Geology of the Raukumara Region
Encyclopedia
The Raukumara Region of New Zealand
corresponds to the East Cape of the North Island, and associated mountain ranges.
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 Ranges. Successively newer rocks have been accreted to the east coast.
The Raukumara Region used to be adjacent to Northland, before being shifted to its current position, and many rocks of the two regions match.
To the North of the Raukumara Range
s 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.
(hardened sandstone and mudstone), that accumulated in a deep marine environment. These rocks are exposed to the North of the Raukumara Range, from Wakatane to Papatea Bay.
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.
s are common between Opotiki and East Cape, and at Mahia Peninsula.
For example, a magnitude 6.8 earthquake hit the Gisborne area on 20 December 2007.
There were also two major tsunamis on 26 March 1947 and 17 May 1947.
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.
GNS provides a free Map of New Zealand's Geological Foundations.GNS Map of New Zealand's Geological Foundations
The main maps are the 1 : 250 000 QMap series, which will be 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 for free. The map for the Raukumara Area was published in 2001.
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
corresponds to the East Cape of the North Island, and associated mountain ranges.
To the east of the North Island is the Hikurangi Trough, a collision zone between the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic 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
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...
, and a series of SSW-NNE trending basins and ranges, including the Raukumara Ranges. Successively newer rocks have been accreted to the east coast.
The Raukumara Region used to be adjacent to Northland, before being shifted to its current position, and many rocks of the two regions match.
To the North of the Raukumara Range
Raukumara Range
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 greywacke, argillites, siltstones and sandstones. The North...
s lies the Bay of Plenty
Bay of Plenty
The Bay of Plenty , often abbreviated to BOP, is a region in the North Island of New Zealand situated around the body of water of the same name...
, formed of Torlesse (Waioeka) Greywacke
Greywacke
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...
. The central ranges are covered by in-place and allochthonous (displaced) Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
to Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
rocks. To the South are more recent, mainly Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
and Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
, rocks.
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 GreywackeGreywacke
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...
(hardened sandstone and mudstone), that accumulated in a deep marine environment. These rocks are exposed to the North of the Raukumara Range, from Wakatane to Papatea Bay.
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 terraceMarine terrace
A marine terrace, coastal terrace, raised beach or perched coastline is a relatively flat, horizontal or gently inclined surface of marine origin, mostly an old abrasion platform which has been lifted out of the sphere of wave activity . Thus it lies above or under the current sea level, depending...
s are common between Opotiki and East Cape, and at Mahia Peninsula.
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.
There were also two major tsunamis on 26 March 1947 and 17 May 1947.
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
- Drive the loop from Gisborne, through the Wioeka Gorge, looking at the greywacke, to Opotiki, then around East Cape, looking at the Matakaoa volcanics between Cape Runaway and Tokata (e.g., Haupara Point, near Hicks Bay), visiting East Cape itself, then back to Gisborne, though Miocene sandstones and mudstones.
- Try white water rafting down the Motu River.
- Visit Waimata Valley (Y17/508894) and Waitangi Station (Y17/383016), north of Gisborne, to see the mud volcanoes and oil seeps. (Maybe these are difficult to find.)
- Have a hot swim at Te Puia Springs, and see the sinter deposits.
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.GNS provides a free Map of New Zealand's Geological Foundations.GNS Map of New Zealand's Geological Foundations
The main maps are the 1 : 250 000 QMap series, which will be 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 for free. The map for the Raukumara Area was published in 2001.
Further reading
- Graham, Ian J. et al.;A continent on the move : New Zealand geoscience into the 21st century - The Geological Society of New Zealand in association with GNS Science, 2008. ISBN 9781877480003