Tsunami Stratigraphy in a Salt Pond on St. Croix, US Virgin Islands

Home , Anegada Passage, Southern Caribbean

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

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.

Dedication

I dedicate this to the faculty of ORCA, who first sparked my passion for science.

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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.

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

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

List of Tables

Table 1. Criteria of Tsunami and Hurricane Deposits…………………………………………………………………………….10

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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

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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

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 record is critical.

The US Virgin Islands (USVI) is one area in the Caribbean that is particularly lacking in studies on tsunami deposits, both from paleoevents and tsunamis in the historical record (Engel et al.

2016; Fuentes, Tuttle, and Schmidt 2017). In 1867, the USVI experienced the largest tsunami recorded in the historical record that originated in the Caribbean Basin. With waves of over 9m recorded in the city of Frederiksted, St. Croix, and 12m in the on Water Island, St. Thomas, the tsunami caused wide-spread destruction and resulted in over 50 fatalities (O’Loughlin and

Lander 2003). As the most populated of the islands at the time, St. Croix received most of the damage, and also had the highest number of eyewitness accounts (Watlington and Lincoln 1997).

However, scientific examinations of the event, particularly on St. Croix, are lacking. With the exception of one study looking at both historical and paleotsunamis on St. Thomas (Fuentes,

Tuttle, and Schmidt 2017), deposits left by the 1867 event have not described in the scientific literature.

The purpose of this study is to identify sedimentary deposits that were left by the 1867 tsunami on the island of St. Croix. Sediment cores were taken from the West End Salt Pond, a large low lying coastal salt pond on the western end of the island. A sand/shell layer was identified in these cores and linked to the 1867 event. The identification of this layer is the first evidence in the literature of the deposits left by the 1867 event on St. Croix, and also point to the possibility of deposits left by older tsunamis deeper in the stratigraphic record.

Figure 1. US and British Virgin Islands. The island of St. Croix, where this study took place, is the southernmost island shown

Chapter 2: Background

St. Croix Geologic Setting

St. Croix is an island part of the U.S. Virgin Islands in the Lesser Antilles.

Approximately 207 km2 in size, the island is separated from the rest of the Lesser Antilles by the

Anegada Passage and the St. Croix and Virgin Islands Basins. Geologically, It is underlain by folded Upper Cretaceous sedimentary rocks that have experienced regional metamorphism and slightly folded Early Tertiary sedimentary rocks (Whetten, 1961) (Figure 2). The mountainous northern half of the island is composed of the Cretaceous siliclastic and intrusive igneous Mt.

Eagle Group. The southern half is a gently sloping coastal plain composed of several Tertiary carbonate platforms overlain by alluvial deposits. The city of Frederiksted on the far western side of the island rests on the Miocene Kingshill Marl Formation.

More than 1km to the southwest of the city of Frederiksted, the West End Salt Pond, located within the Sandy Point National Wildlife Refuge dominates the southern western tip of the island. The salt pond is surrounded by Pleistocene to Holocene carbonate beach deposits. On the northern and southern sides, the salt pond is separated from the ocean by 90-150 m wide berms that do not exceed 2m in height, and by a 1 km wide raised carbonate platform on the west. To the north, the ocean floor quickly descends towards the Anegada Passage, while the southern side consists of limestone platform 12-30m deep that runs for 2-4km south of the coastline (Adey et al. 1977). The western area of the island possesses no reef structures that might weaken a tsunami or storm surge wave approaching from the north or south (Adey et al.

1977; O’Loughlin and Lander 2003).

Figure 2. A geological map of St. Croix. The West End Salt Pond is located in the south western corner. Taken from Whetten,

1961. The study area is shown located in the red box.

The 1867 Virgin Island Tsunami

The Caribbean experiences a steady drumbeat of tsunamis, with 124 tsunami or tsunami- like events being recorded since 1498, the start of the historical record (O’Loughlin and Lander

2003). Twenty seven of these events have caused fatalities, with a recurrence interval for fatal tsunamis of 19±22 years (Proenza and Maul 2010). The historical record is likely incomplete, even for the time period it does cover. Tsunamis could not be recorded unless colonial administrators witnessed or heard of it, and were not usually recorded unless it significantly

5 impacted the day-to-day operations of the colony (Engel et al. 2016; O’Loughlin and Lander

2003).

The USVI is in a relatively geologically active region of the Caribbean, located approximately 250km south of the Puerto Rico trench. There have been earthquakes of a 7+ magnitude felt in the region in 1690, 1787, 1867, 1918, 1943, and 1946, as well as a 6.4 magnitude event in 2014 (Fuentes, Tuttle, and Schmidt 2017). Of these earthquakes, the 1867 quake was the only tremor that produced a major tsunami in the USVI.

Figure 3. The 1867 Dual Event. The earthquake epicenter and wave path, as well as the San Narciso Hurricane path are shown.

Hurricane path data is from NOAA and the earthquake epicenter is from Barkan and Brink, (2010)

The 1867 event, which occurred on November 19th, less than two weeks after Hurricane

San Narcisco hit the Virgin Islands, was triggered by a ~7.2 magnitude earthquake in the

Anegada Trough (Barkan and Brink 2010). Eyewitness accounts from St. Croix and St. Thomas report two large shocks approximately 10 to 15 minutes apart. It produced the largest tsunami in the Caribbean historical record, and generated run ups of 7.6-9.1m on the western end of St.

Croix, 6m in St. John and St. Thomas, and 12m on Water Island, located just south of St.

Thomas (Fuentes, Tuttle, and Schmidt 2017). Smaller waves also struck Puerto Rico,

Guadeloupe, the Windward Islands, and Grenada (O’Loughlin and Lander 2003). Although casualties were limited, with less than 50 total deaths, several ships were sunk and damaged, including the USS Monongahela which was swept ashore on St. Croix, as depicted in an iconic image published by Harper Weekly. There was also significant material damage, with the city of

Frederiksted being almost completely destroyed.

The only published descriptions of the deposit left by the 1867 tsunami were derived from cores taken in salt ponds and lagoons on the island of St. Thomas, USVI (Fuentes, Tuttle, and Schmidt 2017). These layers had erosional contacts, marine and terrestrial sediments, and occasional mud caps and were dated to the 1867 tsunami using radiocarbon dating of leaves and other woody debris present in the core. Fuentes et al also identified several other tsunami and storm deposits at deeper depths, including one tsunami that occurred in 1200-1440 that has also been described by researchers in the British Virgin Islands (Atwater et al. 2012, 2017; Engel et al. 2016).Inundation events that were tentatively identified as originating from tropical storms and hurricanes were also identified in salt ponds on the island of St. John (Brooks et al. 2015).

However, there has been no detailed work on identifying potential deposits from the 1867 event done on the island of St. Croix (Engel et al. 2016), despite it suffering the most damage from the tsunami.

Tsunami Deposit Identification

There is a good understanding in the literature of the distinguishing features of fine- grained (sand-sized or smaller) tsunami deposits compared to the surrounding non-tsunami deposits. The most common fine-grained deposits are widely spread shelly sand sheets that pinch out away from shore (Atwater et al. 2017). Depending on the local geology and inundation dynamics, these sand layers can include sharp erosional contacts, buried plants, poorly sorted, landwards fining sediment packages, laminated layers, rip-up clasts, a wide variety of fossil remnants, fining upward packages topped with mud caps and sediments that are sourced from the nearby marine sediment sources instead of the island (Atwater et al. 2012; Engel et al. 2010,

2016; Morton, Gelfenbaum, and Jaffe 2007; Reinhardt, Pilarczyk, and Brown 2012; Shaw and

Benson 2015). Preservation of these deposits depends on local geomorphic and climatic conditions, as well as the degree of later disturbance due to bioturbation from local fauna as well as anthropogenic factors. Tsunami deposits that are dominated by carbonate sediments, as is typical in the Caribbean, have low preservation potential due to bioturbation and carbonate dissolution and reworking processes (Engel et al. 2016). Salt ponds act as a good preserving agent for inundation deposits due to their low energy and biological activity (Fuentes, Tuttle, and

Schmidt 2017; Larson 2011; Myhrvold et al. 2014). They also offer little recreational or economic value, meaning the degree of human disturbance should be low.

Separating the deposits left by tsunamis from those left by other inundation events, such as hurricanes, has been a significant problem in the scientific community. Both processes leave fining-upward beds composed of marine sediments that are often laminated. However, several different criteria for separating the two have been suggested (Table 1).

Table 1 Criteria of Tsunami and Hurricane Deposits Sedimentary Tsunami Deposit Hurricane Deposit Criteria Source Predominately beach and marine sediments Beach and terrestrial (Engel et al. 2016) sediment; little to no marine component (Engel et al. 2016) Lower Contact Erosive, sometimes sharp (Hawkes et al. Sharp, sometimes conformal 2007) (Wang and Horwitz 2007) Grading Normal (Goff, McFadgen, and Chagué-Goff Normal (Goff, McFadgen, 2004) and Chagué-Goff 2004) Sorting Poorly sorted (Morton, Gelfenbaum, and Moderate to well sorted Jaffe 2007) (Morton, Gelfenbaum, and Jaffe 2007) Shell Content Sourced from a wide range of depths, more Sourced from near shore broken (Tuttle et al. 2004) areas, less broken (Tuttle et al. 2004) Rip up clasts Present (Goff, McFadgen, and Chagué-Goff Not present (Goff, 2004; Morton, Gelfenbaum, and Jaffe 2007) McFadgen, and Chagué-Goff 2004; Morton, Gelfenbaum, and Jaffe 2007) Thickness Mostly <30cm, locally up to 1.5m (Engel et Up to 1m (Goff, McFadgen, al. 2016; Goff, McFadgen, and Chagué-Goff and Chagué-Goff 2004; 2004; Morton, Gelfenbaum, and Jaffe 2007) Morton, Gelfenbaum, and Jaffe 2007) Table 1 - A comparison of common criteria used to determine if an inundation event is a tsunami or hurricane deposit. Adapted from Engel et al. 2016 and Fuentes, Tuttle, and Schmidt 2017

Each wave of a tsunami event will leave at most two sub-units, one for each run-up and backwash (Engel et al. 2016), which can be identified by grain orientation. Hurricane sediment packages have the potential to be up to 2m thick, where most tsunami deposits are not larger than

25cm (Morton, Gelfenbaum, and Jaffe 2007), although localized deposits of up to 1.5m have been described (Goff, McFadgen, and Chagué-Goff 2004). This difference in vertical extent is due to the specific and differing hydrodynamics of each event. Tsunamis consist of a small number of large waves with a short residence time on land, while hurricanes consist of many small waves and a storm surge that can linger on land. Tsunami deposits can contain mud caps and rip up clasts, which are usually absent from storm deposits (Morton, Gelfenbaum, and Jaffe

2007). Burial of complete shells also tends to be more common in hurricane deposits, as the shells are not as exposed to the same degree of energy as a tsunami exerts (Engel et al. 2016).

Tsunami deposits also tend to have marine sediments from a greater range of depths and a smaller terrestrial component than storm surge deposits (Morton, Gelfenbaum, and Jaffe 2007).

Hurricane deposits will contain a higher percentage of terrestrial sediment then tsunami deposits, as the intense rains associated with hurricanes cause additional erosion and runoff (Fuentes,

Tuttle, and Schmidt 2017).

Figure 4. Tsunamigenic sediment entrainment and deposition in a coastal lagoon

Chapter 3: Methods and Site Information

Site Selection

The West End Salt Pond was chosen as the study location on St. Croix for several reasons. Salt ponds have been repeatedly shown to have high preservation potential for inundation events (Atwater et al. 2017; Brooks et al. 2015; Donnelly 2017; Fuentes, Tuttle, and

Schmidt 2017) in the Caribbean and tropical or semi-tropical regions. This is due to the low amount of biological activity present in salt ponds, which is usually limited to birds nesting on the shore and microbial mats. This prevents significant amounts of bioturbation in the sediment.

Salt ponds, which in the Caribbean are often formed by coral assemblages closing off lagoons

(Myhrvold et al. 2014), are typically low lying, with shallow berms separating them from the sea, allowing for even moderate events to overwash the berm and deposit sediment in the pond.

In the West End Salt Pond, the berm is less then 2m high on the northern, western, and southern sides of the pond, with the eastern side sloping up into St. Croix’s southern coastal plain. With multiple eyewitness accounts putting the height of the 1867 tsunami at 8-9m in the city of

Fredisketd, directly to the northeast of the pond, the wave would be more then high enough to completely cover the salt pond (Figure 5). The West End salt pond also has an extremely small watershed, with the primary source of runoff being from the residential neighborhood to the east.

Height (m)

Figure 5. The areas of the US Virgin Islands that would be directly impacted by a repeat of the 1867 tsunami are shown in light blue. The West End Salt Pond on St. Croix is outlined in red.

Four sites from around the West End Salt Pond were selected for coring. They were located to give both east-west and north-south spatial coverage of the salt pond. Site 1 was the most remote site, located at the far western end of the salt pond off the refuge road. It has a thick biological mat overlaying the sediment, and the edge of the pond is bordered by a thick belt of mangroves. Site 2 is on the southeastern side of the pond, near a constructed berm that the refuge road runs across. The substrate there is significantly rockier than any other site. Site 3, on the north-central path of the pond, is located close to a public pool and beach access, with significantly thinner tree coverage then the other sites, and aside from grass, no vegetation between the road and the beach and ocean. Site 4 was located in the north-eastern area of the pond. It had partially solidified sediments in the top several centimeters of sediments.

Figure 6. Core Locations in the West End Salt Pond. The saltpond’s location on St. Croix is shown in the inset.

Field Methods

At each site, one 4” acrylic core and one 3” aluminum core were taken using push-coring methods to the maximum possible depth. The 4” core was extruded on site in 1cm increments and bagged. The aluminum core was cut to the length of the sediment recovered, drained, and capped with paper towels for transport. Both the aluminum core and the extruded samples were returned to The Ohio State University for lab work.

Lab Analysis

Grain size of the extruded samples was calculated using an OSU College of Engineering

Mastersizer MSS laser particle size analyzer with 5-6g of well mixed sediments from each sample. Generally speaking, the acrylic barrels reached a shallower depth then the aluminum barrels. This resulted in a grain size profile that did not cover all the sediment described from the aluminum barrels. The unsplit aluminum cores were scanned with a Neurologica CereTom x-ray computed tomography (XCT) machine, and then split in half and stratigraphically described.

Areas with sand or larger grain size, as well as high shell content were then identified. These areas were then subsampled from the working half of the aluminum cores, and shells were located and photographed using a microscope and a smartphone. Out of a sample of 50 shell parts, a tally was kept of intact shells vs fragments.

Chapter 4: Results

Core 1

Core 1 was recovered to a depth of 36cm with the aluminum core and 29cm with the acrylic core. It is topped with a microbial mat that is approximately 3.5cm thick. Below the mat there is a 19cm thick section of fine sand, with an average grain size of 102μm. There are several recovery artifacts in this section, most prominently a large vertical channel that can be best seen in the XCT Image. This section also contained small amounts of visible organic matter, mainly leaves and woody debris. Below the fine sands in a 6cm thick section of coarse sands, with an average grain size of 218μm. The color of the sand in this unit is light, almost white in some cases. It contains shell parts, with 100% of the examined parts being fragments. More recovery artifacts are present in this section, as horizontal channel connects to the vertical channel above it. The bottommost section, stretching from 28cm to 35cm deep, is also coarse sand. Direct grain size data does not exist for this section due to a lack of corresponding extrusions from the acrylic cores, however, it is comparable in density to the section immediately above it. It contains a high concentration of shell parts, with 17 of the 50 examined parts being whole shells, with the predominant shell kind being gastropod or bivalve shells.

Figure 7. Description of Core 1 19

Core 2

Core 2 was recovered to a depth of 29cm with the aluminum core and 20cm with the acrylic core. It is topped with a microbial mat of roughly 1.5cm thickness. Below the mat there is a 7cm thick section of fine sands, mixed with some coarser materials. The average grain size of this section is 85μm. Underneath that section there is a 6cm thick section of fine sands with an average grain size of 71.5μm. This layer also has limited horizontal bedding occurring, with each bed being ~2cm thick. Small amounts of leaves and woody debris is present in this section.

Below the fine sands there is a sharp contact, followed by a 4cm thick section of coarse sands with an average grain size of 177.5μm. This section has numerous shell parts, with 3 samples out of 50 being complete shells, all 3 being gastropod shells. The bottommost layer is at least 9cm thick, and is dominated by large cobbles ranging from 1 to 7cm wide. The cobbles are held in place by a coarse sand matrix, for which grain size data is not available. The sand contains large amounts of shells parts, 6 out of 50 being complete shells, with 4 gastropod shells and 2 bivalve shells.

Figure 8. Description of Core 2

Core 3

Core 3 was recovered to a depth of 71cm with the aluminum core and 65cm with the acrylic core. Unlike the other 3 cores, there is no microbial mat at the top of the core. The topmost unit consists of inclined coarse sand layers sloping upwards to the right. Direct grain size data is not available, but density comparisons to units down core give an estimate of 211μm.

From 10cm to 17cm there is a unit of inclined fine sand layers sloping upwards to the left. The average grain size data for this unit is 73.5μm. From 17cm to 21cm there is another unit of inclined coarse sand layers sloping upwards to the right. The average grain size data for this unit is 211μm. Below the coarse sand unit, there is another unit of inclined fine sand layers, this time sloping upward to the right. This unit is 4cm thick, and has an average grain size of 81cm.

Underlying this layer is one last inclined layer of coarse sand, with <1cm thickness. The extrusion it is contained in has a grain size of 150μm. None of the inclined layers show appreciable amounts of either vegetative material or shell parts.

Beneath the inclined layers there is 21cm thick unit of uniform fine sands, with an average grain size 76μm. No shell parts were present in this layer, however there was a small amount of vegetative material. This unit is underlain by a sharp contact at 47cm core depth, under which is a 6cm thick unit of homogenous, coarse sand with an average grain size of

250μm. The sand in this layer is a light, almost white color. This layer has a high concentration of shell parts, 7 out of 50 being complete shells. The bottommost layer, which is at least 17cm thick, is extremely coarse sand, with an average grain 311μm. Shell samples taken from this unit had 19 of 50 being complete shells, including several large (~.5-.7cm) gastropod shells. This unit also displays some bedding in the top 5 cm, with an average bed thickness of 1cm.

Figure 9. Description of Core 3 23

Core 4

Core 4 was recovered to a depth of 39cm with the aluminum core and 27cm with the acrylic core. The top 5cm of the sediment are a brittle, low strength microbial mat that collapses underweight. Underlying the microbial mat are 10cm of fine sands with an average grain size of

75μm. There is some vegetative matter in this unit. At 15cm core depth, there is a 4cm thick unit of horizontally layered sands, capped on both ends by coarse sands with a grain size of 247μm, with small beds, less then .25cm thick, of finer sands with a grain size of 105.5μm. There is no vegetative material or shell parts in this unit. From 19.5-22cm, there is a unit of fine sands, with an average grain of 118.5μm. Immediately underlying there is 1cm thick layer of coarse, shell filled sand. The grain size of thick unit is 326μm, and 11 of the 50 shell part samples were complete shells. The bottommost layer in the core is from 25 to 39cm, and is composed of mixed medium sands. Shell parts are not as abundant here as they are in the preceding coarse sand layer, and only one complete gastropod shell was identified.

Figure 10. Description of Core 4

Chapter 5: Discussion

There are 3 distinct packages of sedimentary units that were identified in the cores, two of which are common across all four cores. Named Unit 1, Unit 2, and Unit 3, they are marked on the core figures with red, blue, and green, respectively.

Unit 1

Unit 1 is present in all four cores, and directly underlies Unit 2. It is characterized by a sharp contact followed by a unit of medium to coarse sand, which in Cores 3 and 4 are lighter in color then the other sands in the core. The sands are underlain by even coarser grains, most often extremely coarse sands, but cobblestones are present in Core 2. The recovered depth of Unit 1 is

19.25±6.7cm, although the true depth is likely deeper, as the push-coring methods proved inadequate to completely penetrate the sand and shell hash layers that were encountered.

Unit 1 is interpreted as being deposited by an inundation event. Supporting evidence for this include fining upwards layers of sediment, a much higher shell content than any other part of the cores, coarser material, and clear horizontal bedding. The lack of vegetative matter present in sediments deposited by normal depositional processes of the salt pond suggests a sudden deposition. Unit 1 is further interpreted as having originated from the 1867 Virgin Island tsunami. There are several lines of evidence that support this interpretation. The presence of complete shells suggests a deeper marine origin then typically found in hurricane deposits (Engel et al. 2016). The poor to moderate sorting of the sediment is also typical of a tsunami deposit

(Morton, Gelfenbaum, and Jaffe 2007), as is the lack of laminated beds. Furthermore, if Unit 1 26 was to be interpreted as a hurricane deposit, an argument must be presented for the lack of deposits from other hurricanes in the record. There have been 11 major storms (category 3 or greater) in the past 200 years (NOAA n.d.) that have passed within 100km of the salt pond, yet there is only one inundation deposit present in the cores. If Unit 1 is assumed to have originated from the last major hurricane to pass over the region, Hurricane Hugo in 1989 (Hurricane Maria occurred in September 2017, after the cores had already been collected), Unit 2 would have to have been deposited entirely in the 28 years since the hurricane. Given an average thickness of

15.75±5.1cm, this gives an average deposition rate for the salt pond under normal conditions of

.563±.182cm/yr, a rate that is over 250% of the maximum sedimentation rate recorded at other salt ponds on the Virgin Islands (Brooks et al. 2015). Using the approximate age given for Unit 2 of 157±26yrs, and assuming there is not an unconformity between Units 1 and 2, the formation of Unit 1 would then be close to the 1867 tsunami, which occurred just under 150 years before the cores were collected.

A tsunami also explains the high heterogeneity that Unit 1 shows across all four cores.

The relatively finer layer of sand is significantly thicker in Cores 3 and 4 (8cm and 10cm, respectively), which were both taken in the northern section of the salt pond, where the tsunami would have first struck. In the southern reaches of the pond, the capping layer is only 3cm thick in Core 2, and is not present in Core 1 (although the area were the layer would exist has been significantly modified by recovery artifacts, which may have removed evidence of the layer).

Deposits thinning landwards is typical of tsunami deposits (Fuentes, Tuttle, and Schmidt 2017;

Morton, Gelfenbaum, and Jaffe 2007), which in the case of 1867 tsunami would be north to south (the position of the salt pond on a peninsula means tsunamis originating from different sources could also travel south to north, or west to east). Unfortunately, due to the lack of 27 complete recovery of Unit 1 in any core, it is not possible to see if Unit 3 as a whole thins in a landward direction. The cobbles present in Core 2 can also be explained by a tsunami, which would have swept them out of the elevated, rocky area immediately to the north of the coring site into the salt pond, whereas a hurricane would have distributed them in a more even manner across the salt pond.

The lack of other inundation deposits in the core can be explained by the limited recovery depth of the core as well as the height of the berm, which is ~2m all around the salt pond. The

1918 tsunami, which is the only other recorded tsunami in the historic record to strike the USVI did not have any notable run up on St Croix. Furthermore, storm surge from hurricanes is limited by the deep seas and narrow shelf surrounded the island (Hubbard 1992), and were limited to .5-

1m during Hurricane Hugo, one of the strongest hurricanes to strike the island in the historical record. While waves were much higher during the hurricane, reaching 11m on some parts of St.

Croix (Hubbard 1992), storm surge is required to create large, consistent deposits (Engel et al.

2016). Deposits from paleotsunamis have been identified in the vicinity of St. Croix, including on St. Thomas (Fuentes, Tuttle, and Schmidt 2017) and Anegada and other islands in the British

Virgin Islands (Atwater et al. 2012, 2017), including a large tsunami between 1200-1480. It is possible that deposits from these events are also present in the stratigraphic record of the West

End Salt Pond, however methods capable of achieving a greater recovery depth would be needed in order to find them.

Unit 2

Unit 2 is present across all four cores, and is directly under the microbial mat, or Unit 3 in

Core 3. It is characterized by fine sands, with some limited horizontal bedding occurring. The average thickness of this layer is 15.75±5.1cm, with an average grain size of 81.9±12.29μm.

Shell parts are not present in this unit, however there is vegetative matter, consisting of small branches and leaves, interspersed with the sediment. This unit is interpreted as being deposited by the normal, non-inundation related sedimentary processes of the salt pond. It does not display any of the signs that would point to an inundation event, such as upward fining layers, shell parts, or coarse sands. All four cores were taken within 20m of the shore, allowing for the gradual accumulation of the vegetative matter present. Accumulation rates in enclosed salt ponds on the Virgin Islands have a range of .0125-.2cm/yr (Brooks et al. 2015). Although a direct measurement of the accumulation rate for the West End Salt Pond is not available, assuming a middle of the road value of .1±.02cm/yr and an average thickness of 15.75cm gives an approximate age of Unit 2 of 157±26yrs through the present day.

Unit 2 does have a high degree of east-west spatial heterogeneity. Cores 1 and 3, taken in the west-central part of the salt pond have a significantly deeper Unit 2 (19cm and 21cm, receptively), then found in Cores 2 and 4 (7cm and 10cm, respectively), which were taken on the eastern side of the salt pond. Assuming the underlying unit (Unit 3) was all deposited at the same time, this suggests a high degree of variability in the sediment supply to different parts of the salt pond, with the west-central section’s getting more than twice the amount of sediment deposited then the eastern reaches.

Unit 3

Unit 3 is only present in Core 3, and consists of the inclined bedded layers from the top of the core to 26cm of depth, in place of the 1-5cm thick microbial mat topping the other three cores. Despite the presence of coarse sands, the unit does not meet several key characteristics of an inundation event. It has inclined instead of horizontal bedding, a complete lack of shell parts, and lacks an upward fining progression. It also sits on a gradational contact with the top of Unit

2, instead of the erosional uniformity that an inundation event would leave.

It also does not match the criteria for the baseline sedimentation that would be expected for a salt pond. The transition between coarse and finer sands, as well as the bedding and the lack of vegetative material do not fit the profile for the slow, gradual accumulation of horizontal layers of sediment that would be expected. I propose that this unit is related to the construction of a road, parking lot, open play area, and public swimming pool located immediately north of the coring site. The road was first constructed sometime between 1958 and 1982 (as seen on USGS topographic maps of the area), and the swimming pool, buildings and parking lot were built sometime between 1991 and 2004. Approximately 100m of trees and vegetation that separated the ocean-facing beach from the salt pond were clear cut and replaced with concrete and a large grassy field. This would allow for winds to entrain sediment from the beach and field, and deposit them in the pond, a process which would not happen at the other three core sites, where belts of thick foliage remain in place. The asphalt would also result in more runoff during rain events, carrying sediment from the parking lot into the pond. The sediment being sourced from beach sands would also explain the lack of shell fragments and vegetative material, both of which would have minimal presence on the beach.

Chapter 6: Conclusions

Tsunamis pose a major threat to the health and safety of those living in the Caribbean. By examining sources and effects of past events, it is possible to help build society to withstand future waves in the region. A key method of conducting this examination is to look at the deposits that these waves leave behind. Tsunami deposits are often poorly preserved and are frequently reworked by other tsunamis, hurricanes, and normal coastal processes, and can also be confused with deposits from hurricanes, which often have similar characteristics. It is therefore critical to look for evidence of tsunamis in a wide variety of locations and depositional environments. One promising candidate for preserving tsunami deposits is coastal salt ponds, which generally have low sedimentation rates and little to no bioturbational activity. This study looked at the West End Salt Pond on the island of St. Croix, USVI, and found a shell and sand layer present in multiple locations across the salt pond that was interpreted to having been deposited by the 1867 tsunami. While absolute age dating from radioisotope dating or other means is not available, relative age dating using accumulation rates from other salt ponds on the

USVI supports attributing the deposit to the 1867 event. This provides the first evidence in the scientific literature of a deposit left by the 1867 Virgin Island tsunami on the island of St. Croix.

Deposits related to other tsunamis that have been identified in the USVI, BVI, and Puerto Rico

(Engel et al. 2016; Fuentes, Tuttle, and Schmidt 2017; Reinhardt, Pilarczyk, and Brown 2012) were not identified,. Addition sampling using methods capable of deeper core recovery could find evidence in the salt pond of these events. However, the large vertical extents of these deposits provide further evidence for the capacity of large, low-lying salt ponds to act as

31 excellent preservers of inundation events that flood into them. This record of historical and paleo-tsunamis is important for assessing the risk from natural disasters that communities in the

US Virgin Islands, the Caribbean, and the eastern coast lines of North and Central America face.

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