The type section of the Canol Formation (Devonian black shale) at Powell Creek: Critical assessment and correlation in the northern Cordillera, NWT, Canada

Abstract

The Canol Formation is only 24.5 m thick at its historic type section at Powell Creek, northern Mackenzie Mountains, whereas in the off-bank sections of the Mackenzie Plain subsurface, where it is considered a high-quality shale hydrocarbon prospect, it thickens to 60–120 m. This paper reviews available lithological and conodont biostratigraphic information from the type section, discusses choices of contacts and subdivisions, and explores the limits of regional correlation using gamma spectrometry proxies. We position the base of the Canol Formation at the top of the lower resistant unit of the “allochthonous limestone beds”, the thick off-reef debris package present in this outcrop but absent in other well-known Canol sections. The base of the formation can be of a latest Givetian age as suggested by the norrisi zone conodont fauna from the “allochthonous limestone”. The top of the Canol Formation is placed at the base of a distinct, 2.1 m thick horizon with concretionary carbonate beds within the thick shale transition between the Canol and the Imperial formations. Limestone nodules from this horizon produced a conodont fauna that can occur in the jamieae to Upper rhenana zones (Frasnian zones 11–12) thereby suggesting a middle to earliest Late Frasnian age for the Canol top. The cross-section tying several outcrop and well sections across the regional facies zonation reveals that the Dodo Canyon Member, a unit erected in thick off-bank Canol sections, is traceable at Powell Creek. In this correlation, the Vermillion Creek Member, which is the lower portion of the Canol Formation in thick off-bank sections, finds its counterpart in the allochthonous limestone beds sensu MacKenzie (1970). This cross-section is the first correlation of the Canol stratotype at member level available in published sources. Thinness of the Canol Formation at Powell Creek, as well as its location in the carbonate bank toe-of-slope setting, are factors impairing its reference value and calling for more representative sections to act as reference sections and constitute a composite-stratotype for the Canol Formation.