Sedimentology and facies analysis of Devonian Rocks, southern district of Mackenzie, North West Territories, Canada

Abstract

The Devonian rock succession in the southern part of the District of Mackenzie consists of interbedded evaporites and carbonates, fossiliferous carbonates and shales. Inthe study areathe Devonian succession unconformably overlies LowerPaleozoic orPrecambrian strata and thins in a northeastward direction because of truncation below the pre-Cretaceous unconformity. The Lower and lower Middle Devonian parts of the succession have a restricted distribution and onlap Tathlina Uplift, a pre-Devonian paleotopographic high area situated in the southern part of the Great Slave Plain. They include detrital, carbonate and evaporitic facies. The upper Middle and Upper Devonian parts of the succession are widely distributed. They consist of carbonates and shales. The Devonian succession includes seven stratigraphic sequences separated by slight angular unconformities or depositional hiati. Sequence 1 includes Lower Devonian evaporites and carbonates of the Camsell and Tsetso formations and their equivalents. The sequence accumulated in Root Basin, a tectonic depression along a shelf edge that formed an embayment northwest of an ancient landmass situated in the Great Slave Plain. The Middle Devonian succession can be subdivided into five stratigraphic sequences, each including fondo-, clino- and undathem deposits. The two older sequences of the Middle Devonian succession (including in sequence 2, the Sombre, Arnica, Fort Norman and Mirage Point formations and in sequence 3 the Funeral and Landry formations) accumulated during periods of relative sea level rise on the northern flank ofTathlina Uplift. The three younger sequences are part ofthe Elk Point Group of Alberta and accumulated in the northern part ofthe Elk Point Basin during subsequent relative rises in sea level. Sequence 4 includes the Headless, Nahanni, Hume and Lonely Bay formations and the Hom Plateau and Pine Point reef mounds. Sequence 5 consists of the Pine Point Assemblage, the lower part of the Undivided map unit and the Muskeg Formation ofAlberta. Sequence 6 comprises the upper part ofthe Undivided map unit, and the Watt Mountain, Fort Vermilion and Slave Point formations. Middle and Upper Devonian strata are included in sequence 7. Only the lower part is preserved in the District ofMackenzie. It includes the Spence River, Fort Simpson, Hay River formations and the formations of the Grumbler Group. Information from regional and structural geology suggests that the Devonian sediments and evaporites accumulated in slowly subsiding embayments. A comparison between the textures and sedimentary structures of Recent sediments and Devonian rocks indicates that the Devonian rocks have undergone many diagenetic changes. It is speculated that these changes were brought about by burial during the Late Paleozoic Era; by the episodes of uplift, erosion, subsidence and renewed burial during the Mesozoic and Cenozoic Eras; and by the effects of the Laramide Orogeny.