A reappraisal of the Ngatutura volcanics and the Plio-Pleistocene boundary in South-west Auckland

Abstract

Alkali-olivine basaltic vulcanism in the Ngatutura-Ohuka area has been considered to occupy a position close to or at the Plio-Pleistocene boundary, dated at 2 · 5 m.y. by correlation with dated centres further south. Re-examination of the stratigraphic position of the volcanic centre at Ngatutura Point shows that both minor late Pliocene and major early Pleistocene phases of activity occurred, none of which was necessarily contemporaneous with the southern centres. Breccias, erupted in the early Pleistocene, contain a cool Nukumaruan microflora. Associated basalts possess ages concentrated in the range 1 · 4—1 · 8 m.y. and have a reversed magnetic polarity. It is concluded that the major eruption commenced during the Matuyama Reversed Epoch between the Olduvai and Réunion events.