Stratigraphic analysis of the Jurassic Ellis Group and paleotectonics in North-Central Montana : deciphering the historically enigmatic "Belt Island"
Porter, John Richard.
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"Belt Island", an influential positive structure during the Jurassic, is widely represented in the geologic literature from 1948 to present. Introduced as primarily a stratigraphic anomaly in isopach maps of the Ellis Group, the inherent spatial and temporal context was loosely constrained. In subsequent literature, it is widely referenced in a structural context, though support is lacking for originating paleotectonics and stratigraphic data is minimal. Do the stratigraphy, sedimentology, and regional tectonics support the theory of a paleogeographic/paleotectonic high in north-central Montana during the Middle to Late Jurassic during the deposition of the Ellis Group? What are the tectonic explanations for such a feature if one did persist throughout said time? This project looked at outcrops and cores of the Ellis Group from around the project area. A lithofacies characterization was assembled based on rock properties. Utilizing previous work on lithostratigraphy, biostratigraphy, and regional unconformities and integrating these data with the lithofacies characterization a sequence stratigraphic framework was assembled. This sequence stratigraphic framework was applied to an extensive subsurface well database to generate a geologic framework over the project area. The distributions of lithofacies do support the theory of a large feature in the project area during the deposition of the Ellis Group. The stratigraphic architecture as well supports this theory, and additionally adds a spatial and temporal constraint to the feature. With the integrated sequence stratigraphic framework, it is apparent there was regional syntectonic activity influencing deposition. With the ability to put a temporal and spatial constraint on the paleotectonic feature it is possible to understand through time how this feature behaved. This allows the research to state that the feature originally known as "Belt Island" was the paleogeographic area of non-deposition and erosion of a larger peripheral bulge. This lithospheric flexure would have been the reaction to the crustal loading to the west during the Western North America Cordilleran Orogeny.