Boulder Deposits from Large Waves during the Last Interglaciation on North Eleuthera Island, Bahamas

Abstract

Seven boulders measuring 100 to 1000 m3are scattered along the coastal ridge of north Eleuthera. Some are situated on ridge crests up to 20 m above present sea level. The boulders were probably deposited during oxygen-isotope substage 5e or 5d, as shown by their stratigraphic setting and by amino acid racemization ratios. D-alloisoleucine/L-isoleucine ratios were determined for land snails, and oolite of both marine and eolian origin was associated with the boulders. Like the boulders, the probable source rocks exposed in the adjacent cliffs are composed of marine and eolian limestone of oolitic and peloidal composition. The source beds are correlated with stage 9 or 11. The largest boulder is about 10 times the size of the largest Holocene ones moved by waves in the area. Tsunamis are a reasonable possibility as a transporting mechanism of the Pleistocene boulders. However, if deposited instead by storms during the last interglaciation, the storms were of much greater intensity than those occurring in the region during the late Holocene.