DEPOSITIONAL SETTING, LITHO- AND SEQUENCE STRATIGRAPHIC ANALYSIS OF THE DEVONIAN-MISSISSIPPIAN TRANSITION IN WESTERN MONTANA

This study utilizes petrographic and outcrop scale observations to map subsurface Devonian-Mississippian strata of western Montana. Strata are interpreted according to the conventions of lithostratigraphy and sequence stratigraphy in order to deconvolve multi-basinal histories. Stratigraphic intervals recording this time include the Exshaw, Banff, Three Forks, and basal Lodgepole formations, and are generally composed of a basal organic-rich black shale, a medial carbonate and siltstone, and an upper organic-rich black shale. Eighteen lithofacies are identified from five outcrop localities, multiple thin sections and one cored interval that are calibrated to multiple petrophysical well logs for extensive subsurface mapping. Lithostratigraphic mapping shows Devonian-Mississippian strata are dominantly preserved in two discreet basins along the paleo Laurentian margin. Reactivation of deep-seated structural lineaments cause changes in topography, and subsequent accommodation for deposition or erosion of strata. Major features include 1) the Sweetgrass Arch that appears to be both a positive and negative feature through time, 2) the Pendroy Fault Zone which acted as a paleo high partitioning basins along strike, and 3) the Big Snowy/Central Montana troughs that are a weak center of deposition during the late Devonian/early Mississippian, but a strong­­ depocenter during the early to middle Mississippian.

Lithofacies associations have been interpreted to represent environments of deposition on a slope to basin profile. The Waltherian migration of lithofacies associations creates a sequence stratigraphic framework. Specifically two sequence boundaries have been interpreted in the Sappington Basin: the first between the Logan Gulch and Trident members of the Three Forks Formation, and a second between the Trident and Sappington members of the Three Forks Formation. A possible third sequence boundary lies above the Sappington Member. Due to a low accommodation setting, few discernable parasequences are observed as shallowing upward cycles in clastic and carbonate strata. Similar energy cycles are observed in the southern Western Canadian Sedimentary Basin, yet are manifested by various lithologic attributes including varying lithology and ichnology.