ORIGIN OF MIDDLE CAMBRIAN, MID-PLATFORM DOLOMITE, SOUTHERN CANADIAN ROCKY MOUNTAINS

Dolomite laminites, replacement dolomite, and saddle dolomite spar in a mid-platform dolomite body of the Cathedral Formation in the Southern Canadian Rocky Mountains are a single genetic unit. Geometrically the dolomite body consists of a stratiform halo of replacement dolomite extending laterally away from a central breccia zone. Dolomite laminites in the breccia zone thin laterally into the halo zone. Homogenisation temperatures in primary fluid inclusions from dolospar and replacement dolomite in the breccia zone average 145°C. Fluid inclusions in the halo zone average 140°C for dolospar and 125°C for replacement dolomite. The oxygen isotope value of dolomitizing fluids is 10 permil enriched in ¹⁸O over Middle Cambrian seawater.

Relationships within the breccia zone and the dolomite laminites along with large scale syndepositional block faulting and the interpreted thermal state of the Cambrian miogeocline, are consistent with dolomitization of the mid-platform Cathedral Formation during deposition. Thermally driven convection of Middle Cambrian seawater channeled through deep-seated basement faults accounts for the anomalous high temperatures and oxygen isotope values. Hot dolomitizing fluids entered the Cathedral Formation in proximity to the breccia zone, migrated laterally into permeable stratigraphic units and caused replacement of limestone with dolomite. Dolomitizing fluids that breached the seafloor moved laterally away from the breccia zone, decreased in temperature and precipitated dolomite laminites. Turbulent flow of dolomitizing fluids through the breccia zone caused erosion and sanding of replacement dolomite at the base of the zone. Eroded dolomite sediment was expelled onto the seafloor, leaving a void space within the replacement dolomite. Collapse into this void space resulted in brecciation of overlying units.