Evolution of the Early-Late Cretaceous foreland basin system in southwestern Montana based on U-Pb detrital zircon geochronology

Subduction of oceanic crust beneath continental crust at collisional plate tectonic boundaries results in the development of mountain chains and associated sedimentary basins that span the length of continents. One of these vast, continental-scale sedimentary basins formed on the North American continent between ~160-60 million years ago. This study focuses the Idaho-Montana sector of this ancient sedimentary basin system. In the mountain chain adjacent to this basin, tens of millions of years of uplift and erosion resulted in removal of stacks of older rock layers, which would have been exhumed to the surface during earlier stages of mountain building. As a result, one of the only remaining records of these earlier stages is contained within the sedimentary rock layers that were directly shed from the mountains during uplift.

For this thesis, three discrete sedimentary rock layers that crop out in the southwestern Montana sector of the ancient sedimentary basin system were analyzed using field and laboratory techniques. In the laboratory, particularly robust mineral grains—referred to as detrital zircon grains—were separated from sandstones and numerous single-grain analyses were performed. These mineral grains are of interest because their isotopic signatures provide the age at which each grain crystallized, which can in turn be used to fingerprint the sources for these sedimentary rock layers. When evaluated in the context of regional and continental-scale sediment dispersal systems, the interpreted sediment sources for the three discrete rock layers analyzed in this thesis allow for refinement of previous paleogeographic reconstructions in southwestern Montana.