The Ngatatura diatreme

Abstract

Cutting the Tertiary sedimentary rocks of Ngatatura Bay is a well-exposed example of a diatreme. The exposure probably lies within 50 m of the Pleistocene land surface through which the explosion crater was punched. The initial explosions were followed by weak strombolian eruptions of basalt lava forming a rewelded spatter deposit, and finally by weak explosions which reworked the tuff and gave it a complex cross-bedded appearance in its upper part. The presence of the basalt suggests that frozen lava lakes which were inferred from gravity anomalies to lie beneath some Auckland maars, may in fact be rewelded basalt spatter like that observed in the Ngatatura diatreme.