The Gowganda Formation in the eastern part of the Cobalt Basin comprises the Coleman Member and the conformably overlying Firstbrook Member. The Firstbrook Member differs from the Coleman Member in that its lithofacies display (i) marked lateral continuity, (ii) abundant diagenetic red colouration, (iii) evidence of tidal deposition, and (iv) a lack of glaciogenic features. The Firstbrook Member is here divided into a mud-dominated lower facies, a silt-dominated middle facies, and sand-dominated upper facies. These are respectively equated with the prodelta, delta slope, and delta foreslope subenvironments of a prograding river-dominated deltaic wedge composed of coalescing deltas (probably braid deltas). The deltaic wedge was likely fed by a fluvial system analogous to the braidplain system that prevailed during deposition of the overlying Lorrain Formation. Uninterrupted delta progradation is suggested by a single coarsening-upward sequence up to 535 m thick. Deposition probably occurred within a marine rather than lacustrine basin.

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