Deposition of the Ediacaran Rim Dolomite as a Diapir Roof Pond Carbonate During the Formation of the Shuram Excursion, Flinders Ranges, South Australia

The Rim Dolomite forms 1–100-m-thick dolostone lentils in contact with three isolated Ediacaran-age allochthonous salt sheets in the Flinders Ranges, South Australia. It locally intertongues with the Ediacaran Wonoka Formation, which records the most negative carbon isotope excursion (Shuram) in Earth’s history. Previous interpretations concluded it formed as either a depositional facies of the Bunyeroo Formation or as carbonate caprock (commonly understood as a diagenetic product of diapir crest halite dissolution followed by organic-derived carbonate replacement of the gypsic residue).

Using new data from geologic mapping and stratigraphic analysis we reinterpret the Rim Dolomite as a carbonate pond facies of the lower Wonoka Formation. This is based on the recognition of sedimentary structures, interstratification of sedimentary detritus derived from the diapir roof rocks and non-evaporite inclusions within the diapir, and local intertonguing of Rim Dolomite pond facies with the lower Wonoka Formation shallow marine facies that record the Shuram excursion nadir. We envision carbonate deposition in a depression (pond/lagoon) locally on top of the allochthonous salt sheets. Erosion of topographic highs on the salt sheets sourced detrital influx as both sheet flow and channelized deposits into the carbonate pond. The carbonate pond facies were progressively transgressed by the Wonoka sea. As the diapirs continued to rise, the Rim Dolomite/Wonoka stratal roof was drape-folded off the top of the diapirs, incorporating the pond deposits within halokinetic sequences. To our knowledge, this is the first example of terrestrial carbonate deposition associated with strata recording the Shuram excursion.