Geological map of New Zealand 1:50,000 Sheet J36 BD and part Sheet J35 Clearwater

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Oliver, P.J.; Keene, H.W. 1990 Geological map of New Zealand 1:50,000 Sheet J36 BD and part Sheet J35 Clearwater. Lower Hutt: New Zealand Geological Survey. Geological map of New Zealand 1:50,000 J36 & pt J35 1 fold. map + 1 booklet

Abstract: The Clearwater Area is dominated by Quaternary glacial deposits in the major valleys surrounded by glacially-sculptured hills of basement rocks of the Torlesse Supergroup. The torlesse rocks are represented by an altering, flysch-like sequence of dark grey siltstone and grey sandstone with rare conglomerate and the red and green siltstone of the Balmacaan Formation (Mount Taylor Group, Triassic) and coarse, grey-white, concretionary sandstone and dark grey siltstone of the Fingers Formation (Mount Taylor Group). Grey sandstone, dark grey siltstone, and thick conglomerate of the Clent Hills Group (Jurassic) contain plant fossils. Hewson Formation conglomerates and sandstones of Torlesse provenance uncomfortably overlie the Torlesse Supergroup. Mount Somers Volcanics (mid Cretaceous) are represented by Barrosa Andesite, Surrey Hills Tuff, Somers Rhyolite, Hinds River Dacite, Rata Peaks Rhyolite and Stew Point Andesite. The andesite and dacite occur as flows with interlayered tephra. The rhyolites, which occur as domes, stratigraphically overlie the andesite. Remnants of a sequence of Tertiary age outcrop in the southwest of the mapped area. This comprises basal Stour Coal Measures (Haumurian to Teurian), grading up into quartzose Blondin Sand (Waipawan to Bortonian). These two members belong to the Broken River Formation and are overlain by the Kauru Formation, Waihao Greensand, and Homebush Sandstone, all of the Bortonian age. The Nessing Greensand and Amuri Limestone (Runangan to Whaingaroan) occur in laces, and are overlain by Kokoamu Greensand and Otekaike Limestone. Eruptions of Brother Basalt were contemporaneous with limestone depositions in Dunstroonian to Waitakian times. The uppermost tertiary formations are Swin Sandstone, Tokama Siltstone , and Curiosity Shop Sandstone. The area is mantled by glacial till and outwash from a series of glacial advances in the late Quaternary. The two oldest are mapped as pyramid formations and Dogs Hill Formation. The three youngest are mapped as deposits from the clearwater lobe of the Rangitata glacier (Trinity Hill Formation, Hakatere Formation, and Spider Lakes Formation), and the Lake Heron Lobe of the Rakaia glacier (Emily Formation (new), Johnstone Stream Formation, and Lake Heron Formation (new)). Recent deposits comprise landslide deposits in lateral moraine on over-steepened hillsides. Solifluction and slopewash deposits of at least early Otiran to Recent age are in many places overlie old glacial drift and are usually thickly mantled by loess. Present day alluvium occurs in most stream and river beds. Several sets of lake silts form terrace surfaces upstream from the Rangitata Gorge as far back as Rawtor Creek. Several active fault traces are mapped; the largest being the Lake Heron Fault with approximately 35m of vertical displacement in the older till deposits. The potential for further movement on active faults in the area is relatively high. Limestone, coal. kaolin clay, and silica sand are the most important economic aspects in the area. (auth)

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