INTEGRATED TEPHROCHRONOLOGY AND MAGNETOSTRATIGRAPHY FOR CYCLOTHEMIC MARINE STRATA, WANGANUI BASIN - IMPLICATIONS FOR THE PLIOCENE-PLEISTOCENE BOUNDARY IN NEW-ZEALAND

The middle Pliocene to earliest Pleistocene part of the Rangitikei Riv er section in Wanganui Basin contains at least six distal tephras-five are rhyolitic (Kowhai, Eagle Hill, Ohingaiti, Waipura and Vinegar Hil l tephras) and one is andesitic (Mangamako Tephra). All tephras occur as macroscopic layers and are enclosed within cyclothemic marine sedim ents that accumulated in shoreface to outer shelf water depths under c onditions of oscillating sea level driven by glacio-eustacy. The glass and ferromagnesian compositions of the silicic tephras imply derivati on from calc-alkaline rhyolitic sources, either or both of Taupo Volca nic Zone and southern Coromandel Volcanic Zone. The Waipuru and Vine a r Hill tephras have isothermal plateau fission-track (ITPFT) ages of 1 .87 +/- 0.15 Ma and 1.75 +/- 0.13 Ma, respectively. Paleomagnetic resu lts allow identification of (i) the Gauss/Matuyama boundary located ca . 70 m below the top of the Mangaweka Mudstone; and (ii) the top of th e Olduvai Subchron, placed al the base of the Waipuru Shellbed. The IT PFT ages of the tephras are consistent with the interpreted magnetostr atigraphy and also with astronomically tuned timescales. Correlation o f the Rangitikei sequences to the ODP site 846 delta(18)O record indic ates that each sequence corresponds to a glacial/interglacial isotope stage coupler, and allows ages to be interpolated for tephras not date d directly. Integration of the new ages for the Waipuru and Vinegar Hi ll tephras, the magnetostratigraphy, and the sequence correlations to the astronomically tuned oxygen isotope timescale, lead us to place th e Pliocene Pleistocene boundary, as defined at the Vrica stratotype, a t the base of the highstand systems tract siltstone of Sequence 17 in the Rangitikei section, 60 m below the Vinegar Hill Tephra. Copyright (C) 1996 INQUA/Elsevier Science Ltd