Stratigraphic and Sedimentologic Analysis of the Bear Gulch Limestone (Mississippian-Pennsylvanian) near Grass Range, Montana

The Bear Gulch is a dolomitic limestone unit of Upper Mississippian/Lower Pennsylvanian age (~320 Ma) occurring in the Big Snowy Trough, an east/west-trending structural trough north of the Little Belt Mountains in central Montana. The Bear Gulch crops out near Grass Range, Montana, and is an informal unit within the Fergus Group. Hydrocarbon source rocks similar in age and composition to the Bear Gulch limestone have played a central role in the recent success of directional drilling and hydraulic fracturing (e.g., Bakken, Eagleford, etc.). Despite the suspected economic potential and previously-documented paleontological wealth of the Bear Gulch, no recent sedimentary analyses of outcropping Bear Gulch or its subsurface equivalents in the Big Snowy Trough have been published. The results of this study provide the basis for assessing the source rock potential of the Bear Gulch and understanding its stratigraphic affinity to other mapped units within the Big Snowy Trough. Six weeks were spent in the field collecting and cataloging rock samples and measuring laminated sections of the Bear Gulch (July-August, 2013). This was followed by laboratory analysis of the samples. Thirteen of the 75 samples collected were made into thin sections and analyzed with a petrographic microscope. Mineralogical composition, carbon content, and sedimentary fabric data were determined through X-ray diffraction, elemental analysis, and SEM studies. Using these combined data sets, sedimentary facies within the stratigraphic measured section of outcrop were defined. This information was compared to gamma ray logs from nearby oil wells and a north/south-trending cross-section across the Big Snowy Trough was constructed in order to explore the lateral variations within Bear Gulch time-equivalent strata across this structure

Early Cenozoic Fluvial Deposits of the Renova Formation in SW Montana: Links to Southern Nevada and Utah?

Examination of the early Cenozoic fluvial deposits of the Renova Formation provides support for the hypothesis that a southern branch of the pre-ice age Bell River of Canada, a river thought to have been the size of the Amazon, may have originated in the southern Colorado Plateau and flowed northward through Nevada, Utah, Idaho, and Montana. The Renova Formation mostly comprises fluvially-reworked and degraded volcanic ash. Radiometric ages of zircon grains from the Renova Formation, reported in the literature, correlate with the ages of zircons from ash-flow tuffs that erupted from mega-calderas in southern Nevada and Utah. There are also older zircons present in the Renova deposits which indicate recycling of zircon grains from Precambrian and Cambrian quartzites of Utah. These results provide evidence of river transport of ash and sand from Nevada and Utah into Montana. Previous research has been reviewed and assessed in the context of the Bell River hypothesis. A field trip was taken to physically observe the composition and depositional features of the Renova. Histograms generated by mass spectroscopy of Renova Formation zircon have been re-analyzed in light of the mega-caldera origin hypothesis. This new model suggests that a major, north-flowing Cenozoic drainage system was present in the western interior of North America before being segmented and destroyed by faulting and volcanism