High-resolution age constraints and fluvial sedimentology of Late Cretaceous to early Paleocene terrestrial deposits of the southwestern USA.

Abstract

A well-constrained age model is a critical piece of information in geologic studies. This dissertation focuses on two successions of fossiliferous fluvial strata within the southwestern US that prior to this work were lacking high-resolution geochronologic control: the Dawson Creek section of Big Bend National Park, Texas and the upper Nacimiento Formation of the San Juan Basin, New Mexico. Establishing age control facilitates interpretations regarding the timing of climatic events, ages of faunas and timing of turnovers, and basin evolution through time. The first case study involves using magnetostratigraphy, biostratigraphy, and detrital sanidine ages to construct the first independent, high-resolution age model for the Late Cretaceous to early Paleocene Dawson Creek section. The generated age model indicates eustatic sea level was the primary driver of deposition, documented warming events represent rapid changes in climate, and the Javelina Formation dinosaur fauna is equivalent in age to the Hell Creek fauna supporting the hypothesis of dinosaur provinciality in the latest Cretaceous. The second case study focuses on the upper Nacimiento Formation of the San Juan Basin, New Mexico and presents a detailed lithostratigraphy and high-resolution age model for the strata using magnetostratigraphy and a detrital sanidine age to constrain the age of Torrejonian mammal localities. Findings from this study indicate the faunal turnover between the Torrejonian 2 – Torrejonian 3 North America Land Mamma age interval zones occurred over ~120 kyr. Comparisons of calculated mean sediment accumulation rates indicate that sedimentation equalized across the basin, suggesting an accommodation minima in the basin. The last study uses the distributive fluvial system model to evaluate the position of four upper Nacimiento sections within a fan system and assess whether the strata were deposited by a prograding system. Results from characterizing pedotype, paleosol maturity and drainage, fluvial facies, and floodplain location indicate the Kutz Canyon and Escavada Wash sections represent medial deposits and the Torreon West and Torreon East sections represent distal deposits. The study interval appears to capture autocyclic migration and aggradation, suggesting the interval is too fine to capture progradation. This work provides improves the chronology of regions important to mammalian and climatic studies in North America.