ABSTRACT: The Carson Creek North Field is an Upper Devonian isolated reef-rimmed buildup in the Swan Hills Formation of the subsurface Western Canada Sedimentary Basin. The Swan Hills Formation belongs to the Beaverhill Lake Group, which contains three regionally defined sequences. The sequence boundaries at the base of the middle sequence (BHL2.1 SB) and the upper sequence (BHL3.1 SB) are present within the Carson Creek North buildup, dividing it into three evolutionary stages (lower atoll stage, upper atoll stage, and shoal stage). The BHL2.1 SB is a locally exposed surface at the top of the lower atoll stage, during which the Carson Creek North buildup evolved from a low-angle ramp to a steep-sided reef margin enclosing a restricted lagoon. The BHL3.1 SB is a regional subaerial exposure surface at the top of the upper atoll stage, also known as the intra-Swan Hills unconformity (ISHU). The upper atoll stage consists of backstepping reef margin cycles, and the shoal stage overlying the ISHU contains backstepping ramps culminating in drowning of the Carson Creek North buildup; therefore, the ISHU occurs within a continuously backstepping margin succession. The ISHU demonstrates ~13 m of paleotopographic relief within the Carson Creek North buildup, yet it is not associated with a regressive margin.
The Beaverhill Lake Group also contains shallow-water carbonates in the southeastern part of the Western Canada basin (Eastern Shelf) that are partly equivalent to the Swan Hills Formation. The Eastern Shelf carbonates are separated from Carson Creek North by more than 100 km across the Waterways shale basin. A regressive facies succession is present just below the BHL3.1 SB in the Eastern Shelf area, followed by an intrabasin lowstand corresponding to a hiatus during which paleotopography developed on the ISHU at Carson Creek North. The Eastern Shelf succession thus contains the missing regression across the ISHU at Carson Creek North. The lowstand was followed by gradual regional flooding that included deposition of the Carson Creek North shoal stage ramps. Stratigraphic comparisons among Carson Creek North, the Swan Hills Formation, and the Eastern Shelf areas indicate that regional differential subsidence patterns and initial basin floor topography were the most likely reasons for the divergent stratigraphic architecture. The origin of the ISHU itself remains unresolved, but it could be linked to a global eustatic event near the base of the Frasnian Stage, enhanced by tectonic activity in the Western Canada Basin.