Sedimentary evidence for tsunami on the northeast coast of New Zealand / Arguments sédimentaires attestant un tsunami sur la côte nord-est de la Nouvelle-Zélande

摘要

Abstract Extensive thin gravel sheets mantle coastal dunes at Henderson Bay (Northland) and Whangapoua Bay (Great Barrier Island) on the Northeast coast of North Island, New Zealand. At each site, the surface distribution of the gravel has been mapped and sampled. At each locality the gravels extend above the 2-3 m (above MSL) limit of storm surge to 14-m elevation at Whangapoua and 32 m at Henderson Bay. The area of gravel exposed is 30,000 m2 at Whangapoua and 120,000 m2 at Henderson Bay. The gravel sheets are single-clast thick and comprise poorly sorted, granule to cobble-sized clasts. Particles are sub-rounded to sub-angular, indicating wave abrasion. A fluvial or colluvial origin for these deposits is discounted for both sites due to their isolation from streams and hillslopes. We also discount the possibility that these deposits are the product of aeolian transport on the basis of clast size (up to 67 mm) and elevation. We propose that they were swept there by tsunami. The freshwater wetlands behind the Henderson Bay dunefield provide further evidence for tsunami run-up. Here, a vibra- core records the landward reworking of beach and dune sand and Loisels Pumice into a freshwater wetland where it is preserved as a 5-cm interbed within peat. Bracketing radiocarbon ages on the enclosing peat provide a maximum age of 5590-5310 cal yr. BP and a minimum age of 1180-930 cal yr. BP for the sand-pumice deposit. At Whangapoua, optical ages on dunes sands from below the gravels range from 6700 ± 500 to 3700 ± 500 BP. However, the most reliable age estimate for the tsunami deposits is derived from Loisels Pumice, which is independently dated to 660-510 years BP. Within this time frame, the most probable mechanism for tsunami generation was the collapse of a submarine caldera (Healy caldera) that occurred circa 700 years BP on the Kermadec ridge to the north of New Zealand.