An Eocene to early Oligocene unconformity-bounded sequence in the Punakaiki-Westport area, West Coast, South Island, New Zealand

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. © 2001 Taylor & Francis Group, LLC.