Cyclostratigraphic calibration of the Frasnian (Late Devonian) time scale (western Alberta, Canada)

Until now, the duration of the Frasnian Stage has remained very poorly constrained, hampering a detailed understanding of sedimentation processes and environmental and evolutionary change. In this study, time-series analyses of high-resolution (10-20 k.y.) magnetic susceptibility data identify sixteen 405 k.y. eccentricity cycles in the magnetic susceptibility stratigraphy of the Frasnian (Late Devonian), derived from carbonate-platform and surrounding slope and basin deposits in western Alberta, Canada. Previous studies demonstrated the generally consistent pattern of magnetic susceptibility change across the Alberta basin and thus demonstrated the utility of magnetic susceptibility stratigraphy as a refined regional correlation tool compared to biostratigraphy. In the present study, we show that the magnetic susceptibility stratigraphy of the Frasnian interval in western Alberta has been significantly influenced by astronomical forcing. Using the sixteen 405 k.y. eccentricity cycles as a geochronometer, we constructed a Frasnian astronomical time scale. This time scale indicates a duration of 6.5 +/- 0.4 m.y. for the Frasnian. Calibrating this duration to the best available Devonian chronology, the absolute age of the Givetian-Frasnian boundary is recalculated to 383.6 +/- 3.0 Ma, and the age of the Frasnian-Famennian boundary is recalculated to 376.7 +/- 3.0 Ma. These new absolute ages take into account the astronomically derived duration of the Frasnian, but they also yield a narrowing of the error margins of the absolute ages by several hundreds of thousands of years.