RECOGNITION OF A CALCITIZED EVAPORITE FROM A HYPERSALINE PLAYA LAKE SETTING: ROSSPORT FORMATION (MESOPROTEROZOIC, ONTARIO)

The Mesoproterozoic Sibley Group of Ontario, Canada comprises a thick succession of mainly clastic sediment, generally interpreted to have been deposited in a rift-related lake. Although the Sibley Group is dominated by siliciclastic deposition, the Middlebrun Bay Member of the Rossport Formation contains a thin (~1 m) carbonate unit. The Middlebrun Bay Member, in exposures on the Channel Islands and along the North Shore of Lake Superior, consists most commonly of cherty, dolomitic microbial laminites and low-relief columnar to conical stromatolites. In contrast to these typical expressions, the Middlebrun Bay Member on Copper Island is a massive, coarsely crystalline limestone unit, lacking stromatolites or microbial laminae. Several features suggest dissolution and replacement of a primary, soluble phase such as an evaporite mineral. The top of the unit is marked by evidence of dissolution and collapse, including large sandstone clasts let down from the overlying bed. At petrographic scale, the Copper Island carbonate comprises mm-scale anhedral spar with abundant stylolites and concentration of insoluble material at grain boundaries, indicating recrystallization from a previous phase. Geochemical data from Copper Island and from correlative stromatolitic carbonate on Channel Island and mainland Ontario are consistent with hypersaline conditions, with elevated concentrations of carbonate-associated sulfate, V, and Ba. The field, petrographic, and geochemical data, taken together, suggest that the massive carbonate exposed on Copper Island is a calcitized evaporite, probably deposited originally as gypsum and replaced by calcite during diagenesis. These data support previous interpretations that the Middlebrun Bay interval was deposited during a period of increased aridity and low clastic influx, and we further suggest that this restricted, hypersaline lake precipitated both carbonate and gypsum, comparable to modern arid playa lakes. The Copper Island carbonate shares several key features with a hypothesized calcitized evaporite from the Mesoproterozoic Atar Group, suggesting a suite of features, including recrystallized textures and elevated carbonate-associate sulfate concentrations, might be useful in recognizing replaced evaporite deposits in the geologic record.