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Geology of the White Lake Area Church, Barry Neil

Abstract

The object of this study is to establish the stratigraphy, structure, and petrology of early Tertiary rocks in the White Lake area near Penticton, British Columbia. This is achieved by field mapping and laboratory work. Early Tertiary rocks of the White Lake area, thought to be mainly Eocene age, consist of five main stratigraphic division; 1. discontinuous beds of basal breccia and conglomerate, 2. a thick and widely distributed succession of volcanic rocks of diverse composition - mainly phonolite, trachyte, and andesite lavas, 3. discontinuous volcanic beds - mainly rhyodacite lava, 4. locally thick volcanic sandstone and conglomerate beds inter-digitated with lahar and pyroclastic deposits, 5. local deposits of slide breccia and some volcanic rock overlain by fanglomerate beds. Each division rests with some angular or erosional unconformity on older rock. Aggregate thickness of the Tertiary strata, where best developed, is about 12,000 feet. These rocks are regionally downfaulted accounting, in part, for their preservation from erosion. Greatest downward movement is near the Okanagan Valley where, in places, it is estimated that basal beds exceed depths of -5,000 feet (m.s.l.). In general, beds are tilted easterly as if rotated downward forming a trap-door-like structure. Locally, folds are developed but these are without regional pattern and may be the result of simple flextures in the basement rocks. Petrographic and chemical data indicates a three-fold division of igneous rocks: 'A' series - mainly plagioclase porphyries; lavas of rhyodacite and andesite composition; 'B’ series - mainly two feldspar porphyries with co-existing plagioclase and sanidine; lavas of trachyte and trachyandesite composition; 'C’ series - mainly anorthoclase porphyries; lavas of phonolite composition and some tephrite. Phase diagrams and subtraction plots indicate that rocks of 'A' and 'C’ series were probably formed by crystal fractionation. In the case of 'A' series, precipitation of mainly plagioclase and pyroxene from andesite produces rhyolite; and for 'C’ series, precipitation of mainly pyroxene and some biotite from tephrite produces phonolite. Rocks of 'B’ series are intermediate in composition to 'A' and 'C’ and were probably formed by mixing of magmas.

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