Geochronology in the Exshaw Formation: correlation with the Hangenberg extinction event.
The Hangenberg mass-extinction event defines the boundary between the Devonian and Mississippian geologic Periods, and is recognized by the common occurrence of black shales associated with marine anoxic environments. Biostratigraphy and radiometric dating have been used to globally correlate these end-Devonian black shales; however, much of the biostratigraphic record is missing and past radiometric ages have not had the resolution necessary to provide clear insight into the causes of the Hangenberg event. Here we apply high resolution U-Pb zircon dating methods to volcanic tuffs collected from four locations in the Exshaw Formation, along the eastern of edge of the Western Cordillera in Alberta, Canada, and compare these results to high-precision ages for the Hangenberg Event (359.3 ± 0.1 Ma) in the type sections of the Rhenish Mountains of Germany. The Exshaw Formation is divided into lower black shale and upper silty carbonate members; the exact location of the Hangenberg event within the Exshaw is unknown due to the sparse fossil occurrence. Two correlated tuffs exposed in the Jura Creek and Rundle Range sections have been dated to 360.0 ± 0.1 Ma, and constrain the position of the Hangenberg event to the upper portion of the lower black shale member of the Exshaw Formation. Ongoing analysis of additional overlying tuffs in the Rundle Range section will further constrain the exact position of the Hangenberg event. Zircon dates of 362.7 ± 0.1 Ma and 363.0 ± 0.1 Ma from Nordegg and Crowsnest sections, respectively, are significantly older than the Hangenberg Event. These new zircon ages confirm and clarify the presence of unconformities in the relatively condensed sections at these locales, and show that anoxic sedimentation began earlier in the Western Canadian Sedimentary Basin compared to correlative strata in Europe.