DEPOSITIONAL AND PRESERVATIONAL ENVIRONMENTS OF THE EDIACARA MEMBER, RAWNSLEY QUARTZITE (SOUTH AUSTRALIA): ASSESSING THE TIMING OF ‘FERRUGINIZATION’

The paleoenvironmental setting in which the Ediacara Biota lived, died and was preserved in the eponymous Ediacara Member of the Rawnsley Quartzite of South Australia is an issue of longstanding interest and recent debate. Over the past few decades, interpretations have ranged from deep marine to shelfal to intertidal to terrestrial. The ruddy color of certain fossiliferous sedimentary packages of the Ediacara Member has been a central tenet in recent debate concerning the paleoenvironmental settings of the Ediacara fossil assemblages.

We examine the evidence in support of and against various paleoenvironmental interpretations of the fossiliferous Ediacara Member, and focus in particular upon the history of ferruginization of these rocks. We find that the coloration of the fossiliferous Ediacara Member at Nilpena strongly reflects post-depositional, late-stage processes. Ferric oxides in the Ediacara Member consist largely of surficial staining which does not follow bedforms; red coloration is patchy and non-penetrative and classic late-stage features such as Liesegang banding are present.

Moreover, uranium isotope analysis of iron oxides associated with fossiliferous beds of the Ediacara Member indicate that these oxides record recent, post-depositional oxidation. Oxides are characterized by ²³⁴U/²³⁸U variations tied to alpha-recoil, which indicate that oxidizing fluids flowed through these rocks sometime in the last couple million years. Therefore, both sedimentological and geochemical evidence unequivocally indicate that the Ediacara iron oxides do not reflect synsedimentary ferruginization and that the presence of iron oxides cannot be used to invoke a terrestrial paleoenvironmental setting for the Ediacara Biota. Conversely, sedimentological, paleoecological and geochemical evidence lend strong support for a shallow marine depositional environment for the fossiliferous Ediacara Member. These findings demonstrate that careful assessment of paleoenvironmental parameters is essential to the reconstruction of the depositional and early diagenetic history of the Ediacara Biota and thus the physical, chemical and biological factors that shaped the development and the fossilization of these early complex ecosystems.