LATE DEVONIAN STRATIGRAPHY OF EAST-CENTRAL IDAHO: FRASNIAN AND FAMENNIAN LEMHI ARCH TO LOST RIVER SHELF MARGIN

Late Devonian strata in Idaho provide analogue to for lower accommodation intracratonic rocks of western Montana. The Jefferson, Birdbear, and Three Forks formations are correlated and differentiated using primary unconformities and internal sequence boundaries. Passive margin Givetian and Frasnian lower Jefferson Banded Dolomite (peritidal members D1 and D2) and Dark Dolomite (open marine D3 Member) are overlain by restricted, mixed lithologies of the latest Frasnian and lower Famennian upper Jefferson Formation. The upper Jefferson D4 – D6 members share facies with the western Jefferson Grandview Dolomite, and on-lap the Lemhi Arch to the east. These units are correlative with the Birdbear Formation and Logan Gulch Member of the Three Forks Formation in Montana. Thick Frasnian Jefferson Formation strata in the central Lemhi Range and Borah Peak area of the Lost River Range were deposited west of the Lemhi Arch with buildups established on ramps near the shelfbreak (Grandview Area). During onset of the Antler Orogeny, before deposition of latest Devonian Three Forks Formation and widespread disconformities, a late Frasnian and younger outer shelf barrier formed with heterolithic, cyclic mixed peloidal facies (~300m thick Grandview Dolomite). Black subtidal carbonates and western latest Frasnian buildups have been identified under a regional basal Famennian sequence boundary (“SB Fm0”). At the same time, the Lemhi Arch of the southern Lemhi Range and Beaverhead Mountains foundered and an intra-shelf basin accommodated sandstones, artifacts after evaporites, and mostly restricted dolostone and limestone (upper Jefferson D4-D6 members). Above a regional sequence boundary and flooding surface younger marginifera-trachytera open marine to local seaway shale and limestone deposits of the Trident Member up to 80m thick were deposited. Variable accommodaton resulted across the Idaho shelf and on paleohighs and lows reactivated on the craton. A major unconformity and hiatus on the outer Idaho paleoshelf was coeval with latest Devonian deposition of the Sappington Formation in the Central Montana Trough. Sappington Formation clastics on the Idaho shelf were either not deposited, or were eroded prior to significant regional Mississippian basin inversion.