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Gravity Survey of a Buried Triassic Rift Basin in Bertie County

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Gravity Survey of a Buried Triassic Rift Basin in Bertie County GRAVITY SURVEY OF A BURIED TRIASSIC RIFT BASIN IN BERTIE COUNTY, NORTH CAROLINA by Cody Shell May, 2020 Director of Thesis: Dr. Eric Horsman Major Department: Geological Sciences The North American rift margin includes of a series of Triassic rift basins along the eastern seaboard of the United States and Canada. This continent-scale rift basin system is comprised of complex and variable geometries that can be generalized into regions with similar structural, deformational, and sedimentary characteristics. Rift basins provided accommodation space for organic-rich Triassic age sediments that may be source rocks for natural gas and petroleum. Most of the known basins are exposed at the surface and relatively easy to access, but a few buried basins have been identified beneath coastal plain strata. I used primarily geophysical methods to study a buried Triassic rift basin in Bertie County, North Carolina, recently discovered from a deep core sample that documented Triassic sedimentary rocks buried underneath approximately 300 meters of Cretaceous and younger, sediments and sedimentary rock. Approximately 30 meters of Triassic strata were recovered from the well, but basement rock was not reached leaving the overall thickness of the basin undetermined. I used a gravity survey to constrain the dimensions and geometry of the basin and surrounding rock bodies at depth. Data processing, modeling, and integration with preexisting data was accomplished using Oasis:Montaj software. The buried basin creates a maximum gravity anomaly of approximately 7 mGal. Modeling of the data suggests the basin is generally elongate, SW to NE, and has maximum dimensions of approximately 15 km wide, 50 km long, and as much as 2.5 km deep (basin infill). In cross section, the basin is asymmetrical and wedge- shaped, with a NW margin that dips steeply SE and a SE margin that dips more shallowly NW. The Bertie basin is deepest to the south and was likely hydrogeologically open in that direction. Previous datasets have been derived from analysis of the cores at the North Carolina Geologic Survey and include whole rock geochemical analysis, thin sections, and magnetic susceptibility. Interpretation of the geochemical data suggests the Triassic strata are derived from a continental island arc, and thin section analysis suggests a provenance of recycled orogenic material. The rocks classified as Triassic tend to have lower magnetic susceptibility than the overlying Cretaceous rock. One interpretation of these data is a change in sediment provenance from late-stage Triassic basin infill to the overlying Cretaceous strata. The Bertie Basin is located in the Southern Segment of the North American rift margin, suggesting that its geometry and stratigraphy should reflect regional trends and exhibit characteristics similar to other southern rift basins. The characteristic geometry of basins in the Southern Segment generally includes narrow to medium size (10 to 25 km across), fault- bounded, half-grabens with no or very subtle growth structures. The Bertie Basin may be part of a series of basins or a sub-basin within a larger basin due to sequential, domino-style faulting during rift migration. Higher extensional rates and faster rift migration within the Coastal Plain province may be related to its reduced dimensions. Burial underneath Coastal Plain strata may also have helped to preserve the Bertie Basin’s original geometry and size which allows for improved constraints on initial tectonic conditions and structures, sedimentary deposition, paleo- environments, and processes related to supercontinent breakup. GRAVITY SURVEY OF A BURIED TRIASSIC RIFT BASIN, BERTIE COUNTY, NORTH CAROLINA A Thesis Presented to the Faculty of the Department of Geological Sciences East Carolina University In Partial Fulfillment of the Requirements for the Degree M.S. in Geological Sciences by Cody Shell May, 2020 © Cody Shell, 2020 GRAVITY SURVEY OF A BURIED TRIASSIC RIFT BASIN IN BERTIE COUNTY, NORTH CAROLINA by Cody Shell APPROVED BY: DIRECTOR OF THESIS:_________________________________________________________ Eric Horsman, PhD COMMITTEE MEMBER:________________________________________________________ Scott Giorgis, PhD COMMITTEE MEMBER:________________________________________________________ Don Neal, PhD COMMITTEE MEMBER:________________________________________________________ Terri Woods, PhD CHAIR OF THE DEPARTMENT OF GEOLOGICAL SCIENCES:___________________________________________________ Stephen Culver, PhD DEAN OF THE GRADUATE SCHOOL:_________________________________________________________ Paul J. Gemperline, PhD ACKNOWLEDGEMENTS I would like to thank East Carolina University and the Southeast Section of the Geological Society of America for funding of my research, Dr. Sarah Titus from Carleton College who lent me the gravimeter used during my fieldwork, Tyler Anderson who helped me to collect data, and Dr Horsman, my advisor, for all the time and effort he has spent to help both in the field and with my analysis/research. Table of Contents List of Tables ................................................................................................................................ vii List of Figures .............................................................................................................................. viii List of Symbols and Abbreviations ................................................................................................ x Chapter 1: Introduction ................................................................................................................... 1 Chapter 2: Evolution of a rift margin .............................................................................................. 4 Heat Flow and Magmatism ....................................................................................................... 13 Extension and Subsidence ......................................................................................................... 16 Chapter 3: Rift Migration ............................................................................................................. 20 Rift Margin and Basin Geometry .............................................................................................. 26 Across-strike Classification ................................................................................................... 26 Along-strike Classification .................................................................................................... 30 Chapter 4: Triassic Rift Basins ..................................................................................................... 35 Faulting...................................................................................................................................... 36 Sedimentology/Stratigraphy ...................................................................................................... 42 Biostratigraphy ...................................................................................................................... 49 Mineral Resources ..................................................................................................................... 50 Base Metals............................................................................................................................ 50 Oil and Gas ............................................................................................................................ 52 Arsenic ...................................................................................................................................... 54 Chapter 5: Regional Geology ....................................................................................................... 56 The Piedmont ............................................................................................................................ 57 Deep River Basin ................................................................................................................... 60 Danville Basin ....................................................................................................................... 65 The Coastal Plain ...................................................................................................................... 67 Offshore ..................................................................................................................................... 76 Chapter 6: Study Site/ Hope plantation ........................................................................................ 80 Thin Sections ............................................................................................................................. 83 Magnetic Susceptibility ............................................................................................................. 86 Local Geology ........................................................................................................................... 89 Chapter 7: Gravity Surveys ........................................................................................................... 92 Gravity Corrections ................................................................................................................... 93 Chapter 8: Forward Modeling: ..................................................................................................... 97 Chapter 9: Fieldwork .................................................................................................................
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