Extremely high-energy wave deposits inside the Great Barrier Reef, Australia: determining the cause—tsunami or tropical cyclone

Abstract

A hydrodynamic approach is used to determine whether tsunami- or cyclone-generated waves were responsible for the deposition of fields of well-imbricated rock boulders (up to 290 tonnes) along the coast of Cairns inside the Great Barrier Reef, Australia. Calculations of the overturning 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 passages each approximately 35 km apart and oriented in the same direction exist in the Cairns region. It is possible that these passages have funnelled and amplified palaeotsunamis. The preferential location of eroded coral boulders up to 3 m in length on reef flats alongside these passages and their absence on other reefs throughout the region provide 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 flats matches closely the ages of coral fragments pinned below the very large rock boulders along the coast. These ages suggest that the Cairns region has experienced large tsunami twice over the last millennium.