital zircon grains, only 31% are younger than 300 Ma, with age modes at 168, 196, and 253 Ma. The remaining 69% of grains are older than 300 Ma, and form a multi-modal age signature characteristic of western North America. Zircon Th/U ratios are mostly 0.1–1, indicating a felsic magmatic signature. A few pre-Mesozoic zircon grains have Th/U ratios either <0.1 or >1, indicating metamorphic or intermediate to mafic magmatic signatures, respectively. Mesozoic zircon yielded abundant Ɛ_Hf values >5 (n=63; 82%), suggesting a juvenile arc source. However, the presence of Ɛ_Hf values ranging from +4.7 to -12.1 (n=14; 18%) indicates a more evolved contribution to the Jurassic arc. 

Colfax Formation sandstone has lower zircon abundance and fewer Mesozoic grains than potentially correlative Mariposa and Galice Formations. However, abundant pre-Mesozoic zircon grains tie the Colfax Formation to North America, and the presence of Late Jurassic zircon and volcanic conglomerate clasts indicate deposition near an active arc. Our data are consistent with east-dipping subduction beneath North America throughout Middle to Late Jurassic time.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6798

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