Granite wash alluvial fans, fan-deltas and tidal environments, northwestern Alberta; implications for controls on distribution of Devonian clastic wedges associated with the Peace River Arch

In the Utikuma and Red Earth fields of the northwestern Alberta subsurface, along the eastern flank of the Peace River Arch, the Middle to Upper Devonian Granite Wash is organized into two systems of partially superimposed clastic wedges that interfinger with, and are onlapped by, the Keg River and Muskeg formations. The lower and upper Granite Wash clastic wedges reach a combined maximum thickness of 70 metres. Granite Wash sediments represent a transgressive-regressive interplay between alluvial fan-delta and tidal environments. In the Utikuma area, alluvial fan-delta massive or cross- to parallel-stratified pebbly sandstone, intercalated with minor conglomerate and mudstone, form fining-upward units. Fan-delta deposits interfinger with estuarine, and intertidal to bioturbated subtidal sandstone and mudstone of the Cruziana ichnofacies. Extensive horizons of pyritic black mudstone was deposited locally under dysaerobic and/or anaerobic conditions. Tidal deposits constitute the finest grained portions of the Granite Wash, and interfinger with supratidal carbonates, algal laminites and evaporites of the Keg River Formation, most widespread in the Red Earth area. Uplift in the central part of the Peace River Arch triggered eastward and northeastward advances of the alluvial fandeltas, and was responsible for westward backstepping of the clastic wedges. Concurrently, the northeast-trending basement highs and lows significantly influenced Granite Wash sedimentation. Consequent paleovalleys divided the alluvial bodies into narrow, confined drainage networks that facilitated the development of fan deltas and estuaries. The resultant irregular paleoshoreline locally restricted water circulation, allowing prolonged periods of fine-grained sedimentation under reducing conditions. The growth of algal mats and precipitation of evaporites occurred under semiarid to arid conditions. The Granite Wash typically blankets the pre-existing passive paleotopography of the Peace River Arch, onlapping basement ridges with characteristic drape and pinch-out geometries. There is no compelling evidence that the northeast-trending paleovalleys were active grabens during Granite Wash sedimentation: large-scale "graben-like" features can be attributed to post-Granite Wash faulting, although the steep escarpments of a few paleovalleys may mark normal faults that were active before and/or during deposition of the Granite Wash.