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Asgar-Deen, M. M.1, C.L. Riediger2
(1) Carleton University, Ottawa, ON
(2) University of Calgary, Calgary, AB

ABSTRACT: Influence of a Silled Basin on the Deposition of the Lower Jurassic Nordegg Member (Fernie Formation) of Western Canada

A silled basin morphology for the deposition of the Nordegg Member was initially predicted on the basis of organic geochemistry. The presence of a sill in the western region of the Nordegg Member basin has been confirmed through a series of subsurface well log cross-sections, isopach maps and detailed core descriptions.
The single largest influence on the deposition of the Nordegg Member was its silled basin morphology. The sill restricted circulation with more marine conditions, particularly during periods of low relative sea-level, and may have contributed to a stratified water column resulting in poorly oxygenated bottom waters. These poorly oxygenated conditions are associated with an absence of bioturbation and benthic fauna. These conditions also facilitated the preservation of organic matter, resulting in total organic carbon contents ranging up to 28 weight percent.
Strata deposited over the western sill have significantly lower gamma-ray signatures and are more calcareous than the typical Nordegg Member basin strata. Both the sill and basin strata have little terrestrial input and are characterized by pelagic sedimentation. The main difference between the two appears to be a paucity of organic matter in the strata deposited over the sill. It is likely that organic matter deposited over the sill was poorly preserved because it was deposited above the zone of poorly oxygenated bottom waters.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004