The layered rocks in this area originated as continental shelf sediments overlain by volcanic arc successions. Now called Yukon-Tanana terrane, they tectonically over-rode the western edge of ancient North America beginning in Middle Jurassic time. Three elements are present in the map area. The west half comprises the Big Salmon Complex; the east half is a separate, in part contemporaneous succession composed of the Dorsey Complex and Swift River Group. Unconformably overlying both these elements are less metamorphosed Klinkit Group and Triassic sediments that are here interpreted as overlap assemblages. The unexposed contact between Big Salmon Complex and Swift River Group is inferred to be an east-side-down normal fault.

The Cambrian–Devonian Bouvette Formation outcrops over large parts of central Yukon. Despite its broad lateral and temporal extent, relatively little is known about its age range, facies distribution, depositional history, and significance for early Paleozoic paleogeographic reconstructions of northwestern Laurentia. At Nadaleen Mountain (NTS 106C/2,3) in east-central Yukon, the Bouvette Formation is remarkably well exposed and provides new insight into the transition between the southeastern Ogilvie platform and northern Selwyn basin. Here, we present preliminary field data collected from this region during 2017 and 2018, including measured stratigraphic sections, biostratigraphy, and detailed imagery acquired from Unmanned Aerial Vehicles (UAVs), in order to test the hypothesis that the Bouvette Formation locally preserves a platform margin reef and forereef succession. These observations not only provide an important new contribution to Yukon’s early Paleozoic depositional history, but also identify an exceptional location to study carbonate platform–margin depositional environments.

Between the Cretaceous granitic rocks (Hake Batholith on the west; Cassiar Batholith to the east) are three belts of metamorphic rocks, collectively part of Yukon-Tanana terrane. These are remnants of oceanic and continental volcanic arcs, and marginal basin sediments of Early to mid-Paleozoic age. At the head of Borden Creek are thick carbonate and andesitic volcanic rocks correlated with Klinkit Group. The Ram Creek fault and Hidden Lake fault are not exposed but deduced to be steeply dipping brittle structures with northeastward thrust or transpressional offset, based upon more complete exposure to the southeast in 105B/3 map area. The former is likely of Cretaceous age; the latter was active between mid-Permian and Early Jurassic time.

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Galena and pyrrhotite occur in manganese-stained quartz-carbonate breccia beneath rusty limestone. Surface geology of the area suggests that the mineralization is a replacement of brecciated limestone in the footwall of a low-angle fault. Results from previous exploration have proved disappointing but the host structure is probably regional in extent and larger deposits may exist along strike.

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

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