Date of Graduation

5-2021

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor

Glenn Sharman

Committee Member

Celina Suarez

Second Committee Member

John Shaw

Keywords

Chemostratigraphy, end-Triassic extinction, Moenave Formation, Sedimentary geology, Triassic-Jurassic Boundary

Abstract

The end Triassic extinction (ETE) is one of the largest mass extinction events of the Phanerozoic, and it has been hypothesized that this event, as well as the Triassic-Jurassic (TJ) boundary, are preserved within the Moenave Formation of the Colorado Plateau. Identification of this boundary within southwest Utah sections of the Colorado Plateau region is critical for better understanding the relationship between climate change and the ETE in terrestrial, low latitude paleoenvironments. The Moenave Formation is well exposed in Blacks Canyon of Zion National Park and in the nearby Warner Valley, where detailed sedimentologic observations and carbon isotope sampling have been conducted. Chemostratigraphic correlation of the bulk organic carbon (δ13Corg) (this study) with carbonate carbon (δ13Ccarbonate) and oxygen isotope (δ18Ocarbonate) chemostratigraphy, detrital zircon U-Pb geochronology, and biostratigraphy provide new constraints on the location of the Triassic—Jurassic boundary and the ETE within the Moenave Formation. This study identifies three negative δ13Corg excursions (NCIE) within the Moenave Formation of Blacks Canyon. The first represents the “initial NCIE” at ~36 m above the base of the Dinosaur Canyon Member (DCM) with a magnitude of ~ -5 ‰, a second NCIE with a magnitude of ~ -6.8 ‰ occurs at ~47 m above the base of the DCM, and the third NCIE has been correlated to the “main NCIE” in the WPM with a magnitude of ~ -5.9 ‰ at ~69 m above the base of the formation. The onset of the ETE is associated with the initial NCIE while the TJ boundary is correlated to the main NCIE. Therefore, these δ13Corg data, in conjunction with detrital zircon U-Pb age constrains from chemical abrasion-thermal ionization mass spectrometry (CA-TIMS), suggest that the ETE occurs in the middle DCM, ~24-36 m above the base of the formation, and the TJB occurs higher in the section in the upper DCM or Whitmore Point Member (WPM). The locations of these boundary events and field- and drone-based observations and measurements are used to quantify the impact of climatic warming across the Late Triassic-Early Jurassic boundary by relating sedimentologic observations to paleohydrologic processes. Observations indicate a trend toward upwards-increasing and then decreasing channelization in the fluvial DCM, with an increase in median and P95 grain size, and possibly fluvial channel depth, near the onset of the ETE. These data are interpreted to reflect an increase in precipitation during, and possibly before, the ETE that ultimately culminated in formation of Lake Dixie, manifested as lacustrine facies of the Whitmore Point Member. This research provides the first direct evidence for the preservation of the ETE within the Moenave Formation and provides insight into the sedimentologic and hydrologic response to the ETE within a low-latitude, arid environment.

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