Testing of the late-ordovician Pre-GICE warm water carbonate hypothesis in Alabama

Author

Brandon Euker, James Madison UniversityFollow
Stacey Law, James Madison UniversityFollow

Creative Commons License

This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Date of Graduation

Spring 2016

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Department of Geology and Environmental Science

Advisor(s)

Stephen Leslie

Steve J. Baedke

John Haynes

Abstract

The Guttenberg Carbon Isotope Excursion (GICE) (uppermost Sandbian-lower Katian, Late Ordovician) has been suggested to represent the transition from a Cambrian-Ordovician greenhouse world to Late Ordovician icehouse world. This transition is thought to coincide with a proposed shift from deposition of warm water carbonate rocks to cool water carbonate rocks in the North American midcontinent. We used oxygen isotopes (d¹⁸O) of conodonts to test the idea that the rocks below the GICE interval represent a consistently warm environment. Conodonts were isolated from samples of the Chickamauga Group collected at the Tidwell Hollow section in Blount County, AL, from the Red Mountain Expressway section in Birmingham, AL, and from Dickeyville, WI. These successions underlie strata that biostratigraphically correlate with the GICE interval. Conodonts at Tidwell Hollow include Appalachignathus sp. and Phragmodus flexuosus near the base of the Chickamauga Group and Oulodus oregonia and Aphelognathus kimmswickensis near its top. These taxa indicate that the interval studied ranges from the Plectodina aculeata Zone to at least the Plectodina tenuis Zone, spanning from below and through the GICE. Conodonts from the Red Mountain Expressway section were collected only to 22m below the Millbrig K-bentonite, where Belodina compressa and Curtognathus spp. dominate the fauna. At 10m below the Millbrig, Polyplacognathus ramosus occurs and the top of the section contains a fauna that includes Phragmodus undatus and Icriodella superba. Results indicate that the sampled interval ranges from the Belodina compressa Zone to the Phragmodus undatus Zone, an interval which is below and up to the GICE. The conodont d¹⁸O values indicate that the rocks below the GICE in Alabama represent a consistently warm environment throughout the Chickamauga Group below the Millbrig K-bentonite Bed, supporting the idea that the pre-GICE environment was relatively stable, and that the GICE, along with the changes in d¹⁸O in the GICE interval, could reflect a perturbation in the Ordovician climate.

Recommended Citation

Euker, Brandon and Law, Stacey, "Testing of the late-ordovician Pre-GICE warm water carbonate hypothesis in Alabama" (2016). Senior Honors Projects, 2010-2019. 142.
https://commons.lib.jmu.edu/honors201019/142