Year of Award

2012

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Marc S. Hendrix

Commitee Members

Rebecca Bendick, George Stanley, James Sears, Leonid Kalachev

Keywords

Eocene/Oligocene Transition, Gravitational Potential Energy, Paleobotany, Paleoclimate, Renova Formation, Southwestern Montana

Abstract

The Renova Formation of southwestern Montana contains an important record of Paleogene floral, faunal, climate and tectonic change in the northern Rocky Mountains. The period between the end of the early Eocene and the early Oligocene (~49-32 Ma) was a time of rapid and far-reaching climate change. This period saw the end of global greenhouse climate and the establishment of icehouse conditions across the Earth. These changes led to profound alterations in both marine and terrestrial ecosystems. This study examines the late Eocene/early Oligocene history of the northern Rocky Mountains by means of an integrated study of the sedimentology, tectonics and fossil content of the Renova Formation.

The first part of this study examines plant fossils found in the Renova Formation in order to examine changes in the composition of the vegetation across the late Eocene/ early Oligocene (E/O) boundary. Plant remains are an effective proxy for climate and are used to estimate multiple climatic parameters across the E/O boundary. The second part of this study examines the paleotopography and paleodrainage patterns of the basins which accumulated the Renova sediments. This is accomplished by a combination of sedimentary facies and detrital zircon analysis. The third part of this study examines the tectonic underpinnings of Paleogene southwestern Montana through a combination of geologic field work and geodynamic modeling.

The results of this study indicate that a seasonal summer dry climate became established in the northern Rocky Mountains by early Oligocene time. This is indicated by the elimination of subtropical plant species, the establishment of dry-adapted species and by paleoclimate parameters calculated from leaf physiognomy. Geodynamic calculations and field data indicate that the Renova Formation was deposited in a series of sub-basins separated by relict paleotopography and inverted topography formed by contemporary lava flows. Normal faulting was not active until the middle Miocene initiation of regional extension. Accommodation space for the deposition of Renova sediments was formed primarily by differential erosion of pre-middle Eocene rocks. Climate change and influx rates of volcaniclastic sediment were also important controls on the evolution of the intermontane basins of southwestern Montana.

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© Copyright 2012 Kevin John Lielke