Semester of Graduation
Master of Science (MS)
Geology and Geophysics
Widespread deposition of phosphate and a transition from tropical to cool water carbonates is seen throughout Middle-Upper Ordovician Nashville Dome area spanning the M4/M5 sequence boundary. Hypotheses explaining the onset of these lithologic changes have included the onset of glaciation, drawdown of CO2 related to the Guttenburg Isotope Carbon Excursion (GICE), and siliciclastic weathering, yet none of these studies have provided definitive evidence that shows any of these are the sole factor driving paleoenvironmental changes across the M4/M5 boundary. In order to test whether rapid subsidence along the southeastern margin of Laurentia may have caused an influx of phosphate and shift in carbonate facies, a multi-pronged approach was applied using REE and minor and trace elemental geochemistry to apatitic fossils or “steinkerns” and petrographic analysis on samples collected from a Middle-Upper Ordovician limestone outcrop located in Tidwell Hollow, Alabama. Elemental concentrations collected using LA-ICP-MS were normalized to the upper continental crust (UCC). Although the REE’s were found to be diagenetically overprinted, minor elements U and Th, as well as Y, were used to aid in interpreting paleo redox conditions. Based on geochemical and petrographic evidence, various stages of carbonate ramp development have been interpreted including a transition from peritidal to deep ramp environment before reverting to proximal shelf facies. These findings highlight the susceptibility of REE’s to diagenesis and the value of U and Th as paleo redox proxies. Also, the findings of this study carry implications for local paleoenvironmental changes and future placement of this area into a regional sequence stratigraphic framework.
Villanueva, Rafael Adrian, "Rare Earth Geochemistry of Apatitic Fossils from the Middle-Upper Ordovician Southern Margin of Laurentia" (2019). LSU Master's Theses. 4961.
Available for download on Monday, May 30, 2022