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Geology of the Upper Triassic Nicola group in south central British Columbia Schau, Mikkel Paul

Abstract

The rocks of the Upper Triassic Nicola Group represent the altered broken remnants of a volcanic island archipelago. About 23,000 feet of Upper Triassic sediments and volcanic rock are present in the thesis area. The group is the product of two cycles of deposition distinguishable on the basis of relative abundance of phenocrysts in the volcanic members. Rocks of the lower cycle (P), characterized by abundant feldspar phenocrysts, are separated into a lower, 6,000 foot thick assemblage of flows, breccias, and tuffs composed mainly of basaltic andesite; and an upper, heterogeneous, 8,000 foot thick assemblage composed of andesitic and rarer basaltic or dacitic agglomerates, breccias, tuffs, flows, and associated sediments as well as coral reef bodies. Rocks of the upper cycle (A), characterized by abundant augite phenocrysts, are separated into a lower assemblage about 7,500 feet thick composed of flows, breccias, tuff, and greywacke with rare limestone; and an upper assemblage about 1,600 feet thick composed of tuff, argillites, and cherts. During the Lower Jurassic, the group was gently folded, variably fractured, and intruded by a sequence of plutons culminating in the near-surface emplacement of huge batholiths. Metamorphism and metasomatism associated with batholiths altered the rocks so that they are now composed of low-grade metamorphic mineral assemblages. By mid-Mesozoic time, the plutons were exposed and a conglomerate laid down. Faulting along previous zones of weakness affected area during late Mesozoic time. Eocene deposits formed in an environment of hills and swamps. By late Miocene, after some jostling of faults blocks, the main topographic features of the Interior Plateau were fixed. Subsequent outpouring of Plateau and Valley basalts as well as glacial erosion and deposits have only mildly affected the main topographic features. The Nicola Group provides a chapter on the evolution of the Cordillera. The relationships provide certain limits to events that occurred outside the thesis area. Equally, relationships within the thesis area cannot all be answered without reference to larger areas.

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