First Recognition of Shallow Water, Low Oxygen Environments in the Devonian of the Canning Basin, Australia: Importance of the Givetian/Frasnian Mass Extinction

The Canning Basin of Western Australia contains one of the best-preserved fossil barrier reef systems known from the Phanerozoic. The reef facies are undeformed, and all reef sub-environments are exposed. It has been long surmised that nowhere in this geographically widespread basin are there strata rich in organic material that could have been deposited under low oxygen conditions in the shallower reef facies. This has made it impossible to recognize any of the late Frasnian anoxic events observed in Europe and elsewhere in strata of this age, including the Kelwasser events. Here we report the presence of such low oxygen, shallow water strata in Givetian, rather than Frasnian-aged strata in the Canning Basin. The section is interpreted to have been deposited during a transition from platform to reefal slope. We have been able to conduct organic carbon isotope analyses as well as magnetostratigraphy through approximately 40m of this section. This section shows several cycles of white, organic poor shallow water limestone grading upward into dark, organic rich limestone and back again, and contains a 2 per mil positive carbon isotope anomaly, consistent with a “Eutrification” event. Tabulate and rugose corals can be found in place with dark strata draped over what may have been living assemblages. While the Frasnian/Fammenian event is considered one of the “Big Five” mass extinctions, the Canning Basin record suggests that this interval of Givetian age may have been more catastrophic, affecting tabulate and rugose corals, and leaving behind Frasnian reefs dominated by stromotoporoids rather than cnidarians