Abstract

The Graminia Formation of the Upper Devonian Winterburn Group is widely distributed in the subsurface of central Alberta. It includes a lower Blue Ridge Member and an upper "Graminia Silt" member. The carbonates of the Blue Ridge Member overlie silty and shaly beds of the Calmar Formation with a gradational contact marked by an upward increase in carbonate content. The Blue Ridge Member is overlain with an erosional contact by silty and shaly beds of the "Graminia Silt" member. The "Silt" Member is, in turn, conformably overlain by anhydritic and peloidal carbonates of the Wabamun Group. In central Alberta the Blue Ridge Member consists of three parts: a lower part composed of silty carbonates: a middle part of fossiliferous and locally reefal carbonates; and an upper part composed of interbedded silty and sandy, in part laminated carbonates, peloidal carbonates and carbonate breccia. The brachiopods and sediment-filled burrows present in the lower Blue Ridge and the colonial corals. locally in growth position, of the middle Blue Ridge, are indicative of a shallow-marine depositional environment. The displacive, nodular anhydrite in the middle and upper parts of the Blue Ridge Member and the stromatolites, peloidal grainstones, dissolution breccia, erosional surfaces and rubble conglomerates in the upper part suggest that during the deposition of the upper Blue Ridge sediments the conditions were peritidal and evaporitic. The "Graminia Silt" member forms a relatively thin marker at the top of the Winterburn Group. It overlies a carbonate breccia bed or includes a basal, shale-rich rubble deposit. In central Alberta it is an interbedded unfossiliferous, greenish grey, green or reddish brown mottled succession of silty carbonates, dolomitic siltstone and shale. The shaly beds locally include sediment-filled desiccation cracks, indicative of ephemeral and evaporitic conditions. The silty beds are massive and have a pseudo-brecciated or bioturbated aspect. Corals and conodonts extracted from fossiliferous carbonates in the middle part of the Blue Ridge Member belong to the uppermost Frasnian Stage. Conodonts from upper Blue Ridge carbonates represent the early Famennian. This suggests that the "extinction" of the late Frasnian coral fauna in central Alberta coincides with a change in the environment of deposition from shallow marine to evaporitic and peritidal. The middle and upper parts of Blue Ridge Member include scattered, dark grey shale partings that contain up to 12.5 per cent of organic carbon.

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