New Zealand Journal of Geology and Geophysics abstracts
An Eocene to early Oligocene unconformity-bounded sequence in the
Punakaiki-Westport area, West Coast, South Island, New Zealand
HELEN LEVER
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
Present address: School of Earth Sciences, James Cook University, Townsville,
Qld 4811, Australia.
Abstract The late Eocene to earliest Oligocene sediments
between Punakaiki and Westport on the West Coast of the South Island, including
the Brunner Coal Measures and the Rapahoe Group, comprise an
unconformity-bounded transgressive-regressive sequence. The lower unconformity,
below the Brunner Coal Measures, is formed on Cretaceous and Paleozoic
granitoids, metamorphic rocks, and the Paleozoic Greenland Group metasediments.
The upper bounding unconformity is located below the shoreline Little Totara
Sand (a part of the Rapahoe Group) around Cape Foulwind, and between other
elements of the Rapahoe Group and the overlying Nile Group limestones south of
Charleston. The upper unconformity is usually erosional, and at Woodpecker Bay
dissolution of a layer of rhodolithic limestone indicates subaerial exposure of
the unconformity surface. An Early Whaingaroan age for the upper unconformity
has been obtained at Gibsons Beach: the more southern exposures of the upper
unconformity are poorly dated but consistent with this age. No correlative
unconformity has been recognised in the Eocene-Oligocene successions at
Greymouth and Murchison, but an exposure at Whitecliffs near Inangahua contains
an unconformity related to sea-level fall, and of a similar age. Breccia
deposition at the Little Wanganui River mouth was initiated in the Early
Whaingaroan, and may be causally related to the upper unconformity. The
restriction of the upper unconformity to the area north of Punakaiki suggests
that a local tectonic event, or tectonic basin controls, may be the cause of
the erosion. Global sea-level charts do not suggest a major sea-level fall at
this time. Whaingaroan unconformities that are observed in the South Pacific
are younger than the age of the upper unconformity. The Brunner Coal Measures -
Rapahoe Group sequence (Rapahoe sequence) is an example of a preserved
unconformity-bounded transgressive-regressive sequence created by tectonic
and/or subsidence controls.
Keywords Eocene; Oligocene; Whaingaroan; sequence; West
Coast; unconformity; Rapahoe Group
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44:
355-363
0028-8306/01/4402-0355 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (1071K); (scanned from paper original: notes about this process)
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