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.