EXTREMELY HIGH-ENERGY WAVE DEPOSITS INSIDE THE GREAT-BARRIER-REEF, AUSTRALIA - DETERMINING THE CAUSE - TSUNAMI OR TROPICAL CYCLONE

A hydrodynamic approach is used to determine whether tsunami-or cyclon e-generated waves were responsible for the deposition of fields of wel l-imbricated rock boulders (up to 290 tonnes) along the coast of Cairn s inside the Great Barrier Reef, Australia. Calculations of the overtu rning moments show that only tsunami are capable of moving such large boulders in this environment. It is hypothesised that large tsunami (> 11 m) have been able to penetrate the Great Barrier Reef through wide (5-10 km) 50-70 m deep passages between individual reefs. Three such p assages each approximately 35 km apart and oriented in the same direct ion exist in the Cairns region. It is possible that these passages hav e funnelled and amplified palaeotsunamis, The preferential location of eroded coral boulders up to 3 m in length on reef flats alongside the se passages and their absence on other reefs throughout the region pro vide further evidence that extremely high-energy waves have been able to penetrate the Great Barrier Reef into the inner channel adjacent to the mainland. Carbon-14 ages of the coral boulders on these reef flat s matches closely the ages of coral fragments pinned below the very la rge rock boulders along the coast. These ages suggest that the Cairns region has experienced large tsunami twice over the last millennium. ( C) 1997 Elsevier Science B.V.