The geology of Bruce Bay-Haast River, south Westland

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Wellman, H.W. 1951 The geology of Bruce Bay-Haast River, south Westland. Christchurch: Caxton Press. New Zealand Geological Survey bulletin 48 36 p.

Abstract: The geology of 250 square miles of rugged country between Haast and Mahitahi rivers is described. The rocks form three well-defined groups: (c) Pleistocene and Recent; (b) upper Cretaceous, lower, and mid-Tertiary; (a) undermass; greywacke, granite, and schist. An old, but still active, major tectonic feature, the Alpine fault, divides the area into two almost equal parts. South-east of this fault a regular belt of probably pre-Permian schist is overlain only by thin Pleistocene and Recent sediments. North-west of the fault a greywacke undermass is strongly deformed, and locally changed to hornfels by discontinuous intrusions of granite. About half the undermass north-west of the fault is overlain by upper Cretaceous and Tertiary beds. The oldest of these beds are breccia and coal measures similar to the Hawk Crag breccia and the Paparoa and Brunner coal measures of the Greymouth district. The coal measures are many thousand feet thick and well indurated, the coal being of medium and high volatile bituminous rank, but either too thin or too disturbed to be economically important. Two distinct Tertiary formations overlie the coal measures. These beds contain few fossils and their age is uncertain. The lower of these two groups of beds, named the Otitia formation, consists of limestone, tuff, agglomerate, and greywacke breccia, with a few fossils of mid-Tertiary age. The unconformably overlying beds of uncertain but probable Miocene age, named the Tititira formation, are composed of sandstone, calcareous mudstone and breccia. No upper Tertiary beds are preserved, but the lower Pleistocene is represented by over a thousand feet of slightly consolidated moraine, fossilferous marine silt, sand, gravel and peat. It is named the Paringa formation and is similar to beds 38 miles north-east at Pug Creek. The Pleistocene beds provide evidence for two periods of glaciation separated by an interglacial period, and for isostatic changes in sea level caused by variations in the weight of ice. (auth)