The "Tar Sands" of the Athabaska River of Northern Alberta are believed to have derived their bituminous content from the underlying coral reef reservoirs of the Upper Devonian which come in contact, or near contact, with the sands at the unconformity between the Lower Cretaceous and the Upper Devonian. The sand was supplied from the pre-Cambrian Athabaska sandstone at the east on the Canadian Shield, and it is suggested that the escape of oil via fissures from the Devonian

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reefs occurred primarily during and also after deposition of the Cretaceous sands. Diagrams illustrating possible conditions are presented.

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