LITHOLOGY, BIOSTRATIGRAPHY AND GEOCHEMISTRY OF THE UPPER DEVONIAN GRAMINIA FORMATION, CENTRAL ALBERTA

The Graminia Formation of the Upper Devonian Winterburn Group is widel y distributed in the subsurface of central Alberta. It includes a lowe r Blue Ridge Member and an upper ''Graminia Silt'' member. The carbona tes of the Blue Ridge Member overlie silty and shaly beds of the Calma r Formation with a gradational contact marked by an upward increase in carbonate content. The Blue Ridge Member is overlain with an erosiona l contact by silty and shaly beds of the ''Graminia Silt'' member. The ''Silt'' Member is, in turn, conformably overlain by anhydritic and p eloidal carbonates of the Wabamun Group. In central Alberta the Blue R idge Member consists of three parts: a lower part composed of silty ca rbonates; a middle part of fossiliferous and locally reefal carbonates ; and an upper part composed of interbedded silty and sandy, in part l aminated carbonates, peloidal carbonates and carbonate breccia. The br achiopods and sediment-filled burrows present in the lower slue Ridge and the colonial corals, locally in growth position, of the middle Blu e 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, dissol ution breccia, erosional surfaces and rubble conglomerates in th upper part suggest that during the deposition of the upper Blue Ridge sedim ents the conditions were peritidal and evaporitic.The ''Graminia Silt' ' member forms a relatively thin marker at the top of the Winterburn G roup. It overlies a carbonate breccia bed or includes a basal, shale-r ich rubble deposit. In central Alberta it is an interbedded unfossilif erous, greenish grey, green or reddish brown mottled succession of sil ty carbonates, dolomitic siltstone and shale. The shaly beds locally i nclude sediment-filled desiccation cracks, indicative of ephemeral and evaporitic conditions. The silty beds are massive and have a pseudo-b recciated or bioturbated aspect. Corals and conodonts extracted from f ossiliferous carbonates in the middle part of the Blue Ridge Member be long to the uppermost Frasnian Stage. Conodonts from upper Blue Ridge carbonates represent the early Famennian. This suggests that the ''ext inction'' of the late Frasnian coral fauna in central Alberta coincide s with a change in the environment of deposition from shallow marine t o evaporitic and peritidal. The middle and upper parts of Blue Ridge M ember include scattered, dark grey shale partings that contain up to 1 2.5 per cent of organic carbon.