EVIDENCE OF TSUNAMI SEDIMENTATION ON THE SOUTHEASTERN COAST OF AUSTRALIA

In coastal regions, the highest magnitude storms cannot always be invoked to account for large-scale, anomalous sediment features. Any coastline in the Pacific Ocean region can be affected by tsunamis, including Australia, which historically lacks evidence of such events. Geologically, tsunamis along the New South Wales coast have deposited a suite of Holocene features that consist of anomalous boulder masses, either chaotically tossed onto rock platforms and backshores or jammed into crevices; highly bimodal mixtures of sand and boulders; and dump deposits consisting of well-sorted coarse debris. In addition, many coastal aboriginal middens were distributed by such events. Within estuaries, tsunamis have left a record of stranded run-up ridges that have been interpreted mistakenly as cheniers. Dating of such deposits indicates that several events have affected this coastline since 3000 BP. In contrast to storm waves, tsunamis can leave a depositional imprint of their passage characterized by chaotic sorting and mixing of sediments either from different coastal environments or of different sediment sizes. The preservation potential of these deposits is high where sediments have been deposited above present sea level or stranded inland.