New Zealand Journal of Geology and Geophysics abstracts

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Re-identification of c. 15 700 cal yr BP tephra bed at Kaipo Bog, eastern North Island: implications for dispersal of Rotorua and Puketarata tephra beds

Phil Shane
Victoria C. Smith

Department of Geology
University of Auckland
Private Bag 92 019
Auckland, New Zealand
email: pa.shane@auckland.ac.nz

David J. Lowe

Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand

Ian Nairn

45 Summit Rd
RD5, Rotorua, New Zealand

Abstract  A 10 mm thick, c. 15 700 calendar yr BP (c. 13 100 ¹⁴C yr BP) rhyolitic tephra bed in the well-studied montane Kaipo Bog sequence of eastern North Island was previously correlated with Maroa-derived Puketarata Tephra. We revise this correlation to Okataina-derived Rotorua Tephra based on new compositional data from biotite phenocrysts and glass. The new correlation limits the known dispersal of Puketarata Tephra (sensu stricto, c. 16 800 cal yr BP) and eliminates requirements to either reassess its age or to invoke dual Puketarata eruptive events. Our data show that Rotorua Tephra comprises two glass-shard types: an early-erupted low-K₂O type that was dispersed mostly to the northwest, and a high-K₂O type dispersed mostly to the south and southeast, contemporary with late-stage lava extrusion. Late-stage Rotorua eruptives contain biotite that is enriched in FeO compared with biotite from Puketarata pyroclastics. The occurrence of Rotorua Tephra in Kaipo Bog (100 km from the source) substantially extends its known distribution to the southeast. Our analyses demonstrate that unrecognised syn-eruption compositional and dispersal changes can cause errors in fingerprinting tephra deposits. However, the compositional complexity, once recognised, provides additional fingerprinting criteria, and also documents magmatic and dispersal processes.

Keywords   tephra; stratigraphy; tephrochronology; correlation; Rotorua Tephra; Puketarata Tephra; Okataina; Maroa; Kaipo; rhyolite; biotite; glass chemistry

G02053; Received 7 October 2002; accepted 30 July 2003; online publication date 13 November 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 591 596
0028 8306/03/4604 0591 $7.00/0 © The Royal Society of New Zealand 2003

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