SILICICLASTIC PRELUDE TO DEGLACIATION OF MARINOAN SNOWBALL EARTH: MAGNETOSTRATIGRAPHY OF THE ELATINA-NUCCALEENA-BRACHINA TRANSITION

While most type-Marinoan sections in South Australia conform to a classic "Snowball Earth" pattern, in which cap carbonate sharply overlies a presumed disconformity and flooding surface on top of conglomeratic siliciclastics, the Elatina Creek section in central Flinders Ranges displays conformable, mixed transition of nonconglomeratic siltstone and carbonate for more than a meter below Nuccaleena cap dolostone. Sohl et al. (1999) demonstrate at least six paleomagnetic reversals within the interval spanned by Elatina Formation. The top of Sohl et al.'s composite section, a convolute-bedded, incisive red sheet sandstone derived from Gawler craton to the west, marks the highest unambiguous unconformity in Elatina Formation at Elatina Creek.

We extend Sohl et al.'s magnetostratigraphy from this red sheet sandstone unit upward through Nuccaleena cap carbonate. High-resolution sampling at two sections (Elatina Creek and Trezona Bore) and moderate-resolution sampling at three more sections (Bunyeroo Creek, Enorama Creek GSSP, and Parachilna Gorge) suggests high dispersion of virtual geomagnetic poles in the topmost Elatina and lowest Nuccaleena interval. Most of Nuccaleena dolostone spans two ensuing polarity chrons, with generally low VGP dispersion but several centimeter-scale VGP departures. Multiple polarity zones characterize overlying, lowest Brachina siliciclastics. Collectively, these results appear to suggest that Marinoan cap carbonate records finescale structure of the earliest Ediacaran geomagnetic field, including excursions, reversal excursions, intervals of geomagnetic instability, and a high reversal frequency.