Bulletin 61: Geology of the White Lake Basin
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Bulletin 61 describes the volcanic geology of the White Lake basin. The object of this study was to establish the stratigraphy, structure, and petrology of Early Tertiary rocks in the White Lake area near Penticton, British Columbia.
This was achieved by field mapping and laboratory work performed mainly as a thesis study for the Department of Geology, University of British Columbia.
Early Tertiary rocks of the White Lake area consist of five main stratigraphic subdivisions, which are from the base as follows:
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 interdigitated with lahar and pyroclastic
5. local deposits of slide breccia and some volcanic rock overlain by fanglomerate beds.
The maximum aggregate thickness of the sequence is about 12,000 feet, and it is all believed to be of Eocene age. Each subdivision rests with some angular or erosional unconformity on older rock. The preservation of the sequence from erosion is partially explained by regional downfaulting. The greatest downward movement is near the Okanagan Valley where it is estimated that basal beds locally exceed depths of 5,000 feet below mean sea level. 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 flexures in the basement rocks.
Petrographic and chemical data indicate 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 'A' and 'C' series rocks 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.
The Dusty Mac gold-silver prospect is the main mineral deposit in the area. A detailed investigation shows that mineralization of the White Lake Formation near Okanagan Falls accompanied a pulse of silicification prior to a period of movement and brecciation on faults and shear zones.
Study of the volcanic rocks of the White Lake Basin leads to consideration of general problems of classification of volcanic rocks, particularly fine-grained rocks, and hence a new classification scheme is presented.
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