Geology of the Auckland Region

The Auckland Region of New Zealand is built on a basement of greywacke rocks that form many of the islands in the Hauraki Gulf, the Hunua Ranges, and land south of Port Waikato. The Waitakere Ranges in the west are the remains of a large andesitic volcano, and Great Barrier Island was formed by the northern end of the Coromandel Volcanic Zone. The main isthmus and North Shore are composed of Waitemata sandstone and mudstone, and portions of the Northland Allochthon extend as far south as Albany. Little Barrier Island was formed by a relatively isolated andesitic volcano, active around 1 to 3 million years ago.

The Manukau and South Kaipara Harbours are protected by the recent sand dune deposits of the Awhitu and South Kaipara Peninsulas. Recent basaltic volcanic activity has produced many volcanic cones throughout the Auckland Region, including the iconic Rangitoto Island.

As with most of New Zealand, the basement rocks of the Auckland Region are composed of greywacke (indurated sandstone, siltstone and mudstone).

Murihiku Terrane Greywacke lies beneath the Auckland Region on the western side, and outcrops south of the Waikato River. The Murihiku Terrane is considered to be an accretionary wedge of mainly volcanogenic forearc sediments. It was formed in Late Triassic to Late Jurassic times (220-145 Ma).

A line of Maitai Terrane rocks are assumed to pass north-south through the centre of the Auckland Region, separating the Murihiku Terrane and Waipapa Composite Terrane, and produce a detectable Junction Magnetic Anomaly (JMA), but do not outcrop. Some Murihiku rocks occur to the east of the JMA, forming the Taupiri and Hakarimata Ranges, but it is assumed that these rocks have been shifted to their present position.

Hunua Terrane Greywacke lies beneath the Auckland Region on the eastern side, as far south as the Hunua Ranges, and outcrops in the Hunua Ranges, many islands in the Hauraki Gulf (Waiheke, Motutapu, Motuihe, Tiritiri Matangi, Kawau, etc.), and Tawharanui Peninsula. It sometimes includes Chert and Argillite. The rocks are generally fine grained and highly deformed. It was formed in Late Triassic to Late Jurassic times (220-145 Ma).

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