2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 7
Presentation Time: 8:00 AM-6:00 PM

Sequence Stratigraphy of the Lower Triassic (Smithian) Thaynes Formation: Framework for Understanding My-Scale Paleoclimate Change

YURCHYK, Stephanie1, ELRICK, Maya2 and ATUDOREI, Viorel1, (1)Earth and Planetary Sciences, University of New Mexico, MSC03 2040, 1 University of New Mexico, Albuquerque, NM 87131, (2)Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, syurchyk@unm.edu

Three 3rd-order depositional sequences (~1-2 My) are documented worldwide establishing their origins related to eustatic sea-level changes. Because the rates of 3rd-order sea-level rise/fall are too fast to attribute to changes in MOR activity and are too slow for typical ~20-400 ky orbital variations, we hypothesize that the My-scale sea-level changes were the result of long-period (~1.2-2.5 My) orbital variations driving glacio-eustasy and/or thermal eustasy. To test this hypothesis, Lower Triassic (Smithian) 3rd-order sequences in the western U.S. were measured and described on a bed-by-bed basis for facies analysis, depositional and sequence stratigraphic interpretations. At these relatively offshore localities, the transgressive and highstand systems tract are carbonate dominated and contain high-frequency subtidal cycles. Maximum flooding zones are shale dominated and lack high-frequency cycles.

Samples were collected for analyzing the δ18O values of conodont apatite to evaluate potential paleoclimate changes in a presumed greenhouse climate. Apatite is used because, unlike calcite, it is very resistant to diagenetic alteration. If the climatically controlled 3rd-order sea-level hypothesis is correct, then δ18O values should decrease within transgressive and maximum flooding intervals and increase and peak within highstand/lowstand intervals.