STRATIGRAPHY AND CHRONOLOGY OF LATE QUATERNARY ANDESITIC TEPHRA DEPOSITS, TONGARIRO-VOLCANIC-CENTER, NEW-ZEALAND

A stratigraphy and chronology of andesitic tephras erupted from Mt Ruapehu, and other volcanoes of Tongariro Volcanic Centre, is constructed from the tephra record preserved on the southeastern Mt Ruapehu ring plain. Here, tephras of late Quaternary age (c. 22,500 years B.P. to present) are found interbedded with local laharic and fluvial deposits, and with distal rhyolitic tephras from Taupo and Okataina Volcanic Centres. Tephras are identified from their field characteristics and stratigraphic positions relative to dated rhyolitic tephra marker beds. The radiocarbon ages of these rhyolitic tephras provide a chronology for the andesitic tephras, dating back to 22,500 years B.P. All tephras erupted from Tongariro Volcanic Centre are grouped into two subgroups: the Tongariro Subgroup (redefined) and the newly defined Tukino Subgroup. Tephras identified on the southeastern Mt Ruapehu ring plain are grouped into seven formations on the basis of lithology: Ngauruhoe Formation [dated c. 1,850 years B.P. - present], Tufa Trig Formation [dated c. 1,850 years B.P. - present], Mangatawai Tephra [dated c. 2,500 years B.P.], Papakai Formation [dated c. 9,700-2,500 years B.P.], Mangamate Tephra [dated c. 9,780-9,700 years B.P.], Pahoka Tephra [dated c. 10,000-9,800 years B.P.], and Bullet Formation [dated c. 22,500-10,000 years B.P.]. Tephras erupted from Mt Ruapehu are grouped into the Tufa Trig Formation, Papakai Formation, and Bullet Formation. Papakai Formation comprises both tephra and volcanic loess. The stratigraphy and chronology of the seven formations are detailed, and revisions to the earlier stratigraphy of Topping (1973, 1974) are discussed. The distributions of principal tephra marker beds are shown, and tephra volumes (all < 0.2 km(3)) calculated from isopach maps wherever possible.