National Park Service U.S. Department of the Interior Natural Resource Stewardship and Science Dry Tortugas National Park Geologic Resources Inventory Report, Revised April 2014 Natural Resource Report NPS/NRSS/GRD/NRR—2014/809 ON THE COVER Surrounded by patchy coral and a deep channel, Garden Key is home to the majority of the park’s infrastructure, including Fort Jefferson, the largest masonry structure in the Americas. View toward the southwest. National Park Service photograph courtesy of the Submerged Resources Center. THIS PAGE Loggerhead Light on Loggerhead Key warns mariners of the keys and shallow carbonate banks fringed with coral reefs in Dry Tortugas National Park. View toward the northeast. US Geological Survey photograph courtesy of Don Hickey. Dry Tortugas National Park Geologic Resources Inventory Report, Revised April 2014 Natural Resource Report NPS/NRSS/GRD/NRR—2014/809 Rebecca Port National Park Service Geologic Resources Division Geologic Resources Inventory PO Box 25287 Denver, CO 80225 NOTE: The Dry Tortugas National Park Geologic Resources Inventory (GRI) report was original completed and distributed in early March 2014 as Port, R. 2014. Dry Tortugas National Park: Geologic resources inventory report. Natural Resource Report NPS/NRSS/GRD/NRR—2014/771. National Park Service, Fort Collins, Colorado. Shortly after publication, the GRI report team was contacted regarding report sections describing construction materials and subsidence of Fort Jefferson. Those sections of the report were based primarily upon the GRI scoping meeting summary, which was completed in 2005 (Thornberry-Ehrlich 2005). Since the writing of the scoping meeting summary, additional research was undertaken regarding subsidence of the fort. This revised version of the report includes updated content and photographs in the “Geologic Materials Used in Fort Jefferson”, “Sediment Erosion and Accretion”, and “Subsidence” sections. The “List of Figures” and “Literature Cited” sections were also updated to reflect the new content. May 2014 U.S. Department of the Interior National Park Service Natural Resource Stewardship and Science Fort Collins, Colorado The National Park Service, Natural Resource Stewardship and Science office in Fort Collins, Colorado, publishes a range of reports that address natural resource topics. These reports are of interest and applicability to a broad audience in the National Park Service and others in natural resource management, including scientists, conservation and environmental constituencies, and the public. The Natural Resource Report Series is used to disseminate high-priority, current natural resource management information with managerial application. The series targets a general, diverse audience, and may contain NPS policy considerations or address sensitive issues of management applicability. All manuscripts in the series receive the appropriate level of peer review to ensure that the information is scientifically credible, technically accurate, appropriately written for the intended audience, and designed and published in a professional manner. This report received informal peer review by subject-matter experts who were not directly involved in the collection, analysis, or reporting of the data. Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect views and policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Government. Printed copies of this report are produced in a limited quantity and they are only available as long as the supply lasts. This report is available from the Geologic Resources Inventory website (http://www.nature.nps.gov/geology/inventory/ gre_publications.cfm) and the Natural Resource Publications Management website (http://www.nature.nps.gov/publications/nrpm/). To receive this report in a format optimized for screen readers, please email [email protected]. Please cite this publication as: Port, R. 2014. Dry Tortugas National Park: Geologic resources inventory report, revised April 2014. Natural Resource Report NPS/NRSS/GRD/NRR—2014/809. National Park Service, Fort Collins, Colorado. NPS 364/124567, May 2014 ii NPS Geologic Resources Division Contents List of Figures ................................................................................................................ iv List of Tables ................................................................................................................. iv Executive Summary ....................................................................................................... v Acknowledgements ..................................................................................................... vii Credits ........................................................................................................................................................................ vii Introduction ................................................................................................................... 1 Geologic Resources Inventory Program ......................................................................................................................... 1 Park Setting ................................................................................................................................................................. 1 Geologic Setting .......................................................................................................................................................... 1 Geologic Features and Processes .................................................................................. 5 Pleistocene Bedrock ..................................................................................................................................................... 5 Holocene Coral Reefs ................................................................................................................................................... 8 Seagrass Beds ............................................................................................................................................................ 13 Unconsolidated Benthic Sediments ............................................................................................................................. 14 Pavement .................................................................................................................................................................. 15 Islands and Sediment Transport .................................................................................................................................. 15 Paleontological Resources .......................................................................................................................................... 20 Geologic Materials Used in Fort Jefferson ................................................................................................................... 20 Geologic Issues ............................................................................................................ 23 Sea Level Rise ............................................................................................................................................................ 23 Hurricanes and Storms ............................................................................................................................................... 24 Coral Reef Rehabilitation ............................................................................................................................................ 25 Recreation and Commercial Uses ............................................................................................................................... 26 Sediment Erosion and Accretion ................................................................................................................................. 27 Subsidence ................................................................................................................................................................ 28 Groundwater Flow and Salinity .................................................................................................................................. 29 Benthic Habitat Mapping ........................................................................................................................................... 29 Geologic History .......................................................................................................... 31 Introduction ............................................................................................................................................................... 31 Mesozoic Era: Birth of the Atlantic Ocean................................................................................................................... 31 Cenozoic Era: Layers of Limestone .............................................................................................................................. 31 Geologic Map Data ...................................................................................................... 35 Geologic Maps .......................................................................................................................................................... 35 Source Maps .............................................................................................................................................................