CRYOGENIAN (MIDDLE NEOPROTEROZOIC) REEFS FROM NORTHWESTERN CANADA

Reefs, among the most environmentally sensitive and complex of marine ecosystems, have evolved and altered dramatically since their Archean origins. A significant potential crisis in reef evolution was the onset of widespread global cooling during the Cryogenian Period of the middle Neoproterozoic (850—635 Ma). This time was characterized by two profound global glaciations with evidence of long-term refrigeration between them. There is scant record anywhere of reefs during the Cryogenian, with the few previous reports mostly restricted to isolated stromatolite mounds.

The Redstone reef complex in the Mackenzie Mountains of northwestern Canada provides the first clear view of the biological components and cement fabrics in Cryogenian (Middle Neoproterozoic) reefs. The reefs occur stratigraphically between glacial diamictites of the Rapitan Group and Ice Brook Formation, and are laterally equivalent to strata bearing pseudomorphs of the cold-water carbonate ikaite. The m-scale reef mounds grew in a high-energy, shallow-marine environment adjacent to a major fluvial system. The reefs consist of stacked, m-scale domes of dendriform and lamelliform stromatolites flanked by cross-bedded pisolitic and intraclastic rudstone. Reefal stromatolites were composed of filamentous calcimicrobes similar to those of early Neoproterozoic, Ediacaran, and Cambrian reefs. The microbial fabrics of the reefs underwent pervasive cementation and replacement by botryoidal aragonite in a near-surface marine environment, a process that has been reported sparingly from other glacial and immediately post-glacial periods in Earth history. In general, these microbial reefs are similar to those of earlier and younger Neoproterozoic age, and show few effects from the extreme glaciations and overall cold-water conditions that characterized the Cryogenian Period.