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Seawater 87Sr/86Sr Stratigraphy of the Middle Ordovician in Central Virginia

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Seawater 87Sr/86Sr Stratigraphy of the Middle Ordovician in Central Virginia SEAWATER 87SR/86SR STRATIGRAPHY OF THE MIDDLE ORDOVICIAN IN CENTRAL VIRGINIA Undergraduate Research Thesis Submitted in Partial Fulfillment of the requirements for graduation with honors research distinction in Earth Sciences in the undergraduate colleges of The Ohio State University By Sean M. Newby The Ohio State University May 2017 Approved by Matthew R. Saltzman, Advisor School of Earth Sciences TABLE OF CONTENTS Abstract .......................................................................................................................................... iii Acknowledgements ........................................................................................................................ iv List of Figures ................................................................................................................................. v List of Tables ................................................................................................................................. vi Introduction ..................................................................................................................................... 1 Geologic Setting and Background .................................................................................................. 3 Outcrop ....................................................................................................................................... 3 Paleogeography ........................................................................................................................... 5 Lithology ..................................................................................................................................... 6 Biostratigraphy ............................................................................................................................ 6 Chemostratigraphy ...................................................................................................................... 7 Previous Work ............................................................................................................................ 8 Methods........................................................................................................................................... 9 Rock Sampling ............................................................................................................................ 9 Rock Processing for Conodont Extraction .................................................................................. 9 Conodont Collection ................................................................................................................... 9 Strontium Isotope Analysis ......................................................................................................... 9 Results ........................................................................................................................................... 11 Conodont Biostratigraphy ......................................................................................................... 11 Carbon Isotope Chemostratigraphy .......................................................................................... 13 Strontium Isotope Chemostratigraphy ...................................................................................... 13 i Discussion ..................................................................................................................................... 15 Biostratigraphic Dating ............................................................................................................. 15 Chemostratigraphic Refinement ............................................................................................... 15 Does the Knox Unconformity represent a global event? .......................................................... 18 Implications for climate and the Great Ordovician Biodiversification Event (GOBE) ............ 20 Conclusion .................................................................................................................................... 21 Suggestions for Future Work ........................................................................................................ 22 References Cited ........................................................................................................................... 23 Appendix ....................................................................................................................................... 26 ii ABSTRACT Throughout most of the past century, the correlation of strata in a region has mainly been defined through biostratigraphy, the dating of rock through fossil succession. However, work over the past several decades has put increasing emphasis on dating rock formations through chemostratigraphy. This process utilizes secular trends in ancient seawater chemistry preserved in fossils and rocks, which can, in some cases, provide more accurate dating of strata. This project utilized the 87Sr/86Sr seawater curve and existing δ13C data to more accurately date the New Market Formation and underlying Knox Unconformity in central Virginia, which are approximately Middle Ordovician in age (~470–458 mya). The age of the New Market Formation has been studied previously near Collierstown, Virginia, primarily through conodont biostratigraphy. The Knox Unconformity forms the base of this formation. This major erosional unconformity occurs through much of eastern North American and represents a fall in relative sea level, and with this work, can be dated at Collierstown to terminate around 462.5 Ma. The cause of this unconformity is either through cooling climate that lowered sea level or regional tectonic uplift and mountain building raising the seafloor above sea level. This Sr isotope study suggests that sea-level changes are more effectively correlated to the unconformity, but the presence of regional uplift is still an important feature towards understanding the processes occurring. Either can have major ramifications for the development of life on Earth around this time, and a more accurate date for this unconformity can better explain the sequence of events that spurred one of the major adaptive radiations in history, the Great Ordovician Biodiversification Event (GOBE), a major ecological change related to the rise of the Paleozoic fauna. iii ACKNOWLEDGEMENTS I would like to thank Matt for providing me with the basis for this work, for his help at Collierstown, and especially for his knowledge on stable isotopes and the Ordovician. I also would like to thank Stig Bergström for teaching me so much about conodonts. Thank you very much to Deanna, Jeff, and Cole for teaching how to work in the clean lab, run the mass spectrometer and answering my questions relating to the lab. Furthermore, I would like to thank Jeff for all of his help in fixing the mass spec whenever a problem came up. I want to thank Mark Kleffner for aiding in the heavy-liquid separation of my samples. In addition, I am extremely grateful to the Friends of Orton Hall Fund for providing me with funding through different parts of my research and the Undergraduate Research Office for the Undergraduate Research Scholarship during my final year of research. I want to thank the School of Earth Sciences for providing me with the Joan Echols Scholarship and Ohio State for their Maximus Scholarship. iv LIST OF FIGURES Figure 1: Collierstown Locality Map .............................................................................................. 3 Figure 2: Photo of Section .............................................................................................................. 4 Figure 3: Middle Ordovician Paleogeographic Map .......................................................................5 Figure 4: Images of Conodonts ........................................................................................................6 Figure 5: Global Seawater Isotope Curves ......................................................................................7 Figure 6: Collierstown Stratigraphic Column and Isotope Curves ................................................14 Figure 7: Chemostratigraphic Correlation of Collierstown Strontium Isotopes ............................17 Figure 8: Ordovician Sea-Level Curves ........................................................................................19 v LIST OF TABLES Table 1: Collierstown Conodonts ......................................................................................12 vi INTRODUCTION The Ordovician was a period of major changes in the evolution of animal life, which began with one of the largest biodiversification events in Earth history and ended with a mass extinction. These events, the Great Ordovician Biodiversification Event (GOBE) and the end- Ordovician (Hirnantian) extinction event, respectively, have major implications on the development of early complex animal life (Sepkoski, 1995). This thesis is primarily concerned with the former, which included a rapid rate of speciation in the fossil record and occupied the early to middle part of the Ordovician. Though the Cambrian Explosion represents an impressive increase in the complexity and diversity of animal life, the GOBE signals the rise of the Paleozoic Fauna that filled unoccupied ecological niches (e.g. Sepkoski, 1995; Trotter et al., 2008; Harper et al., 2015). There were many factors involved in the GOBE, which likely included both
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