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Ministry of Energy Mines and Responsible for Core Review

Geoscience Map 2002-5: Geology of the Penticton Tertiary Outlier,
British Columbia (NTS 082E/5)

by B.N. Church


View Map (PDF 3.66 MB)

Scale 1: 50 000


The Penticton Tertiary outlier is characterized by low mountainous terrain bounded by the Okanagan valley on the east (elev.~350 m) and the Similkameen and tributary valleys on the west (elev.~550 m). From the White Lake basin (elev. ~ 530 m), in the east-central part of the Penticton outlier, slopes rise gently to an almost complete ring of hills that includes Mount Orofino (elev. 1590 m) to the southwest. The concordant summits northwest and southwest of White Lake are a remnant of a once continuous upland surface that comprises the southern extremity of the Thompson Plateau.


The principal mineral resources in the Penticton Outlier are the Dust Mac mine (MINFILE No. 082ESW078) near Okanagan Falls that produced 93 000 tonnes of gold-silver ore from 1975 to 1977 and the Vault prospect (MINFILE No. 082ESW173) west of Skaha Lake containing an estimate gold reserve of 200 000 oz.


In the Okanagan - Boundary region the early Tertiary continental sedimentary and volcanic rocks and associated intrusions fill the Penticton and Kelowna half grabens and the Toroda Creek graben that bisects the Okanagan Highlands in the Midway - Rock Creek area. The Springbrook Formation (210 metres thick) at the base of the succession is a polymictic conglomerate containing clasts derived by stream erosion from a geologically diverse pre-Tertiary metamorphic terrane. This unit is followed by the Eocene Kettle River Formation consisting of rhyolite tuffs sandstones shales and minor conglomerates that are in turn overlain conformably by the Marron Formation (1 700 m thick) consisting of phonolite, trachyte, andesite and basalt lava flows and breccias. In the same area the Marama Formation (300 m thick) forms a series of dacitic lava domes above the Marron sequence. The White Lake Formation (1000 m thick) is a sequence of fluvial lacustrine, lahar and volcanic breccias developed unconformably on the Marron and Marama Formations. Completing the Penticton Group, the Skaha Formation is a mainly chaotic landslide breccia at the top of succession. The Penticton Group is overlain unconformably locally by the (Miocene) Olalla obsidian and rhyolite breccia. In the north Okanagan area near Vernon the felsic Penticton volcanics are replaced laterally by Eocene andesites and basalts typical of the Kamloops Group.


Structural control of the Tertiary outliers is related to northerly trending faults. These are important elements in a north-south stress scheme that is responsible for the many northerly-trending grabens found scattered throughout the southern interior of British Columbia from the Fraser River to the Rocky Mountains. The period 45 to 53 Ma witnessed intense volcanic and tectonic activity across the Cordillera. This period corresponds to northerly movement of the Pacific plate that produced oblique subduction of this plate under the North American craton. This stress engine was active throughout the Cordilleran during the Eocene. This resulted in a complex inter-relationship of shears tension faults and folds and the simultaneous development of grabens folding and thrusting in coastal areas the southern interior including the Penticton area and eastern British Columbia.


The Manuel Creek tuff is a newly discovered unit within the Penticton Group. The unit is a minor one occurring directly above and conformable with the Kitley Lake member of the Marron Formation (K/Ar biotite date 53.1 ~ 1.8 Ma). These rocks are mainly gray or beige, massive and characterized by small scattered black specks of biotite and/or amphibole. In thin section the rock consists mostly of glassy shards replaced by zeolites and clay. Accessory minerals include plagioclase sanidine quartz biotite amphibole and smectite. The Manuel Creek tuff and associated sedimentary facies has been traced southerly from a point 2 km north of the West Kootenay power line to about 3 km south of the power line in the southern extremity of the Penticton Tertiary outlier. The unit ranges up to 10 m thick, strikes on average from 010 to 015 degrees and dips 15 to 30 degrees easterly. The related waterlain facies contain local concentrations of carbonaceous woody trash and fossil leaves. At the base the unit is intercalated with buff coloured siltstone and sandstone eroded from the underlying Kitley Lake member. Laterally the tuff is mixed with lapilli and breccia and interbedded brown sandstone containing lithic clasts derived from the Kearns Creek basalt. The Manuel Creek beds represents an estimate resource of 1.6 million tonnes of medium to high grade zeolitized tuff.


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