Abstract Carbonate strata were widely deposited in the Alberta Basin during the Frasnian. These are well exposed in the Alberta Rocky Mountains and regionally extensive in the adjacent subsurface. This study places many of its classic outcrops from the Cascade (Burnt Timber) Channel to the South Jasper Basin into a single sequence stratigraphic framework for the first time. This framework is correlated from outcrop to subsurface using sequence stratigraphic and biostratigraphic data. Improved confidence in the stratigraphic interpretation is based on new measured sections tied to photographic panoramas, combined with detailed mapping of lithofacies and stratal patterns of continuously exposed platform to basin transitions in outcrop. These data are correlated with new and revised core and well-log interpretations from the Alberta subsurface. Ten third-order composite sequences and their constituent high-frequency (fourth-order) sequences span the uppermost Givetian through Frasnian strata of the Alberta Basin. They reflect stratigraphic architecture typical of a (second-order) depositional sequence: transgression followed by regression, or basin opening and filling. The eight youngest composite sequences are defined from the Cline Channel and Jasper Basin areas using stratal and facies stacking patterns and regional correlation of sequence boundaries and maximum flooding surfaces, integrated with conodont biostratigraphy. Most sequence boundaries observed are subaerial exposure surfaces, seen in outcrop or inferred from onlap of tidal-flat or reef margin deposits onto foreslope facies. The basin was filled asymmetrically by mixed carbonate–clay successions that form dominant east to west prograding strata. Two main types of sediment comprise the basin fill: extrabasinal clay and intrabasinal carbonate. Composite sequences (CSs) and high-frequency sequences (HFSs) can be confidently correlated from outcrop to subsurface. A combination of well-log and outcrop cross sections, integrated with biostratigraphy, support these correlations. These regional (time) surfaces allow better understanding of basin evolution and architecture. The influence of the second-order sequence dominates the accommodation setting and is expressed in the architecture of composite and high-frequency sequences. For example, the tripartite character (lowstand–transgressive–highstand) of CSs in the lower and middle part of the sequence is followed by the appearance of a distinct falling stage component in the upper part of the Frasnian. An increased frequency of truncation surfaces and offlapping strata is consistent with diminishing accommodation. With progressive basin infill and shallowing paleobathymetry, foreslope declivity decreased from a minimum of 10° to less than 1.5° as the depositional system became more ramp-like. This is accompanied by a change of lowstand geometry from wedge to tabular shaped. Deposition of coarser terrigenous clastics was also limited in most of the basin to the lower part of the second-order sequence, except at CS and HFS. Restricted marine circulation onto the carbonate platforms and basin filling in the upper part of the Frasnian coincided with extensive siliciclastic silt deposition in the study area, particularly in the Jasper Basin, where an influx of terrigenous silt formed mixed carbonate–siliciclastic deposits. Silt was deposited during third- and fourth-order lowstands, bypassed into the basin, and reworked during intermittent inundation of the carbonate platforms. Beyond the basic transgressive–regressive architecture of the second-order (Givetian–) Frasnian sequence, we document detailed observations such as (1) controls affecting the onset, cessation, and extent of euxinic shale deposition in the mid-Frasnian and its relation to the second-order maximum flooding surface; (2) the relative speed and distribution of illitic basin fill within the second-order highstand; (3) the effect of basin fill and off-platform sediment transport on regional and local carbonate platform architecture, such as the configuration of in situ carbonate lowstands, initiation of reefs along favorable fairways, and overall margin stacking patterns; and (4) the magnitude of relative sea-level falls associated with the development of sequence boundaries. A comparison to previously established Frasnian sequence stratigraphic schemes within the basin is extended to other basins in Europe and Australia.

ABSTRACT The South Jasper Basin was a major locus of carbonate deposition during the Frasnian, and its sedimentary record is extensively exposed in the Alberta Rocky Mountains. This study places many of its classic outcrops into a sequence stratigraphic framework for the first time. New descriptions of 18 outcrop sections, lateral tracing of stratigraphic geometries, and correlation to a regional sequence stratigraphic–biostratigraphic framework form the basis of the interpretations. The sequence stratigraphic evolution of the study area consists of a second-order, transgressive–regressive depositional sequence, composed of eight composite (third-order) depositional sequences and their constituent high-frequency (fourth-order) sequences. One lowest Famennian third-order sequence is briefly described. The composite sequences are correlated from the northwestern margin of the Southesk Cairn carbonate complex at Toma Creek to time-equivalent strata exposed in the Nikanassin Range. Exposures in the Nikanassin Range include a carbonate shelf prograding southeast into the South Jasper Basin. Stratigraphic architecture of the carbonate platforms was influenced by relative sea-level change within the second-order sequence and timing of basin fill in the Jasper Basin. Extensive euxinic shale deposition occurred in the mid-Frasnian, with its maximum extent coinciding with the second-order Maximum Flooding Surface (MFS), in the Woodbend 2.3 high-frequency sequence. Stratigraphic architecture generally follows the second-order trend, but significant deviations from that trend are observed at both the composite and high-frequency sequence scale. Basinally restricted wedges of shallow-water carbonate occur above third- and fourth-order sequence boundaries during the second-order transgression. Slowing relative sea-level rise in the second-order highstand was reinforced by third- and fourth-order relative falls to produce complex stratigraphic architecture at the platform margins. Offlapping strata with basinally restricted shelf margin deposits and falling stage geometries are uniquely well exposed in the Nikanassin Range, allowing detailed reconstruction of sea-level fluctuations in the second-order highstand. Restricted marine circulation onto the carbonate platforms and basin filling in the late Frasnian coincided with extensive siliciclastic silt deposition in the study area. Silt was deposited during third- and fourth-order lowstands, bypassed into the basin and was reworked during intermittent inundation of the carbonate platforms.

Abstract The southeast and northwest margins of the Frasnian (Upper Devonian) Cline Channel are preserved in excellent and continuous outcrop exposures at both Cripple Creek and Wapiabi Gap in the Alberta Rocky Mountains. Accretionary and interfingering platform margins allow detailed definition and correlation, from platform to basin, of significant sequence stratigraphic surfaces. Eight Frasnian third-order composite sequences are defined using stratal and lithofacies stacking patterns, regional correlation of sequence boundaries, and maximum flooding surfaces, constrained by conodont biostratigraphy. They form part of an upper Givetian–Frasnian second-order transgressive–regressive depositional sequence. Most sequence boundaries observed show subaerial exposure. Others are inferred from stratal architecture, e.g., onlap of tidal-flat or reef margin deposits onto foreslope lithofacies. The Cline Channel was filled asymmetrically from southeast to northwest along the described/studied transect. Progradation of platform margins is on a substrate of platform-derived fine-grained carbonates and extrabasinal clays that form argillaceous carbonates and calcareous shales. Stacking patterns of the composite sequences vary across the Cline Channel. On the southeast side, the second-order Givetian–Frasnian cycle is characterized by initial retrogradation followed by aggradation to retrogradation in the upper mid-Frasnian, and finally, progradation in the upper Frasnian. On the northwest side, the overall stacking pattern is aggradational. With progressive basin filling, platform edges evolved from rimmed boundstone and/or grainstone to mainly grainstone. Foreslope declivity decreased from a minimum of 10° (WD3) to less than 1.5° (WI1) reflecting more ramp-like foreslopes. Coincident with this change, lowstand geometry evolved from wedge shaped to tabular. Where slope gradients were high, lowstands are wedges, less extensive and abutting antecedent highstands. With development of ramp-like geometries, lowstands became tabular and were detached from their antecedent shelf edges with even minor falls of relative sea level. Gentle slope gradients and larger areas for shallow-water carbonate production facilitated extensive lowstand development. Assignment of strata into systems tracts of ramp-like systems is facilitated by subregional correlation. Decreasing accommodation within the second-order highstand is indicated by reduction in composite sequences (CSs) thickness and replacement of open marine with platform-interior strata as the basin shallowed and filled. Composite sequences became more asymmetric, developing thin, offlapping falling stage systems tracts in the late Frasnian, accompanied by a higher frequency of lowstands. Continuous outcrop exposures permitted the amount of relative sea-level fall to be estimated for the bounding surfaces of several CSs. Relative sea-level falls ranged from 9 to ~40 m.

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 Carbon stable isotope data from western Canada, in combination with biostratigraphic control and astrochronologic constraints from magnetic susceptibility data, provide insight into the pace and timing of the Frasnian–Famennian (F–F; Late Devonian) biotic crisis. In much of the world, this event is characterized by two organic–rich shales, which display geochemical anomalies that indicate low-oxygen conditions and carbon burial. These events, commonly referred to as the Lower and Upper Kellwasser events (LKE and UKE), have been linked to the expansion of deeply rooted terrestrial forests and associated changes in soil development, chemical weathering, and Late Devonian climate. The δ 13 C data generated from organic matter record a 3 to 4‰ positive excursion during each event. These data and other geochemical proxy data reported elsewhere corroborate hypotheses about enhanced biological productivity, driven by terrigenous input under exceptionally warm climatic conditions. In this hypothesis,a boom in primary production leads to successive development of anoxic bottom water conditions, decreased biotic diversity, and net transfer of carbon from the atmosphere to the ocean floor. Despite the importance of the F–F events, a precise chronology for the events is lacking due to limited biostratigraphic resolution. Each of the F–F events falls within one conodont zone, with durations estimated on the order of 0.5 to 1.0 Myr. The LKE occurs very high in Frasnian Zone (FZ) 12, while the UKE begins within FZ 13B, just below the F–F boundary. A previous analysis of high-resolution magnetic susceptibility data from the studied sections in western Canada identified 16.5 eccentricity cycles, each lasting 405 kyr, within the Frasnian strata and one in the earliest Famennian. The present study reports δ 13 C anomalies associated with the LKE and UKE in the same sections. The LKE and UKE intervals comprise 7 to 8 and 13 to 13.5 m of stratigraphic section, respectively. Based on our analysis, this implies that they represent only one 405-kyr eccentricity cycle or less.We estimate that the duration of the LKE was a bit more than half of a long eccentricity cycle (~200–250 kyr), while the UKE was more protracted, lasting a full long eccentricity cycle (~405 kyr). The onset of both events is separated by one-and-a-half 405-kyr eccentricity cycles, indicating that they occurred about 500 to 600 kyr apart. This work demonstrates the utility of magnetic susceptibility, or other long time-series proxy data, used in conjunction with astronomical calibration to provide insight into the pacing of significant events in geologic time.

ABSTRACT: The Upper Devonian Grosmont reservoir in Alberta, Canada, is the world’s largest heavy oil/bitumen reservoir hosted in carbonates, with an estimated 400 to 500 billion barrels of “Initial Oil In Place” at average depths of about 250 to 400 m. Our study is part of a more comprehensive effort to evaluate the Grosmont reservoir through geological, geophysical, and petrophysical methods in order to determine the most advantageous method(s) of exploitation. The reservoir is a carbonate–evaporite system. The carbonates of the Grosmont were deposited during the Late Devonian on an extensive platform and/or a ramp in five or six cycles. Evaporites are interbedded with the carbonates at several stratigraphic levels. These evaporites, informally referred to as the “Hondo Formation,” have received scant attention or were ignored in most earlier studies. However, they may play a crucial role regarding the distribution of the most porous and/or permeable reservoir intervals via dissolution, as permeability barriers to compartmentalize the reservoir during or after hydrocarbon migration, and as a source of dissolved sulfate for microbial hydrocarbon degradation. Most Hondo primary evaporites are anhydrite that formed subaqueously as well as displacively and/or replacively very close to the depositional surface. Secondary/diagenetic sulfates were formed from primary sulfates much later and under considerable burial. The locations of primary evaporite deposition were controlled by a shift from carbonate platform or ramp deposition over time. At present the primary sulfates occur in a number of relatively small areas of about 10 by 20 km to 20 by 30 km, with thicknesses of a few meters each. If these areas represent the depositional distribution, the primary evaporites were deposited in a series of large, shallow subaqueous ponds (salinas). Alternatively, the primary evaporites were deposited in a more extensive lagoon, and their present distribution represents the remnants after postdepositional, mainly karstic dissolution. The evaporites would have acted as intraformational flow barriers up until the time of dissolution, which may be a factor in the development of compositional differences of the bitumens contained at various stratigraphic levels. In the eastern part of the Grosmont reservoir the evaporites appear to be dissolved and replaced by solution-collapse breccias and bitumen-supported intervals of dolomite powder. In the western part of the reservoir the sulfates may form effective reservoir seals on the scale of the sizes of former brine ponds. However, it is likely that hydrocarbons bypassed them wherever the carbonates had sufficient permeability and/or where the marls were breached by faults and/or karstification.