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Chapter 13
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 Recognition
and Significance of an Intraformational Unconformity in Late Devonian Swan
Hills Reef Complexes, Alberta
Jack Wendte
Institute of Sedimentary
and Petroleum Geology
Geological Survey of
Canada
Calgary, Alberta, Canada
Iain Muir
Wascana Energy Inc.
Calgary, Alberta, Canada
*
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 lithologic 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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