Interpreting the Origins of Early Triassic (Smithian) 3^rd-Order (My-scale) Sea-Level Changes: Coupling δ¹⁸O Analyses with Sequence Stratigraphy

Stephanie Yurchyk
University of New Mexico, Department of Earth and Planetary Sciences Albuquerque, New Mexico; [email protected]

Three 3^rd-order (My-scale) sequences are documented for the Early Triassic in the Western Interior of the United States, South China Block, the Canadian Arctic, and Pakistan, which establishes their eustatic sea-level origins. The rates of 3^rd-order transgressions and regressions are too fast to attribute to tectono-eustasy. On the other hand, the rates are too slow for the typical ~20-400 ky orbital cycles driving glacio-eustasy. To date, the origins of these 3^rd-order eustatic sea-level changes are not well understood.

This project focuses on understanding the origins of 3^rd-order sea-level changes in the Western and Central Utah Lower Triassic (Smithian) Thaynes Formation. Two stratigraphic sections were measured and described on a bed-by-bed basis for facies analysis, depositional and sequence stratigraphic interpretations. Upward shallowing subtidal parasequences are found within each of the larger 3^rd-order sequences. The roughly symmetric transgressive and highstand systems tracts are carbonate dominated and the maximum flooding zone is shale dominated.

Samples were collected for δ¹⁸O analysis of conodont apatite collected. Apatite is used for the δ¹⁸O analyses because, unlike calcite, it is very resistant to diagenetic alteration. Chemical analysis, when paired with the sequence stratigraphic interpretations, may provide information about whether or not that Early Triassic 3^rd-order sea-level changes are climatically driven by glacio- and/or thermo-eustasy driven by long-period orbital cycles (~1.2-2.5 My-scale). If they are climatically controlled, then δ¹⁸O values should decrease within transgressive facies and increase within regressive facies.

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid