Geology of Whitehorse, Alligator Lake, Fenwick Creek Carcross and Part of Robinson Map Areas (105D/11, 6, 3, 2 & 7)

Three allochthonous terranes, igneous rocks of the Cretaceous and Tertiary Coast Plutonic Complex and associated volcanic complexes comprise the geology between Whitehorse and the Yukon-BC border. The Paleozoic and older(?) Nisling Terrane is composed of the quartz-rich Nisling assemblage, the carbonaceous Nasina assemblage and orthogneiss assumed to be Devono-Mississippian in age. The Nakina sub-terrane of the northern Cache Creek Terrane is composed of Mississippian to Permian spilitized basalt, ultramafite, chert and limestone characterized by Tethyan faunal assemblages and dramatic facies variations. The northern Stikine Terrane is composed of the Upper Triassic Lewes River arc and its plutonic roots.

Upper Triassic to Middle Jurassic sedimentary rocks of the Lewes River and Laberge groups collectively comprise the Whitehorse Trough overlap assemblage. Basal augite-phyric volcanic rocks of the Lewes River Group (Povoas Fm) are unconformably overlain by sedimentary debris derived from the eroding arc (Aksala Fm). These rocks are disconformably overlain by Laberge Group coarse clastics (Takwahoni Fm) deposited in laterally discontinuous submarine fans and conformably overlain by distal fine-grained equivalents (Inklin Fm). Siliciclastic rocks of the Tantalus Formation were deposited in a variety of marine environments in a successor basin to the Whitehorse Trough during Upper Jurassic and Late Cretaceous(?) times.

Plutonic rocks cover approximately 40% of the project area and comprise over 30 individual bodies with a wide range of compositions. Plutonic rocks have been divided into six chrono-lithologically distinct suites: Late Triassic batholiths of the Klotassin plutonic suite; Early Jurassic syn-tectonic(?) foliated diorite; mid-Cretaceous granodiorite of the Whitehorse plutonic suite; mid-Cretaceous granophyric quartz monzonite of the Mount McIntyre plutonic suite; a poorly defined suite of Late Cretaceous intrusions with associated volcanism; and quartz-rich high-level Late Paleocene/Early Eocene intrusions of the Nisling Range Plutonic Suite which form the plutonic roots to Skukum Group volcanism. The Klotassin suite is assumed to form the plutonic roots to the Lewes River arc, but are isotopically dissimilar. The Bennett Granite provides a Late Triassic link between Nisling Terrane and Lewes River arc. Cretaceous and Tertiary intrusions are peraluminous and comprise the Coast Plutonic Complex.

Isolated but widespread accumulations of intermediate mid- and Late Cretaceous volcanic rocks form the Mt. Nansen and Carmacks groups. Four Late Paleocene/Early Eocene volcanic complexes of the Skukum Group are aligned in a northerly trend and represent deeper levels of erosion to the north. Skukum Group activity is also represented by numerous northeast-trending rhyolite dyke swarms representative of a transtensional regime.

Strata of the Whitehorse Trough were deformed into a set of open to tight, northwest-trending folds during Middle and(?) Latest Jurassic/Early Cretaceous compressional event(s). In northern Cache Creek Terrane, much of the deformation took place during Middle Jurassic terrane accretion, but Late Cretaceous southerly verging thrust faults are documented. Nisling Terrane metasedimentary rocks were metamorphosed prior to the intrusion of the Late Triassic Bennett Granite.

The Tally Ho shear zone is a narrow domain of penetrative, ductile deformation with evidence of Late Triassic, sinistral strike-slip displacement. It strikes northwest and dips steeply and marks the easternmost limit of Nisling Terrane exposures. It may represent a terrane boundary between the Lewes River arc and the Nisling Terrane; alternatively it may be a transpressional structure within the arc itself.

Organization: Government of Yukon | Gouvernement du Yukon