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Lateral Facies and Thickness Change of the Devonian-Mississippian Sappington Formation Along Inverted Margins of the Central Montana Trough, Three Forks, Montana


Unconventional hydrocarbon production success in the Williston Basin Bakken Formation has led to rejuvenated interest in the Devonian-Mississippian (DM) of Alberta and Sappington Formation in western Montana. The Central Montana Trough (CMT) was a Late Devonian reactivated Precambrian salient, located east of the Idaho Antler foreland; it later evolved to control deposition of the Madison and Big Snowy groups. Sappington black shales and shoreface siltstones near Three Forks share some intracratonic depositional patterns with the Bakken, although sedimentation was also influenced by differential active foreland subsidence and uplift. The Sappington is 0-30 ft thick on the stable Beartooth Shelf to the southeast, and ∼60 to >120 ft thick in the eastern and western CMT. Farther west along the Idaho-Montana border variable accommodation and overlap by Mississippian turbidites followed active inversion of western DM paleohighs (e.g. Montania, Lemhi Arch). Dependent on the extent of regional pre- and post-depositional erosion, the Lower and Middle Sappington rocks in the study area overlie different older units and underlie the Upper Sappington shale (i.e. Lodgepole Cottonwood Canyon Member) only to the east. A variable transgressive ‘scallion-like’ encrinite succession with hardgrounds regionally overlie the Sappington below the distal ramp Lodgepole facies. The Lower Sappington (unit 1) is comprised of black shales including both carbonaceous, waxy shale of freshwater association and fissile, cherty deep marine shale. The Middle Sappington is composed of a calcareous to shaley siltstone, that coarsens and shallows-upward. The basal fossiliferous, sponge-oncoid silty limestone of the Middle Sappington (unit 2) may represent a basinward shift in facies, however its regional occurrence also over subsiding paleohighs suggests the opposite. Proximal shoreface to rare, sandy foreshore facies (unit 5) are typically truncated across the DM boundary, either directly below basal Lodgepole encrinites or below the Upper Sappington (unit 6). This transitional unit is comprised of a suit of transgressive mixed marine clastics that include a 3 ft thick black shale in sections generally east of the Lombard Thrust (i.e. eastern CMT), and broadly across drowned parts of the craton/paleoshelf. SE-directed synsedimentary folds occur locally below this stratigraphic interval. These relationships suggest latest Devonian western uplift followed by earliest Mississippian inversion.