Wellington Land District
The Wellington Land District comprises the Rimutaka-Tararua Range (south of the Manawatu Gorge) and the sedimentary basins on either side (Palmerston North — Wanganui and Eketahuna-Wairarapa), but it extends to the west of the range north of the Manawatu Gorge to include the Rangitikei area and Tongariro National Park.
The Rimutaka-Tararua Range is composed of greywacke, which is thought to be mainly of Triassic age. The Upper Triassic shell Monotis richmondiana is known from the headwaters of the Otaki River. The entire Wellington Peninsula is also composed of similar greywacke and is evidently of about the same age, as worm tubes and ichthyosaur vertebrae thought to be of Triassic age have been found around the Wellington coast. The Triassic greywacke extends north from the Manawatu Gorge forming the Ruahine and Kaweka Ranges of Hawke's Bay Land District, but in the northern part of Wellington Land District the Kaimanawa Range projects as a fault-bounded block westwards towards Tongariro National Park. The Kaimanawa greywacke is thought to be older than that of the main range and to include the Permian as well as part of the Triassic.
The Rimutaka-Tararua-Ruahine Ranges have been pushed up along major faults (see section B of diagram 5) and so have well defined boundaries (seediagram 2). Sedimentary basins, floored by greywacke, flank the ranges to east and west, and throughout Tertiary time have received the products of erosion of the ranges — first the eastern basin and then the western basin. Filling of the eastern basin began in the Cretaceous and was completed by the Miocene; its contents have now been folded and faulted. Filling of the western basin began in the Miocene and is still continuing, with many large river systems (for example, Wanganui, Rangitikei, Manawatu) draining the ranges and actively extending the coastline seawards with their loads of mud, sand, and shingle.
The Kaikoura Orogeny in the Pliocene and Pleistocene affected both basins, but the sediments filling the eastern basin became greatly crumpled whereas those of the western were broken into blocks by faults. Therefore, as may be seen from section B of diagram 5, the sediments of the western basin are fiat lying or gently dipping, though they have been dislocated by faults to form “highs” and “lows”. In the past some of the “highs” have attracted the attention of oil companies as indicating likely oil traps; an exploratory bore at Mount Stewart, near Feilding, was drilled to basement greywacke, but no oil traces were found.
As is also shown in section B of diagram 5, the sediments filling the eastern basin have been distorted by folding as well as faulting. In many places the older Cretaceous rocks, hard argillites and sandstones, have been broken into a series of slivers which, when exposed at the surface, form prominent serrated dark-coloured hills, contrasting with the low-lying surrounding lighter coloured Tertiary rocks. These hills are features of the Wairarapa and Dannevirke areas and have been locally called taipos (“devils”). To the south of the Wairarapa, in the Cape Palliser area, the basement Triassic-Jurassic greywacke is exposed at the surface, as well as the overlying Lower Cretaceous sandstones and argillites.
Uplift of the sedimentary rocks filling both eastern and western basins has exposed many fine sequences of marine rocks, all richly fossiliferous. The coastal section extending from Wanganui to Waitotara is the type section for the Wanganui Series (Pliocene-mid Pleistocene). Progressive uplift of the Wanganui-Rangitikei area has forced major rivers, such as the Wanganui and Rangitikei, to incise deeply and so form spectacular gorges.
Faulting of the Wellington Land District in the Pliocene and Pleistocene has produced a series of north-east — south-west trending faults, cutting both basement greywacke and young sedimentary rocks alike. Section B of diagram 5 shows some of these faults in simplified form; many are active at the present time and have been the foci of frequent earth-quakes. These active faults are shown in diagram 8.
Several fault zones cut the Wanganui coast and, as may be seen from diagram 8, are the south-western continuation of the Rotorua-Taupo fault zone with its volcanoes and hot springs.
The Wellington Peninsula is traversed by a number of faults, all recently active, which are conspicuous features of the landscape. As seen in diagram 8, the Wellington faults are continuations of South Island faults. The Wairau Fault probably continues off shore along the west Wellington coast, in part accounting for the steepness of the present coastline, and in all likelihood passes between Kapiti Island and the mainland. The Owhariu and Wellington Faults, both continuations of the Awatere Fault, are well expressed in the topography, especially the latter, which for most of its length in Wellington City and the Hutt Valley is marked by a prominent scarp that forms one side of Wellington Harbour. The Wellington Fault continues through the Tararua Range, where its course is marked by scarps, saddles, and valleys, and it forms the eastern boundary of the range from the Mangatainoka River north to the Manawatu Gorge and beyond. Buckling along the southern end of the Wellington Fault has formed a series of basins of which that occupied by Wellington Harbour and the Lower Hutt Valley is the largest.
The Wairarapa Valley is bounded by two major faults, the East Wairarapa, delimiting the coastal Aorangi Mountains, and the West Wairarapa, delimiting the Rimutaka and Tararua Ranges. Both faults are continuations of faults in the Inland and Seaward Kaikoura Ranges (seediagram 8).
The West Wairarapa Fault was last active in 1855, when it moved vertically 10–12 ft and tilted the land to the west as far as the western Wellington coast, causing an uplift of 7 ft in eastern Wellington Harbour and the Hutt Valley and 5 ft in Wellington itself. Movement on the East Wairarapa Fault in 1942 caused severe damage in the Masterton area and many small scarps were formed along the fault line. Both Wairarapa faults continue north-eastwards into Hawke's Bay as a broad fault zone, containing innumerable small faults (seediagram 8).
During the Pleistocene glacial episodes, much of the Wellington Land District, especially the ranges and the Wellington Peninsula, was subjected to a frost climate with freeze and thaw as active erosion agents. Valley glaciers formed in the high parts of the Tararua Range and the entire landscape was smoothed by frost action, with the production of much coarse angular debris which accumulated in the lowlands and mantled many of the higher slopes. With the fall in sea level during the Pleistocene (as water was abstracted from the seas to form ice sheets on land), much of the near-shore part of the sea floor was exposed around the Wellington coastline. This allowed wind to erode the fine sediment of the old sea floor which in some areas was blown inland to form thick deposits of loess mantling the topography.
In the Pliocene marine straits connected the eastern and western sedimentary basins, crossing sags in the rising ranges. One such strait was formed on the site of the present Manawatu Gorge. During the Pleistocene glacial periods the sea withdrew from the Manawatu Strait, which may have been temporarily bridged by gravel deposits derived from erosion of the ranges, but the sea probably returned during some of the earlier interglacials. When the strait was not occupied by the sea the Manawatu River flowed through it from east to west. With the gradual uplift of the ranges throughout the Pleistocene, and the permanent exclusion of the sea, the Manawatu River was forced to cut a deep gorge across the rising country in order to maintain its course. The Wellington Peninsula was probably joined to the South Island in the Pliocene (seediagram 7). During the Pleistocene, however, this land bridge was progressively eroded and lowered so that when sea level rose in post-glacial times an open seaway, the present-day Cook Strait, was formed.
Spilites (altered submarine basalt flows), with their associated red shales and cherts, occur in the Wellington greywackes, though they are of scattered occurrence and do not attain a great thickness in any one locality. Igneous intrusions, probably of early Tertiary age, are found in the Brocken Range — Ngahape area, Wairarapa.
The volcanics of Tongariro National Park are included in Wellington Land District. These consist of five major andesitic volcanoes — Ruapehu, Tongariro, Ngauruhoe, Kakaramea, and Pihanga — together with several associated minor volcanoes and vents. Tongariro volcano is the oldest and dates from the lower Pleistocene. The volcano built itself to some considerable height, but in the middle Pleistocene it exploded violently to form an enormous crater on the rim of which several smaller cones were built, including the active Ngauruhoe on the south rim. Kakaramea volcano, immediately south of Tokaanu, is slightly younger than Tongariro, but became extinct earlier, and Pihanga volcano, immediately south of Turangi, had a similar short-lived history. The Ruapehu volcano was initiated in the late Pleistocene and was rapidly built to a height similar to that of Tongariro prior to its explosive phase, but it also has lost some of its height through explosions. Ruapehu is still active. Extensive ring plains, built up by mud-flows (lahars) down the flanks of the volcanoes, surround the main vents. They can be traced for some distance (for example, to Waiouru, Ohakune, etc.) and are responsible for the barren country traversed by the main highway (Desert Road) north of Waiouru. A lahar derived from Ruapehu swept down the Wangaehu River and caused the Tangiwai rail disaster of 1953.
The volcanic centres of Tongariro National Park Park are aligned along a major crustal weakness marked by faulting, volcanic activity, hot springs, etc. As may be seen from diagram 2 and diagram 8, this weakness continues north-eastwards through the Rotorua-Taupo area and into the south-west Pacific via White Island, Kermadec Islands, Tonga, and Samoa.