TEPHROSTRATIGRAPHY OF HIGH-ALTITUDE PEAT BOGS ALONG THE AXIAL RANGES, NORTH ISLAND, NEW-ZEALAND

Peat bogs occur along the crests of the main axial ranges of the North Island of New Zealand. The bogs provide ideal sites for palynological investigations and have also preserved a number of tephra layers from andesitic and rhyolitic volcanoes to the west and north. The tephra layers have been identified using mineralogy, glass chemistry, and radiocarbon dates. Taupo Tephra and Waimihia Tephra are ubiquitous and easily identified by thickness and grain size. Other Taupo Volcanic Centre eruptives identified are Mapara, Hinemaiaia, Motutere, Opepe, and Karapiti Tephras. Two layers contain cummingtonite, a mineral diagnostic of Okataina Volcanic Centre, and these are identified as Whakatane and Rotoma Tephras. Karapiti Tephra (c. 10 000 yr B.P.) is the oldest rhyolitic tephra layer identified and dates the formation of the deepest bogs. Andesitic tephra is dispersed throughout the northern and western peat bogs and forms several distinct layers. It is characterised by a mineralogy dominated by augite and common hornblende but has a variable glass chemistry. The andesitic tephra grains vary from dense to scoriaceous and opaque to clear vesicular shards. They are intimately mixed with many of the rhyolitic layers, and glass chemistry was often the only means of determining the origin of the tephra. The character and number of layers found in the bogs confirm that several layers were formed by secondary processes, especially wind erosion and transport. The peat bogs generally overlie a layer of weathered andesitic tephra that is clay rich and impermeable and is about 10 000-12 000 years old. Under this, in the higher bogs, is either a thin layer of unweathered angular greywacke fragments over greywacke bedrock or Tertiary siltstone and sandstone. The greywacke fragments are probably remnants of colluvium formed during periglacial conditions 15 000-25 000 years ago.