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 Waitākere 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 Auckland 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 Āwhitu and South Kaipara Peninsulas. Recent basaltic volcanic activity has produced many volcanic cones throughout the Auckland Region, including the iconic and youngest volcano Rangitoto Island.
Basement rocks
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 Dun Mountain-Maitai Terrane rocks are assumed to pass north–south through the centre of the Auckland Region, separating the Murihiku Terrane and Waipapa Composite Terrane. The existence of this terrane at depth is inferred from its distinctive magnetic anomaly (Junction Magnetic Anomaly or JMA) and xenoliths in the Auckland Volcanic Field. 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).
The Morrinsville Terrane Greywacke lies beneath the Auckland Region on the eastern side, south of the Hunua Ranges, and further east to the Coromandel Peninsula. The rocks are generally coarser grained, with massive sandstones and conglomerate. It was formed in Late Jurassic to Early Cretaceous times (160-120 Ma).
Te Kuiti Group coal and limestone
The Te Kuiti Group Rocks overlie the basement rocks, and are present in Northland, Auckland, the Waikato, and King Country, although they have often been eroded or covered. Rocks containing coal were formed from swampland in Late Eocene times (37-34 Ma). The land sank and the sea transgressed, and calcareous sandstone, mudstone, and limestone was deposited in Oligocene times (34-24 Ma).
The coal deposits outcrop near Huntly and Maramarua, where they are mined.
Flaggy limestone bluffs are located in the area south and west of the Waikato River.
Northland Allochthon
See Northland Allochthon.
In Early Miocene times (24-21 Ma), a series of thrust sheets were emplaced over Northland, extending as far south as the Kaipara Harbour and Albany areas. The rocks came from the northeast, and were emplaced in reverse order, but the right way up. The original rocks are of Cretaceous to Oligocene age (90-25 Ma), and include mudstones, limestone and basalt lava. These rocks outcrop around Silverdale, Warkworth, and Wellsford, reaching as far south as Albany.
West Coast volcanism
A volcanic arc became active to the west of the current land in Northland in Miocene times (23 Ma), and gradually moved south down to Taranaki. It produced mainly andesitic strato-volcanoes. Volcanoes were produced in what is now sea to the west of the Auckland Region, near the Kaipara Harbour and Waitākere Ranges (22-16 Ma), and Waikato Heads (14-8 Ma).
The Waitākere volcano has mainly been eroded, but conglomerate from the volcano formed the Waitākere Ranges. The Waitākere Ranges also contain many dikes, volcanic plugs, etc., that formed part of this volcano.
Waitemata sandstone
While the volcanic activity occurred to the west, a rapidly deepening basin developed further to the east, in the Auckland area.
