Geological history of the north-eastern ring plain of Ruapehu volcano, New Zealand

Abstract

Continuous exposures along the Upper Waikato Stream provide new insights into the north-eastern ring plain of Ruapehu volcano, extending the known stratigraphy beyond 22.6 ka. Time control in the sequence is provided by five rhyolitic tephra units, erupted from central North Island volcanoes, comprising Kawakawa, Okaia, Omataroa, Hauparu, and Rotoehu tephras. The sequence is dominated volumetrically by diamictons and fluvial deposits resulting both from volcanic events and periods of instability on the flanks of Ruapehu. Within the sequence are > 60 individual 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 over time and appears to reflect climate change over the Last Glacial period. In periods of severe climatic conditions during marine δ18O Stage 4 (Porewan stadial), and the Last Glacial Maximum of marine δ18O Stage 2 (Ohakean), the north-eastern ring plain aggraded rapidly with deposition of thick continuous diamicton sequences. The other recognized cool period in the southern North Island, the stadial of late δ18O Stage 3 (Ratan), did not appear to induce major aggradation on the north-eastern ring plain. During periods of mild climate within the Last Glacial, deposition on the north-eastern ring plain was dominated by fall accession of either tephra or material reworked from other parts of the ring plain by aeolian processes.