Taranaki Land District
Of the 12 land districts of New Zealand, none has a simpler surface geology than Taranaki, the smallest. At the Awakino River, the Mesozoic strata of the Kawhia Syncline (diagram 11) disappear beneath Tertiary marine beds which extend southwards to form wide lowlands pierced in the west by a line of volcanic vents along which has grown a chain of andesite volcanoes culminating in the giant cone of Mount Egmont (8,260 ft), with its secondary cone, Fantham Peak, on its eastern flank. These Tertiary rocks, eroded to form plains, underlie Taranaki's farm land, and successive eruptions from the volcanoes have mantled them with thick deposits of fertile ash which forms the basis of its agricultural productivity.
Tertiary sedimentation began in Taranaki in the early Eocene with deposition of coal measures followed by limestone and other relatively thin sediments in the Oligocene. During early Miocene times the Taranaki area became a broad, rapidly sinking basin in which accumulated more than 15,000 ft of marine sedimentary rocks. Some of these rocks are shown to the left of section B in diagram 5.
Very late in New Zealand's geological history, these rocks began to be raised above the sea, first in the north, then successively southwards. As a result, the older beds are progressively more deeply buried towards the south, and younger beds are encountered southwards at the surface. The clearest exposures of this fairly continuous sequence of southward-dipping strata are seen south of the Awakino River estuary down the length of the western coast of Taranaki. The oldest strata encountered are Lower Miocene (Pareora Series) alternating beds of sandstone and mudstone. Further south is massive sandstone, followed by coal measures which contain seams that are worked in the Ohura Valley and the Mokau area; these are overlain by mudstones. Much andesite tuff occurs in the succeeding strata. Thousands of feet thickness of upper Miocene sandstones and mudstone are well exposed along the coastal section north of Waitara.
Oil seeps have long been known on the Taranaki coast at New Plymouth, since the early settlers first found petroleum floating on the sea near the volcanic Sugar Loaf Islands. The first drilling rigs were erected in 1865 and for many years a limited supply of petroleum has been won from pumps on the beach near New Plymouth. Many small companies investigated the region for oil, but large-scale operations did not begin until 1939, just prior to the outbreak of the Second World War, when deep exploratory boreholes were put down at Inglewood and Devon. Since 1959 a number of boreholes, reaching depths of as much as 13,000 ft at Kapuni, south of Mount Egmont, have revealed the presence of a reservoir of natural gas below the thick cover of Tertiary marine strata. Examination of fossil pollen grains from samples of this coal shows that the vegetation of these Taranaki coal measures was very similar to and contemporary with the coals of the Waikato.
The volcanic centres of Taranaki are of two ages. The northern and eastern group, comprising the Sugar Loaves, Whareorino, and Pehimatea, are of early Pleistocene age and dacitic composition. The central Taranaki volcanoes, andesitic in composition, are three vents aligned along a single north-west — south-east line, all of upper Pleistocene and Holocene age; Kaitake being the oldest, then Pouakai, followed by Egmont. That Egmont was last active less than 360 years ago is shown by radiocarbon dating of charcoal from a Maori oven covered by the latest volcanic ash from Egmont.
Thick deposits of unsorted masses of volcanic debris which surround these volcanoes, forming the Taranaki Ring Plain, are interpreted as volcanic mudflow deposits (lahars) that swept down the mountain slopes during Pleistocene times. The surfaces of some of the upper lahars are marked by clusters of hillocks composed of huge blocks; these lahar mounds are particularly numerous in the Inglewood and Opunake areas.