Wave-emplaced boulders on Jamaica's rocky shores

Evidence for giant wave events, in the form of boulder fields and unsorted debris ridges, is presented for the first time from six locations in Jamaica and comparisons are made with similar deposits in Grand Cayman which were restudied for this project. All of the sites are located on late Pleistocene raised reef platforms, and face zones where deep water extends relatively close inshore. Most lack the protection of modern reefs. Boulder lithology resembles that of the platforms on which they rest. At Galina, northeast Jamaica, 180 boulders extending one and a half kilometres along the coast and ranging in mass from less than 1 ton to about 120 tons were mapped in detail. Most cluster between 80 m and 160 m from the shoreline, and only 3 are closer than 30 in from the shore. The boulders are backed by a 1-2 m high, heavily vegetated ridge composed of unsorted carbonate sand and coral debris with rare, small boulders. Reconnaissance surveys of other sites indicate similar distributions of boulders and debris ridges. Radiocarbon dates on two boulders from Galina suggest emplacement of one in the early 20(th) Century and the other later than 1950 AD. These may correspond, respectively, to the 1907 tsunami event reported from northeast Jamaica, and to the 1980 passage of Hurricane Allen. Eyewitnesses report boulders up to 15 tons being moved significant distances during the passage of Allen. Extensive movement of large boulders (up to 33 tons), accompanied by wholesale removal of near-shore forest vegetation, also occurred at two sites in southwest Jamaica, following the passage of Hurricane Ivan in 2004, as well as evidence for Ivan-generated movement of boulders on Grand Cayman. Boulder movement resulting from Hurricane Gilbert in 1988 was reported on 12 in high cliffs from the cast coast. There is increasing recognition that submarine landslides play an important, even dominant role in the generation, particularly, of local tsunami. Our identification of such slides in the offshore region of southwest Jamaica, and the fact that the three best-documented tsunami all appear to have been generated locally, indicate a need for more detailed bathymetric surveys of the Jamaican shelf and slope.