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Geology of the northwest quarter of Whitehorse map-area, Yukon, and studies of weathered granite rocks near Whitehorse

University of British Columbia

The northwest quarter of Whitehorse map-area occupies about 1000 square miles in southwestern Yukon. The southwest part of the area lies within the Coast Range and the northern and eastern parts are in the Yukon Plateau but the boundary between these physiographic provinces is very indefinite and the physiography of the area is best classed as transitional. The valleys are mostly broad and steep walled; a few are narrow and U-shaped. The plateau or upland surface, found throughout most of the Yukon Plateau, is here represented by broad ridges and by a few small tablelands averaging 5000 to 6000 feet in altitude, and these are surmounted by groups of mountains up to about 7000 feet in altitude. Some of the mountains have mature profiles but others, showing the effects of glacial erosion, are more rugged. During the Pleistocene the area was overridden by an icesheet to an altitude of at least 6000 feet but, except for cirques on some of the mountains, most evidence of glacial erosion is confined to the valleys and ridges below 5500 feet above sea level. The oldest rocks exposed in the area are Upper Triassic sediments and volcanics of the Lewes River series. They are overlain, apparently with slight unconformity, by Jurassic clastic sediments of the Laberge series. A small area of coal-bearing rocks probably belongs to the Tantalus formation. Acid and basic volcanics of the Hutshi group overlie the Laberge series unconformably along the northern border of the area. Rocks of all the above groups are intruded by the Coast Range granitic intrusions. Greenstones and more highly metamorphosed rocks associated with the main intrusive complex appear to have been derived mainly from the Lewes River series. Flows of fresh basalt exposed in the southern part of the area are believed to be Tertiary in age. The second part of the report deals with weathered granitic rocks found near Whitehorse. The weathering, which in some places extends several tens of feet below the outcrop surface, has resulted mainly in mechanical breakdown of the rocks through the development of fractures, although there has also been a little decomposition such as clouding of feldspars and rusting of biotite. The fracturing and decomposition shown by six weathered samples is described in detail in the report. Evidence regarding the relation of the weathering to the glaciation is inconclusive and hence it is not known if the weathering is connected with the cool dry climate now prevailing at Whitehorse. Certain granitic rocks of the area appear to weather more readily than others. A study of mineralogy, texture and porosity of the main types of granitic rocks of the area shows that those granitic rocks that appear to be especially susceptible to weathering contain larger pores than less readily weathered granitic rocks, and it is suggested that this feature may partly explain the observed difference in weathering.

Text

2012-03-13

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http://hdl.handle.net/2429/41368

Geological Sciences

University of British Columbia

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