Magnetic Susceptibility Cyclostratigraphy of the Ediacaran Wonoka Formation, South Australia, from a Portable Susceptibility Meter

Abstract:

We present the results of a rock magnetic cyclostratigraphy of the Ediacaran Wonoka Fm., from the Flinders Ranges, South Australia. The Wonoka Fm. records the Shuram C-isotope excursion, and the results of this study provide an estimate of its duration. Measurements of magnetic susceptibility (MS) were made in the field with a portable susceptibility meter (GF Instruments SM-20) every 0.4 m over 600 m of dominantly carbonate stratigraphic section, with a 100 meter interval of fine sand turbidites. We filter the data series to pass wavelengths shorter than 300 meters (the Rayleigh frequency for our data series) and use Multi Taper Method (MTM) time series analysis to estimate the power spectrum of the series. We fit a red noise model to the MTM spectrum and calculate 99% chi squared confidence limits to identify cycles with statistical significance. Unambiguous spectral peaks rise above the 99% confidence level with wavelengths of 116.9 m, 45.5 m, 23.1 m, 7.0 m, 2.5 m, 1.7 m, 1.3 m, 1.2 m, 0.9 m, and 0.8 m. Haines (1988, Sed. Geo. V 58) recognizes cycles in clastic sediment delivery to the carbonate portions of the Wonoka with an average wavelength of 8 m and hypothesizes a climate driver related to Milankovitch cycles. The 7 m cycle in MS is likely a more precise measure of the same phenomenon. Furthermore, cycles with wavelengths of 24.1 m, 7.0 m, 1.7m, 1.26 m, and 0.9 m exhibit ratios matching modeled durations of long eccentricity (405 kyr), short eccentricity (123 kyr), obliquity (29 kyr), and the 2 periods of precession (19 and 14 kyr) for the late Neoproterozoic. This interpretation yields a duration of 10 Myr for the 600 m we sampled. Assuming a constant sedimentation rate yields an 11.3 Myr estimate for the ~675 m thick Shuram C-isotope excursion. However, uncertainty lies in applying the MTM-based estimate to the turbidite section. Our estimate will be refined and compared with laboratory rock magnetic measurements in an ongoing study of the Wonoka Fm.