LATERAL FACIES CHANGE OF THE DEVONIAN-MISSISSIPPIAN SAPPINGTON FORMATION (BAKKEN EQUIVALENT) ALONG INVERTED MARGINS OF THE CENTRAL MONTANA TROUGH, THREE FORKS, MONTANA

Hydrocarbon production success from the Bakken Formation of the Williston Basin has led to interest in time-equivalent Devonian-Mississippian strata. One of these Bakken-equivalent formations is the Sappington Formation of western Montana. It was deposited in the Central Montana Trough (CMT), a Late Devonian reactivated Precambrian salient, located east of the Idaho Antler foreland. Differential foreland subsidence and uplift of the CMT resulted in unique depocenter geometries. Despite the tectonic paleogeographic differences between the CMT and the Williston Basin, the lower and middle members of the Bakken and Sappington share similar intracratonic depositional elements.

This study focuses on outcrop Sappington sections in an east-west isopach thick between the Tobacco Root Mountains and the Bridger Mountains. Regionally, the Sappington pinches out to the south and east onto the paleogeographic highs of the Beartooth Shelf and Central Montana Uplift. To the west however depositional thinning was locally accentuated by erosion prior to Lodgepole (Madison Group) carbonate deposition. The subsiding CMT provided accommodation space for a flux of fine-grained sediments in paleoequatorial western Laurentia via punctuated fluvial and eolian processes. Depositional character of the six distinct Sappington sediment packages indicates post basinal deposition sediment reworking in an environment with a mixed assemblage of energies, such as an estuary/embayment. Early Gondwanan glaciation produced widespread equatorial Late Devonian climate effects overprinted by regional tectonic variation and removal of the geologic record. This is reflected in the Sappington by an abundance of sequence boundaries in a relatively thin (10-40m) package of rocks.

Like the Bakken the Sappington can be subdivided into a lower, middle, and upper member. The lower member of the Sappington is comprised of a suite of stacked black shale lithofacies with distinct internal sequences. The middle member of the Sappington is comprised of four siliciclastic units in sharp contact and sharing few facies similarities. The very thin clastic upper member is a package of minimally preserved poorly genetically related facies unconformably overlain by prograding crinoidal grainstone of the lowermost member of the Lodgepole Formation.