New Zealand Journal of Geology and Geophysics abstracts

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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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