Holocene coastal evolution and evidence for paleotsunami from a tectonically stable region, Tasmania, Australia

Abstract

Stratigraphic investigations of three coastal waterbodies in southeastern Tasmania reveal major paleoenvironmental phases related to sea level change and anomalous deposits consistent with tsunami inundation. Twenty-two short sediment cores were examined for their sedimentology and fossil diatom, foraminifera and macrofossil assemblages; nine radiocarbon ages were obtained. Despite diverse Holocene histories at each site, four common phases of Holocene paleoenvironmental evolution can be distinguished. In Phase I (pre-8000 yr BP) terrestrial environments existed. During Phase II (8000–6500 yr BP) ponded freshwater environments formed behind transgressive coastal barriers. In Phase III (6500–2000 yr BP) the sites were subject to varying degrees of marine influence, resulting in environments ranging from current-swept tidal inlets to sheltered brackish-marine lagoons. In Phase IV (2000 yr BP to present) there was a decrease in marine influence, one site changed to a freshwater wetland environment while the other two changed to ephemeral salt pans. This study suggests that postglacial sea level rise culminated after c. 7300 cal. yr BP in southeastern Tasmania and that there was probably a late-Holocene fall in sea level. These paleoenvironmental histories provide a framework within which to identify anomalous deposits and assess them for likely causes. Five anomalous deposits are identified, three of which are considered likely to have been deposited by tsunami occurring at c. 4000 cal. yr BP, c. 2000 cal. yr BP and <2000 cal. yr BP, although deposition by large storms cannot be ruled out.