Microfossil evidence for a possible maar crater and tuff ring beneath Rangitoto Volcano, Auckland, New Zealand

ABSTRACT

Distinctive low diversity foraminiferal and ostracod faunas in Holocene mud from a stratigraphic drillhole through Rangitoto Volcano (∼600 years old) indicate a highly sheltered, shallow-marine environment existed near the middle of the exposed entrance to Waitemata Harbour, Auckland, for at least 7000 years prior to Rangitoto’s eruption. Modern Analogue Technique estimates for these faunas indicate they accumulated at low tidal to shallow subtidal depths (0–11 m). The most similar analogue faunas lived subtidally in the extreme shelter inside the breached Onepoto maar lagoon, Auckland. Therefore, we infer the possible existence of a pre-7500 year-old maar crater and surrounding tuff ring buried beneath Rangitoto Volcano. This Holocene mud occurs in the drillhole 24–10 m lower than where the microfaunas indicate it accumulated. It is inferred to have slid into the adjacent harbour channel, along with 7 m of basaltic tuff and a piece of lava flow, near the start of the second phase of Rangitoto’s eruption.

KEYWORDS:

• Rangitoto Volcano
• foraminiferal paleoecology
• ostracod paleoecology
• basalt glass shard chemistry
• buried maar crater

Acknowledgments

We are grateful to Paul Augustinus for providing access to the mud samples and for encouraging us to undertake this study. He also provided a valuable review but declined the offer of co-authorship. Shane Cronin and Phil Shane obtained the funding for the stratigraphic drillhole on Rangitoto via DEVORA from the NZ Earthquake Commission Biennial Grant Scheme and Auckland Council funded most of the radiocarbon dating. Elaine Smid kindly provided information on Rangitoto Wharf drillholes. We thank Joanne Egan, Catherine Reid and Liz Kennedy for reviewing this manuscript and suggesting improvements.

Disclosure statement

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

Data availability statement

The data that support the findings and interpretations in this study are openly available in zenodo at http://doi.org/10.5281/zenodo.6426921. These include online Appendix A. Tephra glass shard analyses from the Rangitoto drillhole, and EPMA machine set up; online Appendix B. Foraminiferal and ostracod census data from fossil samples from the Rangitoto stratigraphic drillhole (R11/f262); and online Appendix C. Relative abundance of foraminifera in modern sediment samples from Waitemata Harbour used for comparison with Rangitoto drillhole samples.

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This study was largely undertaken without funding support except for Victoria University of Wellington support for JH.