Date of Award
December 2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Geology
First Advisor
Sven Egenhof
Abstract
Mudstones are a major component of red-bed unit deposition in western North America during the Permo-Triassic but have remained understudied due to their lack of economic importance and general lack of attention toward the redbeds themselves. Analysis of fine-grained sediments—including depositional features and fossil content—is crucial for resolving questions regarding the accumulation of such red beds but has remained underutilized. This dissertation thus focuses on red-bed deposits from the Permo-Triassic transition in western Laurentia. The first chapter focuses on the Torrey Member of the Moenkopi Formation, which represents the concurrent evolution of three deltaic systems in present-day Utah. Analysis of the mudstone revealed that deposition in the intertidal zone is defined by suspension settlement, whereas bedload transport dominates in the distal delta. Supratidal mudstone is often “cannibalized” from the intertidal zone or is deposited by flooding of the fluvial system. The vertical distribution of mudstone facies, combined with detailed stratigraphic correlations of unit outcrops, demonstrates that the deposition of the Torrey Member occurred in a single sequence.
The second chapter is a thorough redescription of the Permo-Triassic Spearfish Formation in the Williston Basin. The analysis of mudstone and siltstone sections in the unit presented direct fossil evidence—in the form of Tasmanites—that sediments of the Spearfish Formation record the cyclical growth and evaporation of an epeiric sea with high sediment runoff from the continent. This results in the accumulation of strata that superficially resemble continental deposits, highlighting the dangers of assuming depositional settings based solely on unit color. Fossil content also suggests that deposition occurred during the Late Permian throughout the study area. The Permo-Triassic boundary is placed on stratigraphic and paleontological grounds at the base of a salt bed two-thirds above the unit’s lower contact.
The final study presents a description of various rip-up clasts in low-color-contrast settings, primarily from the Moenkopi and Spearfish formations, supplemented by black shale deposits of the Bakken and Alum formations. These findings revealed that rip-up clasts are more abundant than previously believed and identify a new clast habit that is most strongly associated with intertidal and supratidal mudstones.
Recommended Citation
Zallum, Ibrahim, "Mudstone Deposition From Continental To Shallow-Marine Realms— Examples From The Triassic Of Western North America" (2025). Theses and Dissertations. 8252.
https://commons.und.edu/theses/8252