Investigating the Mesoproterozoic-Paleozoic Great Unconformity of Western Montana: Detrital Zircon Geochronology and Implications for Terminal Evolution of the Belt Basin

This study is focused on determining the depositional ages of geologic formations above and below the Great Unconformity to understand how the rocks changed and what that change means to western Montana’s tectonic past. The Great Unconformity of western Montana represents a depositional hiatus in the stratigraphic record of over half a billion years. The movement of tectonic plates during this hiatus changed western Montana’s sediment source, referred to as a change in provenance. This change in provenance is represented by Meso-Proterozoic rocks of one composition overlaid by younger Paleozoic rocks of an entirely different composition. By using detrital zircon ages and framework grain analysis of the rocks that formed before and after the Great Unconformity we can investigate the change in provenance over this enormous hiatus in deposition. The rock units analyzed include the MacNamara, Pilcher, and Garnet Range formations of the Belt Supergroup (Meso-Proterozoic) and the Lower unit of the Cambrian Flathead formation (Paleozoic). Samples were collected from Alberton, the northern flank of the Flint Creek Range (Porter’s Corner), and the southern flank of the Garnet Range Mountains, all in western Montana. Zircon ages are collected by zapping hundreds of grains with a laser and measuring the decay rates of specific radioactive isotopes (U-Pb) that are blasted off of the zircon grain. The results come in the form of distinctive, provenance-defining zircon age spectra or “barcodes.” By collecting detrital zircon “barcodes” from multiple formations we can reconstruct the geochronology and tectonic evolution of successive stratigraphic units. This study is significant because the Great Unconformity is poorly documented in western Montana and represents one of the longest known depositional hiatuses in geologic history. I anticipate presenting preliminary detrital zircon geochronology spectra for each of the stratigraphic units listed above and interpreting these data in context of published tectonic models to determine terminal evolution of the Belt Basin. In particular, it is expected that detrital zircon “barcodes” from the Belt’s Garnet Range formation will be notably different from those of the underlying Belt units, indicating a completely different and previously undocumented Belt Basin provenance