Paleobrine Temperatures, Chemistries, and Paleoenvironments of Silurian Salina Formation F-1 Salt, Michigan Basin, U.S.A., from Petrography and Fluid Inclusions in Halite

Abstract A 3.7 m stratigraphic section of the Silurian Salina Group F-1 Salt, Detroit Salt Company mine, Michigan Basin, was studied to document the paleodepositional conditions, including brine chemistry and temperature, of evaporite precipitation. Sedimentary features, including rippled surfaces, halite chevrons, cumulate halite, and dissolution surfaces confirm a shallow-water depositional setting. Penecontemporaneous dissolution surfaces indicate that waters undersaturated with respect to halite were introduced into the Michigan Basin, and that the basin brines were nonstratified during flooding, allowing inflow waters to come into contact with halite crystals at the bottom. Fluid inclusions in primary chevron halite have homogenization temperatures between 5° and 25° C, which represent brine temperatures during halite precipitation. These temperatures are lower than expected for a marginal marine basin at a subtropical paleolatitude, and they suggest a climatic cold spell for a period of thousands of years. The major ions in fluid-inclusion brines, analyzed using the ESEM-X-Ray-EDS method, have relatively low concentrations of Na+, Mg2+, and SO42−, and elevated concentrations of Ca2+ compared to modern seawater. Brine-inclusion chemistries from the F-1 Salt compare closely with the major-ion composition of fluid inclusions from other Silurian marine halites (A-1, A-2, and B Evaporites of the Salina Group and Carribuddy Group, Canning Basin, Australia). These results indicate a seawater parentage of the Michigan basin F-1 Salts and that the major-ion chemistry of seawater remained more or less constant during the last 15 million years of the Silurian. Detailed documentation of the temperatures, brine chemistries, and environments of halite deposition is a prerequisite for geobiological study of the entrapment of ancient microbes in salt. The results reported here can help pinpoint the timing of entrapment of microorganisms in fluid inclusions in halite and can be used to argue that brine inclusions in the F-1 halites have been isolated and preserved in an unmodified state for more than 400 million years.