9-2 Conglomerate and Sandstone Composition of the Colfax Formation, California: Implications for Middle-Late Jurassic Subduction

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

Presenting Author:

Authors:

Abstract:

The number and direction of Jurassic subduction zones represented in the Western Sierra Nevada Metamorphic Province (WSNMP) of California remain uncertain, yet understanding Jurassic tectonics influences how we interpret more recent tectonic development of the region. Because the Colfax Formation formed along the continental margin during Middle to Late Jurassic time, characterizing the formation’s sediment sources provides insight into the development of the continental margin.

We completed seven field clast-counts of cobble conglomerates and eight thin-section point counts of pebble conglomerate and sandstone samples. All conglomerates and sandstone samples are dominated by chert, include lesser volcanic lithic clasts and minor quartz, and contain lower metamorphic clast abundance than previously reported. Despite post-depositional low-grade metamorphism, a few chert pebbles show radiolarian ghosts in thin sections, indicating a deep-marine origin for the chert. The presence of numerous volcanic lithic grains, including large andesitic clasts with visible plagioclase laths, contradicts earlier interpretations that the Colfax Formation lacks a proximal volcanic source.

Sandstone samples are generally fine- to medium-grain size, with subangular to subrounded grains that exhibit varying degrees of calcite replacement. Authigenic epidote and chlorite occur in many samples, consistent with low-grade metamorphism. Colfax Formation sandstone samples plot close to the quartz–lithic axis of a quartz-lithic-feldspar (QFL) ternary diagram, and close to the Qm-Lt axis of a monocrystalline quartz-feldspar-total lithics (QmFLt) ternary diagram. These results indicate transitional to (lithic) recycled orogen provenance.  

The Colfax Formation was deposited on the chert-rich Clipper Gap Formation, which has been interpreted as part of an accretionary complex, and includes a transition unit of mixed Clipper Gap and Colfax in some regions. The compositional similarity between Colfax and Clipper Gap Formations suggests that material derived from the subducting oceanic plate into an accretionary complex was later recycled into the Colfax basin. In addition, the presence of intermediate volcanic lithic clasts in the Colfax Formation indicates proximity to a magmatic arc. These data collectively support a tectonic setting involving an accretionary subduction margin during Colfax Formation deposition, consistent with east-dipping subduction beneath North America in Middle to Late Jurassic time.

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

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.