Abstract

The Middle Triassic Doig Formation of the Wembley area, in west-central Alberta, contains several north–south oriented, elongate, narrow, thick sandstone bodies encased in a predominantly shale succession. Downdip from the thick sandstone body and the enclosing shale-rich beds at Wembley is a succession of sandy siltstones (informally named the sandy siltstone unit). Published interpretations of these coarse clastic units include incised valley-fill, or the fill of contemporaneous growth-fault grabens for the elongate sandstone bodies, and healing-phase deposits (i.e. transgressive beds) for the sandy siltstone unit. A re-examination of the stratigraphic setting and contained facies suggests that the incised valley-fill model seems unlikely, based on the predominance of offshore and lower shoreface beds within the entire length of the elongate bodies, with little evidence for tidal influence or facies normally associated with such settings. Stratigraphic correlations indicate that the elongate bodies formed after deposition of the enclosing shale succession, hence contemporaneous growth faulting is unlikely. However, a slump event, followed by sediment fill is a possible alternative explanation. Correlations also indicate that the downdip sandy siltstone unit is a continuation of progradation that formed the younger shale-rich beds, located landward of the sandy siltstone unit, and are not transgressive deposits. The sandy character of these beds is interpreted to result from deposition during sea-level fall and lowstand, when a regressive surface of erosion formed on the shelf platform. Only the uppermost beds of the sandy siltstone unit can be considered transgressive.

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