Geology of the Ngaruawahia subdivision

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Kear, D.; Schofield, J.C. 1978 Geology of the Ngaruawahia subdivision. Wellington: Department of Scientific and Industrial Research. New Zealand Geological Survey bulletin 88 168 p.

Abstract: The most ancient rocks of the Ngaruawahia subdivision belong to the Hokonui System, and are of contrasting facies in the west and east. In the west some 18,000 feet of marine, tuffaceous sediments and volcanic greywackes are mapped within the Newcastle, Rengarenga, and KiriKiri Groups. They range in age from Oretian-Otamitan to Heterian. They are essentially normal shelf sediments deposited on the continental shelf margin of the New Zealand Geosyncline. In the east, rocks of the Manaia Hill Group are more indurated than those of the same age in the west and have increased startigraphic thickness. They are deposited nearer to the geosynclinal axis than those to the west, probably just beyond the foot of the continental slope, and they lack certain typically axial features. The Rangitata Orogeny was responsible for most of the present fold structures observable in Hokonui rocks. The next period of sedimentation did not commence until upper Eocene (Arnold) times, when the Waikato Coal Measures were deposited. Locally the Tertiary and Quaternary periods are represented by rocks of the shelf facies. totalling probably no more than 5,000 feet in thickness. The Te Kuiti Group, of upper Eocene to upper Oligocene, is the oldest and exhibits a classical sequence from coal measures pssing up into estuarine mudstones and sandstones, then open-water marine sandstone, mudstones and limestones which become more calcareous towards the topmost beds of the Group. The succeeding Waitemata Group comprises sandy Miocene sediments that become less calcareous upwards. Volcanism, hitherto locally absent during the Tertiary, appears as minor acidic volcanic glass culminating with an andesitic outbreak. During the subsequent deposition of the Tauranga Group, basalts, basaltic andesites, and andesites were erupted to form Pirongia Mountain (Alexandra Volcanics). However, the Tauranga Group is dominated by terrestrial sediments derived from rhyolitic provenances, from the Coromandel Range during the Waitotaran and mainly from the Rotorua-Taupo region in subsequent times. Deposits described include road metal, ceramic and bloating clays, pumice, cement materials, limestone, rare sulphides, and the possibility of oil and geothermal resources.