The transition from open marine to nonmarine conditions in Atlantic Canada during the middle Carboniferous was of great consequence to global ecosystems. The Carboniferous Mabou Group consists of the Hastings Formation, comprising grey-green carbonates and stromatolites, and the overlying Pomquet Formation, comprising red calcareous sediments. The Hastings and Pomquet Formations interfinger at Broad Cove Inlet, Cape Breton Island, Nova Scotia. We present bulk carbonate content, carbonate mineralogy, and ⁸⁷Sr /⁸⁶Sr isotopic data for 215 meters of Hastings and Pomquet sediments from Broad Cove Inlet, and we develop a model for their deposition in a hypersaline environment. 

We recognize two primary sedimentary facies in the Mabou Group at Broad Cove Inlet: 1) grey-green dolomitic limestone that ranges between <1%-56% carbonate 2) red calcitic shale that ranges between <1%- 24% CaCO₃.  Six stratal alternations are recognized throughout the interval. With respect to temporal stratigraphic trends, the grey facies are prominent at 0-14 meters, 41-81 meters, and 91-163 meters.  The red facies are prominent at 14-41 meters, 81-91 meters, and 163-215 meters. 

We consider several deposition models: 1) a peritidal environment (restricted marine system); 2) a hypersaline lacustrine environment (nonmarine); and 3) a “salinas” environment (where an arm of a marine basin has been isolated from its source).  Hypersalinity is indicated by the presence of stromatolites throughout the grey-green facies.  Preliminary ⁸⁷Sr/⁸⁶Sr ratios are not consistent with deposition in a marine marine influenced basin, also supporting the hypersaline lacustrine model. A climate forcing mechanism most likely influenced the observed stratal cyclicity.