Chapter 12 geologic time section 12.3 dating with radioactivity
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As a result, the details of basin-filling and their relations to reef and carbonate shelf development are complex. During Woodbend deposition, the major geological elements that directly or indirectly influenced deposition of the group were: In general, maximum subsidence occurred over the old central Elk Point Basin north and west of the Meadow Lake Escarpment.
Relative subsidence was much less to the south and southeast of the escarpment, which also marks the change from dominantly carbonate-clastic cycles in the northwest to carbonate-evaporite cycles in the southeast. The Peace River Arch, the most topographically prominent element, continued to be an emergent landmass. However, throughout Woodbend and Winterburn deposition the landmass area diminished in size as a result of gradual subsidence and onlapping of the arch by a series of backstepping fringing reef complexes Dix, In addition, normal block-fault movements in this area affected both Woodbend and Winterburn deposition O'Connell et al.
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The West Alberta Ridge, largely inundated by the end of Beaverhill Lake sedimentation, continued to affect deposition, although at a progressively reduced level during Woodbend sedimentation. Initially, however, it was a site of major Leduc reef complex development. The Tathlina High, although completely buried by the end of Middle Devonian Elk Point deposition, resumed its influence on Woodbend deposition. Relative to adjacent areas, the Tathlina High and Ft. Nelson Uplift remained submarine topographic highs against which basin-fill shales downlapped and thinned.
Although block faulting of the basement influenced Woodbend deposition in the vicinity of the Peace River Arch, it was most dramatic in the vicinity of the Tathlina High, especially where fault movements accompanied by Elk Point salt removal produced large increases in accommodation space during deposition of the Fort Simpson Formation see Fig. This feature had little significant effect on Winterburn sedimentation. The Meadow Lake Escarpment, a pre-Devonian erosional and structural feature well known for its tremendous effect on Middle Devonian sedimentation, also appears to have influenced many of the depositional facies changes in Upper Devonian strata.
For example, the southern Alberta Leduc shelf edge closely coincides with both the underlying southern bank edge of the Beaverhill Lake Group Oldale and Munday, this volume, Chapter 11 and the Meadow Lake Escarpment. As described by Ross and Stephenson , a large-scale basement trend, known as the Rimbey Arc, coincides with the southern half of the km long Leduc-Homeglen-Rimbey-Meadowbrook reef chain. In addition, this lineament, in part, areally coincides with a change in subsidence and accommodation space during various phases of Woodbend basin-filling see discussion of subdivisions within Cooking Lake-Majeau Lake deposits.
Both the Woodbend and Winterburn groups were subjected to post-depositional dolomitization and structural movements Mountjoy, ; Machel, ; Dix, ; O'Connell et al. At a detailed level, basement fault movements and the removal of salt at differing times from both within and below Woodbend-Winterburn strata, created locally complex stratigraphy and structure. Despite these localized complexities, the present-day structure of the Woodbend-Winterburn strata Fig. Dips increase both toward the west, due largely to downwarping during the Laramide Orogeny, and toward the centre of the Williston Basin, in Saskatchewan.
To illustrate effectively the depositional history and paleogeography of such a thick and complex stratigraphic succession, the Woodbend and Winterburn groups are herein subdivided into seven major chronostratigraphic intervals. These intervals are designated using central Alberta stratigraphic terms. They are, from youngest to oldest:. With the exceptions of the base of the Cooking Lake, the top of the upper Graminia and the Leduc, each of which require special discussion, these intervals are bounded and defined by correlatable borehole log markers, which are interpreted as chronostratigraphic surfaces throughout the basin.
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Interval boundaries are shown as heavier lines on all cross sections. To achieve consistency of treatment, more than 10 borehole logs were copied, made into cross sections, correlated and, where appropriate, the major bounding chronostratigraphic markers for the chosen intervals were recorded for inclusion in the Atlas database.
Many of the distinct log signatures can be traced confidently over vast areas of the basin, while others will require additional core work, coupled with biostratigraphic or other dating data, before a higher level of confidence in correlation can be attained. As shown in Figure However, some intervals depart dramatically from the recognized formal lithostratigraphic units. In some examples e. In areas where the log markers are difficult to pick, the magnitude of error is generally considered to be less than 30 m. Errors of this magnitude are likely confined to areas with thick continuous basin-filling shales, where bounding surfaces do not exhibit a distinctive log character e.
Given the possible alternative picks, the shift in resulting isopach values is less than the isopach contour interval of the Atlas maps. Thus, the pattern of deposition inferred from these isopach maps is relatively insensitive to minor miscorrelation. Notwithstanding these uncertainties, the correlation of these bounding surfaces, in most cases, is consistent with published and available albeit limited biostratigraphic control see McLaren , ; McLean and Sorauf ; Marchant and McLean In addition, these surfaces also are consistent with some of the distinctive breaks in deposition noted from published and non-proprietary core studies e.
Designation of these chronostratigraphic events assists in highlighting the transgressive-regressive cyclicity of the Woodbend-Winterburn groups. This cyclicity is schematically shown in Figure The basic pattern for each cycle is: The initial stage is dominated by carbonate platforms.
The maximum transgressive stage has the best developed organic-rich shales and isolated carbonate complexes. The regressive stage is dominated by basin-filling shales and carbonates. In general, the Woodbend Group is characterized by transgressive deposits while the Winterburn Group is dominated by regressive deposits.
The Woodbend Group comprises the first five chronostratigraphic intervals identified in the Woodbend-Winterburn succession i. Each interval is interpreted as consisting of a relatively rapid upbuilding phase aggradation of shelf and reefal sediments, followed by a phase dominated by the progressive infilling of basinal areas by shales and marlstones progradation.
Reef and shelf aggradation occurred during relative sea-level rises, whereas the progradational phase of basin-filling was restricted to periods of relative sea-level fall or stillstand. It should be noted that for mapping purposes a distinction between post-reef and reef-equivalent basin-fill deposits was generally not made. The intervals and details of their paleogeography, isopach and lithofacies variations, and areas of problematic correlation, are discussed in stratigraphic succession. The Cooking Lake interval comprises extensive sheet-like shelf carbonates and an equivalent basin-filling shale.
During deposition of the Cooking Lake interval a gradual deepening and variable increase in accommodation space occurred in different parts of the basin. This represents a significant change from the more subdued basinal relief observed within the underlying Beaverhill Lake Group. Three major paleogeographic realms were established during the Cooking Lake interval. For mapping purposes Fig. This type section lies within the eastern shelf area and was designated by the Geological Staff, Imperial Oil Limited in the Calmont Leduc 3 W4 well see inset, Fig. Elstow W3 type section well Kent, An extensive carbonate shelf developed in the east over southern Alberta, Saskatchewan and Manitoba, where underlying Beaverhill Lake sediments had reached a suitable bathymetry for subsequent prolific carbonate production.
The shelf thickens to 90 m directly beneath the Rimbey-Meadowbrook Leduc barrier reef-trend and then thins abruptly to less than 6 m immediately west of this trend Figs. The Cooking Lake carbonates consist of peloidal and skeletal limestone brachiopods, crinoids, stromatoporoids, bryozoans. Unlike the majority of the younger Woodbend carbonates within the eastern shelf area, the Cooking Lake interval is relatively undolomitized.
Thick, extensively dolomitized Cooking Lake strata are found only beneath the Homeglen-Rimbey-Meadowbrook Leduc reef trend and its southerly extension into the Caroline-Cheddarville area Figs. The Cooking Lake shelf edge extends in a north to northeast direction from Township 38 to subcrop at Township 92, a distance of km. South of Township 38, the orientation of the shelf edge changes to an east-west direction and persists into the foothills under the Brazeau and Bighorn thrust sheets.
Whether this long, linear shelf-edge is controlled by a change in subsidence across a deep-seated hinge line, or is simply a function of antecedent Beaverhill Lake depositional topography, has yet to be resolved. As described by Ross and Stephenson , the southern half of this shelf edge closely coincides with the Rimbey Arc, a major linear basement trend.
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It is also noteworthy that the northern half of the Cooking Lake shelf edge closely follows the margins of carbonate banks developed within the underlying Mildred and Moberly members of the Beaverhill Lake Group. The position and thickening of the Cooking Lake shelf edge could in fact reflect both structural and inherited topographic controls, which might themselves be genetically related. Portions of the eastern shelf have been described in detail by Andrichuk , Kent , Wendte and Dunn They have shown that shelf strata can be divided into three units, consisting of an upper and lower carbonate cycle separated by an argillaceous carbonate see for example Fig.
These units are remarkably consistent in thickness and lithofacies over a broad area of the basin, suggesting that extremely stable conditions persisted during Cooking Lake deposition. Even in the vicinity of the underlying Meadow Lake Escarpment, there is no apparent effect on Cooking Lake deposition. The lower Cooking Lake cycle represents a westerly i. With only local exceptions, the lower carbonate cycle is conformable with the underlying Beaverhill Lake Group. As such, the lower bounding surface of the Cooking Lake interval is not considered to be a regional chronostratigraphic event, although in parts of eastern Alberta this contact may be abrupt Wendte, pers.
The separation between the Beaverhill Lake and Cooking Lake becomes less distinct toward the eastern portion of Saskatchewan, where no clear change in log signature is detected see Fig. Despite this limitation, isopach mapping using an approximate Beaverhill Lake top in Saskatchewan can constitute only an insignificant error, given that the transition from Beaverhill Lake to Cooking Lake occurs over a narrow interval.
This approximation of the isopach does not significantly affect the interpretation of the Cooking Lake interval map Fig. The pattern of basin-filling observed in the underlying Beaverhill Lake Group continued without any distinct break into the lower cycle of the Cooking Lake Formation.
This feature is well noted in Saskatchewan where the upper portion of the Beaverhill Lake and lower Cooking Lake-equivalent are recognized as the Saskatoon Member of the Duperow Formation see Fig. The top of the Saskatoon Member and lower Cooking Lake carbonate cycle appear to represent a basinwide, chronostratigraphically significant event see Fig. This event marks the onset of gradual deepening of the basin and areal restriction of subsequent shelf and reef development within the overlying Leduc Formation.
The middle argillaceous unit of the Cooking Lake shelf strata has been designated as the Elstow Member in Saskatchewan Kent, It rests sharply and disconformably on the underlying carbonate cycle, and represents the last significant occurrence of shale deposited within the shallow-water carbonate complexes of the Woodbend group until late in Leduc shelf development see Figs. The thickness of this shale unit varies from less than 3 m in southern Saskatchewan to as much as 30 m beneath the Bearberry, Ricinus, Strachan and Phoenix Leduc reefs near the mountain front Fig.
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The source of the shale has yet to be proven but probably was introduced from the northeast. The middle argillaceous unit is largely confined to the extensive Cooking Lake shelf; it does not appear to make up any significant component of basin-filling shale to the west.
These studies form an important foundation to the regional perspective provided by this chapter. The presence of an overlying progradational stage is not obvious and it is probably very thin. The Zeta Lake pinnacle reef trend at West Pembina is situated approximately between the 80 and m contours on the southeastern side of the Cynthia Basin Fig. Uplift at the end of the Frasnian may have been in response to tectonic activity occurring along the continental margin of the Western Canada Sedimentary Basin. The sedimentology of these sandstones has not been broached in the published literature.
However, it appears to be contiguous with the lower part of the shaly facies of the upper Cooking Lake cycle in the northeast Andrichuk, The upper Cooking Lake carbonate cycle is transitional with the underlying shale unit, but the top is abruptly terminated and capped by a submarine hardground surface Wendte, This cycle correlates with the lower Wymark Member in Saskatchewan Kent, The nature of the bounding surface or contact at the top of both the lower and upper Cooking Lake carbonate cycles has not been described in all parts of the basin, particularly toward the eastern subcrop edge.
Unlike the ramp profile of the lower Cooking Lake carbonate cycle, the upper cycle has an abrupt, steeply dipping western shelf edge. This edge is recognized by an abrupt thinning in the Cooking Lake isopach Fig. It is the termination of the upper cycle that actually defines the Cooking Lake shelf edge. The upper Cooking Lake cycle is more areally restricted than the underlying cycle. Carbonate shoal development within the upper Cooking Lake cycle was the main contributor to thickness variations in the eastern shelf.
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Andrichuk , Klovan and Wendte noted that the configuration of these shoals was important in localizing subsequent Leduc reef growth. The top of the Cooking Lake interval appears to be a synchronous surface throughout the basin. In some areas it can be confidently traced into the basin-filling Majeau Lake shale. Consequently, only the lower Majeau Lake basin-fill belongs within the Cooking Lake interval.
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Although the lower Majeau Lake shale lies within this interval it appears to postdate the development of the Cooking Lake shelf. As noted by Stoakes and Wendte , it is difficult to distinguish the Cooking Lake Formation over the Swan Hills reef complexes of western Alberta from younger Leduc reef and platform development.
If present, it is likely less than 30 m in thickness. As shown in Figures More detailed work will be required to map Cooking Lake strata that fringe the Peace River Arch and extend over the Swan Hills reef complexes of the Beaverhill Lake Group before the evolution of carbonate growth in these areas will be satisfactorily resolved. North of the Peace River-Athabasca Arch the lower Majeau Lake unit either thinned and changed facies into bituminous shales of the Muskwa Formation or was not deposited.
To the south of this arch, this unit thins westward onto the Swan Hills bank.