SPATIAL DISTRIBUTION AND SEDIMENTARY FEATURES OF TSUNAMI DEPOSITS IN PUMICE- AND CARBONATE-DOMINATED BEACH ENVIRONMENTS

For distinguishing tsunami and storm deposits, it is important to understand the diversity in characteristics of the deposits due to the deposit materials and environmental settings. For example, spatial distribution and sedimentary features of pumice-dominated and carbonate-dominated tsunami deposits are significantly different from those of heavy mineral-dominated tsunami deposits. The pumice-dominated deposit (pumiceous sand) is a mixture of pumice and beach sand that are transported and deposited inland by a tsunami. This unique sediment layer is often observed in near-source tsunami deposits of volcanic origin. Landward thinning and landward fining, those are commonly recognized for sandy tsunami deposits, are not typical features of these volcanism-induced tsunami deposits. For some cases, the deposit was partly covered by the following ash or pumice falls. It provides opportunity to observe the original structure precisely and to consider post-depositional process by comparing the covered and exposed deposits.

Local tsunamis were generated by the 1994 Vulcan eruption and the 2006 Tavurvur eruption, both in Rabaul Caldera, Papua New Guinea. The tsunami heights were 3-6 m and inundation distances were 30 - 200 m from the shore for the two events. The tsunami inundation area is covered by pumice-rich tsunami deposit continuously. The origin of the pumice is mainly drift pumice as there are no pumice fall-out layers landward of the tsunami limit. The pumice tends to be accumulated in the uppermost part of the layer, and the base of the deposits is sandy. The tsunamis eroded the top of the ash layer deposited before the tsunami inundated. Along the inundation boundary, a distinct belt of accumulated pumice and wood branches was created. Because density variations are essential in the formation of sedimentary features of tsunami deposits, this case history of tsunami deposition is informative to aid in the identification of paleotsunami deposits in pumice- and carbonate-dominated beach environments.