Tsunami Stratigraphy in a Salt Pond on St. Croix, US Virgin Islands
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Tsunami Stratigraphy in a Salt Pond on St. Croix, US Virgin Islands Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Paul Russell, B.S. Graduate Program in Earth Sciences The Ohio State University 2018 Thesis Committee: Derek Sawyer, Adviser Audrey Sawyer Matt Saltzman Copyright by Paul Russell 2018 Abstract Tsunamis pose a major threat to human health and safety in the Caribbean and coastal communities around the world. In order to constrain the recurrence interval of these natural disasters and help build resilience, sedimentary deposits from both paleotsunamis and those in the historical record must be identified and examined. Preservation of tsunami deposits is often poor, but coastal salt ponds, a common occurrence in the Caribbean, have been recognized as having the potential to capture tsunamites. This study examined <1m cores from the West End Salt Pond on the island of St. Croix, US Virgin Islands for evidence of inundation events. One deposit that showed several characteristics of an inundation event and was present in multiple locations across the pond was identified, and interpreted as having originated from the 1867 Virgin Island Tsunami. Deeper cores may reveal deposits from other tsunamis that have been identified in the region. ii Dedication I dedicate this to the faculty of ORCA, who first sparked my passion for science. iii Acknowledgements I would like to acknowledge my advisor Dr. Sawyer, who funded my time at Ohio State and provided me with the opportunity to travel to the Virgin Islands to do research. The Friends of Orton Hall provided financial support both for field work and conference travel. Claudia Lombard and the Fish and Wildlife Service allowed me access to the Sandy Point Wildlife Refuge and the Westend Saltpond. Donny Nelthropp lent us a roof to sleep under and put up with constant requests for equipment. Dr. Brooks, Bekka, and the whole team at Eckerd provided valuable coring equipment and ran radioisotope dates for me. Finally, my family and friends provided invaluable advice and support over the last two years. iv Vita June 2012……………………………………Lakewood High School June 2012……………………………………A.A., Everett Community College June 2014……………………………………B.S., University of Washington August 2016-Present………………………...Graduate Research Assistant, Department of Earth ____________________________________ Sciences, The Ohio State University Fields of Study Major Field: Earth Sciences v Table of Contents Abstract ......................................................................................................................................................... ii Dedication .................................................................................................................................................... iii Acknowledgements ...................................................................................................................................... iv Vita ................................................................................................................................................................ v List of Tables .............................................................................................................................................. vii List of Figures ............................................................................................................................................ viii Chapter 1: Introduction ............................................................................................................................. 1 Chapter 2: Background .............................................................................................................................. 4 St. Croix Geologic Setting ......................................................................................................................... 4 The 1867 Virgin Island Tsunami ............................................................................................................... 5 Tsunami Deposit Identification ................................................................................................................. 9 Chapter 3: Methods and Site Information ............................................................................................. 13 Site Selection ........................................................................................................................................... 13 Field Methods ......................................................................................................................................... 16 Lab Analysis ............................................................................................................................................ 17 Chapter 4: Results ..................................................................................................................................... 18 Core 1 ..................................................................................................................................................... 18 Core 2 ..................................................................................................................................................... 20 Core 3 ..................................................................................................................................................... 22 Core 4 ..................................................................................................................................................... 24 Chapter 5: Discussion ............................................................................................................................... 26 Unit 1 ...................................................................................................................................................... 26 Unit 2 ...................................................................................................................................................... 29 Unit 3 ...................................................................................................................................................... 30 Chapter 6: Conclusions ............................................................................................................................ 31 References ................................................................................................................................................... 33 vi List of Tables Table 1. Criteria of Tsunami and Hurricane Deposits…………………………………………………………………………….10 vii List of Figures Figure 1. The US Virgin Islands location in the Caribbean…………………………………………………………………………3 Figure 2. Geologic map of St. Croix………………………………………………………………………………………………………….5 Figure 3. The 1867 Dual Event………………………………………………………………………………………………….……………..7 Figure 4. Tsunamigenic sediment entrainment and deposition in a coastal lagoon………………………………..12 Figure 5. 1867 Tsunami inundation of the US Virgin Islands…………………………………………………………………..14 Figure 6. Core locations in the Westend Salt Pond…………………………………………………………………………………16 Figure 7. Description of Core 1………………………………………………………………………………………………………………19 Figure 8. Description of Core 2………………………………………………………………………………………………………………21 Figure 9. Description of Core 3………………………………………………………………………………………………………………23 Figure 10. Description of Core 4……………………………………………………………………………….……………………………25 viii Chapter 1: Introduction Tsunamis are a major threat to the islands of the Caribbean Basin, with 27 fatal tsunamis since 1510 Common Era (CE) (Mercado-Irizarry and Liu 2006). The Caribbean has several potential tsunamigenic sources in the form of volcanic activity, submarine landslides, and earthquakes, as well as significant exposure to tsunamis originating from other points in the Atlantic Ocean (O’Loughlin and Lander 2003). With a population of 35.5 million people and an average of 300,000 beach visitors per day, the Caribbean represents one of the areas of highest risk for fatal tsunamis in the world (Proenza and Maul 2010). In order to build regional resilience to tsunamis, it is necessary to examine the cause, scale, and timing of past events (Engel et al. 2016). Looking only at the historical record, as recorded by colonists and indigenous peoples, is not sufficient however, as there may be tsunamigenic sources with recurrence intervals of longer than the 500 years that tsunamis in the region have been documented (Brill et al. 2012; Engel et al. 2016). If such sources do exist, regional planners are operating with incomplete information, which puts lives and property at risk. Tsunami deposit identification in the USVI, and in the Caribbean in general, is complicated by the prevalence of hurricanes and tropical storms in the region. In addition to potentially reworking older deposits, hurricanes leave sedimentary packages that can closely resemble those of tsunamis (Morton, Gelfenbaum, and Jaffe 2007). While significant progress has been made for distinguishing the deposits characteristics of a tsunami from other sources (Brill et al. 2012; Engel et al. 2010; Scheffers, Scheffers, and 1 Kelletat 2005), the work on separating tsunami stratigraphy from that of hurricanes has not been as successful, although recent advances are providing more clarity (Atwater et al. 2012; Engel et al. 2016; Fuentes, Tuttle, and Schmidt 2017) Building a tsunami record that stretches beyond the limited historical