Depositional Sequences, Facies and Evolution of the Upper Devonian Swan Hills Reef Buildup, Central Alberta, Canada
The Swan Hills reef buildup, 60 m (197 ft) thick and covering an area of I 10 km2 (42 mi2) is Upper Devonian (Frasnian) in age. Reef interior, reef flat, reef margin, reef foreslope and basin environments formed during distinct growth stages of the stromatoporoid reef. The first stage, representing progradation of the reef margin and terminated by subaerial exposure, is recognized throughout the buildup. The second and third stages, developed only on the windward (northeast) side of the buildup, represent backstepping of the reef margin during storms. Both of these stages are capped by shallow-subtidal marine hardgrounds. Vertical accretion of the reef occurred during the fourth and fifth stages, that are recognized throughout the buildup. These stages are capped by subaerial exposure and the removal of part of the windward side of the buildup. A widespread sand shoal with little facies differentiation is the last stage of reef growth. It is capped by a hardground and drowning. Porosity in limestone is present in high-energy facies such as reef margins, sandy parts of reef flats, sand aprons and locally in slope deposits. Porosity in all but the last stage is not laterally continuous due to reef interior muddy facies.