Hawke's Bay Land District
This land district separates readily into three regions of geological structure; the main forms of the eastern Wellington district are continued into Hawke's Bay. The western boundary is formed by high, rugged mountain ranges running south-west — north-east and composed of Triassic-Jurassic greywacke. The east is characterised by broken, hilly country formed from Cretaceous and Tertiary sediments. Here complex earth movements have upthrust small ridges or tilted blocks, exposing anticlinal cores of late Cretaceous rocks. The third region, which is a continuation of the wide Wairarapa trough from Woodville through the Takapau Plains to Mohaka and Wairoa on the shore of Hawke Bay, is filled with thick Upper Tertiary and Pleistocene sediments. The land district lies within the zone of active crustal instability.
The massive greywacke ranges of the first region cover less than a quarter of the Hawke's Bay Land District. The greywackes are here largely unfossiliferous but are probably of Triassic and Jurassic age. They were probably first pushed above the sea in the Rangitata Orogeny (late Jurassic and early Cretaceous) but the high country they then formed most likely had only a temporary existence, being periodically reduced by erosion and reconstituted by earth movements throughout Cretaceous and Tertiary times (seediagram 7). The Kaikoura Orogeny of the Pliocene and early Pleistocene gave the Ruahine Range its present development. The Kuripapango Trough, which is a sag in the range to the west of Napier, was probably formed in much the same way and at the same time as the Manawatu Gorge.
The range as a whole has been pushed up along major faults to the east and west (seediagram 5). The active Wellington Fault, continuing from further south, lies along the eastern edge of the range, while numerous other small faults, also active, are present. Earthquake tremors are frequent in this area and surface movements have occurred on many of the faults.
Greywacke similar to that of the Ruahine Range probably underlies most of the area to the east of the range, as is shown in diagram 5. Tilting movements along faults have exposed this basement greywacke in a number of places (for example, Waewaepa and Wakarara Ranges) (seediagram 12), but here it is only moderately folded and faulted and is possibly younger than the Ruahine greywacke. Further east again (for instance, in the Otane Range) (seediagram 12), the basement greywacke is not itself exposed, and the oldest rocks at the surface are Lower Cretaceous fossiliferous sandstones with local patches of grits and conglomerates, deposited in basins developed after the Rangitata Orogeny.
Spilites, with their associated red shales and jaspers, occur widely in the Hawke's Bay greywackes, and at Maharahara they contain small quantities of copper, which was worked for a short time.
The coastal series of low ranges and hilly country forming the second region run parallel to the Ruahine axis, from Mahia to Palliser Bay in the Wairarapa, bordering and projecting into the Wairarapa — Hawke Bay — Wairoa trough. Structurally the much folded and faulted late Cretaceous and Tertiary sediments of the coastal zone form a complex fold system trending north-east — south-west (diagram 12). The Akitio Syncline lies inland, running parallel to the southern Hawke's Bay coastline and ending at Hastings. Along the seaward side are complexly faulted, tilted, and folded strata, mainly of massive mudstones which form a number of parallel anticlines and synclines. The striking hogback of the Silver Range north of Elsthorpe is composed of alternating sandstones and mudstones of Miocene age. Kahuranaki is a complexly faulted mass of upper Pliocene cemented shelly limestone, and the well known Te Mata Peak is part of a prominent dip slope in the same limestone on the west flank of the Elsthorpe Anticline. The limestone forms prominent dip slopes throughout central Hawke's Bay and is often quarried for agricultural use; very occasionally some layers are pure and are used in glass manufacture (for example, Pakipaki, Waipawa Gorge, etc.). Fossils are abundant in most of the Tertiary rocks of the district; the Dannevirke Series is based on the sequences exposed near Te Uri and Waipawa. The late Cretaceous rocks do not contain many fossils, but New Zealand's largest ammonite (2 ft in diameter) was found near Porangahau.
The complex landscape of the region is thought to have resulted mainly from earth movements during the Kaikoura Orogeny, which have continued to the present day. Hence the characteristic broken ridges and non-conformity of sediments laid down during this period — shearing and faulting forming deep depressions which became filled with thousands of feet of rapidly deposited sandstones and conglomerates, grits, and claystones. Those of the Dannevirke Series are extremely fine grained and form bentonitic clays which have marked swelling properties due to the mineral montmorillonite and outcrop throughout the area where the Tertiary-Cretaceous boundary is exposed. Bentonite is quarried from coastal deposits south of Porangahau and is used as a drilling mud and as a binder, filler, and plasticiser in many industries, notably ceramics. It is also valuable as an absorbant, emulsifier, chemical water softener, and suspension medium. Due to its peculiar rheological properties, it is responsible for much local slipping in Hawke's Bay, and it has been suggested that much of the overthrusting of Upper Tertiary beds was made possible by the bentonite “greasing” the underlying Cretaceous rocks. Other sediments with economic value include some of the Tertiary sandstones, which have been found useful as moulding sands and are used in small foundries in Napier and Dannevirke. Oil companies have surveyed much of Hawke's Bay area, but so far without promising results, mainly because of the complex, small-scale faulting and folding. Pliocene lignites have been found in the Dannevirke area, but are of very poor quality and too thin to work.
The central lowlands lie between the two preceding regions. Structurally they consist of two shallow basins (the Ruataniwha Depression and the Heretaunga Plains) where the underlying Tertiary and lower Pleistocene are covered with younger alluvium which is still being deposited in the broad flood plains of the rivers. Ignimbrite and pumice pebbles abound in the upper Pleistocene and Holocene gravels of the lowlands, and ash layers are abundant in some of the Wanganui Series sediments, especially those of the middle Pleistocene (for instance, near the base of the Clifton — Cape Kidnappers coastal section).
The Ruataniwha Depression (which includes the Takapau Plains) is filled with Pleistocene and Holocene terrace deposits and is crossed by the river system of the Tukituki and its tributary, the Waipawa. Most of the present topography results from tilting and river terracing under frost-climate conditions during the Pleistocene glaciations. Towards the foot of the Ruahines the surface is tilted and terrace systems may be seen in the harder calcareous silt-stones and conglomerates of the earlier Pleistocene.
The Heretaunga Plains is a strongly alluviated area, as it lies in the lower reaches of three large rivers (Ngaruroro, Tukituki, Tutaekuri). Fossiliferous marine Pliocene rocks form the margin of the plains and these rocks also outcrop at Scinde Island, Napier city.
That Hawke Bay is probably a structural feature is evident from the inward dip of the surrounding rocks, especially the complex of anticlines and synclines of the coastal highlands which plunge into the depression from the south, apparently to reappear on the northern side.
The enormous quantities of rock carried down stream by the rivers into the lowland areas have left many thick beds of conglomerates and greywacke pebbles which are suitable for road metal. The Heretaunga Plains form a very important ground-water system, containing sufficient water to irrigate the entire fertile area of the plains and provide for ancillary industries. Drilling has shown that the water-bearing beds are thick and large reserves have been proven.
The Wairarapa Fault Zone continues into this region of Hawke's Bay District and many active faults are known (seediagram 8). Earthquakes of considerable intensity are frequent in the region and the Napier earthquake of 1931, which devastated the city of Napier, raised the sea bed at Ahuriri 7–9 ft to form dry land, now used by industry and housing. The Pahiatua earthquake of 1934 occurred in the same fault zone and the Wairoa earthquake of 1932 in the continuation of the zone on the northern side of Hawke Bay.