Kahurangi National Park is geologically one of the oldest and most complex areas in New Zealand with basement rocks dominating the landscape. The Paleozoic Buller and Tākaka terranes, separated down the middle of the park by the Anatoki Fault, form the geological basement.
The Buller terrane contains Ordovician metasedimentary turbidites, extensively intruded by Late Devonian granitoids and to a lesser extent, Early Cretaceous granitoids. To the east the Tākaka terrane comprises an older sequence of Cambrian volcanic and volcaniclastic arc-related sedimentary rocks, and a prominent mélange unit. The oldest fossils found in New Zealand, trilobites, are present in the sedimentary rocks. Overlying the arc-related sequence is a Late Cambrian to Early Devonian passive margin succession of clastic and carbonate sedimentary rocks. The Tākaka terrane has been intruded by Late Devonian mafic/ultramafic rocks and Early Cretaceous granitoids. The Buller and Takaka terranes were amalgamated in the early Middle Devonian.
A regional unconformity separates Late Cretaceous and younger sedimentary rocks from the basement. Late Cretaceous to Early Eocene sediments were deposited in basins in the north of the park. After a period of erosion the Late Eocene saw deposition of coal measures followed by calcareous sediments and limestone in the Oligocene. Renewed tectonic activity in the Miocene-Pliocene resulted in emerging land and sediments becoming terrigenous. The rapid uplift of the Southern Alps in the Pliocene-Pleistocene led to substantial volumes of gravels being deposited, particularly in the Karamea district.
Ongoing tectonic activity caused by the convergence of the Pacific and Australian plates has resulted in a mountainous topography. Uplifted Early Ordovician limestone, extensively altered to marble, has led to significant examples of karst topography. Some of the longest and deepest cave systems found in the Southern Hemisphere are located in the park. Extensive glaciation during the Pleistocene created many “U” shaped valleys and glacial tarns. In a few places Oligocene limestone has been sculpted by water and ice to form the best examples of uplifted mesas in New Zealand. Landslides are a common feature in the steeper areas of the park and many were triggered by earthquakes. Within the park lies part of New Zealand’s deepest and most intensely deformed Paleogene-Neogene sedimentary basin.
Overview of stratigraphy
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. It contains steeply plunging folds in strongly sheared quartzite, sandstone and siltstone derived from Golden Bay Group rocks. The zone appears to be the scar left behind by a stratigraphic excision of unknown tectonic origin.
Intruding the metamorphosed sedimentary sequence in the western half of the Buller terrane is the Karamea Batholith comprising late Middle Devonian to Early Carboniferous ( Ma) granitoids. The bulk of the granitoids mapped in the park are known as the Karamea Suite granite emplaced Ma. 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.
Early Cretaceous ( Ma) Separation Point Suite granite, consisting of biotite granite and granodiorite, also intrudes the Buller terrane. 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.
Tākaka terrane
The Tākaka terrane is a complex sequence of Middle Cambrian to Late Cambrian ( Ma) igneous and volcaniclastic arc-related rocks mixed with sediments from the southeastern margin of Gondwana, and a ā margin sequence from Late Cambrian to Early Devonian ( Ma) consisting of clastic and calcareous sedimentary rocks. 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. The terrane is structurally complex with up to 11 fault-bounded slices in the arc-related sequence and two in the passive margin sequence. The fault slices, with their different stratigraphies, appear to have juxtaposed parts of the arc-related sequence that were originally some distance apart. Three of the fault slices contain broken formation and mélange and together are mapped as the Balloon Melange. All pre-Devonian rocks have undergone low to middle greenschist facies metamorphism.
Volcanic arc-related sequence The oldest rocks in the park are represented by the Junction Formation consisting of Middle Cambrian ( Ma) turbiditic sandstones, siltstones and conglomerate debris. Outcrops occur as narrow north-south slices intersecting the east end of the Cobb Valley and a slice located about northwest of the western end of the Cobb Reservoir. Eroded Junction Formation material is found in rocks of the younger Haupiri Group.
Haupiri Group The oldest rocks dated by fossils in the Tākaka terrane, and in New Zealand, are found within the Haupiri Group. Calcareous siltstone and conglomerate beds contain trilobites of possible Middle Cambrian age ( Ma). These beds are confined to a small area about south of the Cobb Reservoir where they are interbedded with Mataki Volcanics.
The Haupiri Group is best represented by the Tasman Formation in a north to south slice at the west end of the Cobb Valley. This slice is dominated by siliceous siltstone and sandstone, with lenses of limestone and debris-flow conglomerate. The olistolithic limestone lenses contain trilobites, brachiopods and molluscs of late Middle Cambrian age ( Ma). 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).
Haupiri Group sediments were derived from both continental and volcanic arc sources in a back-arc basin intra-oceanic arc setting. The earliest Haupiri Group sediments are interbedded in their upper part with the back-arc basalts of the Mataki Volcanics while the youngest Haupiri Group sediments overlie and are interbedded with the youngest Benson Volcanics.
Devil River Volcanics Group The Devil River Volcanics Group includes all volcanic and plutonic rocks of Middle to Late Cambrian age ( Ma). The volcanic and plutonic rocks interbed and intertongue with sediments of the Haupiri Group and are grouped into the following (earliest to latest) formations:
Mataki Volcanics dominated by basaltic flows, pillows and pillow breccias, volcaniclastic breccia, conglomerate and sandstone and interpreted to represent back-arc basin tholeiitic basalts. Mataki Volcanics are interbedded with early Haupiri Group sediments up to early Benson Volcanics and Tasman Formation. Cobb Igneous Complex contains gabbro and serpentinised ultramafic rocks. Interpreted to be layered shallow-level intrusions of boninitic andesites in a back-arc environment. Numerous late-stage mafic and felsic dikes intrude the gabbro and ultramafic rocks. Cobb Igneous Complex intrudes early Haupiri Group sediments and lower Mataki Volcanics. Benson Volcanics consists of volcaniclastic breccia conglomerate and sandstone, tuffs and flows representing at least 9 volcanic suites that generally changed with time from basaltic to andesitic with some dacite and rhyolite at the top of the stratigraphic succession. They were erupted in a subduction-related setting. Geochemistry of the rocks suggests deposits formed on and adjacent to an island arc. The earliest Benson Volcanics are interbedded with upper Mataki Volcanics while the youngest Benson Volcanics are overlain by and interbedded with the youngest Haupiri Group sediments.
At around the latest Cambrian to early Ordovician ( Ma) dolerite dikes and sills intruded virtually all of the fault slices. Their geochemistry is similar to modern rift-related volcanic rocks.
Balloon Melange Three fault slices are characterised by diamictite, broken formation and melange: Balloon Slice, roughly long and up to wide intersecting the Cobb Reservoir; Anatoki Slice beside the northern half of the Anatoki Fault; and the Tunnel Slice at the eastern end of the Cobb Reservoir. The bulk of the melange is derived from Junction Formation material with additional material from the Tasman Formation, Lockett Conglomerate and Devil River Volcanics Group. It includes exotic blocks from a few metres up to possibly in length of chert, limestone, conglomerate, siltstone, volcanic sandstone, andesite, basalt and mafic intrusives set in a sparse to abundant matrix of blue-grey sandy to silty mudstone. The matrix is interpreted to be derived from turbiditic layers of sandstone and mudstone from the Junction Formation. Blocks from the latest Cambrian to Devonian passive margin sequence are absent and the melange is therefore inferred to have formed in the Late Cambrian.