The Tsunami and Co - seismic subsidence history of the Orowaiti Estuary, Westport, New Zealand.

The Orowaiti Estuary is located east of the town of Westport on the west coast of the south island, New Zealand. Surface and sediment records were recovered and analysed in an attempt to identify evidence of tsunami and co – seismic subsidence in the estuary. Proxies utilised for identification were sedimentology, palaeontology and geochemistry, along with historical records for correlation. The transect representing the modern environment behaved in a manner typical of New Zealand estuarine environments. It supported a gradient of sedimentology which correlated with tidal heights. It also exhibited a number of different genera of the microbenthic Foraminiferida. The dominant genera observed in the transect were repeated in the core at depth, suggesting a period of either erosion or subsidence in the estuary to prevent a gradual decrease in mean water level over time. Caesium isotope dating revealed evidence of relatively shallow peaks in radioactivity in the core, placing the 1930’s above the suspected subsidence or flooding event influencing foraminiferal distribution. ICP – MS analysis highlighted a section of the core below this where small increases in all common pollutants, including mercury appeared. This was correlated with the middle 1800’s, where the gold rush was at its height. Due to bioturbation in the lower core sections, it was concluded that co – seismic subsidence was most likely responsible for the lack of speciation in the core compared to the transect. The suspected seismic event responsible was thought to be the potential 1717 Alpine Fault rupture. It was chronologically fitting, and was the only documented major rupture before the gold rush. Flooding evidence was thought to have occurred further up the core, in an undisturbed, sand dominated section that could be correlated to a 1979 avulsion of the Buller River into the Orowaiti. No evidence was found the supported the occurrence of tsunami in the Orowaiti estuary