New Zealand Journal of Geology and Geophysics abstracts
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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