High-resolution U-Pb Geochronology of Terrestrial Cretaceous-Paleogene and Permo-Triassic Boundary Sequences in North America

High-resolution geochronology provides a means to evaluate the timescales of responses to major shifts in Earth history, such as ecosystem recovery following a major impact event or a mass extinction. Additionally, geochronology can be used to correlate sections across the marine and terrestrial realm and around the world. Changes in ecosystems or isotopic composition of deposited rocks will be influenced by local effects, but can also have a global signal. With geochronlogy, the same time interval can be found in distant regions, and if a phenomenon (e.g. a carbon isotope excursion) was global in scope, many different localities each from the same time interval would show the same signal.Here I present high-resolution uranium-lead geochronology pertaining to two mass extinctions: the Cretaceous-Paleogene mass extinction (around 66 million years ago, Ma) and the Permo-Triassic mass extinction (around 252 Ma). In the first chapter, I introduce some of the complexities involved in using the rock record to piece together a picture of life on Earth back in time. Dating volcanic ash deposits (tephra) with uranium-lead geochronology is an important piece of the puzzle. In the second chapter, the chemical, analytical, and statistical methods used are presented in detail. Analyses of several reference materials are included, which illustrate the precision and accuracy attainable with these methods. The third chapter applies these high-resolution dating techniques to the Cretaceous-Paleogene boundary in Northeastern Montana. Although one of the volcanic ash deposits in the area has been dated by 238U/206Pb, more than forty distinct volcanic ash deposits have been identified. Here I present new data on 12 samples representing at least 10 distinct tephra, at a precision and accuracy much higher than in the previous uranium-lead work. In the fourth chapter, 238U/206Pb techniques are again applied to volcanic tephra, this time in a section in the Texas Panhandle believed to be of latest Permian age, though possibly earliest Triassic. With extremely few fossils preserved, determining a precise age can only be done through radioisotopic geochronology. One such study used the K-Ar method, but the uncertainties in age were 4 million years. The new ages I have determined place this section within uncertainty (≤500 thousand years) of the global stratotype section and point at Meishan, China, and one tephra is distinctly younger than the boundary.This work lays the foundation for other studies. Paleontologists studying the latest Cretaceous and earliest Paleogene in Northeastern Montana can use the dates provided here to constrain the age of fossil localities and develop a regional biochronology. It also allows for comparison with 40Ar/39Ar geochronology, and the combination of the two can be used to calibrate the geomagnetic polarity timescale. Determining the age of strata in the Texas Panhandle will enable stratigraphers to determine whether shifts in the carbon and oxygen isotope records are occurring at the Permo-Triassic boundary. Paleontologists will also be able to make use of these new dates to guide their interpretation of rare microfossils found in the rocks surrounding the volcanic ash bed