Date of Graduation
12-2023
Document Type
Thesis
Degree Name
Master of Science in Geology (MS)
Degree Level
Graduate
Department
Geosciences
Advisor/Mentor
Sharman, Glenn R.
Committee Member
Suarez, Celina A.
Second Committee Member
Dumond, Gregory
Keywords
dating techniques; sedimentation rate
Abstract
The “law of detrital zircons” states that a sedimentary unit cannot be older than its youngest detrital zircon. The youngest U-Pb date(s) from detrital zircon may thus be used to infer a given sedimentary deposit’s maximum depositional age (MDA). MDAs may be calculated in various ways using dates from zircon U-Pb geochronology. Common dating methods used for U-Pb geochronology are laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS), a low- and high-resolution instrument type, respectively. Chapter 2 will systematically compare paired LA-ICP-MS and CA-ID-TIMS dates from the same zircon grains to assess the agreement between these two methods. Previous case studies have noted that U-Pb dates acquired via both dating methods from the same zircon crystals are often different and can result in MDA calculations that are not within uncertainty of one another. This work is motivated by the growing number of MDA and detrital zircon studies since the 1980s. Chapter 3 will consist of a case study where paired LA-ICP-MS and CA-ID-TIMS dates are applied to the Elliot Formation of South Africa with the goal of better constraining the stratigraphic placement of the end-Triassic mass-extinction (ETE) and Triassic-Jurassic boundary (TJB). In Chapter 2, I investigate the degree of agreement between zircon U-Pb dates analyzed via LA-ICP-MS and CA-ID-TIMS by compiling data from published literature. I compiled a database of 49 publications with 1528 paired LA-ICP-MS and CA-ID-TIMS analyses from 16 laboratories. The database shows LA-ICP-MS 206Pb/238U dates to be systematically younger than the corresponding CA-ID-TIMS dates, with the mode of the age offset distribution shifted by ~2-2.5%. This shift hints at the possibility of “matrix effects”, Pb loss, and/or inheritance. Chapter 3 focuses on the Elliot Formation in South Africa, a paleontologically significant Triassic-Jurassic fluvial system. Unlike the overlying Clarens Formation, the upper section of the Elliot Formation is taxonomically diverse. The lower section of the Elliot Formation is poorly sampled and appears to be less diverse than the upper section. The precise age of the Elliot Formation, including the stratigraphic placement of the ETE, TJB, and Central Atlantic Magmatic Province (CAMP) is poorly constrained. This research will focus on the type section, Barkley Pass, of the Elliot Formation. A detrital zircon study of the Elliot Formation at Barkley Pass, the first of its kind at this locality, would constrain the stratigraphic position of the ETE and the TJB and place paleontological studies into a new chronologic framework. I present analysis of 18 detrital zircon samples from the Elliot Formation at Barkley Pass. I was able to constrain the MDA of the Elliot Formation to 208.80 ± 0.14 Ma near the base of the section and 194.65 ± 0.13 Ma near the top. I have found that the uppermost Lower Elliot Formation is Early Jurassic in age and not Late Triassic as previously thought, which is supported by preliminary C-isotope data placing the onset of the ETE to 154 m from the base of the Elliot Formation.
Citation
Howard, B. L. (2023). Maximum Depositional Age Analysis using Paired LA-ICP-MS and CA-ID-TIMS Detrital Zircon U-Pb Geochronology. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/5182
