The role of porosity in discriminating between tsunami and hurricane emplacement of boulders - a case study from the Lesser Antilles, southern Caribbean

Coastal boulder deposits are a consequence of high-energy wave impacts, such as storms, hurricanes or tsunami. Parameters useful for distinguishing between hurricane and tsunami origins include distance of a deposit from the coast, boulder weight and inferred wave height. In order to investigate the role of porosity on boulder transport and elucidate the distinction between tsunami and hurricane impacts, we performed Archimedean and optical 3D-profilometry measurements for the determination of accurate physical parameters for porous reef and coral limestone boulders from the islands of Aruba, Bonaire and Curaçao (ABC Islands, Netherlands Antilles, Leeward Islands). Subsets of different coral species and lithotypes constituting the boulders were sampled, the physical parameters of boulders were analyzed, and each boulder component was attributed to a certain range of porosity and density. Lowest porosities were observed in calcarenite (5–8%), whereas highest porosities were measured for serpulid reef rock (47–68%). Porous serpulid reef rock (0.8–1.2 g/cm3) and the coral Diploria sp. (0.6–1.0 g/cm3) possess the lowest bulk densities, while less porous calcarenite (2.0–2.7 g/cm3) and the coral Montastrea cavernosa yield the highest bulk density values (1.6–2.7 g/cm3). The obtained physical parameters were used to calculate boulder weights and both hurricane and tsunami wave heights necessary to initiate transport of these boulders. Boulders are up to 5.6 times lighter than given in previously published data, and hence required minimum hurricane or tsunami waves are lower than hitherto assumed. The calculated wave heights, the high frequency of tropical storms and hurricanes in the southern Caribbean and the occurrence of boulders exclusively on the windward sides of the islands, implicate that for boulders on the ABC Islands a hurricane origin is more likely than a tsunami origin.