Environmental interpretation and a sequence stratigraphic framework for the terminal Proterozoic Ediacara Member within the Rawnsley Quartzite, South Australia

Abstract

Fossils of the Ediacara biota are confined to preservational windows in the Ediacara Member of the Rawnsley Quartzite (Pound Subgroup), from the Flinders Ranges, South Australia. The base of the Ediacara Member is a type 1 sequence boundary incised into the partly lithified Chace Quartzite Member of the Rawnsley Quartzite. The Ediacara Member and the upper half of the Rawnsley Quartzite comprise the Rawnsley depositional sequence, bounded above by the Early Cambrian Uratanna depositional sequence. The Rawnsley sequence developed over an erosional surface cut into the Chace Quartzite Member, with some 250 m of relief. Southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone. At lowstand, local channels were filled by massive sand. The highstand systems tract consists of several parasequence sets of laminated siltstone and channelized, amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well-bedded, clean sandstone. Impressions of soft-bodied Ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward-shoaling, delta-front environments between storm- and fairweather wavebase. The Ediacara Member is, in places, conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member. Distinctive microbial-mat-bound sand laminae, described as petee structures, are characteristic of the Rawnsley Quartzite above and below the Ediacara Member. During deposition of the Ediacara Member, colonization of the deeper substrates by microbial mat communities enabled preservation of the Ediacara biota. The erosional surface at the base of the Ediacara Member is one of at least four incision events recorded in the terminal Proterozoic and Early Cambrian succession of the Adelaide Geosyncline. The similarity of incision-filling lithofacies of the Rawnsley and Uratanna sequences has, in condensed sections, led to confusion between the depositional sequences. Characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the Proterozoic–Cambrian boundary.