Bonaire's boulder fields revisited: evidence for Holocene tsunami impact on the Leeward Antilles

Supralittoral boulders and blocks are prominent sedimentary features along rocky shorelines worldwide. In most cases, their deposition is attributed to high-energy wave events (tsunamis, severe storms). Even though tsunami waves are expected to have higher transport capacities compared to storm waves, megaclasts of up to 100 t were observed to have been moved laterally by the latter waveform. The deduction of certain extreme wave events (tsunamis, severe storms) from the boulder record thus remains a major challenge in palaeo-event research; the debate on their differentiation is ongoing. At the eastern coast of Bonaire (Leeward Antilles) in the southern Caribbean, numerous limestone blocks and boulders (up to c. 130 t) are distributed on top of a 3–6 m a.s.l. (above mean sea level) palaeo-reef platform. Disagreement exists among a number of authors concerning the transport processes involved in the formation of the boulder fields. In this paper, state-of-the-art modelling approaches of coastal boulder entrainment and transport were applied in order to provide new and more reliable data to support/challenge the working hypothesis of tsunami deposition. To improve the reliability of the boulder transport model, more realistic input parameters were provided by new DGPS measurements of the boulder dimensions and the calculation of bulk densities by taking into account the heterogeneity of the reef-rock boulders. Existing hydrodynamic equations were modified to allow for the irregular shape and real dimensions of the boulders. The results indicate that (i) boulder weight and dimension, and thus (ii) calculated wave energy and wave heights were overestimated in most of the previous studies, where calculations of boulder volume were based on multiplication of the main axes. The results of this study and wave heights observed during recent severe tropical cyclones seem to rule out storm-generated waves for the dislocation of the largest blocks. However, the majority of coarse-clast deposits may have been generated by periodic hurricane swells. The results underline the significance of more realistic field data in modelling boulder transports by waves.

► We used coastal boulders and existing transport models to test the tsunami hypothesis for Bonaire.
► Results are based on boulder-specific volume and density measurements.
► Boulder volumes measured by DGPS are realistic and amount c. 50% of multiplied main axes.
► Largest boulders on Bonaire were not transported by hurricane waves.