Mountain building influenced the evolution of the Alberta BasinWhat
The AGS has undertaken a structural and geochronological study in order to establish the correlations between the tectonic processes in the Rocky Mountains and Foothills and the changes in depositional processes and sediment input in the Alberta Basin. We have dated major thrust faults in the southern Canadian Rockies and offered new and independent constraints confirming that deformation of the foreland fold-and-thrust belt (Main and Front ranges of the Rocky Mountains and in the Foothills) propagated eastward, with thrusting taking place in distinct pulses that were separated by intervals of relative tectonic quiescence. Work is underway to date major thrusts in the West Ranges of the Rocky Mountains.
The Alberta Basin and the adjacent Rocky Mountains constitute a foreland system. To understand the stratigraphy and depositional processes in the Alberta portion of the Cordilleran Foreland Basin it is critically important to understand the tectonic processes that lead to the rise of the Rockies. Figure 1 illustrates the direct relationship between thrusting in the Rockies and its consequences in the Alberta Basin: flexural subsidence and uplift are the primary controls on the creation or destruction of accommodation space in the foreland, thus controlling, to a large extent, the type of depositional system and the thickness of depositional sequences.
|Rocky Mountains Foreland Belt||Alberta Foreland Basin|
Figure 1: Depositional changes in the Alberta foreland basin were triggered by orogenic pulses in the Rocky Mountains fold-and-thrust belt
The formation of the Rocky Mountains’ fold and thrust belt had a profound effect on Alberta's hydrocarbon and coal potential:
1) Within the deformed belt, fold and thrust faults
- produced structural traps, some of which are filled or partially filled with hydrocarbons, mainly gas (e.g., commercial fields in thrust faulted Paleozoic carbonates on the Alberta Foothills), and
- brought at or near surface thick and extensive coal-bearing sections.
2) In the Alberta Basin, tectonic loading caused by this deformation depressed the western edge of the North American craton and resulted in
- a regional westward dip, which inclined the stratigraphic traps into a favourable orientation so that they could be filled with hydrocarbons generated farther west;
- preservation of a westward-thickening foreland basin succession containing important coal-bearing horizons; and
- increased burial and thus maturation of the sedimentary section to the west in front of the deformed belt; in the ‘deep basin’ of west-central Alberta, increased maturation caused by burial beneath foreland basin deposits resulted in Devonian to Lower Cretaceous strata passing through the oil window into the dry gas zone.
Methodology and Results
Thrust faults in the Main and Front ranges as well as thrusts in the Foothills formed at shallow structural level, in the brittle regime. Such faults are marked by gouge that is notoriously difficult to date. We have employed a new analytical technique, which consists of independent 40Ar/39Ar dating of four grain size fractions of illite separated from each fault gouge sample. In the West Ranges where thrusts developed at greater depth, we have used the classical 40Ar/39Ar dating technique of phyllonite.
Gouge dating of thrusts in the Main Ranges showed that the Pyramid (163.0 Ma), Simpson Pass (161.7 Ma), and Johnson Creek (145.7 Ma) thrusts were partly contemporaneous with the development of the first clastic wedge (Fernie-Kootenay/Nikanassin/Minnes strata) in the foreland basin. Phyllonite dating along the western margin of the Rocky Mountains indicated that Early Cretaceous dextral oblique slip, with a peak between 136-123 Ma, corresponds to a major Valanginian-Barremian depositional hiatus in the foreland basin followed by the deposition of the Barremian(?)–Aptian Cadomin conglomerate sheets. In the Front Ranges, the emplacement of the Greenock thrust (103.1 Ma) and Broadview–Snake Indian thrust (99.2 Ma) was contemporaneous with development of Cenomanian Dunvegan deltaic deposits in the immediate foreland (Fig. 2).
Figure 2: Thrusting in the Alberta portion of the fold-and-thrust belt was contemporaneous with dextral transpression and igneous activity along the northern Rocky Mountain Trench, which triggered the development of a major fluvial to marginal-marine depositional system in the Alberta Basin.
Thrusts in the Front Ranges, the Rocky Pass (74.8 Ma), Sulfur Mountain (75.6 Ma), and Clearwater (74.2 Ma) thrusts, define a Campanian phase of tectonic loading that led to the last major transgression in the southern portion of the Alberta foreland basin (Fig. 3). Along the eastern margin of the Front Ranges, the McConnell thrust (54.0 Ma), together with the Muskeg (52.4 Ma), Brule (53.9 Ma), and Nikanassin (52.1 Ma) thrusts in the Foothills, recorded the last phase of regional contraction. The Late Jurassic, Early Cretaceous, mid-Cretaceous, Late Cretaceous, and early Eocene deformation pulses are separated by relatively long periods of tectonic quiescence.
Figure 3: A major Campanian thrusting pulse in the Alberta portion of the fold-and-thrust belt triggered subsidence and associated accommodation, which was marked by the last major transgression—the Bearpaw Sea—in the Alberta Basin.