Pacific Rim Research Program

Foraminiferal and diatom assemblages in 11 cores (3–7.5m deep) of Holocene sediment from brackish marine Ahuriri Inlet in southern Hawke’s Bay, New Zealand, provide a record of 8.5m of subsidence followed by 1.5m of uplift in the last 7200 cal years, in a region overlying the subduction zone between the Australian and Pacific Plates. Modern Analogue Technique was used to estimate paleotidal elevation of the 97 richest foraminiferal assemblages. The most precise estimates are for high-tidal salt marsh assemblages cored in marginal settings in the north and south of the former inlet. The least precise estimates are from low-tidal and subtidal assemblages from cores in the middle of the inlet. These paleoelevation estimates combined with sediment thicknesses, age determinations (from tephrostratigraphy and radiocarbon dates), the New Zealand Holocene sea level curve, and estimates of compaction, identify the Holocene land elevation changes and earthquake-displacement events in each core. The following major, earthquake-related displacements are inferred: ca 7000 cal yr BP (40.6m displacement); ca 5800 cal yr BP (40.5 m; ca 4200 cal yr BP (ca 1.5 m); ca 3000 cal yr BP (1.4 to 1.8 m); ca 1600 cal yr BP (ca 1.7 m); ca 600 cal yr BP (ca 1 m); 1931AD Napier Earthquake (+1.5 m). Further smaller events involving regional subsidence or earthquake-shake compaction are indicated during the 7000–3000 yr BP interval, but cannot be identified precisely. The six (possibly subduction interface) subsidence events in the last 7000 years have had a return time of 1000–1400 years. Identified displacement events have a range of sedimentary expressions, from an eroded and burrowed hiatus surface, to an abrupt lithologic switch from mud to sand, or peat to shelly mud, or in some places no change in sediment character whatsoever.