ABSTRACT

The Fairholme-Sassenach succession is represented by three distinct facies that were deposited: a) along the locus of a southwest-facing reef front that probably marks the periphery of the southern Alberta shelf, b) in a basinal area to the southwest, and c) along the margin of the basinal area adjacent to the locus of the reef front.

The earliest Fairholme sediments consisted of a sheet-like unit of chemically precipitated carbonate muds with variable amounts of evaporites, clay, silt, and sand (Hollebeke Formation). They were deposited above an erosion surface of very low relief developed on Cambrian and older rocks, following a period of gentle southeastward epeirogenic tilting. Subtle facies variations within this unit outline an area of slightly more rapid subsidence and deeper water deposition that lay along the southwestern margin of an extensive shoal where carbonate muds and evaporites were deposited under conditions of restricted circulation.

An abrupt regional increase in subsidence resulted in normal circulation over the shoal and the previously subtle contrast between the shallow and deep water environments of deposition became accentuated. Dark, organic-rich, shallow-water, skeletal limestones were deposited as a series of biostromes and related calcarenites (Borsato Formation) in the turbulent shallow water above the shoal, while dark muds (Perdrix Formation) accumulated beyond its margin. A slightly emergent zone on the shoal that had been intermittently populated by algae, stromatoporoids, and Amphipora became a locus of biohermal reef growth. Subsequently, progressive increase in subsidence resulted in drowning of most of the biostromes and restriction of the environment favourable for the growth of lime-secreting organisms to the locus of the biohermal reef front proper. Light coloured, open-textured, skeletal limestones (normal facies of the Peechee Member of the Southesk Formation) accumulated in the turbulent, well-aerated, shallow water along the reef at a rate commensurate with the rate of subsidence, while dark organic muds and silts (Mount Hawk Formation) were deposited much more slowly in the poorly aerated deeper water beyond the margin of the reef. Coarse, organic-rich, skeletal detritus (dark transitional facies of the Peechee Member) that was swept off the reef accumulated in a narrow transition zone along the margin of the basin at the foot of the reef. As reef growth continued local relief in the immediate vicinity of the reef front increased to a maximum of approximately 100 feet.

Upward growth was eventually arrested by a decrease in rate of subsidence. Skeletal detritus and organic debris derived from the reef and discharged into the adjacent parts of the basin formed a widespread blanket (Grotto Member of the Southesk Formation) over the off-reef muds and obliterated most of the local relief in the immediate vicinity of the reef front. In the final stages of Fairholme deposition under conditions of very slow subsidence upward reef growth was essentially static and clean skeletal calcarenites (Arcs Member of the Southesk Formation) accumulated in the turbulent and well-aerated shallow water environment that extended out over what had formerly been the marginal zone of the basin. Fairholme deposition terminated with a regional regression that left most of this reef margin complex emergent. Between 300 and 500 feet of relief existed between the top of the reef margin complex and the deeper parts of the basin floor to the southwest at this time. Deposition in the basin appears to have continued locally without interruption during the regression. A sequence of quartzose clastic sediments (Sassenach Formation) filled the basin while the reef margin complex was emergent and accordingly the remaining relief was obliterated. Subsequently, a thin sequence of shallow water evaporitic carbonates (upper part of the Sassenach Formation?) was deposited over the whole region as a prelude to deposition of the Palliser limestone.

1 Published by permission of the Director, Geological Survey of Canada, Department of Mines and Technical Surveys, Ottawa.

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The geometry of the reef margin complex was conspicuously modified as a result of differential compaction in post-Sassenach time. The thickness of the off-reef muds has been reduced by as much as 40 per cent more than that of the carbonates in the reef margin complex. Fluids expelled from the muds and some of the older sediments during compaction are probably responsible for the complete dolomitization of the highly permeable skeletal limestones of the Borsato Formation and the Southesk Formation in the reef margin complex.