COMPARISON BETWEEN GEOCHRONOLOGIC AND GEOCHEMICAL PROVENANCE MODELS FOR CRETACEOUS SEDIMENTARY ROCKS IN SOUTHWESTERN MONTANA

This research is investigating the regional tectonic evolution and sediment source areas of Cretaceous clastic rocks in southwestern Montana. This research compares the major and trace element geochemistry of the Cretaceous mudstone and sandstone samples from southwestern Montana with existing detrital zircon U-Pb geochronology. This comparison allows for conclusions to be drawn concerning the provenance models for this area. This geochemical approach is being used because some source rocks have minimal zircons which can limit our understanding of source areas and tectonic evolution. Existing detrital zircon U-Pb data suggest that both the Valanginian-Aptian Kootenai Formation and the Albian Blackleaf Formation demonstrate an up-section change from primarily recycling of older sedimentary rocks to increased input from igneous rocks. In contrast, the Cenomanian-Santonian Frontier Formation primarily exhibits input from igneous sources with minimal sedimentary recycling. Geochronology also suggests that the Coniacian-Maastrichtian Beaverhead Group primarily demonstrates sedimentary recycling. Geochemical analysis of sedimentary samples from these areas allows for the evaluation of these up-section changes in provenance from predominantly recycled to recycled and arc and then back to a recycled provenance. Additionally, the application of the Index of Compositional Variability, as well as trace element abundance and ratios, provides insight into provenance. Geochemical analysis of the samples has been completed using XRF analysis for major elements and LA-ICP-MS for trace elements. The major element and trace element composition of each of the samples is being aggregated and interpreted. The trace-element and major-element data will enable the determination of sediment source areas by giving a broad sense of the various rock types eroding to form the sediments we are analyzing. That data can then be compared and contrasted with the detrital zircon U-Pb dating data obtained from similar samples. This research will give insight into the source areas of the sediments, provide a more comprehensive provenance model, and give a broad sense of the regional tectonic evolution of southwestern Montana during the Cretaceous.