Tsunami or storm deposit? A late Holocene sedimentary record from Swamp Bay, Rangitoto ki te Tonga/D’Urville Island, Aotearoa – New Zealand

ABSTRACT

Informed by Māori oral histories that refer to past catastrophic marine inundations, multi-proxy analysis of stratigraphic records from Swamp Bay, Rangitoto ki te Tonga (D’Urville Island) shows evidence of an anomalous deposit extending some 160 m inland. The deposit includes two distinct lithofacies. The lower sand unit is inferred to have been transported from the marine environment, with corresponding increases in the percentages of benthic marine and brackish–marine diatoms, and geochemical properties indicative of sudden changes in environmental conditions. Radiocarbon dating indicates the deposit formation is less than 402 yrs BP, and pollen indicates it is unlikely to be younger than 1870 CE. Core stratigraphy age models and co-seismic chronologies point to the marine unit most likely being emplaced by tsunami transport associated with rupture of the Wairarapa Fault in 1855 CE. The overlying unit of gravel and silt is inferred to be fluvial deposit and slope-wash from the surrounding hills, loosened by ground-shaking following the earthquake. These findings indicate the 1855 CE earthquake may have been more complex than previously thought and, or, available tsunami modelling does not fully capture the local complexities in bathymetry and topography that can cause hazardous and localized tsunami amplification in embayments like Swamp Bay.

KEYWORDS:

• Māori
• tsunami
• earthquake
• sedimentology
• microfossils
• geochemistry
• Rangitoto ki te Tonga
• D’Urville Island
• Raukawa Moana
• Cook Strait

Acknowledgements

The authors acknowledge Mr Gus Forgan (Patuki Station Farm) for permission to enter and survey Swamp Bay as well as Ngāti Koata Trust and Te Rūnanga o Ngāti Kuia for their involvement and support of this work. Dr Marcus Vandergoes is thanked for palynology advice, Jenny Dahl for helping with radiocarbon sample selection, and Dr William Power and Ms Gaye Downes for bringing our attention to the 1894 newspaper article about historic tsunami observations. Mr Arne Pallentin is thanked for his assistance with cartography and Ms Georgina Griffiths (MetService) is acknowledged for assistance with obtaining renders of mean sea level pressure analyses for the Aotearoa-NZ region during ex-tropical Cyclone Gita at the end of February 2018. Finally, we would like to acknowledge the two anonymous reviewers who provided constructive comments on an earlier version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The supplementary data files that support the findings of this study are openly available in zenodo at https://doi.org/10.5281/zenodo.6585466.

Notes

1 Although Aotearoa is a/one Māori name for New Zealand’s North Island, Aotearoa–NZ is commonly used to refer to all of New Zealand.

Additional information

Funding

This research was funded by the Resilience to Natures Challenges National Science Challenge – Vision Mātauranga Science Programme (grant agreement no. 28378), the NIWA Strategic Science Investment Fund – Hazards, Climate and Māori Society (grant agreement no.C01X1702), and the GNS Science It's Our Fault Programme (jointly funded by the Earthquake Commission, Wellington Regional Council and Wellington City Council).