ABSTRACT: Devonian Carbonate Sequences in the Southern Canadian Rocky Mountains

GELDSETZER, H. H. J., Geological Survey of Canada, Calgary, Alberta, Canada

Spectacular exposures along 1100 km of the southern Canadian Rocky Mountain thrust belt contain a composite of five Devonian carbonate sequences. The distribution in this western exposed part of the "Western Canada basin" is controlled by a passive high, the West Alberta ridge (WAR), which merges northward with the Peace River arch (PRA). The first three carbonate sequences progressively onlap the WAR while the last two overstep the WAR with exposures straddling the western flank of the WAR. All sequences are bounded by pronounced unconformities that become less obvious in adjacent intracratonic basins and disappear in the mioclinal basins to the west where anoxic sediments (Road River, Besa River) accumulated throughout the Silurian and Devonian. All sequences except for the fourth o e have remarkably uniform lithologies and are therefore each represented by one formation only.

The first carbonate sequence--Muncho-McConnell Formation of Ludlovian to Lochkovian age--is a thick peritidal quartzose dolomite resting unconformably on Ordovician to Silurian carbonates and wedging out to the south against the WAR. The second sequence--Stone Formation of Emsian age--consists also of peritidal, mostly sandy dolomite overstepping the first sequence by 150 km. The third sequence--Dunedin Formation of late Eifelian to mid-Givetian age--is represented by open-marine carbonates overstepping the second sequence by 120 km. Prominent basal sandstones and successive overstepping by the first three sequences document gradual southward onlap of the WAR interrupted by periods of uplift or eustatic sea level fluctuations.

A strong regression--Watt Mountain hiatus in late middle Givetian time--caused regional emergence and erosion of the PRA. A transgression of probably global proportions--Taghanic Onlap--began at about the Givetian-Frasnian boundary and continued with only two minor interruptions to the end of the Frasnian. This transgression is represented by the fourth sequence--the Fairholme Group--a complex assemblage of carbonates and siliciclastics deposited at varying water depth. The main carbonate lithosomes are a widespread basal platform (Flume), overlain by buildups ranging from small patch reefs to massive reef-rimmed platforms (Cairn-Peechee; Leduc) and two post-buildup banks centered above the buildups and prograding progressively outward. Both banks are bounded by minor unconformities a d both the lower (Arcs-Grotto; Nisku of late Frasnian age) and the upper (Ronde-Simla; Graminia-Blueridge of latest Frasnian age) host patch reefs within the prograding margins.

The origin of the buildup phase (the well-known hydrocarbon-bearing Frasnian reefs of the Western Canada basin) is ascribed to a sudden change of the sea water chemistry from aerobic to dysaerobic terminating carbonate production on the Flume platform except for scattered elevated areas where shallow-marine carbonates continued to accumulate.

After a brief emergence of the carbonate banks during earliest Famennian time, a regional carbonate shelf--Palliser Formation of early and middle Famennian age--was established. This last carbonate sequence ended with the deposition of late Famennian black shale (Exshaw Formation). During a prominent regression prior to the deposition of upper Tournaisian sediments (Banff Formation), the Exshaw and Palliser formations were partly or entirely eroded in the northern area.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)