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

Until now, the duration of the Frasnian stage is 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 kyr) magnetic susceptibility (MS) data identify sixteen 405-kyr eccentricity cycles in the MS 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 MS change across the Alberta basin and thus demonstrated the utility of MS stratigraphy as a refined regional correlation tool, compared to biostratigraphy. In the present study, we show that the MS stratigraphy of the Frasnian in western Alberta is significantly influenced by astronomical forcing. Using the sixteen 405-kyr eccentricity cycles as a geochronometer, a Frasnian astronomical time-scale is constructed. This time-scale indicates a duration of 6.5 ± 0.4 Myr for the Frasnian. Calibrating this duration to the Kaufmann's (2006) 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 to 376.7 ± 3.0 Ma. These new absolute ages take into account the astronomically derived duration of the Frasnian, but also yield a narrowing of the error margins of the absolute ages by several hundred thousand years.