Salt dissolution tectonism and microbial origin of drainage-line silcrete in the Lower Cretaceous Athabasca Oil Sands, foreland Alberta Basin

Abstract

Abstract The Beaver River Sandstone is a 10 s cm to 1.7 m thick silicified interval emplaced enigmatically within unconsolidated fluvial sand beds of the Lower Cretaceous McMurray Formation, the principle host rock of the Athabasca Oil Sands in northeastern Alberta. The silicified interval was partially eroded into during the Late Pleistocene as glacial meltwaters enlarged and deepened the Athabasca River Valley. The silcrete consists of a 10 s km long linear trend of discontinuous caprocks that sourced the innumerable quartzite boulders along the floor of the Athabasca River Valley. Siliceous cementation was concurrent with Aptian deposition of the McMurray Formation. The silicified bed extends along the disconformity between the lower and middle intervals of the McMurray Formation. The silicification at the end of lower McMurray deposition was spatiotemporally linked to regional dissolution of halite-anhydrite beds in Devonian Prairie Evaporite substrate, 200 m below. A sulphate-rich brine migrated up-section into the uppermost sand beds of the lower interval accumulated along topographic lows of the ancestral Athabasca River Valley, a segment of the regional Assiniboia Paleovalley. Near surface redox of the seep occurred by sulphate reducing microorganisms sourced from groundwater emanating from widespread peat mires. Strongly acidic groundwater resulted in corrosion of the quartz sand grains and silica saturated groundwater in the shallow subsurface. A pH shift occurred at onset of the middle interval, triggering silica cementation as the acidic seep mixed with meteoric-charged oxygenated groundwater along topographic lows of the river valley. Microbial filaments of unknown taxonomic affinity, now silicified, are the first recognized in the McMurray Formation oil sands. These microfossils consist of silicified hollow branching sheaths covered with nanoscale bulbous protuberances that preserve stepped transition textures of hydrated silica into quartz crystallites. They are evidence of partial microbial controls on the sulphate redox process resulting in this unusual type of silcrete.