Late cenozoic geology, Ancient Pacific Margin NATMAP Project, report 3: a re-evaluation of glacial limits in the Stewart River basin of Stewart River map area, Yukon Territory

Investigation of stream-gravel lithology, distribution of erratics and high terraces, and natural exposures of terrace gravels along Stewart River have generally corroborated the all-time limits of glaciation mapped by H.S. Bostock in the Stewart River basin of the Stewart River map area. Stream gravels beyond these limits completely lack lithologies associated with the Selwyn Basin. Placer deposits beyond these glacial limits formed from fluvial concentration of gold sources occurring in those basins. The upper reaches of streams spanning glacial limits may contain undisturbed placers, whereas lower reaches may contain placers formed following glaciation, and buried placers. Bostock’s unpublished notes have proven to be a valuable aid in mapping the surficial geology of the area. 2001-A3 L.E. Jackson, Jr. 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 Terrain Sciences Division, Geological Survey of Canada, 101-605 Robson Street Vancouver, British Columbia V6B 5J3 3 Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6 INTRODUCTION R surficial geology mapping and investigation of the late Cenozoic stratigraphy of Stewart River map area (115-O and N) and the immediately adjacent areas of Stevenson Ridge map area (115 J and K) continued during the 2000 field season. One of the goals of the 2000 field season was to resolve the all-time limit of former glacial cover in the portion of the Stewart River basin contained in Stewart River map area centered on 1:50 000 NTS map areas 115-O/6 to115-O/11 (Fig. 1 and 2). Placer gold has been mined from Stewart River tributaries such as Barker, Black Hills, Maisy May, and Scroggie creeks for up to 100 years (Cairnes, 1916). Investigation of the distribution of glacial erratics and the composition of placer gravels in the Barker Creek area during the 1999 field season indicated that this area was never invaded by the Cordilleran Ice Sheet (Jackson and Huscroft, 2000). This conclusion CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter 2001-A3 L.E. Jackson, Jr. et al. 2 Résumé L’étude de la lithologie des graviers alluvionnaires, la répartition des blocs erratiques et des hautes terrasses et les affleurements naturels de graviers de terrasse le long de la rivière Stewart corroborent en général les limites de glaciation de tous les temps cartographiées par H.S. Bostock dans le bassin de la rivière Stewart dans la région cartographique de la rivière Stewart. Les graviers alluvionnaires rencontrés au-delà de ces limites ne comportent aucune unité lithologique associée au bassin de Selwyn. Les placers présents au-delà de ces limites se sont formés suite à la concentration fluviatile de l’or provenant de sources présentes dans ces bassins. Les tronçons amont des cours d’eau se trouvant à l’intérieur de la limite maximale d’avancée glaciaire pourraient contenir des placers non remaniés alors que les tronçons aval pourraient contenir des placers formés après la glaciation et des placers enfouis. Les notes inédites de Bostock se sont avérées être une aide précieuse pour cartographier la géologie de surface de la région. contradicts glacial limits mapped in this area by Duk-Rodkin (1999; and glacial limits in Gordey and Makepeace (1999)), but corroborates those established by Bostock (1966). Figures 2a and b are plots of these conflicting glacial limits. Glaciation has the capability of either destroying placer deposits through erosion, or preserving them through burial. An understanding of the geomorphic history of the Stewart River map area, including glaciation, is essential to understanding the origin and evolution of gold placers and exploration for new placer resources. This paper describes field investigations of glacial limits in the portion of the Stewart River basin contained in the Stewart River map area. REGIONAL SETTING T portion of the Stewart River basin within the Stewart River map area is underlain by high-grade metamorphic rocks, intrusions or structurally emplaced bodies of ultramafic rock, and erosional remnants of regionally erupted mafic lava of the Carmacks Group (Bostock, 1942). Bedrock exposures are rare. Thick, residual, colluvial overburden covers hillsides and most ridge tops, and gravel, loess, and organic fills have accumulated in many valley bottoms. Permafrost is nearly continuous throughout the area. Vegetation is dominated by dense boreal forest and brush that mantles most slopes from toe to ridge-top. This thick vegetation commonly makes foot traversing difficult and helicopter landings nearly impossible over much of the area. 2001-A3 L.E. Jackson, Jr. et al. 3 CURRENT RESEARCH RECHERCHES EN COURS 2001 Article Article Titles Titres Purchase Information Information pour acheter