STRATIGRAPHY OF THE DEVONIAN - CARBONIFEROUS BOUNDARY UNITS, WEST-CENTRAL MONTANA

Samples from the Devonian-Carboniferous (D/C) upper Three Forks and Sappington (Bakken) at the Logan Gulch Devonian type section near Three Forks, Montana were processed for palynomorphs. Additional sampling included the “false Bakken shale” from within the lower Lodgepole Formation at Pole Canyon (Horseshoe Hills). Green shales of the Three Forks Formation Trident Member and black to green shales of the Sappington Formation were deposited in the cratonic Central Montana Trough (CMT). These strata represent ~40m of Middle and Late Famennian rocks that were later buried under the Mississippian Madsion Group. The Lower Sappington overlies seaway limestone and shale of the upper Trident Member and is characterized by deepwater to restricted black shale subunits, changing abruptly upward to open marine fossiliferous, silty limestones, a unique burrowed shale (Sappington Unit 4), and siltstone/sandstone of shoreface association. The Sappington Formation at the Logan Gulch reference section is easily divided into 6 widely occurring lithostratigraphic units or lithosomes, however prospective basal unit 1 shales at this location are anomalously thin (<30 cm). Unit 4 of the middle Sappington is a Strunian shale, well developed in the central part of the CMT. The D-C boundary occurs above unit 4 within coarser unit 5 clastics that are bound by a relatively sharp basal surface (indicative of a basinward shift / sequence boundary, or other?) and a disconformity below the base of unit 6 (upper Sappington of Cottonwood Canyon Fm.). Tournaisian unit 6 is comprised of a basinal lag with ~1m of black shales overlain by burrowed siltstones, overlain in turn by another sharp contact beneath glauconitic, crinoidal packstones of the Lodgepole (“Scallion”). Sequence boundaries and hiatus are common throughout the Three Forks-Sappington-Lodgepole transition.