The Devonian Horn River Group and the basal Imperial Formation of the central Mackenzie Plain, N.W.T., Canada: multiproxy stratigraphic framework of a black shale basin

Abstract

This study proposes six new lithostratigraphic units and redefines two others within the Horn River Group (HRG), a shale-dominated succession recently prospected for shale hydrocarbons. Its lower Hare Indian Formation rests on a drowning unconformity. In the SOB area (the area between Norman Wells and the Keele tectonic zone), this formation consists of the Bluefish black shale, Francis Creek grey shale, and Prohibition Creek black shale members. The latter two are lateral equivalents of the grey shale Bell Creek Member (updated definition) in the northern part of the study area. The overlying Canol Formation in the SOB area is divided into the Vermillion Creek and Dodo Canyon members. The Prohibition – Dodo Canyon package is ≥100 m thick and composed of brittle siliceous mudrocks with median SiO2 68%–79%, Al2O3 6%–9%, varying amount of calcite and dolomite, and type I–II kerogen with median TOC 4.5%. The thin Canol Formation blanketing Kee Scarp carbonate banks is identified as its Dodo Canyon Member. The latter is a “sweet bed” for fracking with median SiO2 75%–79%, TOC 5.1%, and trace-metal signatures indicating strongest anoxia in the section. The Mirror Lake Member in the base of the overlying Imperial Formation consists of soft illite-rich grey shales with minor siderite and dolomite. The overlying Loon Creek Member is composed of Canol-like black shale facies in its basal part and less organic-rich, lighter-colored shales and siltstones above. The Loon Creek Member is partly coeval with an overlying succession of sandstones and siltstones of the Canyon Member in its updated definition. Conodont data do not indicate significant time gaps between lithostratigraphic units. The majority of standard conodont zones from the uppermost Eifelian (ensensis Zone) up to the middle Frasnian (punctata Zone) are recognized. Assemblages of the hemiansatus and semialternans/latifossatus zones are missing probably owing to scarcity of conodont samples. The base of the HRG approximates the Eifelian–Givetian boundary, and the Hare Indian Formation is mostly Givetian. The Vermillion Creek Member spans the Givetian–Frasnian boundary. In places where thick Kee Scarp carbonates aggrade on thick sections of the Bell Creek Member, the Kee Scarp – Canol contact is found within the transitans Zone and the Middle Frasnian punctata Zone conodont assemblage is found in the basal part of the Dodo Canyon Member. The main part of the Dodo Canyon Member and basal beds of the Imperial Formation remain undated.

Stratigraphic Unconformities: Review of the Concept and Examples from the Middle-Upper Paleozoic

“…The upper few decimeters below the top are chertified and also very pyritic in core or rusty in outcrops. Phosphatic crusts characteristic of hardgrounds at other drowning unconformities did not develop, which is explained by overall phosphorus-lean sedimentary system [45]. Four other sections show 0.5-2.6-m-thick transitional interval of argillaceous bioturbated micritic limestones and shales.…”

“…Nevertheless, this hiatus survived in the territorial table of formations until recently [44]. Decisive in retiring this hiatus are (1) updates in conodont data showing Canol base time gliding from the Frasnian transitans-punctata on top of Kee Scarp carbonate banks to the upper Givetian norrisi in offbank depressions [44]; (2) carbonate-bank slope depositional setting of allochthonous bioclastic debris interfingering with laminated black shales; and (3) absence of any evidence of subaerial exposure or vadose processes, like oxidation of pyrites and organic matter and characteristic redistribution of Fe and Mn, prior to the onset of Canol deposition [45].…”

“…The Hume/Bluefish contact has been measured in three cores from Canol Shale exploration wells and accessed in three outcrops of the Norman Range and northern Mackenzie Mountains [45,109]. The coral-stromatoporoid facies composing the main part of the upper Hume Formation occurs in direct contact with the Bluefish shale in two of six sections, and in both cases, it shows a rugged corroded top with deep (8 cm in core) solution pockets filled with black shale from the overlying anoxic facies.…”