New Zealand's Most Easterly Palaeotsunami Deposit Provides Supporting Evidence for Major Regionwide Event

Previous research on Chatham Island, part of New Zealand's easternmost island archipelago, has successfully identified evidence for the large, historically documented AD1868 tsunami and up to two earlier events (probably AD1575 or AD1604 and AD1280-1390). However, the thin sediment cover throughout most of Chatham Island appears to provide little accommodation space for the accumulation of multiple events, with more recent tsunamis probably eroding the sedimentary records of their predecessors. In such situations it is invariably only the unusually large predecessors that may still be preserved in the sedimentary record. Here we report on preliminary work from Okawa Point on the NE side of Chatham Island where there is evidence for at least three possible events, one historical (thin sand unit) and two palaeotsunamis (boulders/sand). While the timing of these events still needs further work, it seems likely that the historical event relates to the AD1868 and the two palaeotsunamis are probably the AD1575/AD1604 and a much earlier, and larger one, most likely laid down around 2800-3000 years BP. Evidence for this earlier event consists of a widespread boulder deposit and associated sand layer that can be traced at least 1.5 km inland. The most likely candidate for this event appears to be a regionwide palaeotsunami reported from around the SW Pacific with the proposed source being associated with subduction along the Tonga Trench-Hikurangi Trough. Photo taken looking SW and showing part of boulder field at Okawa Point, Chatham Island.