Proterozoic basins of Canada
Authors: Campbell F H A 
Published: August 2013

The Wernecke Supergroup is subdivided, from oldest to youngest into the Fairchild Lake Croup, the Quartet Croup and the Gillespie Lake Croup. The Fairchild Lake Croup consists of at least 4 km of light-grey weathering, thin bedded to laminated siltstone, mudstone
and fine sandstone with some intercalated carbonate. Locally these rocks have been transformed into phyllites and schists. The Fairchild Lake Croup is tentatively subdivided into five formations. Carbonate members near the middle and at the top of this group are important stratigraphie markers.
Although most of the Fairchild Lake Croup was deposited in deeper water, by predominantly southerly flowing currents, a shallow marginal marine regime characterized the depositional setting of the second youngest formation. The Quart et Croup, which generally conformably overlies the Fairchild Lake
Croup, consists of at least 5 km of monotonous dark-grey-weathering, wavy to lenticular to flaser bedded siltstone, carbonaceous siltstone, mudstone and fine sandstone; two subdivisions are recognized in this group. Although the basal part of the Quartet Croup probably accumulated in a
sediment-starved stagnant basin, the greater part of the group was deposited in a shallow marine environment. The contact between the Quartet Croup and the overlying Gillespie Lake Croup is transitional. The Gillespie Lake Croup consists of more than 4 km of buff- to orange- to grey- to locally
maroon­weathering dolomite, silty and clayey dolomite, siltstone, mudstone and fine sandstone. In the type area, seven subdivisions are defined in the Gillespie Lake Croup. The lower part of this group is fine grained terrigenous sediments and terrigenous-carbonate admixtures. The upper part of the
Gillespie Lake Croup consists of shallow water dolomite characterized by stromatolites, cryptalgal laminates, oolites, and pisolites. The Wernecke Supergroup hosts numerous spectacular breccia complexes which locally contain significant enrichments of U, Cu, Ba, Co or Fe. These bodies are of
variable size and shape; their most spectacular development occurs in the upper part of the Fairchild Lake Croup. Individual breccia complexes are commonly composed of three genetic varieties of breccia: fault breccia, stope breccia and channel-way breccia. Severa! types of alteratibn are associated
with the breccia complexes including Na-feldspathization, silicification, hematitization and carbonatization. The breccia complexes are located on or near splays of the Richardson Fault Array, a system of west- to northwest-trending faults of great antiquity. The breccia complexes are envisaged as
having developed in a dilational tectonic regime. Dykes and sills of diorite, gabbro and peridotite intrude the Wernecke Supergroup. These intrusive bodies are probably of several ages. Strata of the Wernecke Supergroup have been deformed into west-northwest trending open folds, although the
structural style is locally more complex, involving faults and the effects of intrusive breccia complexes. Some of this deformation is related to the Racklan orogeny, which probably culminated between 1.2 and 1.3 Ga ago. The Wernecke Supergroup is correlated with the lower three groups of the
Belt-Purcell Supergroup of the southern Cordillera. Paleocurrent analyses coupled with limited facies information indicate that in the Middle Proterozoic, an east-west trending depositional margin lay to the north of the present day Wernecke Mountains. lt is hypothesized that the formation of this
margin is related to a continent-continent collision event recorded in the Aphebian Coronation Geosyncline.