ANOMALOUS CARBONIFEROUS ACCOMMODATION IN IDAHO

Compared to coeval strata in Alberta and Nevada, Idaho’s Carboniferous succession shows anomalous accommodation and fill. Consisting of approximately 3 km of compacted carbonates, sands and flysch, the stack has at least two periods of accommodation (subsidence), influx of craton sand, and ultimately eustatic-controlled carbonate buildups. Beginning with the McGowan Creek western-derived flysch, there is compelling evidence for a rapidly developed foreland trough, owing to crustal loading by the Antler allochthon. This was in Kinderhook and Osage time. A problem with this is that the Roberts Mountains thrust has not been identified in Idaho. Rather, some suggest local pull-apart basins, as suggested for coeval units in the Canadian cordillera (Eisbacher, 1983). The flysch shows deposition well below the photic zone (and deep water anoxia), with progradation and aggradation proceeding until the carbonate bank gradually reached shallower water and oxygenation. With the highly fossiliferous Scott Peak Formation (Meramec) came shallow water depostion, although high energy grainstones are largely lacking. The second accommodation event begins in early Chesterian time and anoxia (South Creek Fm.) once again yields to oxygenated mud banks of the Surrett Canyon Fm. Identifying the source of voluminous carbonate mud in the Surrett Canyon Fm. remains elusive to interpret. The two Idaho Mississippian events correspond to the Roberts Mountains and Christina Peak subsidence events in Nevada (Trexler et al. 1991). Latest Mississippian Arco Hills and Bluebird Mountain formations indicate eustacy with craton-derived sand inputs. No work has been done on the Lower Pennsylvanian Bloom Member of the Snaky Canyon Fm., but significant accommodation is required, establishing a third event. However, the overlying Juniper Gulch Member, with its successions of coral/algal/Palaeoaplysina buildups. Compared to adjacent regions in Nevada and Alberta, the Idaho section is anomalously thick and represents deposition within more extensive crustal instability. Furthermore, coarse deposits of the Diamond Peak Formation (Nevada) are absent in Idaho.