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Abstract

Swan Hills reef complexes are isolated buildups up to 75 m thick that occur on an underlying drowned carbonate platform (approximately 60 m thick) in the subsurface of west-central Alberta and were studied in detail at the Judy Creek and Snipe Lake oil fields. Although these two reef complexes are 85 km apart, 8 to 10 m thick megacycles can be correlated between them. The top of the fourth reefal megacycle is a widespread subaerial unconformity (the intraformational Swan Hills unconformity [ISHU]) that separates an underlying rimmed-reef complex from an overlying ramp-bounded shoal complex.

Emergence at the ISHU was a result of a low-magnitude, relative sea level fall. This is substantiated by the following observations: (1) this surface exhibits a lithified nature continuously across both reefs; (2) shallower-water, mainly tidal-flat deposits overlie relatively deeper-water subtidal limestones at the contact; (3) solution vugs filled with marine sediments occur down to 2.3 m below the ISHU; and (4) oxidation of sediments occurs in some cores immediately beneath the unconformity

Distinct and unique lithologie changes occur in lagoonal successions in the fourth megacycle below the ISHU. The middle and upper parts of this megacycle consist entirely of shallow lagoonal deposits and totally lack the "deep"-water lagoonal deposits that typify portions of the first three reefal megacycles. These distinct changes record the gradual and progressive loss of accommodation space prior to emergence and suggest that the withdrawal of the sea was not due to a Pleistocene-like, glacial eustatic lowering of sea level. This sea level fall and resulting emergence had little effect on reservoir quality of the limestones underlying the ISHU.

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