GEOLOGICAL HISTORY OF THE NORTH-EASTERN RING PLAIN OF RUAPEHU VOLCANO, NEW-ZEALAND

Continuous exposures along the Upper Waikato Stream provide new insigh ts into the north-eastern ring plain of Ruapehu volcano, extending the known stratigraphy beyond 22.6 ka. Time control in the sequence is pr ovided by five rhyolitic tephra units, erupted from central North Isla nd volcanoes, comprising Kawakawa, Okaia, Omataroa, Hauparu, and Rotoe hu tephras. The sequence is dominated volumetrically by diamictons and fluvial deposits resulting both from volcanic events and periods of i nstability on the flanks of Ruapehu. Within the sequence are > 60 indi vidual andesitic lapilli units, derived primarily from Ruapehu volcano via mostly sub-plinian eruption mechanisms. An average eruption rate of more than one lapilli eruption per 1000 years is estimated for the ca. 60 ka record. The style of deposition on the ring plain changes ov er time and appears to reflect climate change over the Last Glacial pe riod. In periods of severe climatic conditions during marine delta(18) O Stage 4 (Porewan stadial), and the Last Glacial Maximum of marine de lta(18)O Stage 2 (Ohakean), the north-eastern ring plain aggraded rapi dly with deposition of thick continuous diamicton sequences. The other recognized cool period in the southern North Island, the stadial of l ate delta(18)O Stage 3 (Ratan), did nor appear to induce major aggrada tion on the north-eastern ring plain. During periods of mild climate w ithin the Last Glacial, deposition on the north-eastern ring plain was dominated by fall accession of either tephra or material reworked fro m other parts of the ring plain by aeolian processes. Copyright (C) 19 96 INQUA/Elsevier Science Ltd