CYCLOSTRATIGRAPHIC CALIBRATION OF THE FRASNIAN (LATE DEVONIAN) TIME-SCALE (WESTERN ALBERTA, CANADA)

Currently, only few U-Pb isotopic dates constrain the Devonian geological time scale, hampering a detailed understanding of rates of sedimentation processes, environmental changes and key-events in the evolution of life on Earth. One possible way to reduce uncertainties in the Devonian geological time-scale is the recognition of astronomical cycles in its stratigraphy. Therefore, this study reports frequency analyses of high-resolution (10-20 kyr) magnetic susceptibility (MS) data 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 demonstrated the utility of MS stratigraphy as a refined regional correlation tool, compared to biostratigraphy. In the present study, it is shown that the MS stratigraphy of the Frasnian in western Alberta is significantly influenced by astronomical forcing. Cyclicity in the studied sections is ascribed to different Milanković astronomical parameters. The astronomical interpretation of observed periodicities is supported by the presence of several amplitude modulations consistent with astronomical theory, and by average sedimentation rate patterns that agree with the existing lithostratigraphy. Sixteen 405-kyr long eccentricity cycles are recognized in the Frasnian MS stratigraphy. By using these 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, Earth Sci. Rev. 76, pp. 175-190) 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, and yield a narrowing of the error margins of the absolute ages by several hundred thousand years. Therefore, this study demonstrates that the recognition of astronomical cycles can also significantly refine the Paleozoic geological time-scale.