Scheme F, DG3 : General geology of the affected area

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Thomson, R. 1977 Scheme F, DG3 : General geology of the affected area. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 296 20 p.

Abstract: The proposed scheme F DG3 dam will create a 3 – armed reservoir within the marginal to the existing valleys occupied by the Clutha and Kawarau Rivers. The segment upstream from the dam will be confirmed to the narrow Cromwell Gorge, a steep-sided valley incised in the flanking mountain ranges during their elevation by tectonic movements some two million years ago. Tilting of the faulted and elevated blocks at this time created parallel, asymmetric, topographic lows, one of which is occupied by the valley near Cromwell. Tertiary lacustrine sediments previously deposited on the schist basement were protected from erosion within the depressions and subsequently mantled by fluvioglacial alluvium derived from a series of glacial advances in the catchment heads. The upper two arms of the lake will occupy narrow to broad valleys recently cut in the outwash gravels by the degrading river systems. A number of rock types will be influenced by the lake. The predominant varieties are schist, outwash alluvium, lacustrine sediments, and mass movement deposits. Some shoreline erosion is expected in the latter three categories and slumping may occur within the Tertiary sediments in the Kawarau Arm, but no areas have been defined where large-scale landsliding will be induced or accelerated by the reservoir. Investigations to-date at the DG3 damsite suggest that the foundations will be largely in competent schist. One fault has been detected along the river bottom but other defects along the preferred centre line appear to be insignificant. Small faults and many crush zones of indeterminate origin have been encountered in drill holes in the vicinity and an area of weak rock downstream from the site and upstream from the right abutment; geometry and rock properties are being investigated by surface and subsurface excavations. An area of recent faulting exists 5 km north-east of the damsite. The latest movement may have been about 30,000 years ago. During the last 150,000 years the rock ridge north of Clyde has apparently been uplifted 230 m as indicated by a displaced terrace remnant of outwash alluvium. Earthquakes currently being generated by tectonic movements in Central Otago are small in magnitude and are unlikely to produce significant damage to the dam or the reservoir margins. It is expected that the greatest risk of shaking will come from large events in Fiordland. Reservoir - induced seismicity is unlikely due to the small volume and low head of the DG3 reservoir, but cannot be discounted. Some comment is included on the effect of the reservoir on the potential mineral resources of the region, although time has not permitted any detailed study to be made of specific minerals that may prove economic in the future. (auth)