| dc.description.abstract | This dissertation uses ichnological assessment to reevaluate depositional environments and the physicochemical controls on benthic organisms. I am assessing environments from icehouse and greenhouse global climates, and from polar and near-equatorial positions. This comparison originates from recent trends in the literature which suggested that the physicochemical controls on bioturbation are significantly different between climatic regimes and latitudinal positions. We identified these major controls from the high-latitude Permian Mackellar Fm, the mid-latitude Cretaceous Dakota Group, and the mid-latitude Cretaceous Greenhorn and Niobrara formations. In addition to interpretation of physicochemical controls, each chapter explores an application of ichnology: reevaluation of depositional environments (Mackellar Fm), identification of biogenic modification of porosity and permeability (Dakota Group), description of bioturbation trends through Oceanic Anoxic Event (OAE) strata (Greenhorn and Niobrara Fms), and the use of bioturbation intensity and ichnocoenoses to identify Milankovitch cyclicity (Greenhorn and Niobrara Fms). The Mackellar Fm was previously interpreted to had been deposited in a lacustrine system, but this study refutes this interpretation based on the identification of ichnogenera that only known from marine systems. Mackellar Fm ichnogenera have diminutive morphotypes, shallow penetration, low individual bed and high overall ichnodiversity and are interpreted as a short-lived benthic community that was stressed by freshwater input and high sedimentation rates in a river-dominated deltaic to fully marine system. The study on the Dakota Group assessed physicochemical parameters at the time of deposition using ichnocoenoses and ichnofacies to reinterpret depositional histories, reevaluate sequence stratigraphic models, and evaluate the influence of bioturbation porosity and permeability in these formations. Dakota Group depositional environments are interpreted as: fluvial to deltaic (Lytle Fm), paralic to nearshore (Plainview Fm), distal to proximal shoreface (Glencairn Fm), and continental to offshore marine depositional environments (Muddy Fm). Lastly, analysis of Greenhorn and Niobrara fm ichnology identified a high diversity with abundant bioturbation throughout oxic and OAE intervals. I interpreted that the anoxic events were not long-term events but a period of dysoxic conditions with punctuated periods of anoxia. Additionally, based on observed cyclic patterns in ichnocoenoses and bioturbation, identified harmonic cycles that match Milankovitch cyclicity (eccentricity, obliquity, and precession). | |
