QUARTZ SOURCE DETERMINATION USING PANCHROMATIC CATHODOLUMINESCENCE AND PETROGRAPHY - A USEFUL TOOL IN PROVENANCE ANALYSIS

Provenance analysis of sedimentary rocks is an essential tool for basin analysis and paleogeographic reconstructions. Numerous tools and analytical methods have been developed over the years to investigate the source of detrital constituents in sedimentary rocks. The Late Cretaceous (Turonian) Turner Formation in the eastern Powder River Basin (PRB), WY and the time equivalent Frontier Formation in the western PRB are commonly described as the remnants of a marginal marine depositional system. However, contradictory evidence exists about the source of those deposits.

Here, we present results from a comprehensive scanning electron microscope panchromatic cathodoluminescence (SEM-PCL) analysis of >1000 individual sand-sized quartz grains from core and outcrop across the PRB. We combined SEM-PCL analysis with classic QFL investigation to provide insights on the provenance of these contemporaneous units.

Our results show a clear demarcation between quartz types observed in the Frontier Formation and quartz types in the Turner Formation. The Frontier Formation comprises 70% quartz with an abundance of microfractures (Type A quartz), 15% monocrystalline quartz, commonly zoned, and without any notable fractures (Type B quartz), and 15% polycrystalline quartz with undulous extinction (Type C quartz). In contrast, the Turner Formation samples comprise, on average, 22% Type A quartz, 55% Type B quartz, and 23% Type C quartz. However, when separated by stratigraphic position, samples from the upper Turner almost uniquely contain type B quartz, while the rest of the Turner comprises a mix of quartz that is near the average.

Our preferred interpretation, also supported by additional field data, links these contrasting quartz type distributions to distinctly different source areas for the Turner and the Frontier Formations.

Impacts from this study are intended to assist with paleogeographic reconstructions during the critical phase of basin development along the evolving Rocky Mountains and help shed light on a longstanding question about the depositional relationship between these two formations. Further, they can educate basin modeling efforts, and provide a better understanding of diagenetic pathways. Lastly, we demonstrate the usefulness of an often overlooked and easy to use method in provenance studies.