Abstract: Surficial geology mapping of the lower Stewart River valley has revealed sets of high level terraces formed when the paleo-Stewart River and the paleo-Yukon River were at higher base levels during the Pleistocene glaciations and during preglacial time. These terraces are composed of bedrock and a variety of gravelly alluvial fill, some of which are glacial in origin, others appear to be nonglacial in origin. The terraces are dominated by pebble and cobble gravel deposits which are typically covered by aeolian sand and silt deposits.

The age of these high level terraces was determined by paleosol development, height, and relationship to glacial limits. At best, these are crude correlations which serve to model drainage evolution. For example, inferences can be made about the aggradation of these main valleys. A regional base level was established possibly in late Tertiary time which is identified as a bedrock terrace level above the present flood plain level. The timing of this feature is probably older than White Channel gravel age (Pliocene-Early Pleistocene). Subsequent aggradation of valley fill gravel in the Stewart River drainage followed. The style of deposition is considered nonglacial for this highest alluvial surface. The main rivers incised their valleys due to a lowering of base level; perhaps due to tectonic uplift and/or isostatic readjustment subsequent to a long period of stability.

Placer gold distribution on the terraces appears to be widespread. Gold grains are typically flat, smooth and smaller than 1 mm. Gravel sampling of lithofacies was conducted on these high level terraces followed by concentration of heavy minerals by sluicing and panning. Gold is present on many of these terraces. Favourable targets for placer exploration may be in tributary valley terraces which grade to the main trunk stream high level terraces or basal gravel overlying bedrock. Erosion of outwash terraces is believed to contribute gold to modern bars along the Stewart River.

Authors: Fuller, E.A.

Map Scale: 1 : 0

NTS Mapsheet(s): 115I, 115J, 115O, 115P

Citation: Fuller, E.A. , 1994. High level terraces along Lower Stewart River and parts of Yukon River. In: Yukon Exploration and Geology 1993, Yukon Geology Exploration, Indian & Northern Affairs Canada/Department of Indian & Northern Development: Exploration & Geological Services Division, 15-28.

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