Preliminary results from the Stewart River stratigraphy and surficial mapping suggest a minimum of four glaciations and two interglacial periods. Methods of stratigraphy, paleomagnetics, soil analysis, tephra chronology, and relative geomorphic preservation are employed to differentiate and describe Quaternary events. Glaciations, from oldest to youngest, are the pre-Reid (multiple early to mid Pleistocene glaciations), Reid (> 200 000 years), and McConnell (14 000 - 29 600 years). Interglacials are represented by organic deposits from Stirling Bend and Ash Bend, in addition to Wounded Moose and Diversion Creek paleosols preserved on pre-Reid and Reid surfaces. During their maximum extent, pre-Reid ice sheets inundated the study area leaving isolated nunataks on Klondike Plateau and the northern part of Stewart Plateau near Syenite Range. North trending intervalley channels on Stewart Plateau represent confined ice flow in Stewart and McQuesten River valleys from ice obstructions in Tintina Trench. Undifferentiated pre-Reid surficial materials are thick in the lowlands of Klondike Plateau and Tintina Trench, areas proximal to the terminus of multiple pre-Reid glaciations. Reid ice terminated at Reid Lakes in the Tintina Trench. The McConnell ice sheet impinged into the east boundary of the study area, terminating approximately 20 km northeast of Stewart Crossing. Petrographic analysis of woody material from the oldest pre-Reid deposit at Stirling Bend (unit A), suggests a late Tertiary age. Paleomagnetic measurements from overlying loess (unit B) and glaciofluvial sediments (unit C) have undetermined polarity. Remaining pre-Reid glacial and interglacial units from Stirling Bend have normal polarities and represent deposits from either a subchron within the Matuyama reversed chron or early Bruhnes normal chron. Reid deposits underlie Sheep creek tephra at Ash Bend suggesting a minimum age of 200 000 years. McConnell deposits are late Wisconsin age. The distribution of surficial materials, related to multiple glaciations, physiography, and fluvial order contrasts, may govern the distribution of placer occurrences in McQuesten map area. Placer deposits occur anomalously in areas around Klondike Plateau, coinciding with the terminus of pre-Reid glaciations, Further exploration in pre-Reid ice terminal environments may yield significant placers through a better understanding of sediment distribution and genesis.

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.

New geochronological and geochemical data from the ‘Windy-McKinley’* terrane provide insight into the age, correlation and paleotectonic settings of the various subdivisions of the terrane. U-Pb zircon age determinations for felsic meta-volcanic rocks of the White River formation and gabbro intrusions are Late Devonian and late Middle Triassic respectively. These new age determinations substantiate the proposed correlation of these components of ‘Windy-McKinley’ terrane with the succession on strike to the northwest which hosts the volcanogenic massive sulphide deposits in the Delta District, Alaska. Trace-element geochemical data from Triassic gabbro intrusions into the Mirror Creek and White River formations, and diabase and gabbro of the Harzburgite Peak-Eikland Mountain ophiolite suggest that magmatism in both subdivisions occurred in supra-subduction zone settings. However, the age of the ophiolite is not known, therefore mafic magmatism may not be coeval across the terrane and may have formed above different subduction zones at different times. *Quotes are used to indicate that the assignment to Windy and McKinley terranes is obsolete, but a new name has not yet been assigned.

Glacial history reconstructions and geomorphic mapping in the North McQuesten River, Dublin Gulch, and Keno Hill map areas indicate a succession of less extensive glaciations. From oldest to youngest, the main glacial episodes are the pre-Reid (multiple glacial episodes), Reid and McConnell glaciations. The surficial geology of the study area is dominated by deposits of the Reid and McConnell glaciations. Pre-Reid glacial deposits are mostly confined to infrequent erratics on plateau areas above the Reid glacial limit. Glacial limit mapping indicates that ice flow patterns were similar in both the Reid and McConnell glaciations. Valleys aligned parallel with glacial ice flow are broad and U-shaped with significant glacial deposits in valley bottoms. In contrast, valleys aligned transverse to glacial ice flow are narrower and have a more V-shaped morphology. This relationship appears to be a controlling factor on the distribution of placers in the study area. Numerous drainages were analyzed for their placer potential in each of the three map areas. Their potential was based on geomorphic evaluations, glacial history, geochemistry, bedrock geology, and historic records. A concentration of potential placer creeks were identified in the Keno Hill/Mayo Lake area. Fewer prospective creeks were identified in the Dublin Gulch and North McQuesten River map areas.

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.

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.

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. 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. This thesis is available online at https://open.library.ubc.ca/cIRcle/collections/ubctheses/831/items/1.0053519. A copy of this thesis is available at the EMR library – QE195 F9.

Ice accumulations in the Coast Mountains of southwestern Yukon and the Cassiar Mountains of south-central Yukon during the late Wisconsinan were responsible for glaciation of the Whitehorse area. Cirques in the Coast Mountains likely supported the first glaciers that advanced out of the mountain valleys ahead of the more distal Cassiar accumulation. Glacial maximum is characterized by topographically unconstrained ice flow trending northwesterly over most of the map area. Ice thickness over the city of Whitehorse exceeded 1350 m during full glacial conditions. Deglaciation is characterized by frontal retreat punctuated by periods of dynamic equilibrium and readvances. Differential retreat of the Cassiar and Coast Mountain ice lobes enabled the Cassiar lobe to penetrate, and at times readvance, up-gradient into Coast Mountain valleys. This pattern of deglaciation created ice dams and a series of proglacial lakes that submerged valleys under as much as 300 m of meltwater.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.