Print this pageComparative Evolution of Late Devonian Swan Hills Reef Complexes, Western Canada
MUIR, IAIN D., FRANK A. STOAKES, and JACK C. WENDTE
Late Devonian Swan Hills reef complexes are isolated buildups between 35 and 75 meters thick, that occur on an underlying drowned carbonate platform. Many of these buildups have produced in excess of 100 million barrels of oil. Three of these complexes (Judy Creek 'A', Snipe Lake, Goose River) are 10's of kilometers apart and consist of correlatable 7 to 10 meters thick cycles.
The underlying platform consists of cycles arranged in a backstepping stacking pattern, resulting in the formation of isolated shoals. The initially more areally restricted reef complexes nucleated on these depositional highs. The first reef cycle is strongly progradational, resulting in lateral expansion of the reefs. Deposits of reef cycles two through four form rimmed-reef complexes, and exhibit variable upbuilding to backstepping stacking patterns. The top of the fourth cycle shows evidence for subaerial exposure due to a low-magnitude relative sea-level fall. A forestepping phase of reef development prior to exposure as predicted by contemporary sequence stratigraphic models does not occur. Accumulations above the subaerial exposure surface form ramp-bounded shoal complexes with backstepping cycle stacking patterns. The ultimate drowning of the reef complexes occurs at different levels, with smaller buildups such as Goose River being terminated at earlier stages of reef development.
The results of our studies show that these reefs evolved during well-defined and correlatable stages corresponding to fluctuations of relative sea level, although stacking patterns within and between complexes may vary.