Evaporites deposited within the Michigan basin during late Wenlockian time appear to record a significant drop in sea level, whereas coeval carbonates deposited on the topographically higher Wabash platform of Indiana do not. The conflicting evidence from basin and platform can be resolved by a hypothesis of isolated evaporative drawdown within the basin. This hypothesis postulates that high evaporation rates and constriction of ocean inlets between the Michigan basin and North American epeiric seas caused sea levels inside the basin to drop significantly white external sea levels remained nearly constant. During drawdown, supratidal Silurian carbonates on the basin margin acted as a rim separating normal and hypersaline seas allowing recharge of seawater and discharge of brine through restricted inlets and through restricted inlets and through the subsurface. Geologic evidence for sea-level drawdown includes subaerial exposure of preevaporitic carbonates within the basin , extensive dolomitization of carbonates at the basin margin, and reduced deposition of sulfate and magnesium salts in the basin center.