Compaction in halite-cemented carbonates – the Dawson Bay Formation (Middle Devonian) of Saskatchewan, Canada



Halite-impregnated carbonates in the Dawson Bay Formation of Saskatchewan lie between beds of halite and are buried to a depth of 1 km. They exhibit two different diagenetic styles – some resisted compaction and had high pre-salt porosities; others contain compaction-broken fossils and pressure-solution seams. The uncompacted rocks, together with the difficulty of explaining how halite cement could enter the Dawson Bay after overlying bedded halites were deposited, suggest that halite cementation occurred early with only a few tens of metres of overburden. Early diagenetic compaction is suggested by the presence of unbroken, displacive skeletal halite crystals, which cross-cut compaction structures, and by the difficulty of explaining how (1) later compaction could occur in halite-cemented rocks and (2) how pore-fluids could be expelled after surrounding rocks lost their permeability. The organic-rich nature of many carbonates may explain why compaction was both early and extensive, but this explanation fails to explain how similar compaction developed in horizons with lower organic contents. Chemical compaction may also have been enhanced by aragonite dissolution during seawater evaporation or brine dilution. Early chemical compaction in Dawson Bay carbonates indicates that compaction in other carbonates need not signify deep burial diagenesis; neither can compaction be used indiscriminately to identify other diagenetic events as being of deep burial origin. Early halite cementation, as in the Dawson Bay Formation, preserves carbonates at early diagenetic stages and may thus preserve geochemical information unmodified by later diagenesis.