Ediacaran high reversal rate in the Johnnie Formation - Ken Kodama - 2021 MagIC Workshop (1/21/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 2021-01-27

"A High Reversal Rate (12.7 reversals/Ma) in the Ediacaran Johnnie Formation (Rainstorm member) Calibrated by Rock Magnetic Cyclostratigraphy" by Ken Kodama, Dept. Earth and Environmental Sciences, Lehigh University

Lightning poster talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.

Abstract:
A High Reversal Rate (12.7 reversals/Ma) in the Ediacaran Johnnie Formation (Rainstorm member) Calibrated by Rock Magnetic Cyclostratigraphy

K.P. Kodama, Dept. Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015

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 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. 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 rock magnetic cyclostratigraphy provides high resolution time control to calculate the reversal rate. Our combined magnetostratigraphic section shows 11 polarity intervals in 865 ka of time, yielding 12.7 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.