Microbial carbonates as indicators of environmental change and biotic crises in carbonate systems: examples from the Late Devonian, Alberta basin, Canada

Abstract

Microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the Proterozoic. We report here the varied roles that microbial carbonates played in Late Devonian carbonate platforms in the Alberta basin, Canada. We recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the Frasnian–Famennian mass extinction. Detailed sequence stratigraphic analysis of two isolated platforms in the Canadian Rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation, aggradation and backstepping, and renewed progradation related to rates of second-order and third-order sea-level change and basin infill. The carbonate system was reorganized following annihilation of many carbonate-producing biota during the Frasnian–Famennian mass extinction. Microbial carbonates figure prominently in both Frasnian platform development and the Famennian recovery of the carbonate system following the Frasnian–Famennian mass extinction. During transgressive platform phases deeper-water facies with a crinoid, rugose coral, and microbe dominated biota abruptly overlie prograding stromatoporoid framestones. Microbial carbonates consist of abundant Girvinellid oncoids that nucleated around other fossil clasts. The temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply. Microbes played a significant role in highstand reef margin facies where Renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral/stromatoporoid mounds. Microbially laminated carbonates also form stromatolitic mats in peritidal shallowing-upward cycles and repetitive stromatolitic intervals commonly indicate sea-level lowstands associated with the development of sequence boundaries. Stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats. An end-Frasnian sea-level lowstand exposed the isolated carbonate platforms. Transgression and mass extinction characterize the Frasnian–Famennian boundary event and microbial carbonates occur at this horizon in western Canada. Stromatolites, oncoids, and large-scale microbial thrombolites appear to be opportunistic taxa temporarily filling unoccupied ecological niches immediately above the extinction boundary. The paucity of megafauna implies that few metazoans remained to exploit available food and nutrient sources and the stratigraphic position of these microbial carbonates supports their interpretation as ‘disaster forms’. Microbial carbonates thus play several significant roles in the development of Late Devonian carbonate systems in western Canada. Microbial carbonates occur at important sequence stratigraphic and paleoecologic horizons indicating changes in sea level, nutrient supply, and biotic assemblages. These examples indicate that microbial carbonates can be important indicators of environmental and ecological change within carbonate systems.