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Bare Bedrock Erosion Rates in the Central Appalachians, Virginia

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Bare Bedrock Erosion Rates in the Central Appalachians, Virginia W&M ScholarWorks Undergraduate Honors Theses Theses, Dissertations, & Master Projects 5-2009 Bare Bedrock Erosion Rates in the Central Appalachians, Virginia Jennifer Whitten College of William and Mary Follow this and additional works at: https://scholarworks.wm.edu/honorstheses Part of the Geology Commons Recommended Citation Whitten, Jennifer, "Bare Bedrock Erosion Rates in the Central Appalachians, Virginia" (2009). Undergraduate Honors Theses. Paper 326. https://scholarworks.wm.edu/honorstheses/326 This Honors Thesis is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Undergraduate Honors Theses by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. BARE BEDROCK EROSION RATES IN THE CENTRAL APPALACHIANS, VIRGINIA A thesis submitted in partial fulfillment of the requirement for the degree of Bachelors of Science in Geology from The College of William and Mary by Jennifer Whitten Accepted for ___________________________________ (Honors, High Honors, Highest Honors) ________________________________________ Gregory Hancock, Director ________________________________________ Christopher Bailey ________________________________________ James Kaste ________________________________________ Scott Southworth Williamsburg, VA April 30, 2009 Table of Contents Abstract......................................................................................................................................................3 Introduction...............................................................................................................................................5 Hack’s Theory of Dynamic Equilibrium..................................................................................................6 Recent work.............................................................................................................................................8 Study Area................................................................................................................................................14 Methodology.............................................................................................................................................16 Field Sampling.......................................................................................................................................16 Laboratory Methods...............................................................................................................................22 Cosmogenic Radionuclides......................................................................................................................24 Theoretical Background.........................................................................................................................24 Calculating Erosion Rates......................................................................................................................26 Results......................................................................................................................................................30 Erosion Rates.........................................................................................................................................30 Episodic Erosion....................................................................................................................................34 Slope and Curvature..............................................................................................................................39 Discussion.................................................................................................................................................42 Relief Change........................................................................................................................................42 Location and Position of Sampling Sites...............................................................................................45 Time Scales...........................................................................................................................................49 Conclusion................................................................................................................................................53 Acknowledgements..................................................................................................................................55 References................................................................................................................................................56 Appendix A: Field data and pictures of sample locations.....................................................................61 Appendix B: Measured chipping lengths...............................................................................................79 Appendix C: Tables with all collected samples included......................................................................80 2 Abstract Long term average erosion rates were determined by measuring in situ concentrations of the cosmogenic radionuclide 10Be in the Blue Ridge of central Virginia. The time scale of the erosion rate measurements is on the order of 104-105 years. Several landscape variables were also investigated to determine which processes control erosion. Curvature and slope measurements taken at sample locations using ArcGIS showed slight correlations with erosion rate. Higher erosion rates had low slope values of <5% and curvature values that converged near 1.2. Location of the sample site was taken into consideration because many sites only had bare bedrock outcrops just off of their summits. Whether the sample was collected from a ridgeline or from a side slope had no effect on erosion rates. A slight negative correlation was observed between erosion rate and elevation from sea level throughout Shenandoah National Park, such that higher erosion rates were found at lower elevations. For sampling sites taken down the side of an individual peak no correlation was found between elevation and erosion rate. The dominant erosional processes present at the summits do not seem to have a significant effect on the erosion rates. Areas dominated by episodic events had lower erosion rates than sites dominated by grain-by-grain erosion. In contrast to previous studies, erosion rates found here were most heavily influenced by rock type. The metasandstone sampled had the lowest average erosion rate while the shale had the highest erosion rate average. The mean summit erosion rate for 20 bare bedrock samples is 9.72 m/My, but individual measurements varied from 2.46 to 41.00 m//My. Nearby long term fluvial incision rates are at least an order of magnitude larger than the summits erosion rates, indicating 3 that the landscape is in disequilibrium, more specifically that the relief in the central Appalachians is increasing. The increase in relief is likely due to either high frequency climate changes during the late Cenozoic era or the local change in the Blue Ridge drainage network during the late Miocene epoch. 4 Introduction The Appalachian Mountains are one of largest and most studied ancient orogenic belts, located on the east coast of North America ranging from Newfoundland, Canada southwest to Alabama. They were formed by a series of collisions during the Alleghanian orogeny in the Paleozoic Era and further uplifted in the Late Triassic Period by an extensional event. Initially, summits reached heights of about 3500 to 4500 meters (Pazzaglia and Brandon, 1996). Over time the mountain range has experienced significant erosion, with modern summits just reaching 2000 meters. The longevity of mountain ranges has presented geologists with an interesting paradigm. Models have been developed to determine the relative importance of tectonic and surficial processes on orogenic evolution and some indicate that mountain ranges should be completely eroded in 107 to 108 years, yet the Appalachians still have significant relief even after hundreds of millions of years, suggesting that we lack an understanding of the rate at which surface processes operate on mountain ranges or the influence these surface processes have over the landscape (Ahnert, 1970; Beaumont et al., 2000; Whipple, 2001). One of these landscape models by Ahnert (1970) found that mountain landscapes should be almost completely eroded in approximately 107 years. His landscape model is based on the relationship between local relief in topography and mechanical erosion using data collected from 20 mid-latitude river basins around the world. Many different environmental factors, such as stream erosion, climate, precipitation and temperature, isostatic and tectonic uplift, and vegetation, were evaluated to determine their effect on denudation rates. Precipitation and temperature were found to have the least effect on the mountain denudation rates. Taking uplift into account, Ahnert (1970) found that relief is reduced by 10% every 18.5 m/My. Without uplift the values change to 11 m/My (Ahnert, 1970) and the mountain chain erodes more quickly. 5 Thus, a combination of environmental factors work together to create a very dynamic condition of mountain decay where observed mountains chains are still able to have significant topography on time scales of 107-108 years. Hack’s Theory of Dynamic Equilibrium In 1952 Hack began work in the central Appalachians
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