New Zealand Journal of Geology and Geophysics abstracts
Geochronological constraints on Cretaceous–Paleocene volcanism in
South Westland, New Zealand
Carolyn J. Phillips
Alan F. Cooper
J. Michael Palin
Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand
Simon Nathan
Institute of Geological & Nuclear Sciences Ltd
P.O. Box 30 368
Lower Hutt, New Zealand
Abstract Cretaceous and Paleocene sedimentation in
South Westland, New Zealand, is recorded in the Otumotu Formation,
Tauperikaka Coal Measures, Whakapohai Sandstone, Arnott Basalt,
Buttress Conglomerate, and Tokakoriri Formation, originally named and
mapped by Nathan in 1977. Within this stratigraphic sequence, the name
Buttress Conglomerate was used to describe volcanic conglomerates at
Porphyry and Buttress Points that contained rounded clasts of
plagioclase-phyric intermediate volcanic rocks. Stratigraphically, the
volcanic conglomerate at Porphyry Point forms sharp contacts with the
underlying Arnott Basalt (Haumurian) and overlying Tokakoriri Formation
(Teurian). The volcanic conglomerate at Buttress Point, however, is
entirely fault-bounded. Clasts from each unit were collected and U-Pb
zircon dated using the TIMS and ELA-ICP-MS methods. A trachyandesite
clast collected at Buttress Point gives an age of 96.9 ± 1.6 Ma,
whereas a rhyolite clast collected at Porphyry Point gives an age of
61.4 ± 0.8 Ma. Petrological, geochemical, and stratigraphic data
suggest that erosion of the clasts closely followed volcanism, and that
these ages accurately reflect the depositional ages of the
conglomerates. Conglomerates at Porphyry and Buttress Points have been
formally renamed the Porphyry Point Member of the Tokakoriri
Formation and the Buttress Point Conglomerate, respectively.
Keywords Cretaceous; Paleocene; stratigraphy;
volcanism; zircon; geochronology; Porphyry Point Member; Buttress Point
Conglomerate; new stratigraphic names
G03009; Received 11 April 2003; accepted 5 July 2004; Online
publication date 23 March 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
1–14
0028–8306/05/4801–0001 © The Royal Society of New Zealand 2005
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