Identifier

etd-02292016-141246

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

The Mohawkian series of the Middle Late Ordovician was a time of great lithological and faunal changes that coincide with a positive ä13C excursion known as the Guttenberg Isotopic Carbon Excursion (GICE). Two prevailing hypotheses have been proposed to explain the cause of these changes. The “cold-water” hypothesis suggests that global cooling caused the tropical belt to shrink and cold water to invade the epicontinental sea while the “tectonic-forcing” hypothesis suggests that increased weathering rates led to basinal deepening, resulting in cooler oceanic waters invading the midcontinent sea. This study tests if regional tectonics caused by the Taconic Orogeny led to an invasion of cool ocean water into the epicontinental sea using rare earth elements (REEs). Additionally, this research analyzes the potential REE enrichment sources to determine the level of diagenesis and to better determine paleoceanographic conditions. The hypothesis of this research states that REE values from the Dickeyville, Wisconsin section will exhibit a shift through time from “HREE-depleted” patterns to “HREE enriched” patterns. The “HREE-depleted” pattern would indicate that the shallow carbonate platform experienced LREE and MREE enrichment due to sediment deposition dominating the seawater signal. The transition to a “HREE enriched” REE pattern would coincide with a decrease in sediment deposition as basinal deepening occurs and seawater became the dominant REE enrichment source. Rare earth element concentrations were measured using an LA-ICP-MS on four conodont species prevalent in Dickeyville, WI with attention to the three types conodont tissues as well. The Dickeyville conodonts experienced REE enrichment from high terrigenous influx, which overprinted the “true” seawater REE signal. Additionally, REE profiles indicate that K-bentonite deposits played a role in REE enrichment in the stratigraphically associated intervals and that sediment from a mix of sources is the primary source for REEs with occasional inputs from weathered magmas originating from the Transcontinental Arch and Precambrian shield. Due to the terrigenous overprinting and ashbed REE enrichment, the redox and oceanographic changes at the Dickeyville section could not be definitively interpreted. Therefore, REE analysis from the Dickeyville section does not indicate that tectonics played a role in the Upper Ordovician changes.

Date

2015

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Herrmann, Achim

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