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Geology of Mount Kobau Okulitch, Andrew Vladimir

Abstract

Rocks of the Kobau Group occur between the Okanagan and Similkameen Valleys in southern British Columbia and northern Washington. The Group consists of quartzite, phyllite, greenstone and minor limestone deposited within a synorogenic, eugeosynclinal environment in pre-Cretaceous, possibly post-Devonian, time. Intrusion and extrusion of basic igneous rocks accompanied deposition. The observed succession has been divided into nine units with total original thickness under 5,000 feet. Earliest recognized deformation of the Group formed tight recumbent folds with easterly trending axes. Transposition of compositional layering to foliation and extensive shearing occurred at this time and was accompanied by regional dynamothermal metamorphism which attained the middle subfacies of the greenschist facies. Later (second phase) deformation produced overturned and normal folds with steep axial planes and south-easterly trending axes, and refolded early recumbent structures. Several quartz latite dykes cut the Group during or shortly after late folding. Emplacement of granitic and dioritic stocks with radiometric ages of 144 x 10⁶ years or less followed second phase folding. Contact metamorphic zones of varying extent are present around larger intrusive bodies and attain the hornblende-hornfels facies. A latest (third) phase of deformation about poorly defined northerly trending axes may be related to this intrusive episode. A number of dacite and basalt dykes intruded both stock and country rocks. Extensive fracturing during Tertiary time broke preexisting structures into numerous blocks and wedges. Fractures parallel axial planes of early and late folds as well as faults in the Okanagan Valley. Relationships between the Kobau Group and rocks in adjacent areas are unknown. The Group possesses lithologic and structural similarities to parts of the Shuswap Complex and may share some of the complex's history. Part of the southerly adjacent Anarchist Group may be correlative with the Kobau Group.

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