Paleoenvironments and growth of early Neoproterozoic calcimicrobial reefs: platformal Little Dal Group, northwestern Canada

Abstract

Although the calcimicrobial reefs of the basinal Little Dal Group (<1080 Ma; >780 Ma) are well-described, little is known about related shallow-water facies to the southeast. Study of these platformal carbonates shows that they represent inner, mid- and shallow outer ramp zones of a homoclinal, storm-influenced ramp. Seven depositional sequences are recognized and correlated with reef-bearing strata in the Little Dal basin and in the Shaler Supergroup of Victoria Island. Reef distribution is distinctly depth partitioned across the Little Dal ramp. Inner and mid-ramp carbonates contain meter-scale mounds, patch reefs, and biostromes composed of fine-grained stromatolites including Tungussia, Baicalia, and Jurusania. These reefs are strikingly similar to those reported from coeval shallow-water strata in the Shaler Supergroup of Victoria Island and the Bitter Springs Formation of Australia. Multi-storey stromatolitic biostrome/bioherm complexes greater than 100 m in height, seem to be restricted to the mid-ramp zone of the Little Dal ramp and probably reflect high rates of subsidence in the Mackenzie Basin. Calcimicrobial reefs occur only in the deepest water strata of the study area, associated with outer ramp sediments. These reefs contain calcified cyanobacterial filaments in varying degrees of preservation, and are petrographically indistinguishable from the kilometre scale basinal calcimicrobial reefs to the northwest. The association of calcimicrobial reefs with deep, low-energy environments suggests initial establishment of reef-building calcimicrobes in deep-water benthic environments.