Devonian-Mississippian antler foreland basin carbonates in Idaho: Significant subsidence and eustasy events
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Idaho's mid- and late Paleozoic sedimentary successions are carbonate and mixed carbonate-siliciclastic units showing >5 km subsidence combined with several organic buildups in downslope and eustasy-influenced foreland basin settings. Overall, Silurian through Late Carboniferous sediments average 6-10 km thickness. Idaho's margin was passive in Early to Middle Devonian time, and transitioned into a foreland basin in the Late Devonian with additional large tectonic and sedimentary loading accommodation events in the Carboniferous. The Middle and Late Devonian Jefferson Formation (> 2 km thick) is composed of downslope marine units that pass upward into downslope buildups comprised of stromatoporoids and disphyllid corals. A drowning event stopped buildup growth, followed by eustatic fall that subaerially exposed the buildups. Abrupt high amplitude sea level changes in the late Frasnian to Famennian were a result of coeval initiation of glaciation in Gondwana. Coincident with the first Late Devonian to Carboniferous tectonic loading event, the resulting foreland basin received flysch derived from western uplands. To the east, a >3 km thick succession of Mississippian carbonate was deposited in the distal foreland basin. The first Carboniferous foreland subsidence event is in Kinderhookian time, and a second is during early Chesterian time, with eustatic overprinting in the mid- to late Chesterian. The carbonates are subdivided into two packages: 1) uppermost Devonian - lower Mississippian shale and middle Mississippian progradational carbonates containing abundant pelletal and ooid grainstones; and 2) upper Mississippian carbonates dominated by muddy skeletal banks that grade upward into mixed carbonates and siliciclastics. Although the general history of these units is similar to that of adjacent, on-strike regions, accommodation appears to be more significant than what occurred in Nevada, where crustal thickening through Late Devonian thrust faulting is clearly demonstrated; in Idaho coeval thrusting has yet to be defined.
