A HYPERSALINE ENVIRONMENT FOR STROMATOLITE GROWTH IN THE MESOPROTEROZOIC SIBLEY GROUP (ONTARIO, CANADA)

The Mesoproterozoic Sibley Group is a dominantly clastic red bed succession, exposed in the Nipigon area along the north shore of Lake Superior in Ontario. Within the Rossport Formation, the Middlebrun Bay Member comprises a meter-scale carbonate-chert unit, containing both stromatolitic and massive carbonate. Previous work has established that the overall environment was lacustrine; this study examines the particular environment in which the Middlebrun Bay carbonates formed, in order to understand the relationship between stromatolite growth, lake history, and water chemistry.

On Channel Island and mainland Ontario, the Middlebrun Bay Member is characterized by cherty, dolomitic stromatolites. These stromatolites are stratiform to columnar, with synoptic relief typically less than 2 cm. Both black chert and dolomite preserve depositional carbonate textures, including fine-scale laminae and clotted and precipitated stromatolitic textures. Trace element concentrations, measured by ICP-MS, are strongly elevated compared to average Proterozoic carbonates. In contrast, the Middlebrun Bay Member on Copper Island is a coarsely crystalline limestone, lacking chert or internal stromatolitic lamination. This massive white limestone has a texture similar to chicken-wire evaporite and contains sandstone clasts derived from the overlying bed, reminiscent of collapse-related brecciation. In thin section, this unit has large subhedral to euhedral crystals, abundant stylolites, and zonation apparent by cathodoluminescence. Both stylolites and crystal boundaries contain accumulations of insoluble residue, indicating that dissolution and reprecipitation processes were important in generating the final texture. Trace element concentrations are generally elevated compared to average Proterozoic carbonates.

Observations of both the massive limestone and stromatolitic dolomite-chert facies of the Middlebrun Bay Member are consistent with deposition in a strongly hypersaline environment with low clastic influx. The massive limestone is best interpreted as a calcitized evaporite, based on field, textural, and geochemical evidence. The stromatolites likely formed in very shallow areas laterally adjacent to evaporite deposition, as observed in some modern hypersaline lakes.