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Cretaceous and Cenozoic sedimentary basins of Northland, New Zealand

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    Isaac, M.J.; Herzer, R.H.; Brook, F.J.; Hayward, B.W. 1994 Cretaceous and Cenozoic sedimentary basins of Northland, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 8 230 p.

    Abstract: This monograph describes the geology and petroleum prospects of the Northland region, an area of onshore and offshore New Zealand between latitudes 34deg.S and 37deg.22'S. The region is the northernmost to be studied for the basin studies programme of the Institute of Geological and Nuclear Sciences; it lies northwest of the active plate boundary between the Australian and Pacific plates. The Northland peninsula covers about 17 000 km2. Metasedimentary and metavolcanic rocks of Permian to Early Cretaceous basement terranes outcrop in the east. East of the peninsula, the sedimentary cover over most of the area is apparently thin, although seismic coverage and data quality on which this is assessed are poor. West of the basement outcrop, the thickness of Cretaceous and Cenozoic strata increases to 3-4 km near the present west coast; offshore, a large area has more than 5 km of Cretaceous and Cenozoic strata, and locally the maximum thickness exceeds 8 km. West of the peninsula, the Northland region includes an area of 35 000 km2 where the water depth is less than 1000 m. The sequence present west of the Northland peninsula is similar to that of the adjacent and contiguous Taranaki Basin which covers New Zealand's only proven commercial oil and gas fields. Basement is overlain by a near-continuous blanket of mid to Late Cretaceous terrestrial and paralic facies 600-1400m thick, with greater thicknesses present locally in half-grabens. The unit is inferred to be equivalent to both the Taniwha Formation and Pakawau Group, as mapped in Taranaki. Terrestrial, paralic and shallow marine facies are onlapped and almost completely buried by a bland seismic unit, believed to be mainly open marine sandstone and mudstone. Subsidence continued throughout the Paleogene, an bathyal sediments were deposited over large areas. Shallow marine facies and at least one Paleogene deltaic system are present close to the west coast; in eastern Northland, Late Eocene coal measures and glauconitic sandstone rest unconformably on basement, and outcrop. Direct well ties confirm that the strong seismic reflector of regional significance west of the Northland peninsula is equivalent to the Oligocene to earliest Miocene Ngatoro Group limestone in the Taranaki Basin. Onshore, the Te Kuiti Group limestones are the lateral equivalent. Earliest Miocene rocks of onshore Northland record rapid subsidence from shelf to bathyal depths. In earliest Miocene time, a series of nappes (the Northland Allochthon) was emplaced southwestward over what is now the Northland peninsula, with later southeast-directed movement onto southern Northland. North of Hokianga Harbour (35''36'S), the allochthon front reached up to 80 km offshore, but further south the margin is closer to the coast. The allochthonous strata include mid and Late Cretaceous proximal and distal sandstones and pelagic mudstone, Paleocene to earliest Miocene glauconitic sandstone, calcareous mudstone, and micrit ic limesto ne, and Cretaceous to Paleocene ophiolites (the Tangihua Complex). Subduction-related Early Miocene calc-alkaline volcanism began as the nappes were emplaced. Igneous rocks are present in two 350 km long, northwest-trending belts 20-80 km apart; numerous large stratovolcano complexes are inferred from outcrop mapping (the Northland peninsula) and geophysical surveys (western offshore). Radiometric dating indicates the volcanoes were active over the period 16-22 Ma; activity ceased by the end of the Early Miocene. Thick sequences of conglomerate and sandstone derived from the volcanoes and the eroding Northland Allochthon accumulated in the deep bathyal Waitemata basin south of the allochthon margin. Onshore there are almost no sedimentary rocks younger than middle Miocene, other than Pliocene to Recent subaerial dune complexes. Gradual uplift of the Northland peninsula and southwestward tilting has led to deep erosion on land and accumulation of a thick clastic sequence offshore to the west. The hydrocarbon resources of the Northland region are almost untested. There has been no drilling offshore, and only two deep (> 650 m) tests have been drilled onshore. The mid to Late Cretaceous strata present west of the Northland peninsula are probably coal-bearing, and by analogy with Taranaki are believed to have considerable source potential. Overlying marine mudstones also have source potential. Modelling suggests many of the potential source rocks are thermally mature. (auths)

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