Brine mixing in the Middle Devonian of western Canada and its possible significance to regional dolomitization

Abstract

The desiccation that most large evaporite basins experienced in their history created large elevation heads. These heads must have induced flows of ground- or formation-waters into the evaporite basins. The consequences of such groundwater flows are believed to be (1) provision of the necessary hydrodynamic drive to allow dolomitization of carbonates beneath and adjacent to the evaporite basin, and (2) the formation of anomalously distributed evaporites in the basin adjacent to sites of groundwater inflow: sites where the groundwaters mixed with marine-derived brines. Such brine mixing may also have been responsible for depleting the marine-derived brines of their contained sulfate, so explaining later development of sulfate-impoverished potash deposits. Anomalous developments of anhydrite surrounding older carbonate buildups in the distal parts of the Middle Devonian Elk Point Basin of western Canada may represent such products of brine mixing and provide evidence of dolomitization of underlying Lower Palaeozoic and Devonian carbonates coevally with the filling of the evaporite basin. Regional dolomitization in the Williston Basin is stratigraphically confined beneath the Middle Prairie Evaporite suggesting a genetic link between the dolomitization and evaporite deposition. Dolomitization by refluxing brines generated in the evaporite basin appears unlikely because of the presence of an aquitard at the base of the evaporite succession. Developments of laminar and pisolitic carbonates adjacent to the carbonate buildups are believed to indicate the former presence of subaerial springs that carried calcium-rich groundwaters into the evaporite basin. Anhydrite envelopes around the buildups are interpreted to be reaction products that formed when calcium-bearing spring waters mixed with sulfate-rich but calcium-depleted marine brines. The calcium in the spring waters is believed to have entered the groundwaters as a by-product of the dolomitization of carbonates underlying the evaporite basin. Precipitation of gypsum (now anhydrite) as a consequence of brine mixing must have depleted the sulfate content of the marine-derived brines. It is possible that the sulfate-depleted potash deposits in the Prairie Evaporite originate by this mechanism. On the other hand, the stratigraphic position of these deposits lends little support for this hypothesis.