Stratigraphic architecture and diagenetic evolution of the late Devonian to early Mississippian strata in western Montana
Borski, Jack Busse
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Late Devonian to early Mississippian strata have been extensively studied in geologic literature both for academic and economic purposes. Lithostratigraphic intervals spanning this time in western Montana include the Exshaw, Banff, Three Forks and Lodgepole formations. These formations are generally composed of a lower organic-rich black shale, a medial calcitic and dolomitic siltstone to sandstone, and upper organic rich shale. This siliciclastic-dominated rock package is anomalous in the carbonate-dominated Paleozoic of western North America. This study aims to address the following research question: where and what were the environments of deposition throughout the Late Devonian to Early Mississippian in western Montana and what is the diagenetic evolution of these rock packages? To answer this question, multiple datasets were utilized including core, outcrop and geophysical well logs. A lithofacies scheme comprised of eighteen lithofacies was constructed based on rock attributes to interpret and correlate lithostratigraphic intervals as well as to establish a sequence stratigraphic framework from outcrop. The stratigraphic framework constructed from outcrop was then mapped throughout the subsurface of the study area where outcrop is absent. Finally, a diagenetic analysis was conducted according to significant sequence stratigraphic surfaces and rock bodies. Lithostratigraphic and sequence stratigraphic surfaces are generally concurrent due to the low-accommodation epicontinental basinal setting of western Montana during this time. Sequence stratigraphic distribution indicates a long-term regression followed by a long term transgression that is overprinted by tectonically emergent features and third order cycles that are the primary concern of this project. Prominent structural features affecting sedimentary dispersal patterns include the Beartooth shelf, Sappington basin (that may be the incipient central Montana trough), the area near the Pendroy fault zone (possibly the incipient central Montana uplift), and western Canadian sedimentary basin.