| Popis: |
The Late Ordovician has been recognized as one of the most substantial periods of intracratonic flooding in the history of the North American continent and one of the most dynamic periods of climatic and sea level fluctuations in Earth history. Paleoceanographic changes in water temperature, turbidity, and nutrient supply during this time were accompanied by an episode of major ecological change known as the Richmondian Invasion. During this event, over 50 warmer water taxa were introduced to the Cincinnati Arch region in several waves over the course of 500,000 years, resulting in substantial reorganization of the ecologic community structure. Although the overall structure of this invasion is well known, the details and timing of individual invasion events are largely unknown, and the proper study of these attributes requires well a constructed high-resolution stratigraphic framework. I have identified a series of discrete horizons and faunal epiboles within the previously undifferentiated Bull Fork Formation and the Rowland Member of the Drakes Formation in central Kentucky that are characteristic of the Waynesville Formation in southwestern Ohio. In doing so, I have recognized multiple erosive surfaces representing sequence boundaries (one being the “Mid-Richmondian unconformity”), as well as distinct lithologic and biologic gradients which have allowed us to trace the Waynesville and its component units for approximately 200 km from its type area in southwestern Ohio to central Kentucky. These correlations, supported by historical studies and limited carbon isotopic data, provide a high-resolution stratigraphic framework within which to study changes in paleoecologic structure.This framework was utilized to determine the absolute depth ranges of benthic assemblages along the Waynesville depositional ramp. I first constructed a depth-calibrated ramp model of the study interval. The proximal end-member (shoreline) is indicated by desiccation cracks. The distal end-member was determined by the occurrence of a microendolith ichnospecies known to inhabit the shallow euphotic zone in modern seas. Extrapolating this data allowed us to establish absolute depths of each depositional environment along the ramp. Faunal census counts were taken at representative stratigraphic sections and subjected to quantitative analysis in order to analyze biofacies composition and underlying ecological gradient structure. The depth ranges of the ramp model were then applied to the results of the quantitative analysis, which allowed us to determine the absolute depth ranges of each biofacies along the ramp. Ultimately, the research herein seeks to improve our understanding of the interplay of Late Ordovician climate, oceanography, stratigraphy, and paleoecology. More specifically, this study seeks to 1) rectify the complex lithostratigraphic nomenclature of the Waynesville Formation at the member, submember, and bed level; 2) improve stratigraphic resolution and revise the third- and fourth-order cyclicity of the study interval; 3) extend these correlations from extensively studied distal faces into relatively understudied proximal facies; 4) construct a depth-calibrated ramp model of the study interval from shoreline to deep subtidal environments; and 5) examine and quantify lateral gradients of lithologic and faunal change along this depositional ramp during a major phase of the Richmondian Invasion. |
