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Interpreting the Origins of Early Triassic (Smithian) 3rd-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 3rd-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 3rd-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 3rd-order eustatic sea-level changes are not well understood.

This project focuses on understanding the origins of 3rd-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 3rd-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 3rd-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