24-3 Reevaluation of the Mississippian Copper Basin Group in south-central Idaho and implications for the enigmatic Antler orogeny

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part I

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Abstract:

The Late Devonian to Mississippian Antler orogeny created thick sedimentary sequences along the western margin of Laurentia. Outcrops interpreted to record tectonism associated with the Antler orogeny are present in the Copper Basin Group in south-central Idaho and the Roberts Mountain Allochthon in central Nevada. However, the tectonic setting and associated mechanism of basin subsidence are still poorly understood, with previous research suggesting deposition during either east-dipping subduction, west-dipping subduction, or along a strike-slip plate boundary. To address this debate, we measured and characterized over 6 km of strata in the Mississippian Copper Basin Group in south-central Idaho. Results corroborate previous research that show the Copper Basin Group is dominantly a siliciclastic system characterized by thick packages of marine mudstones, mature to super mature sandstones, chert pebble-cobble to rare boulder conglomerates, and rare limestones with decompacted and backstripped sedimentation rates ranging from 200 m/Myr to 500 m/Myr. There is considerable lateral variability in the grain size of conglomeratic units, which grade into sandstones and mudstones, and the stratigraphic group only outcrops/ occurs over a small spatial area (<4000 square kilometers). New and published U/Pb detrital zircon geochronology shows that Devonian strata below the Copper Basin Group contain 500-600 Ma and 1.0-1.8 Ga zircons in the Milligen and Jefferson formations, whereas samples from the Copper Basin Group lack these age modes, and contain dominantly 1.8-2.0 Ga zircons. Permian strata above the Copper Basin Group contain 1.0-1.8 Ga zircon grains, but not the 500-600 Ma zircons present in the Devonian strata. Syn-depositional zircons are not present in any of the samples. Anomalously high sedimentation rates and a potentially regionally restricted basin geometry suggests that the Copper Basin Group was deposited along a releasing bend or extensional step over along a strike-slip plate margin with sediment sourced from the proximal Ordovician Kinnikinic Formation through marine feeder canyons. The source of the chert pebbles has been proposed to be the Antler highlands to the west, but there is little evidence for its existence in Idaho. These results challenge early interpretations that the Copper Basin Group was deposited in a flexural foreland basin system and is consistent with more recent interpretations of large scale left-lateral translation along western Laurentia during the Late Devonian through Mississippian periods.

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

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