Year of Award

2024

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Geosciences

Committee Chair

Marc S. Hendrix

Committee Co-chair

Michael H. Hofmann

Commitee Members

Marc S. Hendrix, Michael H. Hofmann, Kent D. Sugden

Keywords

Provenance, Quartz Typing, Cathodoluminescence, Powder River Basin

Publisher

University of Montana

Subject Categories

Geology

Abstract

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 Sandstone 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, I present results from a comprehensive scanning electron microscope panchromatic cathodoluminescence (SEM-PCL) analysis of >1200 individual sand-sized quartz grains from core and outcrop across the PRB. I combined SEM-PCL analysis with classic petrographic QFL investigation to provide insights into the provenance of these contemporaneous units. Along with this, a comprehensive diagenetic study of the Turner Sandstone was performed using numerous analytical techniques to evaluate present phases and relative timing of formation. Results show a clear demarcation between quartz types observed in the Frontier Formation (west) and quartz types in the Turner Sandstone (east). The Frontier Formation comprises 70% quartz with an abundance of microfractures (Type I quartz), 15% monocrystalline quartz, commonly zoned, and without any notable fractures (Type II quartz), and 15% polycrystalline quartz with undulous extinction (Type III quartz). In contrast, the Turner Sandstone samples comprise, on average, 22% Type I quartz, 55% Type II quartz, and 23% Type III quartz. However, when separated by stratigraphic position, samples from the upper Turner almost uniquely contain type II quartz (up to 71%), while the rest of the Turner comprises a mix of quartz that is near the average. My preferred interpretation, supported by additional field data and literature, links these contrasting quartz-type distributions to distinctly different source areas for the Turner and the Frontier Formations. The Turner is likely sourced from reworked sediments being eroded from lower Cretaceous deposits in the Black Hills of South Dakota. Results from the diagenetic study show a high abundance of quartz overgrowths, carbonate cement phases, and grain coating clays. My preferred interpretation is that these events occurred early during the initial burial process of the Turner Sandstone. 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, I demonstrate the usefulness of an often overlooked and easy-to-use method in provenance studies.

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