Geology of the Raukumara Region

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The Raukumara Region of New Zealand corresponds to the East Cape of the North Island, and associated mountain ranges.

New Zealand Country in Oceania

New Zealand is a sovereign island country in the southwestern Pacific Ocean. The country geographically comprises two main landmasses—the North Island, and the South Island —and around 600 smaller islands. New Zealand is situated some 2,000 kilometres (1,200 mi) east of Australia across the Tasman Sea and roughly 1,000 kilometres (600 mi) south of the Pacific island areas of New Caledonia, Fiji, and Tonga. Because of its remoteness, it was one of the last lands to be settled by humans. During its long period of isolation, New Zealand developed a distinct biodiversity of animal, fungal, and plant life. The country's varied topography and its sharp mountain peaks, such as the Southern Alps, owe much to the tectonic uplift of land and volcanic eruptions. New Zealand's capital city is Wellington, while its most populous city is Auckland.

East Cape coastal location in New Zealand

East Cape is the easternmost point of the main islands of New Zealand. It is located to the north of Gisborne in the northeast of the North Island.

North Island The northern of the two main islands of New Zealand

The North Island, also officially named Te Ika-a-Māui, is one of the two main islands of New Zealand, separated from the larger but much less populous South Island by Cook Strait. The island's area is 113,729 square kilometres (43,911 sq mi), making it the world's 14th-largest island. It has a population of 3,749,200.

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To the east of the North Island is the Hikurangi Trough, a collision zone between the Pacific Plate and the Australian Plate. The Pacific Plate is being subducted under the Australian Plate, compressing the east of the North Island, and causing the North Island Fault System, and a series of SSW-NNE trending basins and ranges, including the Raukumara Range. Successively newer rocks have been accreted to the east coast.

Pacific Plate An oceanic tectonic plate under the Pacific Ocean

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres (40,000,000 sq mi), it is the largest tectonic plate.

Australian Plate A major tectonic plate, originally a part of the ancient continent of Gondwana

The Australian Plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately 100 million years ago when India broke away and began moving north. Australia and Antarctica began rifting 85 million years ago and completely separated roughly 45 million years ago. The Australian plate later fused with the adjacent Indian Plate beneath the Indian Ocean to form a single Indo-Australian Plate. However, recent studies suggest that the two plates have once again split apart and have been separate plates for at least 3 million years and likely longer. The Australian plate includes the continent of Australia, including Tasmania, as well portions of New Guinea, New Zealand, and the Indian Ocean basin.

North Island Fault System

The North Island Fault System or North Island Dextral Fault Belt is a set of southwest-northeast trending seismically-active faults in the North Island of New Zealand that carry most of the dextral strike-slip component of the oblique convergence of the Pacific Plate with the Australian Plate. They include the Wairarapa Fault and Wellington Fault to the southwest, the Ruahine and Mohaka Faults in the central section and the Waimana, Waiotahi, Whakatane and Waiohau Faults to the northeast. Most of the fault system consists of dextral strike-slip faults, although towards its northeastern end the trend swings to more S-N trend and the faults become mainly oblique normal in sense as the zone intersects with the Taupo rift zone. This fault zone accommodates up to 10 mm/yr of strike-slip displacement.

The Raukumara Region used to be adjacent to Northland, before being shifted to its current position, and many rocks of the two regions match.

Northland Region region at the northern end of New Zealands North Island

The Northland Region is the northernmost of New Zealand's 16 local government regions. New Zealanders often call it the Far North or, because of its mild climate, the Winterless North. The main population centre is the city of Whangarei, and the largest town is Kerikeri.

To the north of the Raukumara Range lies the Bay of Plenty, formed of Torlesse (Waioeka) Greywacke. The central ranges are covered by in-place and allochthonous (displaced) Cretaceous to Oligocene rocks. To the south are more recent, mainly Miocene and Pliocene, rocks.

The Raukumara Range lies north of Gisborne, near East Cape in New Zealand's North Island. It forms part of the North Island's main mountain chain, which runs north-northeast from Wellington to East Cape, and is composed primarily of Cretaceous greywacke, argillites, siltstones and sandstones. An epoch of the New Zealand geologic time scale lasting from 95.2 to 86.5 Mya is named the Raukumara Epoch after the range.

Bay of Plenty Region in North Island, New Zealand

The Bay of Plenty is a bight in the northern coast of New Zealand's North Island. It stretches 260 km from the Coromandel Peninsula in the west to Cape Runaway in the east. The Bay of Plenty Region is situated around this body of water, also incorporating several large islands in the bay. The bay was named by James Cook after he noticed the abundant food supplies at several Māori villages there, in stark contrast to the earlier observations he had made in Poverty Bay.

Greywacke A hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix

Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The term "greywacke" can be confusing, since it can refer to either the immature aspect of the rock or its fine-grained (clay) component.

Basement rocks

All basement rocks beneath the Raukumara Region belong to the Torlesse Composite (Waioeka) Terrane, of late Jurassic to early Cretaceous age (150-100 Ma). They are largely composed of Greywacke (hardened sandstone and mudstone), that accumulated in a deep marine environment. These rocks are exposed to the north of the Raukumara Range, from Whakatane to Papatea Bay.

Torlesse Composite Terrane

Torlesse Composite Terrane contains the Rakaia, Aspiring and Pahau Terranes and the Esk Head Belt. Greywacke is the dominant rock type of the composite terrane, argillite is less common and there are minor basalt occurrences. The Torlesse Composite Terrane is found east of the Alpine Fault in the Southern Alps of New Zealand. Its southern extent is a cryptic boundary with the Caples Terrane within the Haast Schists in Central Otago. It is named for the Torlesse Range in Canterbury. The Rakaia Terrane rocks, of Permian to late Triassic age, occur south of Rangiora.

The Late Jurassic is the third epoch of the Jurassic period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.

The Early Cretaceous or the Lower Cretaceous, is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 146 Ma to 100 Ma.

In-place Cretaceous to Oligocene rocks

Sandstones and mudstones were deposited over the region in Cretaceous to Oligocene times (100-24 Ma). These rocks still cover the Raukumara Range.

The East Coast allochthon

In Early Miocene times (24-21 Ma), when the Raukumara region was still adjacent to Northland, a series of thrust sheets was emplaced over much of the East Cape area. Large portions of the Cretaceous to Oligocene rocks have been displaced by tens to hundreds of kilometres, forming what is known as the East Cape Allochthon. The rocks came from the Northeast, and were emplaced in reverse order, but the right way up. The original rocks are of Cretaceous to Oligocene age (100-24 Ma), and include mudstones, limestone and basalt lava.

Most of the rocks east of the line from Papatea Bay to Waipiro Bay, and rocks just north of Whatatutu are allochthonous. Allochthonous rocks are assumed to underlie the more recent rocks to the south as well.

In the north, from Cape Runaway to Tokata, are allochthonous basaltic rocks, believed to represent sea floor, that has been obducted onto the land. These rocks correspond to the Tangihua rocks of Northland.

Miocene and Pliocene rocks

During Miocene and Pliocene times, sandstone, mudstone, and some limestone was deposited over much of the southern area of the Raukumara region.

Marine terraces

Marine terraces are common between Opotiki and East Cape, and at Mahia Peninsula.

Hot springs

Sinter deposits occur around the Te Puia Springs.

Geological Resources

The area to the south of the Raukumara region contains oil seeps (Waimata Valley and Waitangi Station), and is believed to have potential commercial gas and oil reserves. While the region has been explored, and minor reserves found, no major commercial reserves have yet been found.

Geological hazards

The Raukumara Region, and the Hikurangi Trough are prone to earthquakes, with the consequential risk of tsunami. For example, a magnitude 6.8 earthquake hit the Gisborne area on 20 December 2007. [1] There were also two major tsunamis on 26 March 1947 and 17 May 1947. [2]

Much of the land is composed of soft mudstones, that are easily eroded, particularly in areas without bush cover.

The region is also prone to flooding, when tropical cyclones manage to come sufficiently far south.

The region can also receive minor falls of volcanic ash from the Taupo Volcanic Zone.

Geological sites worth visiting

Maps

Geological maps of New Zealand can be obtained from the New Zealand Institute of Geological and Nuclear Science (GNS Science), a New Zealand Government Research Institute. [4]

The main maps are the 1 : 250 000 QMap series, which was completed as a series of 21 maps in 2010. Low resolution versions of these maps (without the associated booklet) can be downloaded from the GNS site. [5] The map for the Raukumara Area was published in 2001. [6]

Alternatively digital GIS layers are available using the free GNS map viewer.

See also

Related Research Articles

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Geology of New Zealand

The geology of New Zealand is noted for its volcanic activity, earthquakes and geothermal areas because of its position on the boundary of the Australian Plate and Pacific Plates. New Zealand is part of Zealandia, a microcontinent nearly half the size of Australia that broke away from the Gondwanan supercontinent about 83 million years ago. New Zealand's early separation from other landmasses and subsequent evolution have created a unique fossil record and modern ecology.

The Auckland Region of New Zealand is built on a basement of greywacke rocks that form many of the islands in the Hauraki Gulf, the Hunua Ranges, and land south of Port Waikato. The Waitakere Ranges in the west are the remains of a large andesitic volcano, and Great Barrier Island was formed by the northern end of the Coromandel Volcanic Zone. The main isthmus and North Shore are composed of Waitemata sandstone and mudstone, and portions of the Northland Allochthon extend as far south as Albany. Little Barrier Island was formed by a relatively isolated andesitic volcano, active around 1 to 3 million years ago.

The Waikato and King Country regions of New Zealand are built upon a basement of greywacke rocks, which form many of the hills. Much of the land to the west of the Waikato River and in the King Country to the south has been covered by limestone and sandstone, forming bluffs and a karst landscape. The volcanic cones of Karioi and Pirongia dominate the landscape near Raglan and Kawhia Harbours. To the east, the land has been covered with ignimbrite deposits from the Taupo Volcanic Zone. Large amounts of pumice from the Taupo Volcanic Zone have been deposited in the Waikato Basin and Hauraki Plains.

The Taranaki Region of New Zealand is built upon the Median Batholith in the West, and Greywacke Rocks in the East. However, no rocks older than Miocene times are visible at the surface. The dominant feature of the Taranaki Region is the andesitic stratovolcano of Mount Taranaki, which is only about 130,000 years old. The dissected hill country to the East of the Taranaki Peninsula, and West of the Central Volcanic Plateau is composed of soft Miocene to Pleistocene sandstone and mudstone. The coastal lowlands around Wanganui to the South have well-developed Quaternary marine terraces, and coastal sand dunes.

The Wellington Region of New Zealand has a foundation of Torlesse Greywacke rocks, that make up the Tararua and Rimutaka Ranges, that go from Wellington in the south to the Manawatu Gorge, where they are renamed as the Ruahine Ranges, and continue further north-northeast, towards East Cape. To the west of the Tararua Ranges are the Manawatu coastal plains. To the east of the Ruahine Ranges is the Wairarapa-Masterton Basin, then the Eastern Uplands that border the eastern coast of the North Island from Cape Palliser to Napier.

Geology of Canterbury, New Zealand

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Tahora Formation

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Geology of the Northland Region

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Geology of the Tasman District

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Atacama Fault

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Caballo Mountains landform

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Taranaki Basin

The Taranaki Basin is an onshore-offshore Cretaceous rift basin on the West Coast of New Zealand. Development of rifting was the result of extensional stresses during the breakup of Gondwanaland. The basin later underwent fore-arc and intra-arc basin development, due to the subduction of the Pacific Plate under the Australian Plate at the Hikurangi Subduction System. The basin covers approximately 100,000 km2 of which the majority is offshore. The basin contains mostly marine sediment, with significant terrestrial sediment from the Late Cretaceous to the Eocene. The majority of New Zealand's oil and gas production occurs within the basin, with over 400 wells and approximately 20 oil and gas fields being drilled.

The Mindoro block is a microcontinental block located in the Philippine Mobile Belt and the east side of North Palawan Block. It has comprises a metamorphic basement of unknown but probably pre-late cretaceous age, overlain locally by upper cretaceous basalts, and more regionally succeeded by a probable upper eocene sequence of basinal clastic rocks, plus local basalt intercalations and carbonates. These rocks are exposed over a broad area of northern and west-central Mindoro as well as on the Lubang Islands in Verde Island Passage. The Mindoro block is bounded on the west by the Mindoro Suture Zone, and on the north by the Verde Passage Suture, which separates it from the Zambales Ophiolite terrane of Luzon. The eastern terrane boundary may be the East Mindoro Fault Zone, a probable transcurrent boundary that has not yet been studied, but which displays evidence of recent activity. Late Miocene and Pliocene basinal clastic strata lie east of this fault zone, but it is not known if subjacent rocks are related to rocks of the Mindoro block, or if they are part of a third terrane on Mindoro.

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The geology of the United Arab Emirates includes very thick Paleozoic, Mesozoic and Cenozoic marine and continental sedimentary rocks overlying deeply buried Precambrian. The region has extensive oil and gas resources and was deformed during the last several million years by more distant tectonic events.

Geology of the West Coast Region

The geology of the West Coast of New Zealand's South Island is divided in two by the Alpine Fault, which runs through the Region in a North-East direction. To the West of the fault Paleozoic basement rocks are interluded by plutones and both are unconformably covered in a sedimentary sequence. To the East of the Alpine Fault are the Mesozoic Alpine Schist and Greywacke of the Southern Alps. There are numerous active faults throughout the region.

References

  1. ↑ 2007 Gisborne earthquake
  2. ↑ The Gisborne Tsunami
  3. ↑ DA Feary & EA Pessagno 1980. An early Jurassic age for chert within the Early Cretaceous Oponae Melange (Torlesse Supergroup), Raukumara Peninsula, New Zealand. NZ Journal of Geology & Geophysics 23: 623-628.
  4. ↑ New Zealand Institute of Geological and Nuclear Sciences (GNS Science) - a New Zealand government Crown Research Institute.
  5. ↑ GNS New Zealand Geological Maps.
  6. ↑ Geology of the Raukumara Area, Mazengarb, C., Speden, I.G., GNS Science, 2001. (1 : 250 000 map).

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