Gisborne Land District
The western boundary of this land district lies along a continuation of the south-west–north-east-trending North Island mountain chain, composed mainly of hard Jurassic and Cretaceous greywacke, argillites, siltstones, and sandstones. To the north in the East Cape area lies the volcanic region of Matakaoa, a complex mass thought to have been erupted from local volcanoes in Jurassic-Cretaceous times. The central and eastern part of the district is composed of Tertiary sediments forming a north-eastern continuation of the Hawke Bay–Wairoa trough.
The greywacke ranges form the main divide between the Bay of Plenty and the East Coast. The Raukumara Range reaches over 5,000 ft and the Huiarau over 4,000 ft, forming a series of massive ridges, densely forested and deeply dissected by precipitous gorges. The area contains some of the most inaccessible country in the North Island.
The greywacke ranges of the Gisborne district contain rather more fossils than those of the Wellington and Hawke's Bay districts and these give a better indication of age. Lower Jurassic fossils are found in the greywacke of the Ikawhenua Range near Taneatua and Upper Jurassic fossils from that of the Raungaehe Range near Awakeri; it is therefore likely that the greywacke on the western boundary of the Gisborne district is dominantly of Jurassic age. Toward the east Lower Cretaceous fossils occur in greywacke in the upper Waimana Valley and it is likely that this Lower Cretaceous greywacke extends still further eastwards to form the rugged hills of the Urewera country, immediately inland from the Bay of Plenty coast east of Opotiki. The Raukumara Range, forming the central axis of the district, is composed mainly of sparsely fossiliferous argillites, sandstones, and conglomerates of Cretaceous age which have been folded and faulted to form a series of prominent domes. In many of the deeply cut river valleys of the area, for example, the Motu and its tributary the Mangaotane, the rocks of these domes have been exposed; these have been mapped and used as the basis for the Taitai, Clarence, and Raukumara Series (Lower and Upper Cretaceous). All strata contain the fossil clam Inoceramus in varying amount, including some giant specimens up to 1½ ft long.
Taitai Series rocks are well exposed in the Tapuwaeroa Valley (where they extend to within 10 miles of the east coast) and in the centre of domes in the Te Puia area. They are of two types: a compacted sandstone and siltstone (Mokoiwi beds) and a hard dark sandstone, with pockets of conglomerate containing well rounded pebbles of quartz porphyry, feldspar, and greywacke (Koranga beds). The latter are very resistant to erosion and form some of the most striking peaks in the Raukumara Range, including Mount Hikurangi (5,753 ft) the highest non-volcanic mountain in the North Island, Mount Aorangi (4,091 ft) and Mount Taitai (2,012 ft), whose rugged tops tower precipitously over the surrounding ridge country.
Clarence and Raukumara Series rocks occur widely and form high-level hilly country in the Motu and Mata areas. Siltstones and mudstones are the main rock types, but hard sandstone beds may occur. Carbonaceous sandstones and mudstones, greensands, and bentonitic clays of the Mata Series are exposed in the Mata and Tapuwaeroa Valleys.
The Matakaoa volcanics take the form of a plateau broken by the east-west fault-bounded Wharekahika trench, which is filled with Tertiary sediments. The plateau stands at about 1,000 ft and extends from Matakaoa Point to Cape Runaway. The volcanics are mainly basaltic and extensive deposits of pillow lavas (submarine basalt flows) provide evidence of submarine eruptions. The rocks appear to correspond closely to the Tangihua Volcanics of Northland and are of about the same age, as Upper Jurassic or Lower Cretaceous fossils are found in a small body of limestone in the basalts near Cape Runaway. A smaller area of the same volcanics occurs south of the sedimentary trench, in the Hicks Bay–Te Araroa area.
The Matakaoa area was probably a volcanic island in the New Zealand Geosyncline during the Rangitata Orogeny but had a varied history throughout the Tertiary (seediagram 7) and was the source of the volcanic pebbles found in conglomerates at various levels in the Gisborne Cretaceous and Tertiary sediments. In the Pliocene the sea invaded the Wharekahika trench to form a strait between the main land mass and Matakaoa. The Matakaoa area, along with the main ranges of the North and South Islands, was uplifted, folded, and faulted during the Kaikoura Orogeny.
The eastern part of this land district is occupied by a vast thickness of Tertiary sediments filling a continuation of the Hawke Bay–Wairoa trough. The Miocene and Pliocene beds attain a thickness of nearly 4 miles south of Gisborne (see section A of diagram 5) and somewhat less to the north. The sediments have in general been less strongly deformed than in the geologically similar regions to the south of Hawke's Bay. Two main basins may be recognised, the Wairoa and Tutamoe basins, though both are complicated by faulting. They are bordered along the coast by a complicated system of depressions separated by steeply pinched anticlinal zones (for example, at Waitangi and Whangara), where Miocene mudstones and sandstones are broken through by cores of hard Cretaceous rocks to form high jagged peaks similar to the Taipos seen in the Wairarapa-Dannevirke areas.
The Wairoa basin shows a succession of gentle inward-dipping folds in the Tertiary sandstones and mudstones. The western boundary is formed by the fault-bounded greywacke of the Huiarau Range and the eastern by the Mahia Peninsula, where basement rocks are known to approach the surface from geophysical surveys (see section A of diagram 5). Lake Waikaremoana (elevation 1,970 ft) lies on the western flank of the basin and is drained towards Hawke Bay by the Waikare Taheke River. The lake appears to have been formed in Holocene times by landslides damming a gorge cut by the river through the Miocene sandstone of the Ngamoko and Panekiri Ranges immediately to the south. The sandstone rests on soft clays whose erosion and subsequent undercutting have produced the landslides which form a natural rock-fill dam some 1,200 ft high extending 3 miles down the valley from the lake outlet. The lake is the reservoir for three power stations in the Waikare Taheke Valley.
The Tutamoe basin flanks the Raukumara Range and is a shallow saucer-shaped structure with slightly folded Tertiary strata dipping into it, the central resistant Miocene sandstone with conglomerate beds forming a tableland which separates the headwaters of the Waipaoa from those of the Waiapu. These conglomerates are thought to be as much as 7,000 ft thick in places.
The coastline shows a complicated series of structural depressions which may be drowned or filled with fine recent deposits of alluvium layered with thick ash and pumice bands (for example, the Gisborne-Matawhero depression) derived from eruptions in the Taupo-Rotorua area. Four main ash showers have been traced, these may be up to 20 ft in thickness (for example, Te Arai Valley), or they may form prominent escarpments as in the Waimata Valley (Gisborne) where the ash consists of coarse gritty tuffs containing much mica and quartz. All the rocks exposed towards the south are comparatively soft; they show raised beaches where there has been local uplift.
The Cretaceous and Eocene beds of the Gisborne Land District have been faulted and folded along north-west–south-east axes. These lie at a marked angle to the folds and faults in the younger Tertiary sediments, which lie north-east–south-west, parallel to the east coast. Two periods of folding have been suggested–the first trending north-west and the second north-east.
The Wellington Fault system of active faults can be traced to just south-west of Lake Waikaremoana (seediagram 8) but other active faults continue the same trend north-east and traverse the greywacke between the lake and the Bay of Plenty. Many other faults, not active at present but also trending north-east, have been mapped north and east of Lake Waikaremoana.
The Cretaceous and Tertiary rocks inland of the east coast have widespread occurrences of natural gas vents (as at Te Puia), oil seepages, black shales smelling strongly of oil, and carbonaceous mud pools (covering over 10 acres at Hylands, Waimata). The search for commercial quantities of oil has continued in the district since last century, when the first drillings were made in 1874 at Waitangi Hill, 24 miles north of Gisborne. Recently domes have been drilled in the Gisborne area at Mangaone (to 5,085 ft) and Ruakituri (to 9,005 ft), but the only findings were a little gas and salt water. Difficulty was experienced in penetrating the thick sequences of bentonitic clays. The repeated loss of drilling gear and caving of boreholes finally led to the abandonment of drilling activities.
The prevalence of bentonite in this region and, to a lesser extent, of the soft papa clays, is the cause of frequent slips of surface strata after rain. Characteristic landforms of the district are the sharp “taipos” of harder rocks flanked or interrupted by “slumped” country, where landslides which may involve hundreds of acres have occurred. This causes much engineering difficulty with road and rail foundations and is responsible for the acute erosion problems in many localities now cleared for agriculture.