Stratigraphy and chronology of late pleistocene volcanic ash beds in Central North Island, New Zealand

Abstract

Ash beds of rhyolitic composition older than the Rerewhakaaitu Ash are described (in order of increasing age) for the central North Island of New Zealand. For convenience the ash beds down to the Rerewhakaaitu Ash (c. 14,700 yr B.P.) are included in the Holocene and the older ash beds (Okareka, Te Rere, Oruanui, Mangaonjf and Rotoehu) are referred to the late Pleistocene, the age of Rotoehu Ash being greater than c. 41,000 yr B.P. Okareka Ash and Te Rere Ash, with sources near Mt Tarawera, lack distinctive bedding and are of limited extent. Oruanui Formation, with a source near Taupo, has two distinctive members: Oruanui Breccia; and Oruanui Ash (distributed over 50,000 sq miles). Mangaone Lapilli, erupted from three main sources in the Okataina Volcanic Centre, comprises five major pumice lapilli and block beds. Rotoehu Ash, erupted from near Lake Rotoma, is a distinctive shower-bedded deposit recognisable over 6,000 sq miles. Rotoehu Ash rests without time interval on Rotoiti Breccia and is here considered a member of that formation. The record within the ash column has particular value where ash beds are radiocarbon dated: for example, the Oruanui Formation, erupted c. 20,000 yr B.P., suffered extensive erosion and so provided a local source in the central North Island for wind-transported Mokai Sand and loess; the ash beds also contributed to aggradation gravels and sands of the Hinuera Formation in the Waikato Valley and lowlands. Correlation of persistent but well-weathered formations with soils is attempted for the Waikato, Bay of Plenty, Gisborne, and Hawke's Bay districts. Oruanui Ash is provisionally correlated with Aokautere Ash, which was previously used as a marker bed in loess deposits of the Manawatu district, and has proved a useful tool in pedological interpretation.