Quaternary geology of northwestern Canterbury Plains. Scale 1:100,000

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Wilson, D.D. 1989 Quaternary geology of northwestern Canterbury Plains. Scale 1:100,000. Wellington: Department of Scientific and Industrial Research. Miscellaneous series map / New Zealand Geological Survey 14 1 fold. map + 1 booklet

Abstract: To the west of the Canterbury Plains, the Southern Alps and higher foothills comprise intensely folded and deformed Triassic greywacke and argillite (Torlesse Supergroup). The lower foothills are composed of Cretaceous calc-alkaline volcanic rocks (Mount Somers Volcanics), an Upper Cretaceous to Eocene transgressive sedimentary sequence with interbedded basic volcanics (Eyre Group), thin chalk (Oxford Chalk), and Miocene sandstone, volcanogenic sediments, and interbedded basalt flows (Burnt Hill Group). The northwestern part of the Canterbury Plains consists of predominantly Quaternary age fluvial gravel, sand, and silt deposits (Kowai, Hororata, Woodlands, Windwhistle, Burnham, and Springston formations) derived from the greywacke and argillite of the Southern Alps. Two major river systems, the Waimakariri and the Rakaia, have built out fluvioglacial fans and with progressive downstream aggradation formed a series of surfaces extending over 50 km to Banks Peninsula. Climatic fluctuations with alternating temperate and cold periods (glacial conditions in the Southern Alps) controlled a diverse range of fluvial processes and sediment deposition. The Waimakariri and Rakaia rivers have aggraded across the entire width of the Canterbury Plain during glacial maxima, and incised a seaward-developing trench, typically 2 km wide, during each interglacial period. Flow regimes and river gradients throughout the late Quaternary were controlled by three main factors: sediment load, sea level, and coastline position. Waimakariri River terrace sequences indicate that the dominant control of river gradient during glacial periods was sediment load; but during interglacials, sea level and sediment load were of equal importance. Between the Waimakariri and Rakaia rivers, the Selwyn River and its tributaries drain the foothills and plain. To the north of the Waimakariri River, the Eyre and Cust rivers occupy the trough between the Waimakariri and Ashley rivers. The Ashley River catchment has never been glaciated, but periglacial climatic conditions have contributed to erosion in the foothills, and therefore to high river loading, with consequent gravel aggradation during glacial maxima. Three active fault traces displace Torlesse rocks in the Kowai River - Little Kowai River - Joyces Stream area. All are related to the Porters Pass Fault Zone.

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