New Zealand Journal of Geology and Geophysics abstracts
The nature and age of Ohakuri Formation and Ohakuri Group rocks in
surface exposures and geothermal drillhole sequences in the central
Taupo Volcanic Zone, New Zealand
D. M. Gravley
C. J. N. Wilson
Geology Department
The University of Auckland
Private Bag 92019
Auckland 1020, New Zealand
d.gravley@auckland.ac.nz
M. D. Rosenberg
GNS Science
Private Bag 2000
Taupo 2730, New Zealand
G. S. Leonard
GNS Science
PO Box 30368
Lower Hutt 6315, New Zealand
Abstract The name “Ohakuri” has been
applied both as formation and group names to surface deposits and
deeply buried deposits recorded in geothermal drillholes in the central
Taupo Volcanic Zone (TVZ) of New Zealand. The surface deposits are
dated at 240 ± 10 ka (1 SD) and cap sequences that postdate
the 320–340 ka Whakamaru group ignimbrites, whereas the
buried deposits demonstrably predate the Whakamaru group ignimbrites.
The Ohakuri Formation is here redefined to refer only to the
surface deposits, which are also reinterpreted to be composed
predominantly of primary pyroclastic deposits (mostly ignimbrite with
only minor intercalated fall and secondary mass-flow deposits). The
term Waikora Formation has been used interchangeably with the term
Ohakuri Group in geothermal drill core logs, but was originally defined
to cover only pre-Whakamaru sedimentary units containing abundant
rounded greywacke clasts. We recommend three terms be used: Waikora Formation, which retains its original usage, Tahorakuri Formation
(new) for the other volcaniclastic and sedimentary deposits between the
Whakamaru group ignimbrites and the greywacke basement, and Reporoa Group
(new) as an overall term for all subsurface pre-Whakamaru lavas and
deposits previously and collectively referred to by the name Ohakuri
Group. The Reporoa Group thus includes the Waikora and Tahorakuri
Formations, plus numerous other locally named thick ignimbrites and
lava flows between the Whakamaru group ignimbrites and the greywacke
basement. The term Ohakuri Group should be abandoned.
Keywords Ohakuri Formation; Waikora Formation;
Tahorakuri Formation (new); Reporoa Group (new); geothermal drillholes;
Taupo Volcanic Zone; new stratigraphic names
G05056; Received 16 November 2005; accepted 29 May 2006; Online publication date 24 July 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49: 305–308
0028–8306/06/4903–0305 © The Royal Society of New Zealand 2006
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