Abstract

The Middle Triassic succession of the Western Canada Sedimentary Basin consists of a series of prograding offshore to shoreface sediment lenses of the Doig Formation, overlain by a widespread shoreface sandstone of the Halfway Formation, in turn overlain by lagoonal and supratidal deposits of the Upper Triassic Charlie Lake Formation. Two distinct erosion surfaces are present in the Halfway to lowermost Charlie Lake succession — within or at the base of the Halfway Formation, depending on geographic location, and at the base of the Charlie Lake Formation. Early studies interpreted the erosion surface associated with the Halfway Formation to be an unconformity. At least one author interpreted an unconformity at the base of the Charlie Lake Formation. Subsequently, a conformable facies interpretation became dominant, with the erosion surface associated with the Halfway Formation relegated to be the base of tidal channel deposits. In the conformable facies interpretation, the Doig to lowermost Charlie Lake succession consists of a series of prograding transgressive–regressive sequences. Recently the unconformity interpretation has been resurrected with regional correlations and facies analysis to support the interpretation.

The distribution of facies and stratigraphic relationships do not fit the conformable facies interpretation particularly well and are better explained by the presence of a major unconformity. Of the two erosion surfaces in the Halfway to lower Charlie Lake succession, the one at the base of the Charlie Lake Formation is interpreted to be the unconformity, and the erosion surface associated with the Halfway Formation is interpreted to be a regressive surface of marine erosion.

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