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

Three 3^rd-order depositional sequences (~1-2 My) are documented worldwide establishing their origins related to eustatic sea-level changes. Because the rates of 3^rd-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) 3^rd-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 3^rd-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.