Geological interpretation of Ngawha deep drillhole, Kaikohe, Northland (N15), New Zealand

Abstract

The log of the deep drillhole at Ngawha Thermal Area, Northland, is interpreted as showing at least 1560 ft of Onerahi Chaos-breccia, the top 720 ft containing faunas which are generally older than those of the lower 840 ft. Between this unit and the underlying, highly indurated Permian-Mesozoic argillites of Waipapa Group, there is 170 ft of possibly allochthonous Eocene sedimentary rocks. The thickness of the Onerahi Chaos-breccia is the greatest yet measured for this unit. Resistivity values of 200 ohms may be identified with the base of the Onerahi Chaos-breccia, while seismic refraction profiles indicate a depth to ‘basement’ not significantly greater than the depth of Waipapa Group. A deeper basement inferred from gravity measurements can be neither verified nor refuted from the drillhole or regional geologic data. Minor hydrothermal alteration of the rocks cut by the drillhole suggests a volcanogenic source for some elements and for the heat, but either a rhyolitic or basaltic source is geologically possible. Both permeable reservoirs and the rise of the hot water to the surface are probably controlled by fault zones in the waipapa Group and the Onerahi Chaos-breccia, the latter forming a very impermeable caprock except where shattered by several post-Miocene faults. Present day thermal activity is almost totally concentrated on only one of these faults at the centre of Ngawha; this would be the only reasonable site for further geothermal exploration.