The geology and eruptive history of the Table Mountain region, Coromandel Peninsula

Abstract

The Table Mountain region covers an area of 2,200 hectares, 17 kilometres north-east of Thames, and straddles the main Coromandel Peninsula Divide between the headwaters of the Kauaeranga and Waiwawa Rivers. It is a region of steeply dissected, bush clad slopes and rugged bluffs composed of andesite, rhyolite and sediments. These rocks belong to three Groups. The oldest group of rocks consists of andesite lavas, breccias and sediments that form the upper part of the Beesons Island Volcanics sequence and were erupted during the upper Miocene and lowermost Pliocene. Unconformably overlying these is the mid Pliocene Whitianga Group containing rhyolitic lavas and sediments. In the Table Mt. Region this Group has been divided into the Minden Rhyolites and two informal sedimentary formations. The Wainora Formation contains basal volcanic breccias and freshwater, carbonaceous, epiclastic sediments that were deposited in two lakes on the dissected surface of the older andesites. This formation contains impressions of fresh-water mussels and numerous leaves, as well as considerable amounts of silicified wood. Conformably overlying the Wainora Formation are the thicker and more extensive water and aerially deposited pyroclastic sediments and rarer ignimbrites of the Waiwawa Formation. Many of the water laid deposits are inferred to have been formed by hot pyroclastic flows entering a lake. Minden Rhyolite domes were produced, by endogenous and exogenous growth, towards the end of this phreatic eruptive period. Hydrothermal alteration is inferred to be closely associated with the four Minden Rhyolite domes of this region. During the upper Pliocene to lower Pleistocene, the Omahia Andesite Group was intruded. The narrow Waiwawa Intrusive came up along an old intrusive contact between a Minden Rhyolite dome and the Waiwawa Formation sediments. The large Table Mt. andesite mass is believed to have formed by a combination of upwelling of lava along a fissure and actual intrusion. Both the Waiwawa and Table Mt. Intrusives spilled small amounts of lava out over the surface as lava flows. In the two million years since the cessation of volcanic activity in this region, erosion has greatly altered the landscape and emphasized the harder rock masses.