Geology and geomorphology of Kahurangi National Park

In New Zealand basement rocks are divided broadly into Western and Eastern Provinces – the park is located in the Western Province. The Western Province is subdivided into two tectonostratigraphic terranes: the Late Cambrian to Late Ordovician ( Ma) Buller terrane and the Middle Cambrian to Early Devonian ( Ma) Tākaka terrane. Near the middle of the park the terranes are separated by the inactive north-south trending Anatoki Fault which can be traced from the Wakamarama Range in the north of the park to about from the southern border of the park. Sedimentary deposits cover about 20% of the park and date from Late Cretaceous ( Ma) to Holocene in age. Cover rocks are found mainly in fault-controlled depressions and on top of flat-topped basement ridges. Unconsolidated deposits such as gravels, sand and slope debris are mostly restricted to the Karamea district, inland Mōkihinui River catchment and valleys within the mountainous interior of the park. ==Basement rocks==

Buller terrane The Buller terrane extends from the west coast to alongside the Anatoki Fault. The fault is a narrow zone of breccia, mylonite and cataclasite with fragments of Ordovician limestone commonly present along the fault zone. The oldest rocks are Late Cambrian to Early Ordovician ( Ma) continent-derived quartz-rich turbidites, collectively called the Greenland Group. The Greenland Group passes conformably up into the Golden Bay Group which contains black shales as well as similar rock types to the Greenland Group. Graptolite fossils ranging in age from Early Ordovician to Late Ordovician ( Ma) are present in the black shales. The combined Greenland Group and Golden Bay Group sedimentary sequence is at least thick; has been folded, faulted and undergone low-grade greenschist facies metamorphism. About 2–3 km west of the Anatoki Fault is the Fenella Fault Zone, a zone of folding and faulting that extends from the southern end of the Wakamarama Range to just north of Mount Patriarch. It has been offset in several places by east-west trending faults. Segments can reach up to wide but generally the zone is 100–200 m wide. Karamea Suite granite is predominantly a coarse-grained biotite granite with large pink euhedral crystals of potassium feldspar. The Karamea Suite covers a large area from Kahurangi Point almost continuously to the Mōkihinui River valley in the southwest of the park, and to the southern boundary of the park about northwest of Murchison. Sedimentary rocks overlie much of the Karamea Suite and Greenland Group in the southern third of the park. Several plutons intrude the Karamea Batholith, metasedimentary rocks south of the Wakamarama Range and the far southwest of the park. The largest of the plutons is roughly long and wide and is located in the middle of the Tasman Mountains. The volcanic arc-related sequence, dominated by the Haupiri Group and Devil River Volcanics Group, is largely confined to the western part of the terrane between the Anatoki Fault and the Devil River Fault some further to the east. Overlying the Tasman Formation is the granule to boulder-sized Lockett Conglomerate up to thick containing clasts of volcanic and ultramafic rocks, gabbro, granitoid, quartzite, limestone and sandstone including sandstone derived from Junction Formation. The conglomerate is interpreted to be a fan delta deposit in a shallow marine or possibly terrestrial environment. Other fault slices that contain volcaniclastic conglomerate, sandstone and siltstone generally date from late Middle Cambrian to early Late Cambrian ( Ma). The matrix is interpreted to be derived from turbiditic layers of sandstone and mudstone from the Junction Formation. The onset of carbonate deposition began at the end of the Cambrian to Early Ordovician and led to the Mount Arthur Group succession of carbonaceous and calcareous mudstone, sandstone, and bands of micritic limestone with fossil fragments. Siliceous bands and nodules are present in some areas. The limestone within the group has been extensively altered to marble. Conodonts and trilobites found in the base of the Mount Arthur Group limestone date from the very Late Cambrian to Early Ordovician, while everywhere the top of the limestone appears to be Middle Ordovician ( Ma). A north to southeast band of schist with bedded quartzite and metavolcanic bands lies about 1 km east of Parapara Peak. It is thought to be the stratigraphic equivalent of the Silurian Ellis Group quartzite and overlies Late Ordovician Mount Arthur marble. Overlying the passive margin sequence is an isolated outcrop of Permian to Triassic metasedimentary rocks, the Parapara Group, located in a narrow fault-bounded north-south exposure ending at Parapara Peak. It is approximately long up to wide and the sequence is at least thick and probably lying unconformably on older basement of unknown age. The greenschist-to-amphibolite facies metamorphosed sequence includes basal schist and slate overlain by conglomerate, fossiliferous pebbly sandstone, quartzites, slate and quartz sandstone. It is possible the age range extends from the Carboniferous to Middle Triassic. Dropstones are present, mainly below and above the fossiliferous sandstone, and are interpreted as ice-rafted. The Middle Permian ( Ma) fossils are predominantly bryozoans, brachiopods and molluscs. These fossils suggest a close correlation with Tasmania and eastern Australia around the Middle Permian when continental-arc derived sediments were deposited in a mid-shelf cool water environment. Its elongated exposure is offset in several places by faults. It is composed of ultramafic layered igneous rocks, typically diorite, gabbro and pyroxenite. The complex intrudes Late Ordovician Mount Arthur Group, Ellis Group and Baton Formation. Its age is similar to the age of the Karamea Suite granite and shares geochemical similarities with the diorites of the Karamea Suite. The Riwaka Complex is itself intruded along its eastern margin by Early Cretaceous Separation Point Suite granite with exposures just within the park boundary east of the Arthur Range. A large area of Separation Point Suite granite, some , is exposed in the Hope and Lookout Ranges south of the Wangapeka River. A few small outcrops of Separation Point Suite granite occur about west-northwest of Collingwood, near the park border about southwest of Collingwood and east of Parapara Peak. == Tectonic history of basement rocks ==

Buller terrane sedimentary rocks were formed during the Ordovician adjacent to a continental landmass inferred to be the eastern Australia-Antarctica segment of Gondwana. A significant tectonic event, possibly during the Late Ordovician to Early Silurian ( Ma), led to folding with well developed cleavage and low-grade metamorphism of the Ordovician rocks. Amalgamation of the two terranes post-dates deposition of the Silurian Ellis Group quartzite and probably the early Devonian Baton Formation but occurred before emplacement of the Middle to Late Devonian Karamea Suite granite and Riwaka Complex. It is possible the Karamea Suite and Riwaka Complex were emplaced within an extensional back-arc setting similar to the current tectonic setting of the Taupo Volcanic Zone of New Zealand. == Cover rocks ==

Sedimentary rocks of Late Cretaceous ( Ma) age and younger cover about 20% of the park. They are found in the northern and southern areas of the park, inland from the western boundary in several places and a discontinuous narrow band running roughly from Upper Tākaka to about east of Karamea in the Garibaldi Ridge area. is located about west of Collingwood and extends the entire width of the narrow park extension up to the most eastern point of Whanganui Inlet. Outcrops occur in synclinal structures, such as inland from the Heaphy River and the northern end of the Murchison basin (northern end of Matiri Range), and on plateaus such as the Garibaldi Ridge about 20–27 km east of Karamea. Miocene The Early to Middle Miocene ( Ma) Lower Blue Bottom Group, consisting of blue-grey calcareous mudstone and muddy sandstone, frequently outcrops next to Nile Group limestone. The Lower Blue Bottom Group is mostly preserved in isolated fault-controlled depressions. Outcrops occur along the Ōpārara River, northeast of the Little Wanganui River, along the southern margin of the Wakamarama Range and on the north side of the Karamea Fault. Middle Miocene to Middle Pliocene ( Ma) marine sediments, mapped as Upper Blue Bottom Group, are found only in the Karamea district. Consisting of blue-grey muddy sandstone with shallow-water fossils with a change to massive fine-grained sandstone at about the Miocene-Pliocene boundary. The massive sandstone is often weathered rusty brown in outcrop. Pliocene to present Overlying the Upper Blue Bottom Group in the Karamea district, a few km south of Karamea and similar distance north of Karamea, are outcrops of Late Pliocene to Early Pleistocene ( Ma) Old Man Gravel Group conglomerate. The conglomerate contains weathered clasts of quartzofeldspathic sandstone, schist, granite and other igneous and sedimentary rocks as well as some interbedded sandstone. A few of the landslides have dammed rivers to form small lakes. == Tectonic history of cover rocks ==

Continental rifting began throughout much of future Zealandia after subduction along the Pacific eastern margin of Gondwana ended around 105–100 Ma. Uplifting of the crust due to thermal heating led to erosion from the top and the filling of grabens and half-grabens while at the same time the rifting process stretched and thinned the crust from below. The Alpine Fault began to develop Ma in central Zealandia and had linked up to the Hikurangi subduction margin by Ma. This fault was responsible for the 1929 Murchison earthquake. The 1968 Inangahua earthquake had a epicentre a few miles south of the park. == Geomorphology ==

The park is mostly mountainous with the Tasman Mountains dominating the central area of the park west of the Tākaka River. They rise to approximately concordant summits between (Mount Owen) with the surface defined by these summits gently dipping to the west. The surface may be the Late Cretaceous to Middle Eocene erosion surface, also known as the Waipounamu Erosion Surface, which saw the levelling of the pre-Cenozoic rocks in the park. This earthquake, and to a lesser extent the 1968 7.1 Inangahua earthquake, heavily scarred the landscape in the centre and south of the park. Most of the caves high up in the marble mountains have dry passageways often decorated with stalagmites, stalactites, straws and helectites. Mount Owen's Bohemia Cave is a notable example. As the area rose through tectonic compression during the last 2 million years, groundwaters dissolved out new cave passages along joint planes leading to the immense height difference between the dry upper levels of the cave networks and their resurgence levels hundreds of metres lower. The Riuwaka Resurgence near Takaka Hill is a notable example. The Ōpārara Arch is a notable example of an arch made of limestone although technically it is a natural bridge as it was formed by water erosion. It is located about northeast of Karamea in the upper Ōpārara River valley. It is the largest natural arch in Australasia at high, up to wide and long. The Ōpārara River flows southward for along the contact between Oligocene Nile Group limestone sitting on Devonian Karamea Suite granite before heading west to the coast. The limestone accumulated Ma as a thick shell bank on top of an eroded surface of granite. Thick mud deposits then buried the shell bank and water percolated through the shell bed recrystallising the shell's calcite to form a relatively hard limestone. This sequence was uplifted and the mudstone eroded away during the last 7 million years. As the Ōpārara River valley was forming rainwater filtered through cracks in the limestone and dissolved out a cave network along the contact between the limestone and underlying granite. Eventually the caves coalesced and captured most of the valley's drainage to become part of the Oparara River. The river both dissolved and eroded the caves wider undercutting the roof supports resulting in progressive roof collapse – leaving behind the arches seen today. Further up the valley from the Ōpārara Arch is the Honeycomb Hill Cave, best known for its fossil bird bones. Many specimens of New Zealand's extinct flightless birds, including moa, fell into or got washed into the cave and then preserved by burial in sediment or by getting coated in secondary calcite deposition. Many of the bird bones date back some 20,000 years BP. Bones of native frogs and lizards as well as 40 different land snail species add to the significant scientific importance of this cave. Uplifted limestone mesas In two places there are striking examples of large uplifted mesas, Garibaldi Ridge around east of Karamea, and two mesas located in the Matiri Range: the Devil's Dining Table and the Thousand Acre Plateau to the south of the Devil's Dining Table. The Matiri Range mesas are capped by Oligocene to early Miocene Matiri Formation limestone. During the Miocene the area continued to subside with up to of mud accumulating on top of the limestone. From Ma the park region underwent uplift as the Australia and Pacific plates converged obliquely. As the area got lifted above sea level the soft mudstone was eroded away but the harder limestone was more resistant to erosion resulting in the formation of steep bluffs. During the last ice age water and ice eroded cirques into the limestone at the head of valley glaciers. The Garibaldi Ridge is similar except the capping limestone is Oligocene Nile Group limestone. Longford Syncline Just to the north of Longford, east of Murchison, a narrow section of the park about wide extends around north along the Blue Cliffs Ridge. The ridge is the western limb of the Longford Syncline. It is the most prominent part of one of New Zealand's deepest and most intensely deformed Cenozoic basins – the Murchison Basin. The western limb of the syncline dips steeply down to the axis of the syncline along the Buller River at degrees and the eastern limb dips down towards the river at up to 85 degrees. Up to a maximum of of sediment was deposited from Late Eocene to about Middle Miocene in the basin with the top 3 km lost to erosion. The formation of the Longford Syncline was driven by compression and reactivation of basement faults caused by the convergence of the Australia and Pacific plates, particularly in the last 7 million years. The deep burial of organic-rich sediments resulted in the generation of hydrocarbons with several oil and gas seeps in the Murchison Basin but exploration drilling found no oil and gas reservoirs. == See also ==