Date of Award

Spring 2022

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

William C. Clyde

Second Advisor

Joel E. Johnson

Third Advisor

Julia G. Bryce

Abstract

This work focuses on two terrestrial sedimentary records in the western United States that record the ancient greenhouse interval of the early Paleogene (~ 55 million years ago). The Paleogene, and past warm periods in general, offer a means of understanding potential biotic and environmental response to future warming scenarios. Part One of this dissertation establishes a new stable isotope climate record from the Bighorn Basin of Wyoming that is tied to an extensive fossil mammal assemblage (Chapter 1). This new stable isotope record captures carbon isotope excursions (CIEs) associated with the early Eocene hyperthermals ETM2, H2, and potentially I1 and provides the first record of these hyperthermals from the fossil-rich Fifteenmile Creek area of the south-central Bighorn Basin. Previously identified pulses in mammal turnover and diversity in this area are linked with these global warming events for the first time. These results also highlight spatial variability in the magnitude of these CIEs, with the Fifteenmile Creek area recording smaller-magnitude CIEs than more northerly records in the same basin. Part Two of this dissertation focuses on the Sheep Pass Formation in eastern Nevada. Sheep Pass Formation sediments record an ancient high-elevation lake and fluvial system that provides insight into how high-elevation settings responded to warming in the past. The age of the Sheep Pass Formation remains poorly resolved so rock magnetic, paleomagnetic, and additional geochronological data were collected to develop a better-resolved chronostratigraphy and to further understand how freshwater carbonates acquire a remanent magnetization. Rock magnetic results (Chapter 2) indicate that the Sheep Pass Formation preserves a heterogeneous assemblage of magnetic minerals, including primary and secondary phases. Many samples indicate the presence of goethite and/or maghemite that likely form as alteration products, in addition to a (titano)magnetite remanence carrier likely associated with deposition. New paleomagnetic and detrital zircon U-Pb age constraints on the Sheep Pass Formation (Chapter 3) allow its developing geochemical and sedimentological records to be correlated to the geologic timescale. These data support previous age estimates and place the formation within the Late Cretaceous to middle Eocene interval. The paleomagnetic record from the Sheep Pass Formation records two magnetic polarities, though these directions are influenced by a magnetic overprint producing a noticeable eastern directional bias in the record. Detrital zircon analyses from Member C in the Sheep Pass Formation produce a population of ~62 Ma grains, providing a new maximum depositional age for that part of the formation. The preferred age model based on all the relevant chronostratigraphic constraints suggests that the Sheep Pass Formation ranges from ~ 72 Ma to ~49 Ma with Member B limited to the latest Cretaceous to mid-late Paleocene. Member E is correlated to the nearby Bridgerian Elderberry Quarry Local Fauna (~47.5 – 52 Ma, early-middle Eocene) and correlates to either Chron C22n (~49 Ma) or C23n (~51 Ma). Together, this correlation suggests that the K-Pg boundary would likely have occurred in Member B, early Eocene hyperthermals might be recorded in Member C, and the Early Eocene Climate Optimum (EECO) could be recorded in Member E.

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