Geology of the Northland region

New Zealand's Northland region is built upon a basement consisting mainly of greywacke rocks, which are exposed on the eastern side of the peninsula.

Intra-plate basaltic volcanic activity has occurred around Kaikohe, the Bay of Islands, and Whangārei since late Miocene times.

Sand dunes sourced from volcanoes further south occupy much of the western coast, and the Aupouri Peninsula joins previously separate islands to the mainland to form a large tombolo.

The basement rocks are divided into a number of terranes, that are believed to have been combined together by subduction or strike-slip processes,[1] by mid Cretaceous times (100 Ma).

Murihiku terrane rocks lies beneath the Northland region on the western side, but do not crop out.

[2] Caples terrane rocks crop out to form the Waipapa Horst (Omahuta and Puketi forest area).

The land sank, the sea transgressed, and calcareous sandstone, mudstone, and limestone were deposited in Oligocene times (34–24 Ma).

[3] Limestone crops out around Whangārei, with an interesting display at the Waro Rocks Scenic Reserve, north of Hikurangi.

In Early Miocene times (24–21 Ma), a series of thrust sheets was emplaced over Northland,[2] extending as far south as the Kaipara Harbour and Albany areas.

The original rocks are of Cretaceous to Oligocene age (90–25 Ma), and include mudstone, limestone and basalt lava.

A subduction-related volcanic belt became active to the west of the current land in Northland in Miocene times[2] (23 Ma), and gradually moved south down to Taranaki.

[5] By middle Miocene, Northland was uplifted above sea level, with a slight westward tilt,[6] exposing basement rocks on the eastern side, and resulting in a tendency for rivers to flow in a westerly direction.

[7] The coastal barriers north and south of the entry to the Kaipara Harbour, are essentially consolidated sand dunes, built up over the last few million years.