RECONNAISSANCE INVESTIGATION OF CARIBBEAN 

EXTREME WAVE DEPOSITS – 

PRELIMINARY OBSERVATIONS, INTERPRETATIONS, 

AND RESEARCH DIRECTIONS

Open-File Report 2006-1293

U.S. Department of the Interior

U.S. Geological Survey

Robert A. Morton, Bruce M. Richmond, Bruce E. Jaffe, and Guy Gelfenbaum
Cover photograph – Panorama view on the north coast of Bonaire at Boka Kokolishi in March 2006 showing (from 

left to right), Caribbean Sea, modern sea cliff, scattered boulders and sand deposits on an elevated rock platform, and a 

paleo-seacliff. Wave swept zone and fresh sand deposits were products of Hurricane Ivan in September 2004.
RECONNAISSANCE INVESTIGATION OF CARIBBEAN EXTREME WAVE 

DEPOSITS – PRELIMINARY OBSERVATIONS, INTERPRETATIONS, AND 

RESEARCH DIRECTIONS

Robert A. Morton1, Bruce M. Richmond2, Bruce E. Jaffe2, and Guy Gelfenbaum3

1

U.S. Geological Survey, 600 Fourth St. S., St. Petersburg, FL 33701 

2

U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060 USA

3

U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 USA

Open-File Report 2006-1293

U.S. Department of the Interior

U.S. Geological Survey
Contents

SUMMARY ...........................................................................................................................................................................1

INTRODUCTION...................................................................................................................................................................1

GENERAL GEOLOGIC SETTING.........................................................................................................................................1

EXTREME WAVE EVENTS AND DEPOSITS IN THE CARIBBEAN SEA – A BRIEF OVERVIEW...............................8

Historical Hurricanes ................................................................................................................................................8

Historical Tsunamis .................................................................................................................................................10

Reported Tsunami and Storm Deposits in the Caribbean Region....................................................................10

The Tsunami/Storm Wave Dilemma......................................................................................................................11

SUMMARY OF SITE OBSERVATIONS............................................................................................................................12

Ridges and Ridge Complexes ................................................................................................................................12

Bonaire ......................................................................................................................................................................13

Deposits Near the Shore and Near Sea Level........................................................................................... 13

Elevated Rock Platform Deposits................................................................................................................. 15

Puerto Rico................................................................................................................................................................17

Wetland Deposits............................................................................................................................................ 19

Tidal Flat Deposits........................................................................................................................................... 20

Coastal Plain Deposits ................................................................................................................................... 21

Guadeloupe...............................................................................................................................................................22

Deposits Near the Shore and Near Sea Level........................................................................................... 22

Wetland Deposits............................................................................................................................................ 23

Slightly Elevated Terrace Deposits .............................................................................................................. 23

ORIGINS OF COARSE CLAST AND BIMODAL DEPOSITS ..........................................................................................24

Origins of Ridges and Ridge Complexes ..............................................................................................................25

Origins of Boulder Fields.........................................................................................................................................27

Ocean Swell and Storm Waves.................................................................................................................... 27

Tsunamis........................................................................................................................................................... 30

Bimodal deposits......................................................................................................................................................31

DISCUSSION......................................................................................................................................................................31

Deposit Origins................................................................................................................................................ 31

Elevated Deposits ........................................................................................................................................... 33

CONCLUSIONS..................................................................................................................................................................34

FUTURE ACTIVITIES AND RESEARCH DIRECTIONS...................................................................................................34

Additional Field Investigations ..............................................................................................................................35

Morphology, Sedimentology, and Origins of Caribbean Coastal Deposits.....................................................35

Characterization of Coarse Clast Tsunami and Storm Deposits ......................................................................36

Modeling Coarse Clast Competence, Capacities, and Deposits of Tsunamis and Storm Waves ..............37

Hazard Vulnerability Analysis and Risk Assessment.........................................................................................37

ACKNOWLEDGEMENTS ..................................................................................................................................................37

REFERENCES......................................................................................................................................................................37
ii

Figures

Figure 1. The Caribbean region showing locations of Puerto Rico, Guadeloupe, and Bonaire relative to the major 

physiographic elements and tectonic plate boundaries. ................................................................................................2

Figure 2. Locations of observation sites in Bonaire............................................................................................................................3

Figure 3. Locations of observation sites in Puerto Rico.....................................................................................................................4

Figure 4. Locations of observation sites in Guadeloupe....................................................................................................................4

Figure 5. Tracks of all Atlantic tropical cyclones between 1851 and 2005......................................................................................9

Figure 6. General topographic profile, morphology, and depositional setting of reef-rubble ridge complex.........................13

Figure 7. Reef-rubble ridge complex near sea level on the western coast of Bonaire..............................................................14

Figure 8. Reef-rubble ridge complex near sea level on the eastern coast of Bonaire...............................................................15

Figure 9. Sharp-crested ridge near sea level on the northwestern coast of Bonaire constructed by Hurricane Lenny .....16

Figure 10. General topographic profile, morphology, and depositional setting of boulder field on rock platform.................17

Figure 11. Rock platform on the eastern side of Bonaire.................................................................................................................18

Figure 12. Boulder field on rock platform on the eastern side of Bonaire showing landward decrease in clast size..........19

Figure 13. General topographic profile, morphology, and depositional setting of reef-rubble ridge complex on elevated 

rock platform. ........................................................................................................................................................................20

Figure 14. Mining exposure on the eastern side of Bonaire ...........................................................................................................21

Figure 15. Panorama of landward sloping reef-rubble ridge complex on an elevated rock platform .....................................22

Figure 16. Mining pit on the seaward side of the reef-rubble ridge complex shown in fig. 15 .................................................23

Figure 17. Apron of cobbles deposited by Hurricane Lenny on an elevated rock platform.......................................................24

Figure 18. General topographic profile, morphology, and depositional setting of coastal plain and wetland setting. .........25

Figure 19. Core of sediments beneath wetlands at Bajura, Puerto Rico ......................................................................................26

Figure 20. General topographic profile, morphology, and depositional setting of elevated terrace at Anse Ste. Marguerite 

on eastern side of Grande-Terre, Guadeloupe................................................................................................................27

Figure 21. Coral-rubble ridge constructed by Tropical Cyclone Bebe ..........................................................................................28

Figure 22. Coral-rubble ridge complex on Funafuti Atoll and Tropical Cyclone Bebe deposits ................................................29

Figure 23. Exposure of polymodal reef-derived sediments .............................................................................................................30

Tables

Table 1. Descriptions and data collected at field observation sites in Bonaire...............................................................................5

Table 2. Descriptions and data collected at field observation sites in Puerto Rico ........................................................................6

Table 3. Descriptions and data collected at field observation sites in Guadeloupe........................................................................7
SUMMARY

This report presents an overview of preliminary 

geological investigations and recommended future 

research activities in the Caribbean region pertaining 

to coastal hazards with an emphasis on establishing 

tsunami risk for U.S. territories. Fieldwork was 

conducted in March 2006 on the islands of Bonaire, 

Puerto Rico, and Guadeloupe to evaluate the 

stratigraphic records of extreme wave deposits as 

possible indicators of paleotsunami recurrence. 

Morphological, sedimentological, and stratigraphic 

evidence indicate that shore-parallel coral rubble 

deposits composed of coarse clasts and sand that 

are 10s of meters wide and several meters thick are 

depositional complexes that have accumulated for 

a few centuries or millennia, and are not entirely 

the result of one or a few tsunamis as previously 

reported. The origins of boulder fields on elevated 

rock platforms of the Caribbean islands are more 

complicated than the origins of ridge complexes 

because boulder fields can be constructed by either 

storm waves or tsunamis. What is needed now for 

more conclusive interpretations is a systematic 

sedimentological approach to deposit analysis and 

a set of criteria for distinguishing between coarse 

clast storm and tsunami deposits. Assembling more 

field data from other Caribbean islands, analyzing 

stratigraphic deposits on Puerto Rico and Bonaire, 

and investigating boulder field deposits resulting 

from a historical tsunami can accomplish this. Also 

needed are improved sediment transport models 

for coarse clasts that can be used to estimate the 

competence and capacity of tsunamis and storms 

waves and to determine whether a deposit likely was 

created by a tsunami or extreme storm. Improved 

models may also be useful for reconstructing the 

magnitude of extreme wave events.

INTRODUCTION

The December 26, 2004 Indian Ocean 

earthquake and tsunami were grim reminders of the 

devastating forces associated with these catastrophic 

events and their ability to cause widespread 

death and destruction. Since then government 

agencies, scientists, and engineers have focused 

their attention on other regions of the world where 

active tectonic processes increase the probability 

of generating tsunamis that could cause loss of life 

and property. One such region is the Caribbean 

Sea and surrounding islands where historical 

earthquakes, volcanic eruptions, and associated 

tsunamis have been reported (Lander and others, 

2002). The increased public awareness of coastal 

hazards after the devastating 2004 tsunami follows a 

general scientific trend in recent years to reconstruct 

histories of tsunami hazards from the sedimentary 

record, especially for areas where the written 

histories are short or incomplete. 

This report summarizes our observations and 

preliminary interpretations of coastal sediments 

deposited by extreme waves that were investigated 

on the islands of Bonaire, Puerto Rico, and 

Guadeloupe (Figs. 1-4). This work was conducted 

in early 2006 as part of a regional reconnaissance 

of possible paleotsunami deposits in the Caribbean 

region. We were accompanied in Bonaire by 

Anja Scheffers and Dieter Kelletat (University 

of Duisburg-Essen), in Puerto Rico by Juan 

Carlos Moya (Texas General Land Office), and in 

Guadeloupe by Narcisse Zahibo (Université Antilles 

Guyane). These individuals have published tsunami 

research for the respective islands (see references), 

they were familiar with the local settings, and they 

served as field guides to coastal sites where tsunami 

deposits might be preserved. The field methods and 

types of data collected at all sites included GPS 

locations, photographs, and field notes, while at 

some sites additional data were collected in the form 

of shallow trenches, narrow push cores, and some 

sediment samples (Tables 1-3). 

GENERAL GEOLOGIC SETTING

The Caribbean Sea is one of the active tectonic 

regions of the Earth. Earthquakes, volcanic 

eruptions, and submarine landslides, which are all 




howing locations of Puerto Rico, Guadeloupe, and Bonaire relative to the major physiographic elements and tectonic plate boundaries.
capable of generating tsunamis, are recurrent events 

in the region as a result of the North American, 

South American, and Atlantic plates moving 

obliquely westward relative to the Caribbean plate. 

The zone of oblique subduction between the North 

American and Caribbean plates forms the deep 

Hispaniola and Puerto Rico trenches, which lie 

north of the islands of Hispaniola and Puerto Rico, 

respectively. The deep ocean Mona Passage on the 

western side of Puerto Rico and associated Mona 

Canyon are in the zone of transtension created 

by separation of the Puerto Rico microplate from 

Hispaniola (McCann, 2006). Rupture of a fault in 

the Anegada trough was responsible for the 1867 

earthquake and tsunami that impacted the Virgin 

Islands, whereas failure of the Mona Canyon Fault 

generated the 1918 earthquake and tsunami that 

inundated northwestern Puerto Rico (Lander and 

others, 2002).

Most islands of the Caribbean region owe their 

origins to formation, growth, and emergence of 

submarine volcanoes. As a result, they typically 

have a core of volcanic rocks that is surrounded by 

coral reef limestones and other associated carbonates 

that were deposited in the shallow, warm tropical 

sea. Even today there are many active volcanoes 

along the island arc of the Lesser Antilles between 

Puerto Rico and Venezuela (Pararas-Carayannis, 

2004). Recent major volcanic eruptions that 

generated local tsunamis (Zahibo and Pelinovsky, 

2001; Lander and others, 2002; Pararas-Carayannis, 

2004) include the 1995 explosion of Soufriere Hills 

and 1997 debris avalanche on Montserrat, the 1902 

explosion of Mt. Pelée on Martinique, and the 1990 

explosion of Kick‘em Jenny, an active submarine 

volcano north of Granada. Flank failures and other 

landslides around these volcanic islands have 

produced massive debris flows (Deplus and others, 

2001) that likely have generated local tsunamis.

Details of late Quaternary sea-level fluctuations 

and vertical movement of land masses in the 

Caribbean region are not well known, and yet that 

Figure 2. Locations of observation sites in Bonaire.




Figure 3. Locations of observation sites in Puerto Rico.

Figure 4. Locations of observation sites in Guadeloupe.


onal Setting Data Sediment Composition Comments

mplex near sea level P cobble-boulder coral rubble Pink Beach, H. Lenny and H. Ivan

mplex near sea level P cobble-boulder coral rubble Slave hutsmplex near sea level P cobble-boulder coral rubble Near Willemstoren Lighthousemplex near sea level P cobble-boulder coral rubble Tori’s Reef

rm P isolated boulders H. Lenny erosion of footpathrm P wave-cut notch Interpreted as stage 5erm P isolated boulders Devils Mouth paleokarst

es near sea level P&E sand and coral rubble S. of Goto Meer, low storm ridge and flat-crested ridge, two bed multiple event deposit

mplex near sea level P cobble-boulder coral rubble Salina Tern

dge complex on 

rm P cobble-boulder coral rubble and sand S. of Boka Washikembarm P boulder field and sand Near Boka Washikembarm P organic debris and trash Boka Washikemba, H. Ivan wrack linerm P boulder field S. of Spelonk Lighthouserm P graded boulder field Boka Olivia

dge complex on 

rm P&E cobble-boulder coral rubble and sand S. of Boka Onimarm P cobble-boulder coral rubble Boka Chikiturm P cobble-boulder coral rubble Seru Grandi, low-amplitude bedformsrm P cobble-boulder coral rubble Boka Kokolishi, vermetid rims

dge complex near sea 

P cobble-boulder coral rubble Boka Bartol

rm P small cobble fans Boka Bartol, H. Lenny

sea level, elevated rock 

P cobble ridge, small cobble fans on platform North of Boka Slabaai, H. Lenny

r sea level P sand and cobbles Boka Slagbaai, storm beach

llected at field observation sites in Bonaire. Data key, P=photographs, E=mining exposure. Numbered sites shown on Figure 2.
ng Data Sediment Composition Comments

land P, C1-2, S2 dark brn. sandy peat, brn. muddy sand, brown mud Laguna Tortuguero wetland coreland P, C3, T1-3, S12 dark gray mud with organics and roots Espinar, some fill, wetland core, recent overwashland P, C4-5 brn. and gray sandy mud, sand with heavy min. layer Parque Aguadilla, wetland coreland P, C6-7, T4 dark. brn. sandy peat, tan muddy sand and sand with shell, upward fining Aguada Beach, wetland coreland P, C-8, T5-7 dark brn. muddy sand mottled gray, stiff mud Escuela Epifaunio Estrada, wetland coreland P, C9, S1 gray muddy sand, peat, muddy sand, peat, sand/shell Isabela Beach, wetland coreland P, C10, T8, S4 tan and gray sand, lam. mud with 2 thin sand layers/shell East Bajura, wetland core, recent overwashland P, C11 dark brn. organic sand, tan sand/coral gravel, gray sand Maleza Alta, wetland core

P rocky beach, sea cliff, destroyed lighthouse Point Borinquen 1918 tsunami

ves P, C12, S1 brn. muddy sand grading to gray muddy sand, gray mud with red/brn. organics and muddy peat Punta Cucharas, tidal-flat core

P, C13 brn. organic mud, gray mud, brn. mud, gray sand, coarse white carbonate sand Punta Arenas, tidal-flat core

land P no observations Punta Figuras, beach ridgesland P, C14-15, S7 drk. brn. peat and muddy peat, coarse sand /organics Palmas del Mar, wetland cores, some fillland P, T9 tan mud, gray muddy sand, gray sand, white carbonate sand/ shell frags Rio Blanco, tree harvesting

P, T10, E, S1 multiple sand layers containing rope, plastic, alum. cans Bella Lucia, recent overwash

llected at field observation sites in Puerto Rico. Data key, P=photographs, C=core#, T=trench#, S=# of samples, E=exposure. Numbered sites 
llected at field observation sites in Guadeloupe. Data key, P=photographs, C=core#, S=# of samples, E=mining exposure. Numbered sites shown 

etting Data Sediment Composition Comments

ond P, E S2 sand and cobble/boulders Point Allegre, recent overwash

etland P, S1 sand Plage de Cluny, ridge with high avalanche slopeetland P, C1 S2 tan and gray root-mottled mud Plage de la Perle, mangrove wetlandgoon P no observations Grand Anse lagoonetland P sand and slightly muddy sand Grand Anse park, low beach ridges

ope P covered by development Deshaise, church hit by 1867 tsunami

ain P boulders on beach and rock platform Pointe des Chateaux

P beach rock and sand Anse Tarare

n P covered by development St. Anne, church hit by 1755 tsunamin P, E mixed sand and cobbles, isolated boulders Anse Ste. Margueriten P mixed sand and cobbles, isolated boulders Anse Maurice
history of relative sea-level changes is critical 

to understanding the origins of elevated coastal 

deposits on some of the islands. The relative vertical 

motion of land and sea involves independent tectonic 

processes (subsidence, uplift) and glacio-eustatic sea 

level fluctuations along with subsequent balancing of 

ocean water levels and crustal deformation, known 

as glacial isostatic adjustment (GIA). The GIA 

model results explain the documented monotonic 

rise in Holocene sea level and absence of a midHolocene highstand in the Caribbean region as the 

result of balancing between the glacial meltwater 

component and the equatorial ocean siphoning 

component. The latter component being the principal 

effect in those regions where water flowed away 

from far-field regions toward the collapsed forebulge 

regions to fill the accommodation space created by 

subsidence of the sea floor (Mitrovica and Milne, 

2002). After values for lithospheric thickness and 

viscosity of different regions were adjusted, results 

of the GIA model runs match favorably the postglacial sea level curves from the east coast of South 

America and from the Caribbean, which include 

curves from Jamaica and Curacao (Milne and others, 

2005). The model data for Curacao, in the southern 

Caribbean Sea, suggest there was no Holocene sealevel highstand for the island.

EXTREME WAVE EVENTS AND 

DEPOSITS IN THE CARIBBEAN SEA – 

A BRIEF OVERVIEW

Coastal flooding hazards in the Caribbean region 

are caused primarily by extreme ocean waves, which 

can be generated by either tropical cyclones (storms) 

or tsunamis. These extreme wave events inundate 

low-lying coastal areas with salt water, erode 

beaches, and transport sediments inland where they 

can be preserved as a record of the event. Extreme 

wave events in the Caribbean also are responsible 

for entraining, transporting, and depositing coarse 

clasts, including megaclasts, which have been 

reported on many islands (Jones and Hunter, 1992; 

Taggart and others, 1993; Hearty, 1997; Scheffers, 

2002, 2004; Scheffers and others, 2005, Scheffers, 

2005; Scheffers and Kelletat, 2006; Robinson and 

others, 2006). Following are summaries of the 

general conditions that generate extreme waves 

in the Caribbean Sea, the most recent significant 

storm and tsunami events that affected the islands 

of Bonaire, Puerto Rico, and Guadeloupe, and 

previously reported storm and tsunami deposits in 

the Caribbean region.

Historical Hurricanes

Tropical cyclones (tropical storms and 

hurricanes) are annual events that generate extreme 

waves in the Caribbean Sea. Each year there are 

at least 10 to 12 named hurricanes and tropical 

storms active in the Atlantic Ocean, Caribbean Sea, 

or Gulf of Mexico. That means each year several 

tropical cyclones impact the shores of at least a few 

Caribbean islands. Many hurricanes entering the 

Caribbean Sea originate as classical Cape Verde 

tropical systems in the Atlantic Ocean off the 

northwest coast of Africa north of the equator (Fig. 

5). These intense storms, which track from east to 

west, exhibit counter-clockwise wind circulation 

around a center of low barometric pressure (Simpson 

and Riel, 1981). 

As a result of storm movement and associated 

wind patterns, east- and north-facing shores typically 

experience the greatest impacts from high storm 

surge and wind-driven currents. South facing 

shores are impacted when the storms track south 

of the islands. Extremely rare exceptions to these 

paths and impact patterns are caused by storms 

that form within the region that track from west to 

east, such as Hurricane Lenny in 1999. Because 

the zone of cyclogenesis is centered roughly 10o 

north of the equator, frequency of impacts from 

hurricane-generated waves is highest in the northern 

Caribbean (Puerto Rico, Haiti, Dominican Republic, 

Cuba, Jamaica), and decreases toward the southern 

Caribbean (islands of the Netherlands Antilles and 

the north coast of South America).
Hurricane winds generate and propagate deep 

ocean swell that can cause substantial coastal change 

far from the storm center. This principle of coastal 

change caused by breaking waves from distant storm 

sources was well documented by sandy overwash 

deposits observed by one of us (RAM) in a village 

on Tierrabomba, an island south of Cartagena, 

Colombia. Flooding from Hurricane Lenny, which 

was centered more than 500 km north of the 

Caribbean coast, caused the overwash. The principle 

of distant swell and coastal change is also important 

for Bonaire. A query of the NOAA (2006) historical 

tropical cyclone database shows that since 1851, no 

category 3-5 hurricanes have come within 100 km 

of Bonaire and only 11 tropical storms or hurricanes 

of category 1 and 2 have crossed or passed that 

close to the island. In contrast, Hurricanes Ivan 

(2004) and Lenny (1999), both category 4 storms 

(Saffir-Simpson scale) came no closer than 110 

km and 375 km respectively from Bonaire, and yet 

their waves constructed or reshaped coral rubble 

ridges near sea level and moved megaclasts on rock 

platforms (Scheffers, 2005). The historical data 

demonstrate that storm waves frequently impact the 

shores of Bonaire either as a result of less intense 

storms passing close to the island or the effects of 

deep-ocean swell generated by high intensity storms 

passing farther from the island. 

In Puerto Rico, beaches and adjacent coastal 

plains are regularly flooded by high-energy storm 

waves. Since 1867 there have been 14 category 3 or 

4 hurricanes that have crossed or come within 75 km 

of Puerto Rico. The most recent category 4 events 

to cause widespread beach erosion and overwash 

Figure 5. Tracks of all Atlantic tropical cyclones between 1851 and 2005. Obtained from the National Hurricane Center (http://www.nhc.

noaa.gov/index.shtml).
10

were Hurricanes Hugo (1989) and Georges (1998). 

Like Puerto Rico, Guadeloupe lies within the paths 

taken by many tropical cyclones. As a result, there 

have been 11 category 3 or category 4 hurricanes 

that have crossed or come within 75 km of the 

island since 1871 (NOAA, 2006). Five of those 

storms (unnamed, 1899; unnamed, 1926; Cleo, 

1964; Inez, 1966; and Hugo, 1989) passed directly 

over either Grande-Terre or Basse-Terre. There 

are also accounts of older extreme storms striking 

Guadeloupe, such as the 1776 event that killed 

more than 6000 people (Depradine, 1989). The 

most recent intense storms to impact the east coast 

of Guadeloupe were Georges (1998), Luis (1995,), 

and Hugo (1989). The eye wall of Hugo crossed the 

island at Anse Ste. Marguerite with wind speeds of 

220 km/hr.

Historical Tsunamis

All the most common mechanisms for 

generating local tsunamis (earthquakes, landslides, 

volcanic eruptions) are present in the Caribbean 

Sea. Caribbean island shores facing east toward 

the Atlantic Ocean also are at greatest risk from 

transoceanic tsunamis generated by remote sources, 

such as the one caused by the Lisbon, Portugal 

earthquake in 1755. Lander and others (2002) 

reviewed a 508-year record of written accounts of 

91 anomalous wave events in the Caribbean region 

between 1492 and 2000. Based on their analyses 

they were able to verify 27 historical tsunamis and 

concluded that an additional nine events were likely 

tsunamis. The remaining 60% of the reported wave 

events were attributed to other ocean phenomena 

such as hurricane storm surges, or rogue waves. 

Nearly all islands within the Caribbean region 

have experienced an historical tsunami. However, 

tsunamis generated in the Caribbean Sea tend to be 

highly focused so their impacts are limited to short 

segments of only one or a few islands. Their local 

runup elevations can be high, but they do not have 

the global impact of teletsunamis, such as the 2004 

Indian Ocean event. 

Of the historical tsunamis that impacted Puerto 

Rico in 1867, 1918, and 1943 (Lander and others, 

2002), the 1918 event was the most devastating. 

This event with a reported maximum wave height of 

6 m (Reid and Taber, 1919) inundated the northwest 

coast of the island where its greatest effects were 

felt between the coastal communities of Bajura 

and Aguada (Moya and Mercado, 2006). Mercado 

and McCann (1998) modeled the runup from the 

1918 tsunami along the west coast of Puerto Rico 

and obtained results that reasonably matched the 

observations of eyewitnesses. 

According to historical reports, the island of 

Guadeloupe experienced tsunamis in 1690, 1842, 

1867, 1985, and 2004 (Lander and others, 2002; 

and Zahibo and others, 2005). The latter two 

events were insignificant because wave heights 

were substantially less than one meter. The most 

significant tsunami to strike Guadeloupe, which 

was in 1867, had a reported wave height of 18.3 m 

(Lander and others, 2002), but subsequent modeling 

and reinterpretation of eyewitness reports indicates 

wave runup was probably less than 10 m (Zahibo 

and Pelinovsky, 2001). 

No historical tsunamis have been reported 

on Bonaire but that does not mean that the island 

is exempt from tsunami hazards considering its 

tectonic setting near the northern coast of Venezuela, 

which is an earthquake-generating region. Many 

tsunamis associated with earthquakes have locally 

inundated the northern coast of Venezuela including 

some of the oldest reported tsunamis for the 

southern Caribbean Sea. The catalog of tsunami 

events and analysis of local tsunami impacts on the 

Caribbean coast of Venezuela (Lander and others, 

2002; Schubert, 1994) provide a basis for evaluating 

tsunami risks on Bonaire.

Reported Tsunami and Storm Deposits in the 

Caribbean Region

Despite the large number of historical storm 

and tsunami impacts recorded for the Caribbean 

islands, there are relatively few published reports 
containing adequate geological descriptions of 

the subaerial coastal sediments deposited by 

these extreme wave events. Megaclast deposits at 

nearshore sites on the Grand Caymans, Puerto Rico, 

Bahamas, Aruba, Curacao, Bonaire, Guadeloupe, 

Barbados, St. Martin, Anguilla, and Jamaica (Jones 

and Hunter, 1992; Taggart and others, 1993; Hearty, 

1997; Scheffers, 2002, 2004; Kelletat and others, 

2004; Scheffers, 2005; Scheffers and others, 2005; 

Scheffers and Kelletat, 2006; Robinson and others, 

2006) have captured the most attention because of 

their extraordinary particle size and the requirement 

of extremely powerful waves to transport the large 

boulders. Although some of the authors interpret the 

megaclast deposits as tsunamigenic in origin (Jones 

and Hunter, 1992; Scheffers, 2002, 2004; Kelletat 

and others, 2004; Scheffers, 2005; Scheffers and 

others, 2005; Scheffers and Kelletat, 2006), they 

all acknowledge that either large tsunami or storm 

waves might have been responsible for the preserved 

boulder fields and unsorted debris ridges. 

The scientific focus on boulder fields and coralrubble ridges in the Caribbean region represents a 

bias in terms of nearshore deposits because sandy 

beaches are common around most of the islands. 

For example, sand beaches are so plentiful in 

Puerto Rico that they have been heavily exploited 

for construction aggregate to meet the demands 

for economic expansion in the country. In fact 

mining beach and dune sand near San Juan and 

elsewhere increased shoreline erosion and the inland 

penetration of storm overwash (Capacete, Martín 

& Associates, 1978; Rodríguez and others, 1994). 

Limits of wave inundation associated with winter 

storms between 1962 and 1968 were mapped along 

the north shore of Puerto Rico (Fields and Jordan, 

1972), but the associated sand overwash deposits 

were not investigated. Although sand beaches and 

shorefaces are common in the Caribbean region, 

only two reports have been identified that address 

onshore sand deposits emplaced by hurricanes or 

tsunamis. Both of the articles are for sand deposits 

in Puerto Rico (Bush, 1991; Moya and Mercado, 

2006).

The Tsunami/Storm Wave Dilemma

Students of coarse clast coastal deposits 

commonly are challenged by not knowing if 

tsunamis or storm waves were responsible for 

the coarse rubble and boulder fields that can be 

observed at sites around the world (Bourrouilh-Le 

Jan and Talandier, 1985; Jones and Hunter, 1992; 

Taggart and others, 1993; Hearty, 1997; Nott, 1997; 

Noormets and others, 2002; Williams and Hall, 

2004; Scheffers and others, 2005; Scheffers, 2005; 

Scheffers and Kelletat, 2006; Robinson and others, 

2006). Extreme waves are capable of dislodging 

blocks weighing tens to several hundred tons and 

emplacing those blocks on superjacent intertidal reef 

flats or rock platforms elevated above sea level. 

Many interpretations of coastal megaclast 

origins have relied heavily on equations published 

by Nott (1997, 2003) to determine if tsunami or 

hurricane waves are the most likely mechanism 

for particle entrainment and movement. Results 

of calculations using the Nott equations always 

lead to the conclusion that hurricane waves must 

be approximately four times higher than tsunami 

waves to transport the same size particle (Nott, 

2003). Nott (2003) assumed that the flow velocity 

in storm waves is sqrt(gh) (eq. 15), while tsunami 

flow velocity is 2 sqrt(gh) (eq. 14). The wave height 

required to overturn boulders, the mode of transport 

assumed by Nott (1997, 2003) goes as the velocity 

squared, which is proportional to the height of storm 

waves and to 1⁄4 the height of tsunami waves. As a 

result of the assumed flow velocity to wave height 

relation, a storm wave approximately four times 

the height of a tsunami is required to transport the 

same boulder (Table 1 in Nott, 2003). Also the Nott 

equations are formulated assuming that the forces 

are primarily drag and the mode of transport is by 

particle overturning. The assumption of particle 

overturning may not be correct because there is 

growing evidence from high wave energy coasts that 

megaclast movement, at least on rock platforms, is 

by sliding rather than by overturning (Williams and 

11
12

Hall, 2004; Noormets and others, 2004) when the 

blocks are not fully submerged.

Andrew Moore (2006, personal communication) 

examined the Nott (1997) equations for particle 

overturning and underscored their limitations 

because they don’t include inertial forces that likely 

are important and they incorporate components 

of other wave equations that may have been 

misapplied. Moore also pointed out that the Nott 

equations use the term flow velocity to represent 

wave celerity and two different types of flow 

velocity (depth-averaged versus depth-value) as if 

they all were the same parameter.

Noormets and others (2004) calculated pressures 

and entrainment values for ocean swell (storms) and 

turbulent bore (tsunami) conditions and concluded 

both are capable of quarrying megaclasts from cliff 

edges given sufficient prior fracturing. They also 

concluded that shorter period storm waves have 

higher quarrying capacity and storm surge flooding 

of the rock platform promotes block transport, 

but longer period tsunami waves have higher 

transporting capacities because each wave transports 

the block for a longer time than a storm wave.

SUMMARY OF SITE OBSERVATIONS

Coastal deposits on the Caribbean islands can 

be composed of sand, mixed sand and gravel, or 

extremely coarse clasts including boulders (0.25 

– 4.1 m) and blocks (4.1 – 65.5 m) as defined by 

Blair and McPherson (1999). On some islands, such 

as Puerto Rico, the fringing coastal deposits consist 

predominantly of sand whereas on other islands, 

such as Guadeloupe and Bonaire, coastal deposits 

consist of bimodal sizes including sand mixed with 

cobble/boulder sized coral rubble derived from 

adjacent reefs and megaclasts quarried by waves 

from rock platforms and sea cliffs. 

The following discussion summarizes field 

observations made in March 2006 on the three 

islands on the basis of common depositional settings 

that influence the morphologies of the deposits and 

their origins. Summaries of the site observations are 

presented in Tables 1-3.

Ridges and Ridge Complexes

On many Caribbean islands the coastal deposits 

near the shore are composed of coarse coral reef 

rubble molded into two different morphologies, 

ridges and ridge complexes. A ridge is a narrow, 

wall-like feature up to a few meters high consisting 

of unconsolidated gravel, shingle, or coral rubble 

deposited by waves near the shore. This is also the 

definition of a rampart (Bates and Jackson, 1987), 

a less commonly used geomorphic term. At time 

scales of years to decades, ridges are typically 

unstable landforms that are subsequently reworked 

by moderate energy waves that cause them to either 

migrate landward or to be transformed into sheets 

(Blumenstock and others, 1961; Stoddart, 1965; 

Baines and McLean, 1976). In the literature the 

terms gravel ridge (McKee, 1959), storm beach 

(Baines and others, 1974), storm ridge (Scoffin, 

1993), and rampart (Maragos and others, 1973; 

Baines and McLean, 1976) have been applied to 

some single-event ridge-like deposits. For this 

report the term ridge is restricted to an isolated, 

narrow, low, shore-parallel feature that was 

deposited initially during a single episode but may 

subsequently be modified by a few multiple-wave 

events while still retaining its individual identity.

A ridge complex is a broad, composite, shoreparallel feature generally > 2 m thick consisting 

of unconsolidated gravel, shingle, or coral rubble 

deposited by waves near the shore. A ridge complex 

can exhibit ridge and swale topography, be flatcrested, or be wedge-shaped generally with the 

highest crest elevation near the ocean and surface 

elevations decreasing landward. A ridge complex 

represents an accumulation of many depositional 

events resulting from the long-term reworking and 

coalescence of multiple event deposits including 

former isolated ridges. At time scales of years 

to decades, ridge complexes are generally stable 

landforms that aggrade or prograde seaward as 
additional material accumulates on the seaward side. 

Similar morphological features have been referred 

to in the southern Caribbean Sea as ridges (Schubert 

and Valastro, 1976) and in the Pacific as hurricane 

banks (Baines and others, 1974), shore ridges 

(McKee, 1959), ridge-swale complexes (McLean 

and Hosking, 1991), and beach ridges (Scoffin, 

1993). Along paraglacial coasts similar stormdeposited features are referred to as gravel barriers 

(Orford and others, 2002; Forbes and Syvitski, 

1994).

Bonaire

No historical tsunamis have been reported for 

Bonaire. Consequently sites investigated around the 

island for possible tsunami deposits were diverse 

with respect to their physical settings and to sea 

level.

Deposits Near the Shore and Near Sea Level

All the sites visited on Bonaire (Fig. 2) with 

substantial coarse clasts deposited near the shore and 

near sea level have many common characteristics 

(Table 1). One common attribute is their location 

on the west and south (leeward) sides of the island. 

These west and south facing shores are generally 

protected from high-energy waves generated by 

the predominant trade winds. Deposits near sea 

level are also common around the southern shores 

of the island because the landmass has been tilted 

tectonically so that the southern end has subsided 

and the northern end uplifted (Alexander, 1961). As 

a result of this tilting, sea cliffs predominate along 

the central and northern segments of the island, 

whereas broad tidal flats prevail along the southern 

segments. The deposits near sea level generally form 

ridge complexes a few tens of meters wide that are 

composed of coral rubble with zones of imbricated 

clasts concentrated near the shoreline. A narrow 

barren zone of cemented reef rock or beach rock 

typically separates the rubble ridge complex from 

the sea. The ridge complexes have steep concave 

backbeach slopes that rise to intersect the flat crests 

(Fig. 6), crest elevations are about 2 to 2.5 m above 

sea level (asl), and landward terminations of most 

of the ridge complexes are steep, well-defined 

avalanche slopes (Figs. 6 and 7). 

A typical example of one of these coarse clast 

deposits near sea level is north of Willemstoren 

Lighthouse (Fig. 2, site 2). The ridge complex 

remnant is composed of a bimodal mixture of sand 

and coarse clasts with most of the clasts at the 

surface being cobbles to fine boulders (Fig. 8). A 

wrack line of wood debris and plastic covers the 

seaward side of the flat crest, representing evidence 

of recent storm inundation. Some clasts on the 

surface and near the shore side of the ridge complex 

are imbricated. Scheffers (2005) interpreted this 2 to 

Figure 6. General topographic profile, morphology, and depositional setting of reef-rubble ridge complex near sea level.

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14

3 m high, 40-m wide shore-parallel ridge complex 

as a tsunami deposit. Much of the feature has been 

removed by extensive mining for construction 

aggregate. Before mining, a much broader zone 

of coarse clast accumulation would have formed 

a ridge complex extending inland at least another 

50 m.

North of Boka Slagbaai (Fig. 2, site 22) is a 

ridge (spit) near sea level composed of cobbles 

and fine boulders (Fig. 9). Aerial photograph and 

field evidence (Scheffers, 2005) demonstrate that 

the ridge was constructed by Hurricane Lenny in 

1999 and was modified by Hurricane Ivan in 2004. 

Comparison of our 2006 photographs with the 

post-Ivan photograph of Scheffers (2005) shows 

subsequent ridge aggradation by smaller, nonhurricane waves that break and run up on the ridge 

and deposit fine boulders. In 2006 the ridge was 

about 100 m long, 5 to 8 m wide at sea level, and 

had a narrow, sharp crest that was about 2 m asl. 

At Boka Slagbaai, a low, landward sloping 

barrier beach near sea level encloses a small 

wetland-fringed embayment (Fig. 2, site 23). The 

barrier, which is composed predominantly of sand 

with some cobbles, was formed by storm overwash 

that has transported sediments across the barrier and 

deposited small fans in the adjacent embayment. 

Fresh sand and pebble-size pieces of coral debris 

Figure 7. Reef-rubble ridge complex near sea level on the western coast of Bonaire showing flat crest and steep avalanche slope (Fig. 2, 

site 8).
cover the surface of the barrier, which is barren of 

vegetation. Hurricane Lenny, which was the most 

recent extreme wave event to overwash the barrier, 

damaged buildings that were constructed originally 

in 1868 (Scheffers, 2005).

Elevated Rock Platform Deposits

Of the 13 sites on Bonaire that have coarse clasts 

deposited on rock platforms (Table 1) several meters 

or higher above sea level, eight sites are on the 

eastern side of the island where exposure to highenergy waves and a limestone terrane results in a 

near vertical sea cliff. The surfaces of the platforms 

are covered with microkarst solution cavities. A 

wave-cut notch near sea level promotes active 

cliff face retreat that together with joint-controlled 

fractures make the cliff unstable. Undermining 

beneath the notch causes blocks to fall into the sea or 

allows surging waves to more easily quarry blocks 

by increasing the surface area exposed to uplift and 

air compression (Noormets and others, 2002). 

Coarse clasts deposited on rock platforms above 

sea level consist mostly of: (1) extensive boulder 

fields, (2) thick ridge complexes composed of 

bimodal sediments, (3) small, thin wedge-shaped 

aprons, or (4) isolated megaclasts. At most of 

the elevated platform sites (Fig. 2, sites 4, 6, 10, 

Figure 8. Reef-rubble ridge complex near sea level on the eastern coast of Bonaire showing flat crest, recent storm debris, and clast 

imbrication (Fig. 2, site 2). Largest clasts in the foreground are approximately 0.5 m long.

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16

13,14, 16, 17, 19, 21, 22) the deposits are isolated 

megaclasts or fields of boulders of various sizes 

(Figs. 10-12). At nearly all the sites, the megaclasts 

are separated from the sea-cliff shoreline by a zone 

on the platform where loose sediments are absent, 

having been swept away by repeated storm-wave 

inundation. On the east side of the island, the rock 

platform between the present sea cliff and the paleosea cliff is several hundred meters wide. At site 10, 

blocks having estimated weights of 30 to 50 tons 

(Scheffers, 2005) are located about 110 m from 

the sea cliff where they were quarried by waves 

and emplaced on the elevated platform about 5 m 

asl. The place where boulder field accumulation 

on the elevated rock platform is most consistent 

with that expected by a tsunami is site 14 (Fig. 2). 

There the coarse clasts are graded so that particle 

size decreases away from the sea-cliff shoreline 

(Fig. 12). At rock platform sites 4 and 6 on the west 

coast of the island, the ancestral sea cliff is near 

the shore and the isolated boulders may have been 

derived from the ancestral sea cliff and transported 

down slope to the relatively flat platform by gravity 

processes.

At sites 9 and 15 (Fig. 2), the coral-rubble 

deposits form broad landward-sloping ridge 

complexes composed of bimodal sediments (Figs. 

13-16). Mining exposures show that the clastFigure 9. Sharp-crested ridge near sea level on the northwestern coast of Bonaire constructed by Hurricane Lenny and subsequently 

modified by less energetic waves (Fig. 2, site 22).
supported framework of cobbles and boulders is 

filled with sand that at the surface is unweathered 

(Figs. 14-16). This suggests that the bimodal 

accumulation may be the result of framework 

deposition by extreme waves and subsequent 

framework infiltration of sand delivered by less 

powerful waves. The exposures also show that 

the sediments are internally organized with crude 

stratification, vertical textural trends, and bounding 

surfaces (Figs. 14 and 16). At site 9 (Fig. 2), a shorenormal exposure reveals internally imbricated clasts 

and steep seaward dipping beds that exhibit upwardfining textural trends (Fig. 14). The dip of the beds 

is similar to the slope of the ramp face exposed 

to waves. The juxtaposition of alternating beds of 

coarse and fine clasts forms crude stratification. At 

site 15 (Fig. 2), the ridge complex deposits are about 

80 m wide, 3.5 to 4 m thick at the seaward crest and 

taper landward forming a wedge. Near its seaward 

edge the ridge complex consists of two primary 

stratigraphic units (Fig. 16). The shore-parallel 

exposure shows that the lower unit is composed of 

sand and rounded, disc-shaped clasts that exhibit 

upward-fining textural trends near the top (Fig. 

16A). Equant to platy, subrounded to rounded 

clasts of the overlying upper unit are substantially 

larger, imbricated, and tend to coarsen upward. 

The largest clasts are isolated boulders resting on 

the ridge complex crest (Fig. 16B), which is about 

9 m asl (Scheffers, 2005). The bounding surface 

separating the two units is a disconformity resulting 

from distinctly different depositional episodes with 

different clast sizes deposited. The composition and 

internal organization of the ridge complex deposits 

are similar to those illustrated by Bluck (1967) for 

gravel beaches in south Wales where the framework 

of coarse clasts is filled with smaller particles 

including sand, whereas sand is absent from the 

youngest surficial deposits.

At sites 21 and 22 (Fig. 2) where the rock 

platform is elevated about 6 m asl, aprons of 

rounded light-colored particles that range in size 

from pebbles to small boulders form a thin wedgeshaped deposit (Fig. 17) that is scalloped along its 

seaward edge. The small aprons, which thicken 

landward from individual clasts to a low ridge 

several clasts thick, also coarsen landward. The 

aprons, which were deposited by Hurricane Lenny 

(Scheffers, 2005), are separated from the sea-cliff 

shoreline by a wave-washed zone of non-deposition. 

Puerto Rico

Most of the potential paleotsunami sites we 

investigated in Puerto Rico (Fig. 3, Table 2) were 

in or near reed or mangrove wetlands or on barren 

tidal flats that are separated from the ocean by a 

beach and narrow coastal plain composed of sand 

(Fig. 18). These sites were selected because they 

represent settings with high preservation potential 

Figure 10. General topographic profile, morphology, and depositional setting of boulder field on elevated rock platform.

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18

for paleotsunami deposits. During a tsunami event, 

the beaches and low dunes at the shore would 

provide a source of sand and the low-energy wetland 

or tidal flat environments would serve as traps for 

the high-energy marine sand tsunami deposits. If 

undisturbed after deposition, the sandy tsunami 

deposits also would contrast sharply with the peat 

or muddy wetland or tidal-flat sediments. At several 

sites (Maleza Alta, Fig. 3, site 8; Punta Arenas, site 

11; Palmas del Mar, site 13) coarse gray or white 

carbonate sand was encountered in the bottom of 

the cores. These sand deposits may represent openocean platform sediments that were buried when the 

shoreline advanced seaward during the Holocene 

coastal regression.

Moya and Mercado (2006) investigated several 

field sites on the northwest coast of Puerto Rico that 

according to eyewitnesses were inundated by the 

1918 tsunami. They reported finding thin laminated 

sand deposits with Halimeda sp., shell fragments, or 

heavy mineral concentrations beneath the wetlands 

at Espinar (Fig. 3, site 2), Parque Aguadilla (Fig. 

3, site 3), and Aguada Beach (Fig. 3, site 4) at 

depths of 25, 39, and 25 cm respectively that they 

interpreted as having been deposited by the 1918 

tsunami. The sediment sources or processes that 

would have deposited the overlying 25 to 39 cm of 

sediment in the past 88 years were not evaluated. 

Moya and Mercado (2006) also reported radiocarbon 

dates of event deposits of 630±55 YBP and 2515±70 

Figure 11. Rock platform on the eastern side of Bonaire. Hurricane Ivan overturned the boulder in 2004 (site 10).
YBP from a core at Espinar. The sample dated at 

630±55 YBP is from a depth of about 30 cm, which 

is comparable to or above the laminated deposits. 

Considering the characteristics of these thin sand 

deposits and their contrast with the overlying 

and underlying sediments it is likely that they are 

tsunami deposits, but they may be much older than 

the 1918 event.

Wetland Deposits

Eleven sites visited in Puerto Rico had typical 

coastal plain and wetland topographies (Fig. 18, 

Table 2). A common practice in Puerto Rico, and 

elsewhere, is filling wetlands to elevate and enlarge 

potential coastal construction sites. At Espinar and 

Palmas del Mar (Fig. 3, sites 2 and 13), surface 

sediments consisted of unsorted gravel, sand, and 

mud representing fill used to elevate the wetlands. 

At Parque Aguadilla and Isabela (Fig. 3, sites 3 and 

6), surface sediment compositions and elevations 

indicated that former wetlands had been filled and 

graded, but the cores were taken far enough from the 

altered surfaces that the cores did not penetrate any 

of the fill.

On the north coast at East Bajura (Fig. 3, site 

7), a high, carbonate-cemented eolianite ridge and 

narrow coastal plain separate the ocean from a 

Figure 12. Boulder field on rock platform on the eastern side of Bonaire (Fig. 2, site 14) showing landward decrease in clast size. Oblique 

view toward the sea.

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20

dying mangrove forest. A breach in the eolianite 

ridge has allowed storm washover to penetrate 

about 150 m inland, forming a washover terrace that 

terminates at the wetland edge in an avalanche face 

about 1 m high. Sediments beneath the wetland are 

composed of about 55 cm of tan and gray sand over 

tan laminated mud. Within the mud, at depths of 

79 and 86 cm are two very thin layers of gray sand. 

The upper layer is capped by whole, unabraided 

high-spiral gastropods (Fig. 19). Preliminary 

interpretation of the depositional history of this 

site is: (1) formation of a coastal lake landward of 

the eolianite ridge, (2) aggradation of laminated 

lacustrine muds that was infrequently punctuated by 

tsunamis that overtopped the ridge and deposited the 

thin sand layers. (3) Eventually the eolianite ridge 

was breached and the most recent sand deposition 

has been a result of (4) frequent storm overwash 

by high-energy waves focused in the embayment 

where the ridge was breached. (5) The overwash 

sufficiently filled the lake and increased the salinity 

so that a mangrove wetland formed.

Event deposits that could have tsunami origins 

were identified at two other wetland sites. At 

Parque Aguadilla on the northwest coast (Fig. 3, 

site 3), sediments beneath the mangrove wetland are 

composed of brown and gray sandy mud. Within 

these mostly muddy sediments, at a depth of 127 to 

137 cm, is a layer of gray sand with heavy minerals 

concentrated at the base. At Palmas del Mar on the 

southeast coast (Fig. 3, site 13), sediments beneath 

the wetlands are dark brown peat and muddy peat. 

At a depth of 224 cm the composition changes to 

coarse sand that continues to the bottom of the core 

at a depth of 241 cm. It is uncertain what type of 

depositional environment the basal sand represents 

because the total thickness of the sand and the 

composition of the sediment below the sand are 

unknown.

Tidal Flat Deposits

Sediments beneath tidal flats were investigated at 

two morphologically similar sites on the south coast 

of Puerto Rico. Both Punta Cucharas (Fig. 3, site 10) 

and Punta Arenas (Fig. 3, site 11) are promontories 

that during the Holocene built into the Caribbean 

Sea, forming triangular-shaped landforms. The 

puntas (points) are characterized by beaches and low 

dunes along their outer margins and by tidal flats and 

mangrove wetlands that surround shallow interior 

lagoons. The depositional histories of these features 

are unknown, but they likely involve fluctuations 

in sediment supply, wave climate, and possibly sea 

level.

Sediments beneath a tidal flat at Punta Cucharas 

are brown and gray muddy sand that grade down 

into gray mud. Within the mud, at a depth of 99 

to 101 cm is a thin layer of gray sand with shell 

Figure 13. General topographic profile, morphology, and depositional setting of reef-rubble ridge complex on elevated rock 

platform.
fragments. The sand and shell layer is an event 

deposit of unknown origin.

Coastal Plain Deposits

Only coastal plain sediments were examined 

at the coconut palm plantation at Bella Lucia (Fig. 

3, site 15). This site appears to be dominated by 

storm processes and overwash deposition of sand. 

The evidence of recent beach erosion and frequent 

storm deposition is exposed in a one-meter-high 

erosional wave-cut scarp at the shore where multiple 

sand layers of varying textures are superimposed. 

Distributed throughout these sand deposits are 

strands of plastic rope, pieces of styrofoam, 

aluminum cans, and other trash at elevations from 

the surface to the base of the scarp near the lowtide water level. A trench, which was opened about 

75 m inland from the shore, showed homogeneous 

tan sand with abundant roots. Considering the low 

elevations and wave exposure of this site and the 

track of Hurricane Hugo (1989), at least one of 

the sand layers must be the Hugo deposit, but we 

were unable to distinguish it from other overwash 

deposits. Bush (1991) reported that the Hurricane 

Figure 14. Mining exposure on the eastern side of Bonaire. The exposure, which is on the seaward side of a landward-sloping reef-rubble 

ridge complex (Fig. 2, site 9), shows seaward dipping, upward fining graded beds. Largest clasts in the foreground are approximately 0.6 

m long.

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22

Hugo storm surge elevation at this location was 

about 1 m.

Guadeloupe

Sites visited in Guadeloupe on the northwest 

coast of Basse-Terre were inundated by the 1867 

tsunami and sites on Grande Terre were exposed to 

the 1755 tsunami.

Deposits Near the Shore and Near Sea Level

In Guadeloupe, deposits near the shore and 

near sea level are associated with active beaches 

that are frequently overwashed by storm waves. 

Nearly all of the beaches we visited are composed 

entirely of sand, only a few also contain cobbles 

and small boulders. For example, on the northwest 

coast of Basse-Terre at Point Allegre (Fig. 4, site 1), 

beach sediments consist of sand that includes some 

cobbles and fine boulders of basalt. The cobbles 

and boulders are eroded from the wave-cut scarp 

formed where the sea intersects upland alluvium. At 

lower elevations overwash sand is being transported 

across the beach and deposited into an adjacent 

pond that occupies a topographic low formed 

by upland drainage. On flanks of the uplands at 

elevations above the beach are isolated large basalt 

boulders scattered across the surface. Locally the 

Figure 15. Panorama of landward sloping reef-rubble ridge complex on an elevated rock platform on eastern coast of Bonaire (Fig. 2, site 

15).
boulders have been moved by humans and aligned to 

construct traffic barriers. Whether the boulders were 

initially transported and deposited by storm waves 

or tsunamis is unclear. The relatively small size of 

the boulders and their relatively low elevation and 

proximity to the shore make them likely candidates 

for storm wave emplacement. Scheffers and others 

(2005) concluded that similar boulder fields were 

storm deposits and that there was no morphological 

evidence of impacts from Holocene or historical 

tsunamis on Basse Terre.

Wetland Deposits

A core taken at Plage de la Perle (Fig. 4, site 3) 

provided the only observations of sediments beneath 

a wetland in Guadeloupe. At this mangrove site, the 

core recovered 175 cm of tan and gray mud that is 

mottled and iron stained by root casts. The cored 

sediments exhibited no evidence of any sandy event 

deposit despite the fact that the coring site is about 

60 m inland from the shore and the adjacent mostly 

barren sand backbeach showed evidence of recent 

inundation and landward sand transport.

Slightly Elevated Terrace Deposits

On the embayed east coast of Grande-Terre at 

Anse Ste. Marguerite and Anse Maurice (Fig. 4, 

sites 10 and 11) is a shore-parallel terrace about 

30-50 m wide and 2-3 m asl (Fig. 20). Small sandmining pits in the slightly elevated deposits show 

that they are composed primarily of coarse sand with 

scattered cobbles. Roots are abundant in the mixed 

sand and cobble exposures, which may explain the 

apparent lack of stratification. Boulders and blocks 

are scattered across the elevated terrace surface at 

Anse Ste. Marguerite and some boulder and blocks 

also are located near the shore at the northern end of 

Anse Maurice. An unpaved roughly shore-parallel 

road follows the break in topographic slope that 

separates the shrub and tree covered elevated terrace 

from a lower, presumably younger, shore-parallel 

terrace of sand and cobbles that is covered by dense 

vines and grass. The physical characteristics that 

help distinguish the two terrace deposits are the 

greater elevation, thickness, and width of the older 

deposits, not their compositions. The lower terrace 

thins seaward and pinches out at a wave-washed 

Figure 16. Mining pit on the seaward side of the reef-rubble ridge 

complex shown in fig. 15. The exposure shows (A) clast imbrication 

and sorting, and (B) two beds with a bounding surface and differences in clast sizes. The exposure is approximately 4 m high.

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24

barren zone of a rock platform on which are perched 

scattered cobbles. The frequently inundated barren 

zone of the rock platform merges further seaward 

with the intertidal and subtidal zones of the platform. 

Evidence of recent inundation of the low terrace is 

a wrack line that contains styrofoam floats, plastic 

bottles, and other trash. Whether the most recent 

inundation was by a storm or the low-amplitude 

tsunami of 2004 (Zahibo and others, 2005) is 

unknown. Scheffers and others (2005) interpreted 

the elevated bimodal sediments and scattered 

boulders as a deposit formed by a single tsunami 

2500-2700 YBP.

ORIGINS OF COARSE CLAST AND 

BIMODAL DEPOSITS

The sizes and shapes of cobbles, boulders, 

and blocks at the shore cause them to respond to 

ocean waves differently than finer particles (Bluck, 

1967, 1999; Kirk, 1980; Oak, 1984). As clast mass 

increases, the importance of particle suspension 

decreases and boulders and blocks move primarily 

by rolling or sliding at speeds that are less than the 

flow velocity (Inman, 1949). 

Coarse clast deposits examined on the 

Caribbean islands can be grouped into ridges and 

ridge complexes or boulder fields depending on 

Figure 17. Apron of cobbles deposited by Hurricane Lenny on an elevated rock platform on the northwestern coast of Bonaire (Fig. 2, site 22). 
particle spacing and the resulting degree of clast 

concentration. For ridges and ridge complexes, the 

particles are in contact with one another and exhibit 

fitted (packed) fabrics, whereas in boulder fields, the 

particles are separated, they are single layer deposits, 

and their respective positions do not necessarily 

influence subsequent movement of one another. The 

boulder fields also consist of blocks that are much 

larger than the sizes of clasts in the ridges and ridge 

complexes. Historical accounts of similar features 

formed on the margins of other ocean basins are 

presented as a basis for interpreting the origins 

of coarse clast coastal deposits on the Caribbean 

islands.

Origins of Ridges and Ridge Complexes

Much of the published literature on 

characteristics of cobble-boulder beaches comes 

from two distinctly different climatic and 

depositional settings; either mid- to high-latitude 

formerly glaciated coasts typically with some 

rocky shore segments, or low latitude tropical reefencompassed islands and atolls. On higher latitude 

beaches, clasts commonly are more rounded because 

of prior abrasion during glacial transport. In contrast, 

clasts from reef settings may be rounded by wave 

action or highly angular if they are composed of 

living material, such as coral colonies, that was 

eroded during the emplacement event. 

Observations from mid- to high-latitude cobbleboulder beaches are instructive because they provide 

principles that can be applied to other coarse clast 

deposits, regardless of depositional setting. For 

example, Bluck (1967) found that clast imbrication 

is best developed on the seaward side of beach 

deposits where they form a zone near the height of 

wave runup. After initial deposition and surficial 

clast imbrication of storm ridges as a result of 

unidirectional overwash, reworking by ocean swell 

or waves of less intense storms produces additional 

imbrication and tighter clast fitting (Shelley, 1968). 

The resulting three-dimensional interpenetration and 

interlocking of clasts eventually provides framework 

stability that is more resistant to further reworking 

and helps preserve the older deposits and their 

associated fabrics (Bishop and Hughes, 1989).

Beach ridges and ridge complexes with 

dimensions, morphologies, clast sizes, and 

compositions similar to those observed on Bonaire 

have been reported for several Pacific atolls (Figs. 

21 and 22). For example, Umbgrove (1947) 

described storm-emplaced ramparts composed 

of coral shingle and occasional coral boulders 

up to 1.5 m in diameter that rim numerous atolls 

of the East Indies. Newell (1954) reported that 

on Raroria, ridges comprised of sand and gravel 

and deposited on the reef flat by storm waves 

were derived from the adjacent coral reefs. The 

coarsest and highest ridges coincided with shore 

Figure 18. General topographic profile, morphology, and depositional setting of coastal plain and wetland setting.

25
26

segments that were at a high angle or embayed to 

the direction of storm wave approach. Reef-derived 

blocks up to 9 m long were observed on the reef 

flat and adjacent beach. McKee (1959) described 

coarse clast ridges near the shore and gravel sheets 

on the island interiors that were formed on Jaluit 

Atoll by an intense typhoon (Ophelia) in 1958. The 

pebble to boulder deposits generally were poorly 

sorted, had crude stratification, and exhibited fair 

to good clast imbrication that dipped seaward. The 

ridges were 15-18 m wide and up to 2.4 m high. 

The gravel sheets, which were up to 1 m thick, 

were composed of clasts finer than the ridges, and 

extended inland as much as 90 m. McKee (1959) 

emphasized the winnowing of sand in the storm 

deposits and particle size segregation such that 

sandier deposits represented accumulation on lee 

shores or more normal wave conditions. Baines 

and others (1974) and Baines and McLean (1976) 

documented reef rubble ridges (Figure 21) and 

sheets constructed on Funafuti atoll by waves of 

Tropical Cyclone Bebe (1972) and monitored 

subsequent reworking of the ridges by ocean swell. 

The ridges were up to 4 m high and 37 m wide and 

terminated landward in an avalanche slope of 3-

9o. The largest reef-derived boulder resting at the 

toe of the seaward face was 7x3x3 m. The gravel 

sheets at low elevations extended inland as much 

as 75 m. McLean and Hosking (1991) described 

and illustrated wave-constructed ridges and ridge 

and swale complexes from atolls in Tuvalu. They 

reported that ridge morphologies, elevations, and 

clast sizes were related to wave exposure. On the 

windward sides of the atolls, ridges were composed 

of pebbles, cobbles, and platy coral rubble that were 

imbricated and dipped toward the ocean. Ridges 

with crest elevations of 3-5 m above the reef flat 

were asymmetrical with steeper seaward slopes 

and more gentle landward slopes. Where individual 

ridges had migrated leeward to coalesce with older 

topographically high features, a ridge and swale 

complex as much as 500 m wide was formed. These 

studies showed that ridge crest elevations and ridge 

morphologies depend on heights of wave runup 

and antecedent topography, including prior beach 

deposits. The rubble tracts or veneers form where 

pre-storm elevations near the shore are low and 

consequently wave runup heights are also low.

Although details are lacking, Schubert and 

Valastro (1976) refer to a modern storm deposited 

ridge complex in the southern Caribbean Sea near 

Bonaire. The ridge complex along the northern and 

eastern coasts of La Orchila Island, Venezuela, 

which is composed of coral rubble and shell 

fragments ranging in size from pebbles to boulders, 

is as much as 100 m wide and 3 m high. 

Individual storm rubble ridges and their 

subsequent merging with older storm deposits 

(Figure 22) are part of the long-term sedimentation 

that constructs aggradational and progradational 

depositional complexes on some reef islands (Cloud, 

Figure 19. Core of sediments beneath wetlands at Bajura, Puerto 

Rico (Fig. 3, site 7) showing event deposit of sand and shells 

encased in laminated mud. Scale is in cm.
1952; McKee, 1959; Baines and McLean, 1976, 

Richmond, 1992). After reviewing a number of 

reports of hurricane impacts on reef islands, BaylissSmith (1988) concluded that storm waves were 

the only marine process capable of large volume 

emplacement of rubble ridges and ridge complexes, 

and more frequent less intense waves reworked the 

storm deposits into more stable features. Orford and 

others (1991) described the processes leading to 

subsequent aggradation of storm-constructed coarseclast ridges.

Origins of Boulder Fields

Boulder fields on elevated platforms or upland 

surfaces have the same general characteristics 

and distributions as those formed near sea level. 

An added complication is that extreme waves are 

required to dislodge megaclasts from rock platforms 

and lift them several meters above sea level onto 

the platform before they can be transported inland. 

Also there are only a few published reports of coarse 

boulders and blocks being transported and deposited 

by historical tsunamis or storm waves that can be 

used diagnostically to identify the depositional 

processes. 

Ocean Swell and Storm Waves

The oldest records of boulder movement by 

ocean swell or storm waves are anecdotal accounts 

that provide limited but useful information. For 

example, Stevenson (1864) observed at Whalsey 

Skerries, Scotland, 6-ton blocks that had been 

quarried from their beds by wave action. He also 

reported that a 7.5 ton block moved laterally 22 m 

across a rock platform elevated 6 m asl. Sussmilch 

(1912) reported that storm waves in 1912 dislodged 

boulders and blocks from a rock platform at Bondi 

Beach, Australia. The largest clast dislodged was 

a 6x5x3 m block weighing an estimated 240 tons 

that was transported 50 m across the platform and 

overturned. 

Baines and others (1974) found that ocean swell 

at high spring tide was sufficient to transport large 

boulders from the seaward side over the crest of a 

high storm ridge on a Pacific atoll. According to 

Bishop and Hughes (1989) a 1974 storm in New 

South Wales, Australia transported boulders (1 m b

axis) as much as 100 m inland across a rock platform 

and through a vertical distance of 10-15 m.

McFadgen and Yaldwyn (1984) illustrated a 

growing beach ridge composed of large boulders on 

a sub-Antarctic New Zealand island. The boulder 

ridge was deposited on a 25 m high rock platform 

Figure 20. General topographic profile, morphology, and depositional setting of elevated terrace at Anse Ste. Marguerite on 

eastern side of Grande-Terre, Guadeloupe.

27
28

Figure 21. Coral-rubble ridge constructed by Tropical Cyclone Bebe and adjacent older ridge complex 

on Funafuti Atoll. (A) Air and (B) ground photographs taken by Jim Maragos in December, 1972, only two 

months after Bebe. Compare Bebe ridge with Hurricane Lenny ridge (figure 9) and with reworked Funafuti 

ridge (figure 22).
Figure 22. Coral-rubble ridge complex on Funafuti Atoll and Tropical Cyclone Bebe deposits subsequently reworked 

by less intense storms. The reworked Bebe ridge migrated landward and merged with the ridge complex. Basal 

remnants of the Bebe deposit are lightly cemented by calcium carbonate on the reef flat. (A) Air and (B) ground 

photograph taken by Bruce Richmond in January, 1987, approximately 14 years after Bebe. Compare with figure 21.

29
30

presumably by deep ocean storm waves from the 

Tasman Sea. 

Williams and Hall (2004) and Hall and others 

(2006) described a series of imbricate boulder ridges 

and isolated blocks that were deposited on rock 

platforms as much as 50 m high along the exposed 

coasts of Scotland and Ireland. They relied on 

eyewitness accounts, trapped man-made debris, and 

lack of a tsunami record as evidence that at least 

some of the megaclasts were deposited by extreme 

storm waves of the North Atlantic Ocean. The 

ridges, which are 1-6 m thick and 3-35 m wide, are 

composed of clasts ranging in size from sand to 7 m. 

Clast sorting was also observed with block weight 

decreasing and angularity increasing with elevation.

Saintilan and Rogers (2005) showed that a block 

quarried and emplaced on a rock platform by storm 

swell on the southeast coast of Australia in 1999 

was one of the largest clasts at the site. The newly 

emplaced 15.7-ton block was moved 40 m from 

the rock platform edge and lifted to an elevation 

of 2 m asl. Saintilan and Rogers (2005) used wave 

parameters from nearby gauges to determine that 

the return frequency of the storm waves was every 

four years and that nearly all of the boulders at 

the site could be transported by waves of the same 

magnitudes as those recorded during the boulder 

transport event. The New South Wales site was 

one of those used by Bryant and others (1996) as 

evidence of tsunami boulder deposition.

Tsunamis

Reports of coastal boulders emplaced or moved 

by historical tsunamis are also limited in number and 

in the detail they provide. According to Japanese 

historical documents reviewed by Moore and 

Yoshida (2001), coral blocks up to 5 m in diameter 

were transported by the 1771 Meiwa tsunami that 

struck the island of Ishigakijima. Also the same 

1771 tsunami constructed a set of boulder fields 

on the Sakishima islands of Ishigaki, Tarama, 

Irabu-Shimoji, and Miyako (Andrew Moore, 

2006, personal communication). There are several 

published references to these tsunami boulder 

deposits, but they are written in Japanese. 

Umbgrove (1947) reported that tsunamis 

associated with volcanic eruptions deposited large 

coral blocks on some beaches of the East Indies. The 

tsunami generated by the explosion of Krakatoa in 

1883 carried a block of 300 m3 approximately 100 m 

inland from the shore.

A team of USGS scientists witnessed the 1960 

tsunami wave train as it inundated Hilo, Hawaii 

(Eaton and others, 1961). After the water receded, 

they observed individual boulders weighing up to 

22 tons that had been excavated from a seawall and 

transported inland 120-180 m without visible signs 

of contact with the ground. Apparently the sizes of 

transported boulders were limited by boulder sizes in 

the seawall.

Noormets and others (2002) used sequential 

aerial photographs to constrain the timing of 

emplacement of isolated blocks and rearrangement 

of clusters of imbricate boulders quarried from the 

front of a rock platform on northwest Oahu, Hawaii. 

Dimensions of the boulders and blocks ranged from 

0.5 to 8.8 m. They concluded that emplacement of 

the largest block (96 tons) was by a distant tsunami 

in 1946. The block subsequently moved inland 30 m 

twice, but along different trajectories. At least one of 

Figure 23. Exposure of polymodal reef-derived sediments deposited 

by storm waves on Rakahanga Atoll, Cook Islands. The deposit is 

cemented by calcium carbonate.
those 30-m jumps was caused by long-period swell 

from a winter storm that flooded the rock platform.

Goff and others (2006) investigated a 

discontinuous veneer of 1-3 m boulders and sand 

transported and deposited by a 1975 locallygenerated tsunami on the island of Hawaii. They 

reported that the boulders and sand both showed 

landward fining over the same distance and both 

terminated near the same distance from the shore. 

A landward decrease in clast size from blocks to 

fine sand also was reported for deposits on the reef 

flat of Rangiroa, Tuamotu (Bourrouilh-Le Jan and 

Talandier, 1985).

Tsunami scientists investigating the impacts of 

the 2004 Indian Ocean tsunami on the west coast 

of Thailand observed a field of boulders that may 

have been emplaced and moved across an intertidal 

rock platform by the tsunami at Pakarang Cape 

(Bretwood Higman, 2006 personal communication).

Bimodal deposits

Bimodal coastal deposits are a function of 

available clast sizes in the nearshore sediments and 

the history of transport and deposition of individual 

clasts. They are not diagnostic of a particular 

type of wave process although clast textures 

(size, shape, roundness) and internal organization 

(packing, sorting, and stratification) may provide 

important clues to their origins (Carr and others, 

1970; Williams and Caldwell, 1988). Furthermore, 

interstitial particles within gravel beaches tend to 

be smaller than those at the surface because smaller 

particles infill the larger subjacent clasts that form 

the framework (Bluck, 1967).

The degree of sorting of polymodal deposits 

is related to initial rates of sedimentation and 

subsequent history of sediment reworking. 

Extremely rapid deposition (hours) of large volumes 

of mixed sediment sizes typically results in poor 

sorting because the high concentrations of large 

clasts interfere with the sorting processes and there 

is insufficient time for waves to organize the clasts.

Polymodal heterogeneous coastal deposits 

consisting of clasts ranging in size from boulders or 

cobbles to sand that accumulated as a result of storm 

waves (Fig. 23) are common (Newell, 1954; Bluck, 

1967, 1999; Kirk, 1980; Carter and Orford, 1981; 

Woodroffe, 1994; FitzGerald and others, 1994; Neal 

and others, 2003). In contrast to these examples 

of storm-generated bimodal deposits, Kelletat 

and others (2004), Scheffers and others (2005), 

Scheffers (2005), and Scheffers and Kelletat (2006) 

emphasized bimodal deposits as being diagnostic 

evidence of a tsunami. For example, Scheffers 

and others (2005) interpreted the composition and 

morphology of bimodal deposits on the east coast 

of Guadeloupe as evidence of tsunami deposition. 

Although radiocarbon dates from these deposits span 

a range of nearly two thousand years, Scheffers and 

others (2005) concluded that a single tsunami about 

2500-2700 YBP was responsible for the boulder 

and elevated ridge deposition. The relatively low 

elevation of the bimodal deposits within the range 

of storm surge elevations, their terrace morphology, 

the eastern exposure to frequent hurricane waves, 

and the total volume of accumulated sediment make 

a storm-dominated depositional complex of multiple 

events a likely explanation of their origin.

DISCUSSION

Deposit Origins

It is clear from the preceding discussions that 

extreme waves capable of causing substantial 

coastal erosion and sediment transport are common 

in the Caribbean Sea. What remains unclear are 

the signatures of tsunami and storm deposits that 

are composed of clasts larger than sand and in 

particular clasts that are classified as boulders and 

blocks (Blair and McPherson, 1999). Consequently 

the origins of many of the deposits we observed 

on Bonaire, Puerto Rico, and Guadeloupe are still 

uncertain because the information available to 

interpret the depositional processes was limited 

by rapid reconnaissance. All of the features we 

31
32

observed with known origins were formed by recent 

storms.

Scheffers (2002) used the low frequency of 

hurricanes impacting the ABC (Aruba, Bonaire, 

Curacao) islands to exclude storms as an agent of 

coarse clast deposition. She also concluded that the 

narrow shape and low elevation of the storm ridge 

deposited by Hurricane Lenny were evidence that 

the broad coarse clast ridges near the shore and up 

to 3 m asl were either entirely tsunami deposits or 

a tsunami deposit that was later modified by storm 

armoring. No other physical evidence was presented 

to support the tsunami hypothesis and important 

evidence that supports multiple wave events, such 

as storms, was not considered. Radiocarbon dates 

that supposedly cluster around 400-500 BP, 1500 

BP, and 3500 BP were used as evidence of three 

individual tsunami events. However, the distribution 

of radiocarbon dates grouped by shoreline sector 

around Bonaire (Scheffers, 2005) span thousands of 

years, which is compelling evidence of long-term 

coastal deposition by many extreme wave events. 

Furthermore, in many places mining has destroyed 

the landward parts of the deposits so the entire age 

ranges of the deposits is unknown because the oldest 

sediments were not available for dating. As for the 

boulder and block deposits, Scheffers (2002, 2005) 

concluded that because neither Hurricane Lenny 

nor Hurricane Ivan emplaced blocks on the rock 

platforms of the ABC islands, the boulders were 

quarried and transported by tsunamis. The results of 

the Nott (1997) equations also were used to conclude 

that hurricane waves were incapable of emplacing 

the boulders.

On the basis of similarities in coastal 

depositional features (ridges, ridge complexes, 

boulder fields) and clustering of radiocarbon dates 

for these features, Scheffers and others (2005), 

Scheffers (2005), and Scheffers and Kelletat (2006) 

concluded that since 4500 YBP, (1) 3 to 6 separate 

megatsunamis impacted the Caribbean islands, 

and (2) those individual tsunamis were responsible 

for constructing the coarse clast coral rubble 

ridge complexes and boulder fields throughout 

the Caribbean region. However, the chronological 

tsunami distribution map of Lander and others 

(2002) does not easily support this conclusion of 

regional impacts by individual tsunamis. Instead 

it shows that during the past 500 years only the 

1755 and 1867 tsunamis were reported at multiple 

locations in the Caribbean and there is no evidence 

that they were responsible for massive movement 

and organization of coarse coral-reef rubble. The 

historical locally generated tsunamis have had 

limited impacts, both in their areal extent and in their 

ability to cause significant changes in the landscape.

Large-volume coarse clast accumulations 

and sand blankets interpreted as paleo-tsunami 

deposits, such as those described respectively by 

Moore and Moore (1988), Scheffers (2002, 2005), 

Scheffers and others (2005), Switzer and others 

(2005), and Scheffers and Kelletat (2006) that are 

several meters thick, internally exhibit multiple 

layers with bounding surfaces, and are 10s to 100s 

of meters wide are not likely products of a single 

or a few tsunamis. We conclude they are probably 

the composite record of long-term coastal processes 

spanning centuries to millennia because they have 

morphological, sedimentological, and stratigraphic 

attributes that favor deposition by multiple highfrequency waves. Evidence of frequent waves 

includes ridge complex morphologies, lateral 

and vertical textural trends, internal stratification, 

and the extremely large volumes of accumulated 

sediments. Some of the same physical criteria were 

used by Hayne and Chappell (2001) to establish 

storm origins of beach ridges and to estimate storm 

frequency on the north coast of Australia.

A diagnostic feature of coarse clasts deposited 

by storm waves is the construction of shore-parallel 

ridges and ridge complexes (Umbgrove, 1947; 

Cloud, 1952; Newell, 1954; McKee, 1959; Maragos 

and others, 1973; Baines and others, 1974; BaylissSmith, 1988, McLean and Hosking, 1991; Scoffin, 

1993; McLean and Woodroffe, 1994; Scheffers, 

2005). In contrast, there are no known reports of 

historical tsunamis depositing a high, linear ridge 

of coarse clasts near and parallel to the shore. The 
coarse clast ridges deposited by Tropical Cyclone 

Ophelia (McKee, 1959), Tropical Cyclone Bebe 

(Baines and others, 1974), and by Hurricane Lenny 

(Scheffers, 2005) represent deposit morphologies of 

a single storm on a reef flat that had no antecedent 

topography (prior beach or ridge) to influence 

ridge construction. Although sediment transport 

by strong alongshore currents was demonstrated 

for the Hurricane Lenny ridge, the ridge formed 

and has grown as a result of many breaking waves. 

The roughly linear shore-parallel concentration of 

clasts is a result of maximum storm-wave runup 

and deposition at elevations above runup of nonstorm waves at high tide (Bluck, 1967; Hayne and 

Chappell, 2001). 

Another diagnostic feature of storm-wave 

deposits is the steep avalanche slope (Figs. 7 and 21) 

on their landward sides (McKee, 1959; Baines and 

McLean, 1976; Scoffin, 1993; FitzGerald and others 

1994; Orford and others, 2002). The avalanche slope 

is constructed by bedload transport of clasts during 

overwash across the ridge and flow separation into 

deeper water landward of the ridge.

The external and internal organization of 

coarse clasts and resulting sedimentological 

fabric of the ridge deposits also provide evidence 

of high-frequency wave processes. Although 

clast imbrication is not diagnostic of a particular 

depositional environment or hydrodynamic process, 

imbricated surficial clasts on beaches are common 

products of clast fitting as a result of frequent wave 

swash and backwash (Bluck, 1967). The imbricated 

clasts dip seaward (Fig. 8) and as wave-influenced 

deposits aggrade, an internal imbricate fabric is 

formed (Figs. 14 and 16A). Other evidence of clast 

sorting by multiple waves includes steep seaward 

dipping or vertically stacked graded beds that exhibit 

well-defined stratification (Figs. 14 and 16B). The 

stratified beds are products of lateral accretion 

and aggradation by multiple storm waves that 

periodically add reef rubble to the progradational 

complex. The ridge complex deposits also exhibit 

bedding plane surfaces that separate sedimentary 

units that have distinctly different clast sizes and 

fabrics (Fig. 16B). The bounding surfaces are 

disconformities that may represent substantial 

hiatuses in sedimentation.

Finally, the geometries and overall dimensions 

of the coarse clast deposits and total volumes of 

sediment stored in them are evidence of multiple 

storm-wave events. The coarse clast ridges and 

ridge complexes in the Caribbean region extend 

continuously inland 10s to 100s of meters from the 

shore and they are several meters thick. Although 

precise volumetric estimates of the deposits have not 

been made, it appears that they can contain several 

hundred cubic meters per meter of beach. The total 

volumes of sediment deposited in the ridges and 

ridge complexes likely exceed the capacity of a few 

waves to entrain and transport sediment, especially 

large clasts. A few waves, such as would occur 

during a tsunami, is not a reasonable explanation 

for construction of broad, thick ridges and ridge 

complexes composed of stacked and internally 

organized coarse clasts.

Elevated Deposits

It is uncertain why rubble ridge complexes 

and massive blocks are stranded on rock platforms 

(Bonaire) or banks of bimodal sediments are perched 

on steep slopes (Guadeloupe) several meters above 

sea level and out of the reach of normal waves. 

Some blocks may be remnants of clasts detached 

from the ancestral sea cliff during an earlier 

highstand in sea level. However, that is not a likely 

explanation for most of the boulder fields because 

the largest blocks are closest to the active sea 

cliff and not the ancestral sea cliff, which is much 

farther from the sea. There are several possible 

explanations, some requiring long-term vertical 

movement of the land or sea. 

One possible explanation is that the elevated 

platforms are the result of tectonic uplift that has 

raised the coarse clast deposits above the height of 

normal waves. Long-term uplift has been reported 

for the northern part of Bonaire (Alexander, 1961), 

which might explain the ridge complexes and 

33
34

boulder fields on elevated platforms, but the base 

elevations of 5 to 6 m asl and young ages of the 

deposits would require rapid rates of uplift that are 

unrealistic. The late Quaternary tectonic stability of 

Guadeloupe is undetermined. 

The rubble ridge complexes and some of the 

blocks may be abandoned storm wave deposits that 

formed during a mid-to-late Holocene highstand 

in sea level. GIA model results indicate that the 

mid-Holocene highstand was not manifested in the 

Caribbean region because it is located on the margin 

of the Laurentide forebulge and there are no sea 

level records from the region that show a higher than 

present Holocene sea level. However, the Holocene 

sea level curves showing a monotonic rise that 

have been published for the Caribbean (Milne and 

others, 2005) are based on subtidal elevations that 

preclude establishing a Holocene sea level higher 

than present. Furthermore, it appears that important 

information regarding elevations and ages of coastal 

deposits on the Caribbean islands is being ignored, 

that perhaps should be considered as part of the sea 

level history.

 Another explanation of the perched coarse clast 

deposits is that they are the result of platform or 

slope flooding by extreme wave events under present 

sea level conditions. Although extreme storm waves 

are capable of moving blocks on the platforms (Fig. 

10, Scheffers, 2005) or depositing thin gravel aprons 

(Fig. 16), the elevated ridge complexes do not show 

evidence of active sediment accumulation or recent 

reworking by storm waves.

CONCLUSIONS

Considering our observations from Bonaire and 

evidence from Pacific atolls, we conclude that coral 

rubble ridges near sea level that are a few meters 

thick and a few 10s of meters wide and terminate 

in avalanche slopes were constructed by multiple 

wave events dominated by storms. Similarly, the 

composite rubble features up to 4 m thick and 80 m 

wide on elevated rock platforms also are interpreted 

as ancestral depositional complexes constructed 

by multiple wave events. The origins of extensive 

boulder fields on east Bonaire are uncertain and will 

require additional investigation and analysis before 

their origin can be determined.

Based on our reconnaissance observations in 

Puerto Rico, the wetland sites visited with the most 

promise for additional paleotsunami investigations 

are East Bajura, Parque Aguadilla, Punta Cucharas, 

and Palmas del Mar. Of the deposits examined in 

Puerto Rico, the thin gray sand layer with marine 

shells encased in tan laminated mud that was 

recovered at East Bajura has the greatest potential of 

being a paleotsunami deposit.

Although substantially less time was spent 

examining coastal deposits on Guadeloupe than 

on Bonaire or Puerto Rico during our March 2006 

reconnaissance study, preliminary evidence suggests 

that it does not provide the diversity of settings and 

exposures to extreme waves that will be required 

to establish criteria for recognizing coarse clast 

tsunamis and storm deposits. Despite historical 

accounts of tsunamis striking the northwest and 

south sides of the island, their is no clear evidence 

of tsunami deposits, nor are there accumulations 

that appear to be likely tsunami deposits. The ridge 

of bimodal sediments at Anse Ste. Marguerite has a 

morphology, composition, and accumulation volume 

that are consistent with long-term storm-dominated 

coastal plain deposition, not deposition by a few 

tsunamis. 

FUTURE ACTIVITIES AND RESEARCH 

DIRECTIONS

Our discussions in the field with geologists 

and wave modelers who have published on 

tsunami research, our observations and preliminary 

interpretations of coastal deposits on Bonaire, 

Puerto Rico, and Guadeloupe, and a review of the 

available literature provide a basis for identifying 

future research activities. Following are some of 

the important geological tasks that will improve 

assessment of the tsunami hazard in the Caribbean 

region.
Additional Field Investigations

Data from additional field sites in the Caribbean 

are needed to accomplish two primary objectives. 

The first objective is to increase our exposure to 

and knowledge of Caribbean coastal deposits with 

the intent of understanding better: (1) the evidence 

for paleotsunamis, and (2) the types of reef-derived 

deposits formed by storms and tsunamis. Because 

the Virgin Islands (St. Croix, St. John, St. Thomas) 

are U.S. territories with historical records of tsunami 

impacts, they should receive high priority for future 

field investigations.

The second objective of additional fieldwork is 

to build on the reconnaissance work summarized 

in this report and to begin developing an inventory 

of deposit characteristics that will eventually lead 

to criteria that can be used to identify paleotsunami 

deposits and extend the historical record to improve 

tsunami risk assessment. Based on preliminary work 

in Puerto Rico, the East Bajura site holds the most 

promise for comparing possible tsunami and storm 

deposits. In Bonaire, the diversity of bimodal coastal 

deposits, their surface exposures, potential internal 

exposures related to mining, and their accessibility 

make them ideal candidates for examining coarse 

clast deposits with the intent of understanding the 

mechanisms of emplacement and post-emplacement 

reworking. 

Improved understanding of extreme wave 

processes also could be accomplished by 

reconnoitering other islands, such as Jamaica, 

the Grand Caymans, and the Bahamas, where 

anomalous coarse clast coastal deposits have been 

reported (Robinson and others, 2006; Jones and 

Hunter, 1992; Hearty, 1997) and interpreted by 

some to be the products of tsunamis (Kelletat and 

others, 2004). The Bahamas are not part of the 

Caribbean region and their risk to tsunami hazards 

is substantially lower than their risk to hurricane 

waves. Nevertheless, bimodal sand and boulder 

deposits and megaclasts have been reported on 

Eleuthera (Hearty, 1997) and Long Island that have 

been attributed to tsunami processes (Kelletat and 

others, 2004). These coarse clast deposits present 

an opportunity to broaden the inventory of tropical 

coarse clast deposits, to examine sites where 

paleotsunami deposits have been reported, and to 

test concepts developed for coarse clast deposits 

in the Caribbean Sea. A subtask of this activity 

would be examining reports on inundation heights 

of historical storm surges for the Caribbean islands 

visited and compilation of eyewitness accounts for 

recent storms that are similar to those presented in 

tsunami catalogs.

Because our research in the Caribbean region 

is about tsunami hazards and because evidence 

of past extreme wave events involves boulders 

and blocks, it is important to investigate boulders 

that were transported and deposited by historical 

tsunamis. Several options are available where 

the historical record is unequivocal and where 

colleagues who have already conducted preliminary 

investigations are available for collaboration. For 

example, the 1771 tsunami reportedly transported 

boulders in Japan (Andrew Moore, 2006, personal 

communication), the 1946, 1960, and 1975 tsunamis 

each moved boulders in the Hawaiian islands 

(Noormets and others, 2002; Eaton and others, 

1961; Goff and others, 2006), and the 2004 Indian 

Ocean tsunami may have constructed a boulder field 

in Thailand (Bretwood Higman, 2006, personal 

communication). Additional information about the 

2004 tsunami boulder deposits in Thailand may be 

available in the near future (Wanna Phyo, 2006, 

personal communication).

Morphology, Sedimentology, and Origins of 

Coarse Clast Caribbean Coastal Deposits

Felton (2002) and Felton and others (2006) 

described detailed analytical field techniques 

that allowed them to evaluate widely conflicting 

hypotheses regarding the origins of elevated boulder 

deposits in Hawaii. The thick coarse clast marine 

deposits had been attributed previously by various 

authors to (1) a mega tsunami, (2) uplifted beaches, 

(3) multiple events, (4) surficial slope processes, 

35
36

and (5) artifacts of human occupation. Their 

systematic approach combined deposit composition 

and fabric, facies architecture, and geometries to 

establish a basis for genetic interpretations. Despite 

their thorough approach, it did not include the total 

volume of sediment contained in the deposit and 

comparisons with historical tsunami- and stormgenerated beach deposits. 

Perhaps an even more powerful approach to 

interpreting the origins of coarse clast deposits 

would be combining the sedimentological analyses 

of Felton (2002) and Felton and others (2006) 

with examination of the morphologies, facies 

architecture, and geometries and total volume of 

modern coarse clast coastal deposits produced 

by known tsunamis and storms. Although only a 

few modern coarse clast tsunami deposits have 

been reported, they have common characteristics 

that provide a basis for a provisional depositional 

model. There are many reports of coarse clast storm 

deposits composed of coral rubble. Those from the 

Pacific region (Umbgrove, 1947; Newell, 1954; 

McKee, 1959; Stoddart, 1971; Maragos and others, 

1973; Baines and McLean, 1976; Bayliss-Smith, 

1988; Stoddart and others, 1990; Scoffin, 1993) 

provide important information about coral rubble 

emplacement on reef flats and reworking. Several 

sites from the North Atlantic and North Sea have 

been described where storm waves have detached, 

transported, and deposited megaclasts as ridges and 

fields on cliff tops as much as 50 m asl (Williams 

and Hall, 2004; Hall and others, 2006). 

Additional research is needed to develop a 

systematic sedimentological approach to deposit 

analysis and a set of criteria for interpreting the 

origins of extreme wave deposits. Important 

considerations for the Caribbean region and 

elsewhere are present elevation of the deposits, age 

of the deposits, relative sea-level history (tectonic 

or eustatic motion) since deposition, topography, 

composition, and induration of the antecedent 

surface. Two end-member sedimentation models 

could be used to help evaluate whether the ridge 

complexes on elevated platforms were deposited (1) 

by high frequency waves and abundant sediment 

supply under conditions of prolonged elevated 

water levels (platform flooded by mid-Holocene 

highstand), or (2) episodic extreme wave events 

that rapidly added coral rubble (storm punctuated 

platform flooding and static sea level). If the former 

model is correct, then there should be wide segments 

of the ridge complex with similar textures and 

internal structures and possibly cementation of basal 

deposits, whereas if the latter model is correct, then 

there should be numerous internal discontinuities.

Characterization of Coarse Clast Tsunami 

and Storm Deposits

An ongoing task of the USGS tsunami research 

project is to develop physical criteria that can 

be used to help distinguish storm and tsunami 

deposits in those regions where both processes 

occur, although storms may predominate because 

of their greater frequency of occurrence. As part of 

that effort, a provisional identification of criteria 

that may be useful to differentiate sandy storm and 

tsunami deposits was recently completed (Morton 

and others, in press). What is now needed is a 

comparable first comparison of physical attributes of 

coarse clast tsunami and storm deposits that would 

serve as a field guide.

Kelletat and others (2004) used deposit shape 

(chevrons), deposit composition (mixed sand and 

gravel), and a barren zone between the coarse clast 

deposits and the shore as evidence of a tsunami. 

None of these criteria are diagnostic of a tsunami 

and there are well-documented examples of these 

same features being produced by historical storms. 

Our preliminary examination of the Caribbean 

coastal deposits indicates that deposit morphologies, 

external and internal clast organization, and deposit 

volumes may be a basis for distinguishing between 

coarse clast storm and tsunami deposits. This task 

would be a derivative of comparing modern coarse 

clast storm and tsunami deposits (see previous tasks) 

and would be integrated with those tasks.
Modeling Coarse Clast Competence, 

Capacities, and Deposits of Tsunamis and 

Storm Waves

Considering the common use of the Nott 

(1997, 2003) equation results as evidence for 

tsunami emplacement of coarse clast deposits, 

there needs to be an independent evaluation of the 

basic assumptions of the equations and application 

of the results. This could lead to modification 

of the equations by inclusion of other terms or 

development of improved equations that rely on 

alternative assumptions, such as critical forces 

and mode of transport. For example, Noormets 

and others (2004) carefully compared the dynamic 

pressures and transport characteristics of long-period 

swell (storm waves) and turbulent bores (tsunami 

waves) and applied their results to the historical 

emplacement and movement of megaclasts in 

Hawaii. Their analytical methods may be preferred 

to those of Nott. The equations of Lorang (2000) 

may also have aspects that would improve overall 

prediction of boulder size transport considering 

wave height, wave period, runup elevations, and 

swash velocities.

Other important wave mechanics issues that will 

require additional research and modeling have to do 

with the volume of sediment of different sizes that 

can be emplaced by a single extreme-wave event 

(storm or tsunami) and the most likely geometry 

of the deposit produced by that event. This would 

involve identifying the theoretical upper limits of 

clast size (competence) and volume of sediment 

(capacity) for historical tsunami and storm waves. 

Some aspects of this research may be beyond current 

capabilities because it would require determining 

how particle-to-particle interference of large clasts 

in the bottom boundary layer would influence 

entrainment, transport, and deposition.

Hazard Vulnerability Analysis and Risk 

Assessment

One of the principal objectives of the Caribbean 

Region tsunami project is the integration of field 

and laboratory geological approaches to hazard 

vulnerability analyses and risk assessments. 

Therefore the emphasis of this activity would 

be coordination of the paleotsunami fields 

investigations and land-based hazard risk 

assessments (Geist and Mercado, 1999; Bush 

and others, 2001) with results of tsunami source 

geophysical investigations in the Caribbean Sea 

being conducted by Uri ten Brink (USGS, Woods 

Hole) and tsunami attribute modeling of the 

sources being conducted by Eric Geist (USGS, 

Menlo Park). Model results of expected local 

tsunami characteristics and propagation directions 

from potential earthquakes and landslides in the 

Caribbean would provide a basis for focusing field 

efforts in those areas that have the highest risk 

for tsunami impact and the greatest vulnerability 

considering the local setting.

ACKNOWLEDGEMENTS

We thank Anja Scheffers, Sander Scheffers, 

Dieter Kelletat, Juan Carlos Moya, Narcisse 

Zahibo for scientific and cultural discussions and 

for serving as field guides to study sites. We also 

thank Theresa Fregoso for organizing the GPS and 

photographic data, Russell Peterson for preparing 

the location maps, and Betsy Boynton for preparing 

the illustrations. Uri ten Brink provided the template 

for figure 1. The report was improved by the critical 

reviews of James Goff and David Bush.

37
38

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