The late Cenozoic history of McQuesten map area is characterized by progressively less extensive glaciations and deteriorating interglacial climates. The glaciations, from oldest to youngest, are the pre-Reid (a minimum of two early to mid Pleistocene glaciations), Reid (>200 ka), and McConnell (<29.6 ka BP). Pre-Reid interglacial reconstructions suggest a much warmer and more humid climate than today. The Koy-Yukon interglacial (200 ka) is considered to have a climate similar to a southern boreal forest and the first intact Diversion Creek paleosol, from this period, is documented in the McQuesten River valley. The Stewart neosol (Holocene) is widespread and poorly developed in comparison to past interglacial soils. The distribution of surficial deposits, related to multiple glaciations, physiography, and fluvial order contrasts, may govern the distribution of placer gold occurrences in the study area. Placer deposits occur anomalously in areas outside the pre-Reid limit on Klondike Plateau, and on Stewart Plateau. A copy of this thesis is available at the EMR library – QE195.B65 1997. This thesis is available online at https://www.collectionscanada.gc.ca/obj/s4/f2/dsk2/ftp04/mq21154.pdf

Sedimentological analysis of Quaternary sediments along the Stewart River near Mayo, central Yukon Territory, is used to determine the environmental history of the region. Thirteen sedimentary facies are defined and interpreted. Seven stratigraphic units are interpreted from these facies and they form five informal chronostratigraphic intervals: interglacial, proglacial, glacial, immediate postglacial and Holocene. A copy of this thesis is available at the EMR library – QE696.C376 1993. This thesis is available online at https://doi.org/10.7939/R3222RG09.

The McQuesten River region in the northern part of the McQuesten and Mayo map areas (scale 1:250 000) is underlain by Upper Proterozoic to Mississippian rocks that were deposited in an offshelf setting during the formation of the northern Cordilleran continental margin, deformed during the Mesozoic, and intruded by pre and post-kinematic intrusions. The Selwyn Basin phase of evolution of the continental margin is represented by rock units that correlate with units defined in the eastern part of Selwyn Basin. Dark clastic and rare felsic metavolcanic rocks of the Deconian-Mississippian Earn Group unconformably overlie rocks of the Selwyn Basin phase and are overlain conformably by the Mississippian Keno Hill quartzite. Dark, fine-grained metaclastic rocks of unknown age locally overlie Keno Hill quartzite. Four episodes of plutonism can be distinguished in the area, the earliest probably Early Paleozoic in age, another mid-Triassic in age, and two phases of Cretaceous granitic magmatism. Early Paleozoic bodies are typically metre-scale, fine-grained diabasic dikes and sills intruding rocks of the Hyland Group. Mid-Triassic diorite to gabbro occurs in discontinuous pods of various sizes, primarily in the Tombstone Thrust sheet where they intrude Devonian and Mississippian rocks. The most voluminous and widespread granitic rocks are the early Late Cretaceous Tombstone intrusions (92 ± 2 Ma). Typical Tombstone intrusions are weakly porphyritic, medium-grained hornblende-biotite granite to granodiorite, but they range from syenite to granodiorite and are locally peraluminous. The latest episode of granitic magmatism, the 65 ± 3 Ma McQuesten intrustions, is not yet fully delineated but includes five stocks of peroluminous potassium feldspar megocrystic granite. Paleozoic and Mesozoic structures occur in the region. The Sprague Creek Fault, a pre-Late Cambrian normal fault, is inferred from stratigraphic relationships. A possibly Jurassic phase of shortening is represented by west-northwest-trending, south-vergent folds that pre-date Jura-Cretaceous structures. The most pervasive and important phase of deformation is Jura-Cretaceous in age and kinematically complex. The Robert Service and Tombstone thrusts and Tombstone Strain Zone formed between the Late Jurassic and early Late Cretaceous during northward and northwestward displacement of more southerly hanging wall rocks. The McQuesten River region has numerous mineral occurrences, a long history of mining and mineral exploration and good potential for further discoveries.

This bulletin describes the surficial geology and Quaternary history of the Stevenson Ridge and northern Kluane Lake map sheets, an area covering more than 12 000 square kilometres of southwestern Yukon. The map area straddles the Pleistocene glacial limits that formed the eastern margin of Beringia during glacial periods. As a result, the landforms of this region are diverse and reflect the contrasting geomorphology characteristic of glaciated versus unglaciated terrain. Importantly, work in this region filled a significant gap in our knowledge of central Yukon glacial limits and how adjacent landscapes were impacted by the Cordilleran Ice Sheet. Our goal was to produce an in depth description of the region’s surficial geology that is applicable for a wide range of purposes. While this region is remote, there is growing interest in its mineral endowment, specifically in the Dawson Range, east of the White River, and in the Ruby Range. Knowledge of the surficial geology can be employed in exploration geochemistry programs to improve sampling and can assist in placer exploration. Future development in the region will also benefit from this work by providing a framework on terrain hazards, such as permafrost and landslides, and construction materials like aggregate. From a land use planning perspective, the surficial sediments and associated soils are an important control on the region’s biogeography and therefore, habitat. Hundreds of soil pits were hand excavated during this program and this work is reflected in the detailed descriptions within the bulletin and on the maps.

The tectonic evolution of the Whitehorse trough in central Yukon is largely preserved by the Early to Middle Jurassic Laberge Group, an ~3000-m thick succession of synorogenic clastic strata that unconformably overlies arc and arc marginal rocks of the Lewes River Group. A two-year project was initiated to test a Sinemurian to Toarcian transgression of basal Laberge Group strata westward across the Whitehorse trough and examine the regional relationships between the timing of Jurassic exhumation, sedimentation, and terrane accretion in the northern Canadian Cordillera. Field studies in 2017 targeted basal Laberge Group strata at seven locations in central Yukon. At each field locality, basal Laberge Group strata are known or inferred to unconformably overlie the Povoas formation and multiple units of the Aksala formation. Pre-Early Jurassic unconformities may indicate variable basin topography due to the complex internal stratigraphy of the Lewes River Group, or that regional exhumation and erosion affected the Whitehorse trough prior to Laberge Group sedimentation.

A lithologic succession is recognized in tectonites of the eastern Teslin suture zone in south-central Yukon. Metagraywacke and quartzite, marble, mafic metavolcanics, and interbedded metagraywacke and argillite outcrop on both limbs of an upright northwest-trending syncline at Little Salmon Lake. A body of equigranular granodiorite intrudes the basal stratigraphic units. The granodiorite and its host sediments were penetratively deformed during top-to-the-SW shearing and greenschist facies metamorphism. The granodiorite gives a Devono-Mississippian U-Pb zircon age (353 +1.3/-1.4 Ma) which is interpreted as the minimum age of crystallization. This provides a minimum depositional age for these suture zone protoliths. Based on the sedimentary succession and the age constraints, the eastern suture zone protoliths show a clear genetic link to other pericratonic terranes in the northern Cordillera.

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The study area is underlain by four stratigraphic successions ranging in age from Middle Proterozoic to Early Paleozoic. From oldest to youngest, they are: Middle Proterozoic Wernecke Supergroup; Middle to Upper Proterozoic Pinguicula Group; Upper Proterozoic Windermere Supergroup; and Uppermost Proterozoic to Lower Paleozoic sandstone and carbonate. Together, they represent about a billion years of intermittent sedimentation punctuated by processes such as deformation, uplift, erosion, magmatism and mineralization. Rocks in the study area record eight phases of contractional and extensional deformation, some of which may be related to strike-slip faulting. Two phases of southwest-verging folds and thrust faults may be related to dextral transpression on the Snake River Fault. Mineral enrichments occur in two general forms:: breccia-related (Middle Proterozoic), and veins (Mesozoic to Tertiary). The breccia-related occurrences have enrichments of Cu ± U, Co, Au and Ag, as dissemminations and veinlets in and near intrusive breccia zones (Wernecke breccia). The vein occurrences comprise Zn-Pb-Ag ± Cu and Au, in veins and related lenses and irregular replacements of carbonate.

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Recent field investigations have improved our knowledge of the Quaternary surficial geology, stratigraphy and glacial limits in the McQuesten map area. This information has important applications to surficial geochemical and placer exploration. The Quaternary geology of this area is unique because it encompasses early to middle Pleistocene (pre-Reid) glacial surfaces that are preserved beyond the limit of the Illinoian (Reid) glacial limit. These pre-Reid surfaces have been exposed to long periods of weathering and erosion, which have diminished their original distribution and expression. Stratigraphic exposures examined in the map area provide new evidence for a large glacial lake(s) in the Lake Creek basin (‘glacial lake Coldspring’); the lake developed when pre-Reid ice dammed outlets in the Willow Hills and lower Lake Creek. In addition, there is evidence that another large glacial lake (‘glacial lake Rosebud’) formed on the west side of the White Mountains when a pre-Reid glacier dammed Rosebud Creek. Fieldwork in the White Mountains and on Australia Mountain allowed us to delineate the pre-Reid glacial limit at approximately 1000 m (3300-3400 ft) a.s.l. This elevation is lower than the pre-Reid glacial limit previously mapped for the area by Duk-Rodkin (1999) and is consistent with mapping performed in the adjacent Stewart River map sheet by Bostock (1964), Jackson (2005a,b) and Froese and Jackson (2005).