Geology of the central Otago Peninsula

Laurie, Blair.

The most extensive period of Cenozoic volcanism in the Otago region, New Zealand is the Miocene Dunedin Volcanic Complex (DVC). The DVC has a diameter of 25 km and forms a conspicuous landform on the south-east coast of the South Island. The DVC is split into a western ridge and an eastern peninsula, separated by the Otago Harbour. Present day outcrop indicates the DVC is an eroded remnant of a much larger complex. Mapping still reveals a several hundred meter thick accumulation of alkaline volcanic rocks. The Otago Peninsula is part of the wider DVC. It was formed by volcanic eruptions varying from submarine, to emergent and fully sub-aerial eruption processes from various small vents, over several million years. This has resulted in a complex stratigraphy and has produced a wide range of volcanic and volcaniclastic deposits. The Otago Peninsula, in the mapped area, is characterized by small volcanoes and a volcanic system that includes plumbing, intrusions, volcanic edifices and other accoutrements of volcanic systems with small short lived eruption sites, and local eruptive centers that were active for substantial periods of time or were reactivated. Rocks of the DVC rest on Cretaceous and Tertiary sedimentary rocks, which are underlain by a pre-Cretaceous basement of quartzofeldspathic schist as indicated by clasts in the Port Chalmers Breccia. The sedimentary rocks comprise a non-marine basal unit and marine sandstones, mudstones, calcareous sandstones, bioclastic sandstones and glauconitic sandstones. There is a conspicuous absence of the more mafic volcanics, characteristic of other areas of the DVC. The dominate rock types identified include; widespread lapilli tuffs and diatreme remnants of phreatomagmatic origins, associated with trachytic lavas, and dikes, trachyandesite autobreccias, phonolites and basaltic rocks.

1 v. (unpaged) : col. ill., maps ; 30 cm. + 2 folded sheets, 1 folded map in pocket.

2004Laurie

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