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

DM Gravley; CJN Wilson; MD Rosenberg; GS Leonard

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.

Download Paper: