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.

Abstract Stratal stacking patterns and platform distribution within the Devonian Beaverhill Lake Sequence of northern Alberta were influenced by several factors, those that were primarily external (e.g., climate change, trade-wind flow, terrigenous mud supply) and those that were internal (e.g., carbonate factory, water circulation, basin topography). Within the first-rank Beaverhill Lake Sequence, our study revealed two second-rank (1,2) and 10 third-rank (A-J) transgressive-regressive (T-R) sequences within the Beaverhill Lake Sequence, many of which were progradational and basin-filling, even during a relative rise in sea level. Furthermore, our study reveals three distinct phases of sedimentation during the depositional interval. The first phase of sedimentation occurred during T-R sequence A. During the initial sea-level rise of sequence A, the Peace River Arch fringing platform and Hay River platform initiated along the western margin of the study area. Platforms aggraded, but they did not prograde significantly, likely because detrital carbonate sediment was transported by surface currents into the inner platform and because of the proximity of the platforms to a limited but adequate supply of nutrients. The condensed limestone across the shallow Waterways Subbasin at the end of sequence A was produced by a local carbonate factory within or near the base of the photic zone, but under nutrient-starved conditions. Slope environments near the platforms contained a mixture of locally produced carbonate sediment and transported allochems. Therefore, this first phase of sedimentation during the Beaverhill Lake Sequence contains circulation-and nutrient-constrained carbonate platforms on the western side of the study area and limited carbonate accumulation within a shallow basin. Mixing of sediments between the two environments occurred only within slope deposits. The second phase of sedimentation occurred during the clinoformal infilling of the Waterways Subbasin with the progradation of the carbonate-siliciclastic Eastern Platform, the drowning and burial of the Hay River platform, and the back stepping of the Peace River Arch fringing platform. Lithofacies and faunas found on the Eastern Platform generally grade into deeper-water components, often by the increase in argillaceous sediment in carbonate beds and the loss of shallow-water organisms. Basinal sediments are mainly argillaceous in the thin toes of the clinoforms in the Waterways Subbasin. Evidence of sediment transport by gravity flow or other mechanisms from the Eastern Platform down the slope and into the basin is rare in core and restricted to occasional tempestite-like beds and individual allochems derived from shallow-water organisms. Although the transport of micrite basinward by water currents is likely to have occurred, an in situ fauna inhabited at least the upper portion of the slope environment and produced carbonate sediment. The third and final phase of sedimentation in the Beaverhill Lake Sequence was generally aggradational, with a much-reduced difference in topography between the Eastern Platform and the Waterways Subbasin. Lithofacies and faunas found on the Eastern Platform can be traced into the Waterways Subbasin. Faunas change little, but lithofacies tend to become more argillaceous throughout the study area. The definition of the Eastern Platform margin can only be seen in cross section where carbonates thin abruptly westward.

Abstract The Middle Devonian carbonates of the Slave Point Formation, Hamburg field, northwestern Alberta, are composed mainly of stromatoporoid and Amphipora floatstones and rudstones, with interbedded mudstone and grainstone facies characteristic of deposition in open to slightly restricted marine platform environments. These carbonates have undergone a complex diagenetic history, from shallow to deep burial, as represented by fracturing, calcite cementation, silicification, and dolomitization. Petrographically, four different types of dolomite have been identified (from early to late): (1) fine-crystalline matrix dolomite; (2) pseudomorphic dolomite; (3) medium-crystalline pervasive dolomite; and (4) saddle dolomite. Fine-crystalline dolomite (5–50 (μm) replaces the mud matrix and slightly penetrates the edges of allochems. It occurred in mud-supported facies and was precipitated by marine fluids. Oxygen isotope values range from −11.62 to −9.34‰ (Peedee belemnite), lower than postulated values for Devonian carbonates. The enriched 87 Sr/ 86 Sr isotope value from this phase (0.71002) suggests that later diagenetic fluids may have recrystallized this dolomite. Pseudomorphic dolomite (50–100 μm) replaces crinoids and occurs as single, large dolomite crystals. Its oxygen and carbon isotopic values range from −10.58 to −9.65 and +4.24 to +4.49‰, respectively. Medium-crystalline pervasive dolomite (10–100 μm) occurs along dissolution seams and obliterates all previous fabrics. It is proposed that this medium-crystalline dolomite formed during shallow to intermediate burial because of its association with dissolution seams and high iron content. The range of oxygen isotope values for this dolomite (−11.74 to −9.5‰) suggests precipitation from a warm fluid, possibly in a burial environment, and/or later recrystallization by hydrothermal fluids. The relatively wide range of carbon isotope values (+1.19 to +4.49‰) and enriched strontium isotope ratio (0.710020) suggests recrystallization. Saddle dolomite (250–2000 μm) partially to completely occludes void spaces (both fractures and vugs) and also occurs as a minor replacement mineral. The oxygen isotope values for saddle dolomite (−?13.95 to −?11.97‰), as well as the nonradiogenic to enriched strontium isotope ratios for saddle dolomite (0.70494 to 0.710351), and the fluid-inclusion data (homogenization temperature, T h , range between 125 and 161°C and estimated salinity, between 22.2 and 24.7 wt.% NaCl equivalent) indicate precipitation from hot, highly saline, hydrothermal fluids, which were probably expelled tectonically during the Late Devonian-Mississippian Antler thrust belt development.