U-PB ZIRCON GEOCHRONOLOGY IN THE LATE DEVONIAN EXSHAW FORMATION: GLOBAL CORRELATION WITH THE HANGENBERG BLACK SHALE AND EXTINCTION EVENT

The Hangenberg mass-extinction event occurred at or just before the boundary between the Devonian and Carboniferous Periods, and is recognized by the common occurrence of black shale deposition 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 sparse 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 precision CA-TIMS U-Pb zircon dating methods to volcanic tuffs collected from four locations in the Exshaw Formation, along the eastern 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 but has been proposed as within the upper calcareous black shales of the lower member. Two correlated tuffs exposed in the Jura Creek and Mt. Rundle (Goat Creek) sections, near the main depocenter of the basin, have been dated to 360.0 ± 0.1 Ma. Two tuffs below and above this datum at Mt. Rundle yield ages of 360.2 ± 0.1 Ma and 358.9 ± 0.1 Ma, respectively. These dates establish rock deposition rates and effectively confirm the position of the Hangenberg event and Devono-Carboniferous boundary within the upper portion of the lower black shale member of the Exshaw Formation. In contrast, zircon dates of 362.7 ± 0.1 Ma and 363.0 ± 0.1 Ma from lower Exshaw black shales at the Nordegg and Crowsnest sections, respectively, are significantly older than the Hangenberg Event. These new zircon ages confirm and clarify the presence of unconformities in these locales, and show that sustained black shale sedimentation began earlier in the Western Canadian Sedimentary Basin compared to correlative strata in Europe.