Geology of Okahukura Peninsula, Kaipara Harbour, New Zealand

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Abstract
Okahukura Peninsula, Kaipara Harbour, is underlain by rocks of the West Northland Chaos-breccia (emplaced in the upper Oligocene to lower Miocene), Waitemata Group (upper Oligocene to lower Miocene), Waitakere Group (lower Miocene), and Quaternary dune and terrace deposits. The West Northland Chaosbreccia is inferred to underlie the Waitemata Group; it consists of blocks of chert, argillaceous micrite, and glauconitic sandstone, in a matrix of bentonitic clay. The Waitemata Group is represented by the Timber Bay Formation, thin interbedded mudstones and rippled fine sandstones, up to 1000 m thick; it rests on an irregular surface of the underlying Chaos-breccia. The Waitakere Group is divided into three formations. The new Okahukura Formation contains a prominent, coarse igneous conglomerate, the Matapoura Conglomerate Member, and a wide variety of flysch, flysch-like lithologies, mudstones, cross-bedded sands, fine igneous conglomerates, argillaceous micrite breccias, and slumped blocks. There is much lateral variation, and the Formation rests sometimes conformably but generally unconformably on the Timber Bay Formation. Some rock units described previously from the adjacent Puketotara Peninsula are here reinterpreted and transferred from the Timber Bay and Pakaurangi Formations into the new Okahukura Formation. The overlying Pakaurangi Formation is divided into two portions which are laterally equivalent: in the north the new Oruawharo Hyaloclastite Member, and in the centre and south mudstones and volcanic sandstones with many intraformational discordances, and fairly rich fossil faunas. The Pakaurangi Formation rests in places conformably on Okahukura Formation and in places unconformably on Okahukura and Timber Bay Formations. The topmost formation, Motuouhi Formation, is exposed only in the north. It is the southern extremity of a lens of hard, brown, fine hyaloclastite tuff which is developed more extensively on the adjacent Puketotara Peninsula. The rocks of the Waitemata and Waitakere Groups are derived largely from contemporaneous volcanism on the Waitakere Magmatic Arc, with minor contributions from the underlying West Northland Chaos-breccia, There is an overall westerly dip, and no major faults are known. An extensive development of late Quaternary beach ridges and dunes forms the western portion of the Peninsula, while older Quaternary quartz- rich dune sands are located at a height of c. 105 m a.s.l.

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