Using δ18O of Conodont Apatite and Sequence Stratigraphy to Understand Early Triassic (Smithian) Sea-Level Change

The Early Triassic climate is conventionally interpreted to have been warm and ice-free. During this time, three globally recognized depositional sequences developed in response to My-scale eustatic sea-level changes. The rates of My-scale sea-level rise and fall are too fast to attribute to changes in mid-ocean ridge activity and too slow to attribute to typical ~20-400 ky orbital cycles that drive glacio-eustasy. Previous studies in the Middle Devonian, Late Cretaceous, and Middle Eocene greenhouse climates have suggested that significant glacio-eustatic sea-level changes were responsible for sequence development. This suggests that these particular greenhouse periods were not uniformly warm and ice-free. We are testing the hypothesis that My- and orbital-scale sea-level changes in the Early Triassic (Smithian) were driven by glacio- and/or thermo-eustasy. To test this hypothesis, Smithian marine successions from two localities in the western United States (Lower Thaynes Formation) were described on a bed-by-bed basis to provide facies and depositional environment interpretations, as well as put the sections into a sequence stratigraphic framework. Samples were collected from both locations for high-resolution (~1-10 m) oxygen isotopic analysis of conodont apatite. Conodont elements are excellent biostratigraphic indicators and the apatite is less susceptible to diagenetic alteration than carbonate minerals, making conodont apatite a reliable proxy for determining changes in ice volume and seawater temperatures in deep time. In northeastern Utah (Weber Canyon), the Smithian sequence (~240 m) is composed of a mixed carbonate-siliciclastic lowstand systems tract (>40 m) and transgressive systems tract (~110 m), a black shale maximum flooding zone (~15 m), and a carbonate-dominated highstand systems tract (~75 m). In western Utah (Confusion Range), the sequence is composed of a coarse-grained, carbonate-dominated transgressive systems tract (>40 m) and a mixed siliciclastic-carbonate highstand systems tract (>60 m). Sequences at both locations display high-frequency (104-105 yr), upward-shallowing, subtidal cycles whose subtle facies changes indicate only minor sea-level changes, which is to be expected given the interpreted warm paleoclimate. The high-frequency, subtidal cycles are not present in the Weber Canyon maximum flooding zone because during that time the platform was too deep to record the effects of high-frequency sea-level changes. If our climatically controlled My- and orbital-scale sea-level hypothesis is correct, then δ18O values should decrease within transgressive and maximum flooding intervals and increase and peak within highstand/lowstand intervals and the conventional interpretation of a uniformly warm Early Triassic climate must be re-evaluated.