A High Reversal Rate (10 reversals/Ma) Observed in the Ediacaran Rainstorm member of the Johnnie Formation Calibrated by Rock Magnetic Cyclostratigraphy

Abstract:

A combined magnetostratigraphic and rock magnetic cyclostratigraphic study of the Ediacaran Rainstorm member of the Johnnie Formation indicates a high reversal rate of the geomagnetic field at about 558-559 Ma. The reversal rate reported here is derived from work previously reported from the Rainstorm member of the Johnnie Formation from two localities in Death Valley, CA and from a new study of the Rainstorm member 135 km away from Death Valley in the Desert Range, Nevada. We collected both oriented hand samples and drilled oriented cores from 45.2 m of section above the Johnnie Oolite marker bed at a 2 m stratigraphic spacing. We also collected 122 unoriented samples for rock magnetic cyclostratigraphy throughout 44 m of section at a spacing of 25 cm. Our paleomagnetic measurements revealed a reversal stratigraphy in the Desert Range. Carbon isotope measurements indicated that we had sampled the lowest part of the Shuram carbon isotope excursion including its nadir. The rock magnetic cyclostratigraphy in the Nopah Range of Death Valley and in the Desert Range of Nevada indicated identical sediment accumulation rates (SARs) of 5.2 cm/ka based on the identification of short eccentricity and obliquity in the MTM power spectra derived from the magnetic susceptibility cyclostratigraphy. Average spectral misfit was used to determine the best-fitting SAR for each dataset. The Winters Pass Hills section of Death Valley produced a cyclostratigraphy that had a higher SAR of 8.8 cm/ka. Taking into account the different SARs and tying to the Johnnie Oolite marker bed, we could combine the polarity stratigraphy derived at all three locations. The polarity of a horizon is determined from the great circle distance of a sample’s VGP being either within 30˚or greater than 150˚ from the mean VGP for its locality. There is general agreement of polarity from the three localities throughout the composite section. The rock magnetic cyclostratigraphy provides high resolution time control to calculate the reversal rate. Our combined magnetostratigraphic section shows 9 polarity intervals in 865 ka of time, yielding 10.4 reversals/Ma, a rate that is high, but not as high as the ultra-high reversal rates of 15-20 reversals/Ma indicated by other studies for the Ediacaran and Middle Cambrian.