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Bulletin of Canadian Petroleum Geology, Vol.42, No.1, 1-25, 1994
FRASNIAN REEF AND BASINAL STRATA OF WEST CENTRAL ALBERTA - A COMBINED SEDIMENTOLOGICAL AND BIOSTRATIGRAPHIC ANALYSIS
The Frasnian succession of West Central Alberta is characterized by third order (1-10 m.a.), transgressive-regressive (T/R), depositional cycles, represented by the Beaverhill Lake, Woodbend and Winterburn Groups. Fourth- and fifth-order shallowing upward cycles can be recognized within these T/R cycles. Time correlations between Frasnian carbonate platform and basinal sediments are established by combining sedimentology and conodont biostratigraphy. The depositional history for the Frasnian in the Nordegg area is interpreted and illustrated on cross-sections and paleogeographic maps. Carbonate deposition began with the flooding of the West Alberta Arch and the deposition of the upper Swan Hills Formation during the Lower asymmetrica Zone. Transgression in the Middle asymmetrica Zone initiated the basinal Cline Channel and Duvernay Formation shale deposition, while the time equivalent Cooking Lake Formation was deposited on the drowned Swan Hills platform. The overlying lower Leduc Formation shows backstepping and aggradational reef margin stacking patterns. Maximum relief from the carbonate platform to surrounding Duvernay Formation shale during the Upper asymmetrica Zone was 100 m. Aggradation and backstepping was repeated in the Ancyrognathus triangularis Zone, with syndepositional relief reaching 170 m at the Wapiabi Gap reef margin. Platform-margin profiles were controlled by physical factors such as dominant wind direction and currents. On the Ram Range the margin backstepped, but then aggraded at Cripple Creek. At Wapiabi Gap, to the north on the Bighorn Range, the margin was dominantly aggradational. Ireton Formation shale deposition was also influenced by currents. In the Lower gigas Zone, the Leduc carbonate platform reached a maximum syndepositional relief of 220 m. A change from dominantly biohermal to biostromal platform margins occurred. A prograding wedge of Ireton Formation shale filled much of the relief in the Cline Channel, while the upper Leduc platform was drowned. Finally, the progradational Nisku Formation was deposited during the Upper gigas Zone.