COMPARISON OF COASTAL BOULDER DEPOSITS (HOLOCENE AGE) ON ELEUTHERA, BAHAMAS, WITH STORM-TRANSPORTED BOULDERS ON ARAN ISLANDS, IRELAND

Eleuthera, on the northeastern side of the Bahamas archipelago, has a steep ocean-facing coastline fully exposed to Atlantic storms. Cliffs of Pleistocene limestone rising to about 20 m a.s.l. are topped with modern boulder deposits that include (in order of increasing clast concentration) isolated clasts, scattered-boulder fields, and boulder ridges. Although some sit close to the cliff edge, most have been transported 10s of m inland. The boulders have mass up to several 10s of tonnes.

These deposits have previously been interpreted as tsunamigenic, but recent analysis suggests that they are more likely to have been emplaced by storms. Boulder dimensions were measured in the field, and samples were collected for density measurement in the lab. Volume estimates were checked by photogrammetry: for a subset of the boulders we took suites of images with a GPS-enabled camera, and built 3D models using the Agisoft PhotoScan Pro software. The 3D models returned volumes within 15% of those estimated from XYZ dimensions. We measured elevations and inland distances by laser rangefinder, and corrected the data for tide height to yield topographic positions relative to high water. We compared the boulder masses and topographic settings with analogous data from known storm deposits on the other side of the Atlantic, along Ireland’s west coast.

The Irish data come from boulders transported on the Aran Islands during the winter 2013-2014 “storm factory” events. The largest clasts in the Irish dataset define clear mass-topography relationships with which boulders data from other locations can be compared. The coastal topographies of Eleuthera and the Aran Islands are very similar (steep limestone coasts passing rapidly into deep waters offshore), so they are likely to have similar storm wave dynamics. The Eleuthera Holocene boulders plot along or close to mass-elevation and mass-steepness regression lines defined by the Irish boulder data. This suggests that there is no need to call on extraordinary events such as tsunami to move these clasts.