Middle and Upper Paleozoic Carbonate Sequences in Idaho: Foreland Subsidence, Eustacy and Reefs
Within Idaho's mid- and late Paleozoic succession are carbonates and mixed siliciclastic units showing anomalous subsidence combined with several organic buildups in downslope and eustasy-influenced foreland basin settings. Overall, Devonian through Early Permian sediments comprise 7 km thickness. Idaho's margin was passive in Middle Devonian time, yielding to a significant foreland basin with large accommodation events in the Carboniferous. Starting with the Jefferson Formation (Middle and Late Devonian, over 2 km), downslope rhythmites pass upward into a Nisku-type downslope buildup with stromatoporoids (showing ecophenotypic growth forms) and disphyllid corals. A drowning event occurs, with eustatic lowstand following, subjecting the buildup to subaerial exposure. The abrupt sealevel change is a consequence of coeval glaciation in Gondwana. Coincident with the first Late Devonian/Early Carboniferous tectonic loading event, the resulting foreland basin received western-derived flysch with a 3 km carbonate progradation event following. It is represented by changes from mudstone through packstone, with horizons of pelletal and ooid grainstones. A second tectonic adjustment from sediment loading provided accommodation for a second progradational carbonate package in Bashkirian time. Late Carboniferous carbonates show cyclic development of phylloid algal and hydrozoan(?) buildups, which show an ecologic succession from pelmatozoans (sediment stabilizers), colonial rugosans (colonizers), and diverse shelly faunas and phylloid algae (diversification) and Palaeoaplysina (domination). Buildups show termination by salinity rise (dolomites), siliciclastic suffocation (sands and silts), and brief subaerial exposure.