Late cenozoic geology, Ancient Pacific Margin NATMAP Project, report 4: paleomagnetic and geomorphic evidence for Brunhes-age volcanism, Fort Selkirk and Rosebud Creek area, Yukon Territory

Normally magnetized, valley-filling basalt flows extend more than 10 km down the Yukon River valley from the Fort Selkirk area. These flows are locally overlain by gravel and terminate at the level of the contemporary Yukon River flood plain, suggesting a middle to late Pleistocene age for this previously unrecognized eruptive event. Unlike other valley-filling phases of the Selkirk Volcanic Group, this eruption postdates the Pliocene to early Pleistocene pre-Reid glaciations. Normal magnetism was also determined for basalt flows underlying terraces in the area of the 2001-A4 C.A. Huscroft et al. 1 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 1 Contribution to the Ancient Pacific Margin NATMAP Project 2 Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6 3 Department of Geography, University of Lethbridge, Lethbridge, Alberta T1K 3M4 4 Terrain Sciences Division, Geological Survey of Canada, 101-605 Robson Street Vancouver, British Columbia V6B 5J3 Rosebud Creek–Grand Valley Creek confluence, 60 km to the northwest of the Fort Selkirk area. Based on their unique geomorphic and stratigraphic settings, the Rosebud basalt flows may represent yet another period of Pleistocene volcanism which predates one of the pre-Reid glaciations in the central Yukon Territory. INTRODUCTION T paper reports the results of paleomagnetic sampling of lava flows along the Yukon and Stewart rivers in the Carmacks and Stewart River map areas (115-I and 115 N) during the summer field season of 2000 as a part of the Geological Survey of Canada’s Ancient Pacific Margin NATMAP (National Geoscience Mapping Program) Project (Fig. 1). The objective of this work was to refine the stratigraphy CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 2001-A4 C.A. Huscroft et al. 2 Résumé Les coulées de basalte à aimantation normale qui remplissent les vallées s’étendent sur plus de 10 km vers l’aval du fleuve Yukon à partir de la région de Fort Selkirk. Ces coulées sont recouvertes de graviers par endroits et se terminent près de la plaine d’inondation actuelle du fleuve Yukon, ce qui permet de supposer que cette éruption auparavant non reconnue remonte au Pléistocène moyen à supérieur. Contrairement à d’autres phases remplissant les vallées du groupe volcanique de Selkirk, cette éruption est survenue après les glaciations du Pliocène–Pléistocène inférieur antérieures à la Glaciation de Reid. On a également observé que l’aimantation des coulées de basalte sous-jacentes aux terrasses était normale dans la région de confluence des cours d’eau Rosebud et Grand Valley, à 60 km au nord-ouest de la région de Fort Selkirk. D’après leurs contextes géomorphologiques et stratigraphiques uniques, les coulées de basalte de Rosebud représenteraient une autre période de volcanisme pléistocène qui date d’avant l’une des glaciations antérieures à la Glaciation de Reid dans le centre du Territoire du Yukon. established for Pleistocene sediments and interstratified basalts found in terraces in the Carmacks map area (Jackson et al., 1996) and extend it down the Yukon River valley into the Stewart River map area. The ultimate goal of this study is to constrain the age of Stewart River and Yukon River terraces. The age of Yukon and Stewart River terraces is significant in establishing the geomorphic context of placer deposits found in the study area. River terraces record periods of aggradation, stability, and incision. These changes in local base level have the capability of preserving, enriching, or destroying placer gold deposits in tributary drainages. Hence, an understanding of the fluvial history of an area is an essential tool in identifying geomorphic targets for exploration. Commonly, lack of datable material associated with the terraces within the Stewart River map area precludes the determination of their age. However, near Fort Selkirk and Rosebud Creek (Fig. 2), basaltic flow complexes of the Selkirk Volcanic Group (Bostock, 1936) are interstratified with glacial and nonglacial sediments. This stratigraphy provides a valuable opportunity to compile age control for aggradation of the Yukon and Stewart rivers. REGIONAL SETTING T study area is set between the glaciated and unglaciated environments of the Yukon Plateau, central Yukon Territory. The lava flows associated with the Selkirk Volcanic Group lie 60 km beyond the limit of the Late Wisconsinan age McConnell Glaciation, straddle the limit of the penultimate (Reid) glaciation (thought to have occurred 200–400 ka BP), but are within the limit of pre-Reid glaciations (Bostock, 1966; Jackson et al., 1996; Duk-Rodkin, 1999) that predate the last magnetic reversal ca. 0.8 Ma. The region is bounded to the south by the Dawson Range, and to the north by the Klondike valley and Ogilvie Mountains. The Yukon Plateau is a gently sloping upland consisting of concordant summits, locally termed “domes”, connected by long ridges separated by deeply incised V-shaped valleys. The plateau is considered to be an uplifted erosional surface produced as a result of extensive 2001-A4 C.A. Huscroft et al. 3 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter