PROVENANCE AND PALEOGEOGRAPHY OF NEOPROTEROZOIC-MESOZOIC STRATA, BRIDGER RANGE, MONTANA, USA

Sedimentary strata exposed in Montana’s Bridger Range record the development of the western North American continent from Neoproterozoic deposition of the Belt Basin, passive margin sedimentation through much of the Paleozoic, and the onset of foreland basin deposition during evolution of the North America Cordillera in the Mesozoic. This study presents provenance analyses from siliciclastic stratigraphic intervals exposed in this mountain range illuminating important phases in this evolution. We report ~4,700 U-Pb detrital zircon dates obtained through LA-ICP-MS analysis of sandstones from Neoproterozoic through Cretaceous strata of the Bridger Range. These detrital zircon signatures are compared to those of potential North American source regions and coeval strata of the Cordillera to interpret the provenance of detritus and paleogeography of southwest Montana during this interval of time.

Detritus from the Archean craton is shown to dominate in the Neoproterozoic and early Paleozoic. Sediment sourced from the Appalachian, Pan-African, and Grenville Orogenies to the southeast as well as the Yavapai and Mazatzal Orogenies to the south dominate from the Devonian through the Pennsylvanian. By Jurassic time, the developing North America Cordillera triggers a shift to the dominance of western sources, primarily magmatic arc-derived grains and detritus recycled from uplifted and eroded Paleozoic passive margin strata. Dates from the Morrison and Swift Formations constrain the timing of this transition in provenance, showing dominance of western provenance by Oxfordian time. These units represent remnant back-bulge deposits of the foreland basin system whose foredeep equivalents have been uplifted and eroded. The Kootenai Formation is interpreted to represent the oldest preserved foredeep deposit. Additionally, a shift in provenance during Albian times suggests the foreland basin system evolved from a well-integrated to segmented depositional system. Broadly, this work contributes to a clearer understanding of the evolution of the western North American continent from the Neoproterozoic through Cretaceous.