Graduation Semester and Year
Spring 2026
Language
English
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Earth and Environmental Science
Department
Earth and Environmental Sciences
First Advisor
Majie Fan
Second Advisor
Nathan D. Brown
Third Advisor
Timothy M. Shanahan
Fourth Advisor
Lin Li
Fifth Advisor
Un-Jung Kim
Abstract
The Eocene-Oligocene period in the North American Cordillera represents a period of great intrigue with complex interactions among tectonics, climate, and surface processes. Although this period is generally associated with synorogenic extension associated with gravitational collapse of the Cordillera, the paleotopographic history remains debated. Existing studies offer conflicting interpretations, suggesting either sustained elevations or modest uplift during this time. Notably, recent reconstructions based on volcanic glass δD values have proposed renewed uplift during the early Oligocene, which needs to be tested with a more extensive and robust dataset. Concurrently, global cooling and aridification during this interval may have been amplified by orographic blocking of moisture, promoting development of a Paleogene loess system in the western United States, the extent of which remains poorly constrained. This study integrates lithofacies analysis, hydrated volcanic glass δD data from tuffaceous deposits, zircon U–Pb geochronology, grain-size data, and quartz surface microtexture analysis to understand regional paleoelevation and sediment transport processes in the Sage Creek Basin, southwestern Montana. Our new data show that extended HF pretreatment of volcanic glass does not alter δD values but effectively removes the vii influence of clay alteration and recycled glass. The new δD results do not support the previously proposed early Oligocene surface uplift. The results also reveal significant eolian activity in a mixed fluvial–eolian system in the basin started in the late middle Eocene (48–43 Ma). Constraining paleoelevation and paleoclimate in the North American Cordillera requires more reliable proxies. Leaf waxes are widespread in sedimentary systems and have been successfully applied in other mountain belts and plateaus. However, their application in the North American Cordillera requires a detailed understanding of plant water-use strategies as well as the relationship between leaf wax δD values and elevation. Therefore, I also investigate modern leaf wax and leaf water δD values along two transects in Montana and adjacent regions. The results show that most plants in the study area primarily use groundwater recharged by winter precipitation or high-elevation runoff during hot, dry summers. The study also reports a common biosynthetic fractionation factor (-148 ± 25‰) and an apparent fractionation factor (-79 ± 29‰) for broadleaves, forbs, shrubs, and pines. Grass, however, shows large variability in fractionation factors, with a subset utilizing summer precipitation as a water source. Plant water-use strategies and thw mixing of different vapor sources across the transects lead to a decoupling between leaf wax δD values and elevation, resulting in large uncertainties (~1.5 km) in paleoelevation reconstructions.
Keywords
Eocene-Oligocene transition, North American Cordillera, Volcanic glass hydrogen isotopes, Loess, Grain size analysis, Quartz surface microtexture, Plant source water, Plant leaf wax, Soil leaf wax, Surface water
Disciplines
Geochemistry | Geology | Sedimentology | Stratigraphy | Tectonics and Structure | Water Resource Management
License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Recommended Citation
Nandy, Rijumon, "CONSTRAINING THE TECTONO-CLIMATIC EVOLUTION OF THE NORTHERN ROCKY MOUNTAINS: INSIGHTS FROM VOLCANIC GLASS, LEAF WATER, AND LEAF WAX HYDROGEN ISOTOPES" (2026). Earth & Environmental Sciences Dissertations. 1.
https://mavmatrix.uta.edu/ees_dissertations2/1
Included in
Geochemistry Commons, Geology Commons, Sedimentology Commons, Stratigraphy Commons, Tectonics and Structure Commons, Water Resource Management Commons