North Auckland Land District
For millions of years North Auckland has been tectonically relatively stable and has been worn down to an area generally of low relief. Long-continued weathering on gentle slopes in a warm, moist climate has caused most of the rocks to decay deeply, with the result that the stratigraphy and structure of the region are difficult to decipher. The oldest rocks are complexly folded greywackes and argillites that were deposited in the New Zealand Geosyncline. Some are known to be Permian; others may be Triassic and Jurassic and contain small deposits of manganese ore which have been worked. These rocks, which are exposed only in eastern North Auckland, are broken into blocks by faults.
Other old rocks of North Auckland include basic volcanic and associated intrusive rocks that form the prominent steep hills of central and northern North Auckland, such as the Tangihua and Mangakahia Ranges. They may range in age from late Jurassic to Eocene. Small copper deposits occur at the contact of these rocks with the Cretaceous sedimentary rocks that surround them. At North Cape there is a large mass of ultramafic rocks, including a huge deposit of serpentine at Surville Cliffs that may be quarried on a large scale for use in fertiliser.
Soft, deeply weathered, Upper Cretaceous, Paleocene, and early Eocene sandstones and siliceous clay-stones make up most of the lowland areas of central and northern North Auckland. The details of their stratigraphy and structure are obscure. In many areas these, together with other Tertiary rocks and small masses of serpentine, are chaotically arranged and appear to have been emplaced as large slumps during the mid-Tertiary, when North Auckland experienced its latest period of diastrophism.
Mid and late Eocene and Oligocene strata form small areas of central and northern North Auckland. They include mid-Eocene coal measures, mined mainly from seams at Kamo, Kawakawa, and Hikurangi, and Oligocene limestones. These are of two types, an argillaceous (“hydraulic”) limestone which supplies Northland's cement industry, and a crystalline limestone used mainly for agricultural lime. Much of the southern half of North Auckland is made up of lower Miocene sandstones. These underlie Auckland City.
There has been little geophysical work done in Northland and it is not known how thick are the Cretaceous and Tertiary sediments of eastern Northland, nor what lies beneath them. Quaternary sands form the isthmus of the Ninety-Mile Beach peninsula, the long arms of the entrance to the Kaipara Harbour, and the south arm of Manakau Harbour.
Volcanic activity occurred intermittently in Northland during the Tertiary. Lower Miocene andesitic lavas and agglomerates form the Waitakere Ranges west of Auckland city, and similar rocks of Miocene age occur in eastern North Auckland, forming, for example, the jagged peaks of Manaia and Bream Head near Whangarei. There are small dacite plugs and eroded dacite volcanoes of Pliocene age east of Maungaturoto and near Whangarei.
During the late Pliocene and Quaternary only basalt was erupted in North Auckland. The older of these lavas forms the Tutamoe and Waipoua plateaux. Younger basalt flows, poured out in fissure eruptions in the Pleistocene, form the Kerikeri plateau adjoining the Bay of Islands. Some of these lavas have weathered to form bauxite ore. There are numerous younger flows and scoria cones in the Kaikohe, Kaeo, and Whangarei areas, and hot springs occur at Ngawha, near Kaikohe. At Auckland City basalt has been erupted in late Pleistocene and Holocene times from some 60 centres. Scoria cones, such as Mount Eden, Mount Wellington, and One Tree Hill, are prominent city landmarks, of which the youngest is Rangitoto Island, a lava cone some 850 ft high, probably less than 1,000 years old.