354-2 The challenge of telling geologic time in the aftermath of the greatest mass extinction on the planet: Using carbon and strontium isotopes to refine age estimates of the Lower Thaynes Formation (Early Triassic) in the Great Basin region, western USA

Abstract:

The Permian-Triassic (PT) interval has been well-studied to understand the cause and effect of a mass extinction, but determining the age and tempo of bioevents presents challenges in the aftermath of a mass extinction without good age control. Without biostratigraphic control or the presence of datable radiometric materials, age estimates of some PT intervals may be possible using chemostratigraphy when compared to well-calibrated sections across the globe. Geochemical data must be thoroughly interrogated for diagenetic overprints prior to making chemostratigraphic correlations. Uncertainties due to alteration can be assessed with multiple kinds of geochemical data measured from various materials that behave differently during diagenesis.

            In this study we measured stable carbon (d¹³C) and strontium (⁸⁷Sr/⁸⁶Sr) isotopes from carbonate rock and conodonts preserved in the Great Basin region, western USA. We studied the Permian Gerster Limestone and Triassic Thaynes Fm interval in the Confusion Range (UT), Palomino Ridge (NV), and at Spruce Mountain (NV) where we sampled for d¹³C (n = 182) and ⁸⁷Sr/⁸⁶Sr (n = 115) to characterize the geochemical record and determine how complete the PT interval is here. The basal Thaynes Fm contains a unique succession in the Confusion Range, not found elsewhere in the region, that hosts peritidal carbonates with features characteristic of shallow water conditions (tepees, oncoids, fenestral fabrics), indicative of possible diagenesis and erosion. Based on petrographic analysis, trace elemental concentrations, and comparison with published isotopic records, we developed an age model that suggests that approximately 1.5 Myr of time is missing here from the earliest Triassic (252–250.5 Ma). This omits the entire Induan and lowermost Smithian Stages from the geologic record, consistent with prior paleontological work. Deposition resumes to capture a large Smithian negative d¹³C excursion (d¹³C = -4‰, known as N3) before being capped by a large positive d¹³C excursion at the Smithian-Spathian boundary (d¹³C = 8‰, known as P3). This age refinement helps to answer questions about how (in)complete the PT record is in the Great Basin region, but now poses new questions such as what explains such a large d¹³C excursion (8‰) relative to other Tethyan sections?

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