Additional Contributors:
R. Bezys - Manitoba Energy and Mines, Winnipeg
H.R. McCabe - Consultant, Winnipeg
This chapter summarizes our knowledge of the distribution and character of the Jurassic and lowest Cretaceous (to about Barremian) strata of the Western Canada Sedimentary Basin (Figs. 18.1, 18.2). These strata contain the epicratonic record of the early phases of the Columbian Orogeny. Clastic rocks in the thin, stable-platform pre-orogenic assemblage are mineralogically mature, derived from cratonic sediment sources in the east, whereas the thick sequence of younger, less mature, foreland trough strata were derived largely from orogenic uplifts to the west. Farther east, the Williston Basin was characterized by pronounced downwarping in the Middle Jurassic and eventual filling in the Late Jurassic.
The Jurassic and lowest Cretaceous sedimentary rocks form significant oil and gas reservoirs in central and southern Alberta and southwestern Saskatchewan, and some are hydrocarbon source rocks in Western Canada (Creaney et al. this volume, Chapter 31). The immense coking coal reserves of the southern Canadian Rocky Mountains are found in the Upper Jurassic to lowest Cretaceous succession (Smith et al., this volume, Chapter 33). Probable Jurassic rocks in southern Manitoba host two of Canada's major gypsum mines, and some of the currently subeconomic, yttrium-enriched, phosphate deposits in southeastern British Columbia (Butrenchuk, 1987; Pell, 1991) and southwestern Alberta (Macdonald, 1987) are in Jurassic strata (Christie, 1989).
The Jurassic and lowest Cretaceous strata (as well as the basal Mesozoic redbeds in Williston Basin, which may be either Triassic or Jurassic in age) overlie a regionally unconformable erosional surface that exhibits significant local relief. Karst features are present in some of the sub-Mesozoic limestones. Some of the relief is caused by resistant Paleozoic limestone units that formed cuestas on the sub-Mesozoic surface, and some is due to subsidence in those areas affected by dissolution of Paleozoic salt units. Some of the meteorite impact craters that created relief on the sub-Mesozoic surface in southern Saskatchewan and Manitoba are Jurassic or pre-Jurassic in age (e.g., McCabe, 1971; Sawatzky, 1974).
In western Alberta and eastern British Columbia, the sub-Jurassic unconformity truncates successively older beds eastward onto the craton, from Triassic and Permian in the west, to Carboniferous in the east. In westernmost outcrops, the pre-Jurassic hiatus may have been of short duration. In Saskatchewan and Manitoba, Mesozoic strata lie on rocks as old as Devonian regionally (but as old as Precambrian in some ancient valleys in Manitoba) around the northern and eastern edges of Williston Basin.
Eastward and northward in Alberta, first lowest Cretaceous, then Jurassic strata thin and disappear, partly by shoreward depositional thinning, but largely by truncation below unconformities within the Jurassic and below the younger Lower Cretaceous Mannville Group and equivalents. The approximation of unconformities, and changes in several superposed units to shallower sandstone facies toward the east, cause particular difficulties in accurately identifying and correlating rock units in central Alberta. Jurassic strata are progressively truncated toward the north in Saskatchewan and Manitoba, away from the centre of Williston Basin.
The pre-Aptian Mesozoic sequence includes thin and discontinuous, unconformity-bounded, Upper Jurassic and Lower Cretaceous sandstone units preserved in erosional lows over much of southern and central Saskatchewan and Alberta. The distribution of these units, lying across and beyond the truncated edges of older Jurassic strata, attests to the development of the major "sub-Mannville" unconformity below them in Barremian or earlier time.
New subsurface stratigraphic data and correlations, based partly on new discoveries of ammonites and palynomorphs, are incorporated into this chapter, and further revisions are expected. Local variation in the Jurassic rock units in some areas dictates that a complete understanding of the successions must be based on a large number of closely spaced wells and stratigraphic sections with age-significant fossils. Detailed studies with good biostratigraphic control over broad areas are few, and good core material and fossils scarce. Because of generally poor exposure, deformation and high thermal alteration, the outcrops of Jurassic strata pose special problems. These constraints are reflected especially in the isopach maps for the Rocky Mountains and foothills outcrop areas, which should be considered as generalized approximations of data that are highly variable in quality.
In many areas, identifying the base of the Mannville Group or equivalents where it overlies lithologically similar Upper Jurassic sandstones or shales has caused difficulty, whereas in other areas, a significant chert component differentiates the Cretaceous from the mainly quartzose Jurassic strata. Where Lower Jurassic limestones lie above Carboniferous limestones in central Alberta, or above Triassic limestones in the subsurface of northeastern British Columbia, the contact is commonly difficult to place. Correlations within the Jurassic and lowest Cretaceous succession remain highly interpretive in many areas. In particular, correlations across the Sweetgrass Arch and the Birdtail-Waskada axis (Fig. 18.2b, 18.3), that is, from southern Alberta into southern Saskatchewan, and from there into southern Manitoba, are poorly supported by paleontology and still contain uncertainties.
This compilation has involved re-picking of formation tops over major areas of northeastern British Columbia (Gilchrist), northern Alberta (Tittemore), southern Alberta (Hayes), Saskatchewan (Christopher), and Manitoba (McCabe). The small number of wells controlled by Canstrat studies in southern Saskatchewan and Manitoba places limits on the reliability of the lithofacies maps in these areas.
The Jurassic and lowest Cretaceous strata of the Rocky Mountains and foothills outcrop belt have been the subject of several in-depth studies. A few detailed subsurface studies have been undertaken in small areas of Alberta and Manitoba, and most of southern Saskatchewan. Major summaries and sources for additional reports on the Jurassic and lowest Cretaceous of the Western Canada Sedimentary Basin include reports by Springer et al. (1964), Stott (1970) and Poulton (1984, 1989). Reports synthesizing specific large parts of the region, and providing access to more detailed studies, include those by Frebold (1957, 1969), Stott (1967, 1984), Hall (1984), Stronach (1984), Christopher (1964, 1974, 1984), Gibson (1985) and Poulton et al. (1990).
Jurassic rocks occur in three major epicratonic depositional settings: on western parts of the pre-orogenic, essentially stable cratonic platform or shelf; in the succeeding foreland trough or foredeep superimposed on the same area - the Rocky Mountain Trough; and in Williston Basin (Figs. 18.2 and 18.3).
The pre-orogenic Lower Jurassic sequences are thin and consist of platformal limestones and cherts, and widespread, starved-shelf, phosphatic, organic-rich shales, limestones, and sandstones separated by disconformities (Fig. 18.4). In contrast, the Middle Jurassic strata consist of more localized, slightly thicker siliciclastic units. It is unclear to what extent, if any, deposition of the Lower Jurassic strata over the regional sub-Jurassic unconformity indicates subsidence that might be related to western tectonic events.
The western platform perhaps was less stable during the Early and Middle Jurassic than is generally thought, as indicated by the differences from one place to another in the units deposited and preserved, and the depth of erosion at unconformities within the Jurassic. For example, a major pre-Late Jurassic erosional event removed most of whatever Middle Jurassic strata were deposited in a broad area in northeastern British Columbia and northwestern Alberta, across and well beyond the limits of the much older Peace River Arch (Poulton et al., 1990). Thickening of the Jurassic succession in small areas in the Peace River region suggests localized Jurassic subsidence or differential compaction over Paleozoic fault blocks and grabens (Poulton et al., 1990), but there is no major Jurassic expression of the Peace River Arch or Embayment. Some of the thickness and facies variations might suggest early effects of initial Cordilleran orogenic events underway to the west (e.g., Poulton, 1984).
No western margins or hingelines (with abrupt westward thickening) are preserved for the Lower and Middle Jurassic basin to confirm the presence of the miogeoclinal package that is assumed, in current tectonic models, to have lain west of the platform.
The coarsening- and shallowing-upward succession that begins with Upper Jurassic marine shales (or a locally developed basal sandstone) and is dominated by basin fill, primarily non-marine sandstones, makes up the first undisputed orogenic foreland trough deposits. It is the first of several, westerly derived, orogenic clastic wedges, and extends into the lowest Cretaceous.
The narrow foreland trough (Fig. 18.2c), elongated parallel to the present Rocky Mountain front, was superimposed on underlying, more evenly distributed, Lower and Middle Jurassic rocks. Insofar as the orogenic wedge strata become thicker to the west, the depositional axis of the trough lay west of their western preserved limit.
In southwestern Alberta and southeastern British Columbia, paleocurrent directions in Upper Jurassic strata indicate generally northerly transport (Norris, in Poulton, 1984; Hamblin and Walker, 1979). The interpretation of western orogenic sources, primarily upthrust epicratonic Paleozoic strata (Gibson, 1985), for most of the foreland trough-fill in southern Canada, involves re-orientation of currents in the non-marine, shoreline and offshore deposits to a northward, basin-axial direction, even in the most westerly strata known. Diachroneity at the base of the orogenic sandstones is unproven paleontologically, and accordingly, the figures in this chapter offer alternative interpretations. A few extra-basinal metamorphic clasts have been recognized in the sub-Cadomin conglomerates in the south (Jansa, 1972). In northeastern British Columbia, western source terranes are clearly indicated by metamorphic and exotic chert clasts (M. McMechan in Poulton, 1984; Gabrielse and Yorath, 1989). Uplifted parts of the former Williston Basin may have served as a southern or southeastern source for some of the detritus (Stelck et al., 1972).
Initial orogenic clastic wedges vary in age from Late Jurassic in southern Alberta and British Columbia, to Cretaceous in the northern Yukon (Poulton, 1984). This northward progression of initial foreland trough sedimentation may be an indication of a similar shift in accretionary uplifts to the west, such as the northward motion of westerly source areas along large-scale dextral strike-slip faults lying along the western side of the Omineca Belt (Eisbacher, 1981).
Williston Basin (Figs. 18.2b, 18.3) was rejuvenated and subsided during the Jurassic. The earliest paleontologically dated Jurassic deposits are early Middle Jurassic (Bajocian). More axial parts of the basin underwent increased subsidence rates during the later Middle Jurassic. Williston Basin was filled, and the area was involved in regional uplift, in the Late Jurassic.
Sweetgrass Arch, separating the western parts of the platform from Williston Basin, was a paleotopographic high during most of the Jurassic, and was transgressed progressively beginning in the Bajocian. The broad Swift Current Platform forms the stable northwestern part of the basin, with a heterogenous shallow-marine sequence recording a complex history of sea-level fluctuations.
The Birdtail-Waskada axis (McCabe, 1967, 1971) coincides with the Thompson Boundary Fault of Kent and Christopher (this volume, Chapter 27) and with the eastern part of the Precambrian Trans-Hudson Orogen that separates the Superior and western Archean provinces (Hoffman, 1989). It was a paleotopographic high during the Middle Jurassic, and is characterized by structural and geophysical anomalies at several levels in the geological column. During the early Middle Jurassic the axis separated the northeastern portion of Williston Basin in Manitoba from the main part of northern Williston Basin in Saskatchewan (Figs. 18.2b, 18.3).
In Manitoba, the Jurassic/Triassic evaporite and redbed succession at the base of the Mesozoic is preserved within a major pre-Mesozoic valley system (the Dominion City Channel; see Fig. 18.1 and Corkery, 1987), which was incised as deep as Precambrian Shield rocks. The redbeds and evaporites overlie probable Permian meteorite impact breccias and melt rocks in an outlier in the Lake St. Martin's impact structure (Fig. 18.1). Major regional peneplanation and valley formation occurred over all but the westernmost parts of the Western Canada Sedimentary Basin area during the mid-Early Cretaceous, prior to deposition of poorly understood sandstones in or about Barremian time. These depositional events were, in turn, succeeded by another regional erosional event prior to deposition of the Mannville and Blairmore groups, beginning in about the Aptian.
Last modified: August 15, 2008
