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

Fossils of the Ediacara biota are confined to preservational windows in the Ediacara Member of the Rawnsley Quartzite (Pound Subgroup), from the Flind ers Ranges, South Australia. The base of the Ediacara Member is a type 1 se quence boundary incised into the partly lithified Chace Quartzite Member of the Rawnsley Quartzite. The Ediacara Member and the upper half of the Rawn sley Quartzite comprise the Rawnsley depositional sequence, bounded above b y the Early Cambrian Uratanna depositional sequence. The Rawnsley sequence developed over an erosional surface cut into the Chace Quartzite Member, wi th some 250 m of relief. Southeasterly directed palaeovalleys are filled wi th a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone. At lowstand, local channels were filled by massive s and. The highstand systems tract consists of several parasequence sets of l aminated siltstone and channelized, amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well-bedded, clean s andstone. Impressions of soft-bodied Ediacaran organisms are preserved abov e the valley fill facies on sandstone partings within upward-shoaling, delt a-front environments between storm-and fairweather wavebase. The Ediacara M ember is, in places, conformably overlain by up to 500 m of sandstone of th e same lithofacies as those underlying the member. Distinctive microbial-ma t-bound sand laminae, described as petee structures, are characteristic of the Rawnsley Quartzite above and below the Ediacara Member. During depositi on of the Ediacara Member, colonization of the deeper substrates by microbi al mat communities enabled preservation of the Ediacara biota. The erosiona l surface at the base of the Ediacara Member is one of at least four incisi on events recorded in the terminal Proterozoic and Early Cambrian successio n of the Adelaide Geosyncline. The similarity of incision-filling lithofaci es of the Rawnsley and Uratanna sequences has, in condensed sections, led t o confusion between the depositional sequences. Characteristic trace and bo dy fossil assemblages provide the only clear distinction between deposition al cycles spanning the Proterozoic-Cambrian boundary. Published by Elsevier Science B.V.