Journal of Sedimentary Research, 2008, v. 78, 624–637 Research Article DOI: 10.2110/jsr.2008.068 COARSE-CLAST RIDGE COMPLEXES OF THE CARIBBEAN: A PRELIMINARY BASIS FOR DISTINGUISHING TSUNAMI AND STORM-WAVE ORIGINS ROBERT A. MORTON, 1 * BRUCE M. RICHMOND, 2 BRUCE E. JAFFE, 2 AND GUY GELFENBAUM 3 1 U.S. Geological Survey, 600 Fourth Street South, St. Petersburg, Florida 33701, U.S.A. e-mail: rmorton@usgs.gov , 2 U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, U.S.A. 3 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, U.S.A. ABSTRACT: Coastal gravel-ridge complexes deposited on islands in the Caribbean Sea are recorders of past extreme-wave events that could be associated with either tsunamis or hurricanes. The ridge complexes of Bonaire, Jamaica, Puerto Rico (Isla de Mona), and Guadeloupe consist of polymodal clasts ranging in size from sand to coarse boulders that are derived from the adjacent coral reefs or subjacent rock platforms. Ridge-complex morphologies and crest elevations are largely controlled by availability of sediments, clast sizes, and heights of wave runup. The ridge complexes are internally organized, display textural sorting and a broad range of ages including historical events. Some display seaward-dipping beds and ridge-and-swale topography, and some terminate in fans or steep avalanche slopes. Together, the morphologic, sedimentologic, lithostratigraphic, and chronostratigraphic evidence indicates that shore-parallel ridge complexes composed of gravel and sand that are tens of meters wide and several meters thick are primarily storm-constructed features that have accumulated for a few centuries or millennia as a result of multiple high-frequency intense-wave events. They are not entirely the result of one or a few tsunamis as recently reported. Tsunami deposition may account for some of the lateral ridge-complex accretion or boulder fields and isolated blocks that are associated with the ridge complexes. INTRODUCTION The 2004 Indian Ocean tsunami was a stark reminder of the devastating forces associated with these catastrophic events and their ability to cause widespread coastal change. Since then, attention has been focused 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 associated islands (Fig. 1), where historical tsunamis have been reported (Lander et al. 2002). The islands of the Caribbean region also are frequently impacted by tropical cyclones (Fig. 2), including extreme hurricanes. In recent years, attempts have been made to reconstruct histories of storm and tsunami hazards from the sedimentary record where local histories of these events are either short or incomplete. Because using paleo-event deposits is an accepted method of extending the historical record for coastal-hazard risk assessments (Bourgeois and Minoura 1997; Hayne and Chappell 2001; Jaffe and Gelfenbaum 2002), it is essential to recognize the depositional signatures of the two different extreme-wave processes where both coexist. The distinguishing characteristics of sandy storm and tsunami deposits have been reported recently (Morton et al. 2007a); however, diagnostic characteristics of coarse-clast features deposited by storms and tsunamis have not been compared. To facilitate our assessment of extreme-wave hazards in the Caribbean, field data were collected in Puerto Rico (Isla de Mona), Jamaica, Guadeloupe, and Bonaire (Fig. 3), where historical hurricanes and tsunamis have inundated the coast (Lander et al. 2002) or interpreted paleo-tsunami deposits have been reported (Taggart et al. 1993; Scheffers 2002; Scheffers et al. 2005; Robinson et al. 2006). We used a combination of laser range finder, GPS receivers, field observations, historical aerial photographs, and satellite images to measure distances, elevations, and dimensions of the coarse-clast deposits and to document their morphol- ogies, lateral extents, stratigraphies, and ages. CARIBBEAN EXTREME-WAVE EVENTS Historical Hurricanes Tropical cyclones impact the shores of at least a few Caribbean islands each year. These intense storms, which commonly track from east to west (Fig. 2), exhibit counterclockwise wind circulation around centers of low barometric pressure (Simpson and Riel 1981). As a result of island orientations and 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 pass south of the islands. Extremely rare exceptions to these paths and impact patterns are caused by storms that track from west to east, such as Hurricane Lenny in 1999. Because the zone of cyclogenesis is centered roughly 10u north of the Equator, frequency of impacts from hurricane-generated waves is highest in the northern Caribbean and decreases toward the south. The coasts of Jamaica, Puerto Rico, and Guadeloupe are regularly flooded by storm waves because they lie within the zone of tropical * Present Address: U.S. Geological Survey, 10100 Burnet Road, Austin, Texas 78758 U.S.A. Copyright E 2008, SEPM (Society for Sedimentary Geology) 1527-1404/08/078-624/$03.00