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The geology of the Fraser Valley between Hope and Emory Creek, British Columbia Read, Peter Burland

Abstract

The Fraser Valley from Hope to Emory Creek is underlain by mainly regionally metamorphosed Palaeozoic and (?) Mesozoic sedimentary and volcanic rocks. The Chilliwack group consists of iron- and magnesium-rich pelitic schists and minor basic schists of the staurolite-quartz subfacies and to a lesser extent of the sillimanite-almandine subfacies. Much of the femic component of the original sedimentary rocks was used in the early formation of a biotite-almandine-sericite-plagioclase assemblage. As a result, a later assemblage of minerals (kyanite, staurolite, sillimanite, and muscovite) developed even though they represent minerals too alumina-rich to form in rocks of these bulk compositions. The Hozameen group, which consists of cherts, phyllites and greenstones of the greenschist facies, is composed of schists and amphibolites of the almandine amphibolite facies at its western boundary. The Custer granite-gneiss is an ancient complex of sedimentary and volcanic rocks. During the regional metamorphism of the Custer, the complex formed gneisses and pegmatites, became mobile, and, limited by the structure of the surrounding rocks, moved into an area of less intense metamorphism. The movement caused a shearing of the Custer to form augen gneisses and of the adjacent rocks to form phyllonites. Later release of stresses along definite planes caused a closely spaced faulting of the Custer and the wall rocks. A band of sheared quartz diorite and gneiss separates the Hozameen group from the Custer. The quartz diorite intrudes both units. Unmetamorphosed conglomerates of the Jackass Mountain group unconformably overlie the Custer. A north-trending fault of largely dip-slip movement which formed between Lower Cretaceous to Miocene times has down-faulted the rocks east of the Chilliwack group. The fault extends from Spuzzum to Chilliwack River and possibly into northern Washington. Its trace south of Hope has been partly obliterated by the younger Chilliwack batholith. At least two periods of folding have occurred within the map-area. An early period of folding is associated with the movement of the Custer (Jurassic?) and a later (post-Lower Cretaceous) period with the formation of a northerly trending anticline that occupies the centre of the Fraser Valley.

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