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Late Devonian Depositional Evolution of Western Montana and East-Central Idaho – Jefferson, Three Forks, and Sappington Formations


Late Devonian rocks of western Montana and east-central Idaho record mixed carbonate and siliciclastic deposition (<100 to >1000 m thick) on a dissected ramp that formed in the tectonically active Antler foreland basin. 3rd-order sequence stratigraphy integrated with available biostratigraphy correlates these strata from Idaho over the Lemhi Arch into western Montana. Downramp in Idaho and in the Central Montana Trough (CMT), the Frasnian Jefferson Formation records consistent facies stacking patterns and stratigraphic geometries, including well developed incised channels. 3rd-order high (Idaho) to low (Montana) accommodation sequences are comprised of mixed siliciclastic and carbonate rocks with tidal flats passing into mottled lagoonal to open marine rocks with coral-stromatoporoid biostromes and reefs. Up-ramp tidal flats predominate and thin into stacked TST/HSTs. Subsequently, intrashelf basins in Idaho and Montana filled with evaporite, microbial dolostone, and red-green afossiliferous shale that accumulated in repeatedly flooded sabkhas of the Early Famennian (Jefferson Grandview and Three Forks Logan Gulch members). At least one correlative off-shelf quartz arenite lowstand wedge occurs in Idaho. Small coral-stromatoporoid reefs also developed along the margins of the CMT where accommodation space and open circulation allowed. A series of stromatoporoid reefs occurs behind the Lemhi Arch in the western CMT and Ruby- Snowcrest trough of SW Montana, but they are partially eroded by latest Devonian uplift. Moldic and dolomitic porosity and brecciation often is well-developed in the top of subtidal/ biostromal intervals beneath sequence boundaries recording subaerial exposure. The latest Devonian (Middle and Late Famennian) is marked by paleogeographic reorganization of the shelf with continued differential tectonic loading and deposition of the marine Three Forks Trident Member. It is comprised of 3 shale-limestone sequences in Idaho (up to 90 m thick) followed by Sappington Formation deposition below McGowan Creek Formation turbidites in Idaho and distal ramp Mississipian Lodgepole facies in Montana. The Trident is open marine within the CMT, but it is eroded or was not deposited on the northern and southern margins of the CMT. Likewise this unit was truncated in western Idaho by Antler uplift. Very distal, thin Sappington shale and siltstone were deposited below turbidites in the Lost River Range.