TEMPORAL CALIBRATION OF LATE DEVONIAN DEPOSITIONAL SEQUENCES AND GEOBIOLOGIC EVENTS WITH SPECTRAL ANALYSIS OF MAGNETIC SUSCEPTIBILITY DATA: UPPER DEVONIAN, WESTERN CANADA

Cyclostratigraphy and spectral analysis of high-resolution magnetic susceptibility (MS) data provides unprecedented precision in estimating the timing of geologic events during the Late Devonian. Identification of Milankovitch rhythms within the MS data permitted documentation of 16 long eccentricity (E) cycles (LEC, 400 ky) within the Frasnian stage of the Upper Devonian.

Sequence stratigraphic analysis, of isolated carbonate platforms in western Canada, identified 8 depositional sequences spanning the upper Givetian through the Frasnian-Famennian (F-F) boundary. Based on prior biochronology, and overall duration of the Frasnian Stage, we interpreted the sequences to be 1-2 My duration, i.e. 3^rd order. This is only partially supported by cyclostratigraphic analysis that indicates sequence durations of ~400-1600 ky, i.e. both 4^th and 3^rd order. The eight sequences are nested within a 2^nd order transgressive-regressive (T-R) cycle and durations are mostly 4^th order (~400 ky) during 2^nd order transgression and 3^rd order (~800-1600 ky) during 2^nd order highstand. Higher frequency sea level events during 2^nd order transgression indicate control by LECs while longer duration sea level events during 2^nd order highstand may be controlled by long-term modulation of obliquity (1.2 My cycle).

Several sea level events are associated with significant geobiologic events including the punctata Zone, Lower and Upper Kellwasser events (LKE, UKE). All three are characterized by substantial carbon isotope excursions. The latter two are associated with the F-F biotic crisis. Sequence and spectral analysis indicates that the late Middle Frasnian punctata Zone event is within the transgressive portion of 3^rd order T-R cycle IIc-1, spanning about one LEC. The F-F boundary events are also associated with sea level transgression (LKE, IId-1; UKE, IId-2). Astronomical tuning of MS data implies that the LKE likely occurred during a single short E cycle (100 ky) while the UKE was more protracted, lasting approximately two short E cycles (200 ky). The events are separated by 1.5-2 LEC indicating that they occurred between 600 and 800 ky apart. This work demonstrates the utility of long time series of MS data used in conjunction with sequence stratigraphy to provide insight into the timing of sea level and other events in geologic time.