SEDIMENTARY EXHALATIVE DOLOMITE, WESTERN CANADA

Replacement of limestone by dolomite is common in both Cambrian and Devonian platform carbonates from western Canada. The dolomite bodies appear to be positioned over deep-seated structural elements. In general these dolomite bodies tend to be stratiform. Many have irregular, sharp to diffuse lower boundaries which cut through bedding in adjacent limestone whilst upper boundaries with limestone tend to parallel bedding and be sharp. Fluid inclusion and stable isotope data indicate that dolomite formed at temperatures between 80 and 230^oC, from hypersaline brines with elevated ¹⁸O and radiogenic strontium.

These dolomite bodies vary greatly in size. Vertical extent is from a few tens of meters to over a kilometer; laterally they may extend for several tens of kilometers. These dolomite bodies are zoned. The central zone typically contains angular breccia blocks which float in dolomite sand and/or coarse dolomite spar. Adjacent to this zone, and overlapping with it, saddle dolomite is present in vugs and fractures within coarse-grained, fabric-destroying replacement dolomite. Outer portions of the dolomite bodies contain fabric-preserving replacement dolomite and lesser amounts of dolomite spar. Oxygen isotopes in limestone adjacent to the dolomite tend to be depleted in ¹⁸O relative to limestone further from the dolomite. Some of these dolomite bodies are capped by very fine-grained dolomite laminites. These laminites are dense, do not contain desiccation features, and do not appear to cap tidal flat sequences. The geometric and geochemical relationships of the dolomite within these bodies and surrounding limestone indicate the various dolomite fabrics are part of a single genetic unit.

Sedimentologic observations and geochemical data from the dense dolomite laminites indicate that these formed as flowstones on the seafloor. It appears that as the hot dolomitizing fluids were exhaled onto the seafloor these high density brines formed a stratified was layer. The dolomite in the dense laminites precipitated from this fluid. Away from the exhalative vents the brines cooled and mixed with seawater. As a result, there is a systematic change in the isotopic composition of the dolomite in the laminites laterally.