Whalen, M.T., Eberli, G.P., and van Buchem, F.S.P.
Isolated and attached carbonate platforms developed at near-equatorial latitudes in the Alberta Basin during the late Devonian. These platforms developed atop a regionally extensive Late Givetian-Early Frasnian carbonate ramp. Platform margins are characterized by a rim of stromatoporoid biostromes surrounding a typical back-reef interior dominated by peritidal or subtidal facies. Basinal sediments onlap or interfinger with the buildups. The Miette and Ancient Wall buildups, Canadian Rocky Mountains, western Alberta were chosen for this study because of their well exposured platform margins. Similar subsurface Leduc reefs in central Alberta are important petroleum reservoirs associated with basinal source rocks. Six genetic depositional sequences were defined in both Miette and Ancient Wall based on stratal geometries, platform unconformities, and basinal minearlogic and lithologic trends. Our sequence stratigraphic subdivision, in conjunction with conodont biostratigraphy using the recently developed Montagne Noire zonation, permits more accurate correlation of platform and basin facies and surface and subsurface buildups.
These two buildups evolved from a broad regionally extensive carbonate ramp, to prograding, backstepping, and aggrading isolated platforms, and back to a more-ramp-like phase that prograded across basin fill. The distribution of platform facies at Miette and Ancient Wall reveals a trend toward finer-grained facies both platformward and basinward of the margin where coarse-grained facies are concentrated. A transect from platform interior to basin reveals five facies zones or associations: 1) peritidal platform, 2) shallow to deep subtidal platform, 3) platform margin, 4) forereef and slope, and 5) basin facies associations. The width of the individual facies belts is strongly controlled by the platform/basin topography with wide, gradational facies belts characterizing low-angle prograding platform phases and narrow facies belts along aggrading platform margins. Low-angle prograding platforms generally interfinger with basinal facies while steep-sided aggrading platforms commonly develop bypass margins that are onlapped by thick packages of basinal facies interbedded with coarse-grained platform and slope derived slope facies deposited by gravity flow mechanisms. These units contian packages of highstand and lowstand origin that straddle sequence boundaries. The clast composition of conglomerates and the carbonate content of background sediments can be used to place the sequence boundary accurately. Fine-grained siliciclastic sediments from the north and east, that mixed with platform derived carbonates and organic matter, eventually filled in the basin prior to renewed progradation of the carbonate platforms.
Observed facies distributions form the basis for four facies models that relate facies distribution to platform morphology. Refinement of our understanding of facies distribution with respect to platform evolution enhances our ability to predict platform geometry. These models also provide better control on predicting facies distribution from platform geometries seen on seismic lines including the extent and position of source and reservoir rocks in similar carbonate platforms. Three levels of cyclicity can be documented in the Upper Devonian rocks of Alberta. At the largest scale (second-order) the entire package of carbonate platform and basinal rocks was deposited during a second-order rise and fall of Frasnian sea-level. A medium scale (third-order) of cyclicity is recorded in the six genetic sequences that we have documented in both buildups. Sequences span between 1.5 and 3.5 conodont zones indicating temporal durations of between 0.8 and 2.8 m.y. Sequences are composed of even finer-scale shoaling-upward cycles (parasequences) bounded by marine flooding surfaces. Parasequences durations are difficult to estimate but they appear to represent either fourth or fifth-order cycles. The facies stacking within individual cycles is partially determined by position on the platform. Subtidal cycles along the platform margin or within deep lagoonal settings seldom record shoaling to tidal flat while peritidal platform interior cycles commonly do. Facies evolution within the cycles is also controlled by the long term sea-level trend. Platform parasequences are subtidal dominated during the second-order transgression and more peritidal dominated during the second-order regression. Basinal and slope facies are sometimes non-cyclic because they were deposited at depths that were essentially unaffected by relative sea-level changes. Cyclic basinal facies generally record a change from background sedimentation to gravity flow deposition. The cycles are controlled by sediment input due to sea-level changes and by depositional processes that vary with changes in platform to basin gradient.
Sequence geometries vary systematically within the second-order transgressive-regressive cycle. The transgressive phase records ramp, low-angle prograding, and back-stepping to aggrading styles of platform development, whereas the regressive phase is mainly progradational across late stage mixed carbonate-siliciclastic basin infill. Thus, platform margin geometries are affected by second-order relative sea-level but are also influenced by the interplay between carbonate productivity and basinal sediment supply.
This mixed carbonate siliciclastic system represents a classic example of an uplifted stratigraphic trap. The isolated platform margins are ideal reservoir facies, organic rich basinal facies were productive petroleum source rocks, and the mixed carbonate-siliciclastic basinal facies that onlap and overlie the isolated platforms formed an effective seal for the system. Our sequence stratigraphic analysis reveals that the best source and reservoir rocks were deposited within the same sequences.
*in press in Homewood, P.W., and Eberli, G.P., eds., Sequence Stratigraphy and Hydrocarbon Potential of Mixed Carbonate-Siliciclastic Systems, Elf Aquitaine Bulletin Memoir.
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Last updated on March 7, 1997