Date of Graduation
9-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Geosciences (PhD)
Degree Level
Graduate
Department
Geosciences
Advisor/Mentor
Sharman, Glenn
Committee Member
Suarez, Celina
Second Committee Member
Huang, Xiao
Keywords
Detrital zircon; Geochronology; Maximum depositional age
Abstract
The Early to mid-Cretaceous marks a period of dynamic shifts in tectonism, depositional environments, climate, and biota across the mid-North American continent. However, despite their importance for understanding crucial biotic and geologic transitions during this period, nonmarine Early Cretaceous strata of the Western Interior Basin (WIB) remain poorly constrained geochronologically. Radiometric dating of syndepositional detrital minerals, such as ash-fall zircon, offers a solution to refining geochronologic frameworks. However, a magmatic lull in the Cordilleran magmatic arc during the Early Cretaceous limited the delivery of syndepositional zircon to the WIB, impeding such refinement. In this dissertation, I developed and tested new methods for enhancing syndepositional zircon recovery based on grain morphology and depositional settings within Early Cretaceous deposits of the WIB. I then applied tandem LA-ICP-MS and CA-TIMS zircon U-Pb geochronology to build a geochronologic framework for seven key Early Cretaceous sections within the WIB. In Chapter 3, I show that handpicking sharply faceted detrital zircon increases the likelihood of encountering first-cycle zircon and yields maximum depositional age estimates closer to the true depositional age. This method is evaluated through intra-sample comparisons of randomly selected versus handpicked zircon separates from 30 samples within and outside the WIB. Results show an ~9-fold increase in the abundance of syndepositional zircon in handpicked samples compared to randomly analyzed zircon. In Chapter 4, I evaluate the hypothesis that targeting fine-grained lithologies deposited in low-energy settings enhances the recovery of syndepositional zircon compared to coarse-grained samples. Results from 16 samples of the Early Cretaceous Cedar Mountain Formation indicate that fine-grained samples contain an ~8-fold increase in the abundance of syndepositional zircon, likely due to the better preservation of ash-fall zircon in low-energy depositional settings that lack fluvial reworking or dilution. In Chapter 5, I integrate the results from Chapter 3-4 by using detrital zircon CA-TIMS U-Pb dates and other geochronological constraints to construct Bayesian age models for the Yucca, Cedar Mountain, Cloverly, Kootenai, and Blackleaf formations. These age models constrain depositional timing and reveal key stratigraphic hiatuses within these formations. Notably, these models identify an eastward-propagating depositional slowdown, interpreted as the earliest signal of forebulge migration. Collectively, these findings provide a refined geochronologic and tectonic framework that serves as a foundation for future studies of climatic and biotic change during the Early Cretaceous in the Western Interior Basin. To complement field-based geochronology and address the challenges of growing, unwieldy detrital zircon U-Pb datasets, I applied automated machine learning (AutoML) to classify age distributions based on source similarity in Chapter 2. AutoML outperforms traditional methods in both synthetic datasets and in predicting tectonic settings using a natural dataset from the Triassic–Eocene North American Cordillera. AutoML also identifies diagnostic age ranges, including the 145 – 125 Ma Cordilleran magmatic lull. This work advances the analytical toolkit available to geochronologic researchers and demonstrates machine learning as a predictive and interpretive tool.
Citation
Fekete, J. (2025). Refining the Geochronologic Framework of the Early Cretaceous Nonmarine Western Interior Basin, USA. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/5935
