SEQUENCE STRATIGRAPHIC FRAMEWORK AND DEPOSITIONAL SYSTEMS OF THE LATE DEVONIAN (FRASNIAN) DUPEROW FORMATION IN WESTERN AND CENTRAL MONTANA

Late Devonian rocks in Montana were deposited on the cratonic platform of western North America during a second order sea-level rise punctuated by shorter term relative sea-level falls that resulted in third order depositional sequences and fourth order parasequences. Sequences are comprised of shallow marine framestone reefs and grainstone shoals, restricted back reef wackestones, laminated lagoonal mudstones, and upper intertidal to supratidal sabkha crinkly laminated mudstones and evaporites. Measured sections in the Sawtooth Range, Bridger Range, Little Belt Mountains, Big Snowy Mountains, Little Rocky Mountains, Gallatin Canyon, and at Logan, Montana reveal significant thinning of reef and shoal facies from west to east and southwest to northeast approaching the western edge of the Central Montana Uplift, and thickening of reef and shoal facies along the Central Montana Trough and approaching the shelf break along the Idaho-Wyoming border. Lagoonal and supratidal facies progressively dominate sequences eastward coincident with decreasing accommodation associated with the Central Montana Uplift and Antler orogenic effects. Sequences are correlated using seven measured sections, drill core, and intervening well log data. Individual facies are analyzed using thin sections selected in consecutive order to best compare stacking and facies distribution patterns. X-ray Diffraction is used to compare mineralogy of thin sections as well as Scanning Electron Microscopy to document microbial fabrics and structures within organic-rich lagoonal units. Regional correlation of strata using a sequence stratigraphic framework allows for the construction of paleogeographic maps, facies isopachs, stratigraphic cross-sections, and the mapping of source-reservoir-seal relationships from the shelf break of eastern Idaho to the shelf margin of western Montana and inner shelf depositional environments of western and central Montana. Upper Devonian strata of western and central Montana can be further correlated to their counterparts in the Alberta and Williston Basins.