Date of Award
2025
Document Type
Thesis (Master's)
Department or Program
Earth Sciences
First Advisor
Sarah P. Slotznick
Abstract
The Belt Supergroup of the northwestern United States is one of the most extensive Mesoproterozoic sedimentary successions in North America. Although certain stratigraphic units have been well studied, correlations across the entire outcrop region remain unresolved. This project uses magnetostratigraphy and lithostratigraphy to investigate a potential lateral equivalency between the 1.4 billion-year-old (Ga) Appekunny Formation (Fm.) in Glacier National Park, MT and the 1.4 Ga Greyson Fm. in the Helena Embayment, MT. Both formations consist predominantly of shallow-water mudstones and contain the only North American occurrences of Horodyskia moniliformis, one of the oldest known macrofossils. Paleomagnetic analyses were conducted on two mineralogically distinct sample populations as confirmed through rock magnetism: magnetite-bearing green and gray lithologies and hematite-bearing red beds and carbonates. Magnetite-bearing lithologies dominate both formations, but did not yield a coherent result beyond that of the present local field. Hematite-bearing beds appear intermittently within the stratigraphy, but revealed more coherent and fully demagnetized data. Combining a subset of coherent paleomagnetic directions from the Greyson Fm. and Appekunny Fm. indicates a normal magnetic polarity (225.83!/47.21!, ↵95: 17.50, n = 50), matching previous data from the overlying Spokane and Grinnell Fms. Additional paleomagnetic analyses on the Newland Fm. (underlying the Greyson Fm.) also found a normal magnetic polarity (237.17!/53.63!, ↵95: 23.75, n = 8). Meanwhile, the red beds from the lowest-sampled red bed layer of the Appekunny Fm. record a reversed magnetic polarity (052.94!/-41.49!, ↵95: 22.56, n = 5). While the lateral equivalency of the Appekunny and Greyson Fms. remains uncertain from both paleomagnetic and stratigraphic perspectives, the detection of a magnetic reversal in the lower Appekunny Fm.—but not elsewhere—suggests a potential temporal discontinuity between the two units. This study also expands our understanding of the Greyson Fm. by documenting a new stratigraphic section at Smith River, Montana, where a new occurrence of Horodyskia moniliformis was found. Further study of these exceptionally well-preserved Mesoproterozoic units is vital to understanding early eukaryotic evolution and the origins of multicellular life, and our successful identification of magnetically reliable lithologies provides a foundation for further magnetostratigraphic and paleomagnetic studies.
Recommended Citation
Moehl, Olivia C., "Paleomagnetic Assessment of the Appekunny and Greyson Fms., Lower Belt Supergroup, Montana, USA." (2025). Dartmouth College Master’s Theses. 242.
https://digitalcommons.dartmouth.edu/masters_theses/242
