ABSTRACT

The Leduc Formation (Frasnian in age) forms a northeast-convex, "horseshoe"-shaped dolomite trend comprising mid-shelf through slope facies around what was a large, northeast-trending continental island, the Peace River Arch. Together with inner-shelf siliciclastics (arkose, feldspathic arenite), the Leduc represents an insular fringing reef complex. Geometry and thickness of the Leduc Formation has been controlled by predepositional topography, encroachment of siliciclastics, local tectonics, oceanographic events, and relative changes in sea level. The Leduc comprises three depositional sequences, the youngest (SeqI) being the

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best constrained in terms of facies and geometry. The sequence succession backsteps onto the PRA, with little change in position of the Leduc seaward margin. SeqIII represents a low-relief shelf or ramp, ten's of metres in thickness, facies-equivalent of basinal shales and shoreward siliciclastics. Most of the Leduc carbonate (up to 200 m) accumulated during SeqII time; where cored, the sequence contains shallow-water platform carbonates. The southeast margin of SeqII appears erosional or nondepositional, with relief of ten's of metres, against which was deposited a thin sequence of Ireton strata. The SeqI/II boundary along part of the northern margin (T87, west of 3W6M) is marked by siliciclastics (locally with vadose cement) and indicates across-shelf transport coincident with a short-term sea level lowstand. Influence of siliciclastics throughout the Leduc in this area is controlled by local tectonics. Elsewhere, the I/II boundary is marked by shaly dolomite and shale. SeqI carbonate development (up to ten's of metres thick) is not uniform around the Arch, and appears to vary in geometry from ramp to shelf. The sequence is dominated by deep-water facies with local skeletal accumulations (reefs?) of laminar stromatoporoids and corals along the complex's seaward margin, and contains local shale embayments and siliciclastic bypass channels. The Leduc is overlain by shales and basinal limestones; the upper two sequences suggesting a deepening-upward succession leading to the demise of the reef complex.

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