PETROGRAPHY AND GEOCHEMISTRY OF CONTINENTAL CARBONATES IN SOUTH TEXAS: IMPLICATIONS FOR OLIGOCENE-MIOCENE PALEOCLIMATE AND PALEOENVIRONMENT NEAR SEA LEVEL

Author

Conan Ray Godfrey

Graduation Semester and Year

2017

Language

English

Document Type

Thesis

Degree Name

Master of Science in Earth and Environmental Science

Department

Earth and Environmental Sciences

First Advisor

Majie Fan

Second Advisor

Ashley Griffith

Abstract

The Oligocene-Miocene Catahoula, Oakville, and Goliad formations in south Texas contain abundant continental carbonates which are useful in reconstructing paleoclimate and paleoenvironment in a near sea level region. Field observations and thin section characterizations identify three types of pedogenic carbonates, including rhizoliths, carbonate nodules, and platy horizons, and two types of groundwater carbonates, including cemented beds and carbonate concretions. The pedogenic carbonates display sharp upper contacts and diffuse lower contacts, and contain soil structural elements and biogenic elements related to root action. Under microscope, the pedogenic carbonates are micritic with clotted micrite, micrite laminations, and alveolar-septal structures and display dull cathodoluminescence. The groundwater carbonates display sharp lower contacts and preferential cementation within permeable units, and lack pedogenic profiles that destroy original beddings. Under microscope, the groundwater carbonates contain equant, blocky and sometime drusy and poikilotopic spars, and the spars often display dull to bright orange concentrically-zoned cathodoluminescence. Based on the preservation of microfabrics and the variabilities of carbon and oxygen stable isotopic compostion, we suggest that all the studied carbonates experienced minimal diagenesis, and their isotopic compositions reflect paleoclimate and paleoenviornment in south Texas during the Oligocene and Miocene. The δ18O values of the continental carbonates decreased ~3 ‰ after the earliest Oligocene, which is interpretted to reflect at least a 6° C drop in mean annual temperature in south Texas. The δ13C values of the pedogenic carbonates increased ~4.4 ‰ from the late Oligocene to the middle Miocene, suggesting an expansion of C4 plants in south Texas as early as the early Miocene. This early Miocene expansion of C4 plants is ~16 myr older than what has been documented in Asia, possibly as a result of local climate change rather than global climate change.

Keywords

Pedogenic carbonates, groundwater carbonates, paleoclimate, paleoenvironment

Disciplines

Earth Sciences | Physical Sciences and Mathematics

License

This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 4.0 International License.

Comments

Degree granted by The University of Texas at Arlington

Recommended Citation

Godfrey, Conan Ray, "PETROGRAPHY AND GEOCHEMISTRY OF CONTINENTAL CARBONATES IN SOUTH TEXAS: IMPLICATIONS FOR OLIGOCENE-MIOCENE PALEOCLIMATE AND PALEOENVIRONMENT NEAR SEA LEVEL" (2017). Earth & Environmental Sciences Theses. 180.
https://mavmatrix.uta.edu/ees_theses/180