Discussion: “Were the Ediacaran siliciclastics of South Australia coastal or deep marine?” by Retallack et al., Sedimentology, 59, 1208–1236

Abstract

RICHARD H. T. CALLOW*, MARTIN D. BRASIER , and DUNCAN MCILROY*Department of Geology and Petroleum Geology, University of Aberdeen, Meston Building, Aberdeen,AB24 3UE, UK (E-mail: r.callow@abdn.ac.uk) Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK Department of Earth Sciences, Memorial University of Newfoundland, 300 Prince Philip Drive,St John’s, NL A1B 3X5, CanadaINTRODUCTIONThe late Ediacaran Period is a critical interval inthe history of life on Earth and its poorly under-stood fossils have potential to shape our under-standing of early biosphere evolution. Critical tothis process is an accurate assessment of theenvironments in which these organisms flour-ished, prior to the radiation of animal life duringthe Cambrian explosion. Few successions can bemore historically important for this than theEdiacara Member of the Rawnsley Quarzite inthe Flinders Ranges, South Australia (Glaessner,1984). The depositional settings for these rockshave hitherto been established, and widelyaccepted, as being below fair-weather wave base,delta front, prodelta, shoreface to intertidal(Gehling, 2000). The new interpretation advancedby Retallack (2012), is thought-provoking andincludes a diverse dataset, which is used to arguefor deposition of the Ediacara Member within ashallow marine, coastal, and subaerial suite ofenvironments. This discussion examines a num-berofthe conclusions insupportofthatclaim,andconsiders that the established marine depositionalsetting of the Ediacara Member remains valid.‘DEEP MARINE’ CONTEXT OF THEEDIACARA MEMBERThe title of the paper implies that Gehling (2000)interpreted the Ediacara Member to be a ‘deepmarine’ unit (see also Mount, 1989). The term‘deep marine’ is rarely used in a strict sense bygeologists. It has been used variously to refereither to environments situated beyond the shelfbreak (e.g. Pickering et al., 1989) or to environ-ments affected by processes that are typical ofslope and basin floor environments (i.e. sedimentgravity flows). The depositional environmentsrecognized by Gehling (2000) include sandstonedeposition below fair-weather wave base inincised valley fills (palaeovalleys), delta front topro-delta palaeoenvironments, delta-top sandsheets, braid-deltas and intertidal flats. It is clearfrom this list of palaeoenvironmental interpreta-tions that Gehling (2000) did not consider theEdiacara Member as ‘deep marine’. For example,intertidal environments were identified from theinference of ancient microbial mats and sandgypsum crystals (Gehling, 2000; Gehling & Dro-ser, 2009). Most importantly, the fossiliferousbeds were interpreted as tempestites that formedin delta-front palaeoenvironments between stormand fair-weather wave base (Gehling, 2000).Sediment gravity flows including, but not lim-ited to, turbidites were identified as componentsof the succession by Gehling (2000), as parts ofFacies Associations C, D, and E. However, it wasstated explicitly that the massive channelizedsands (0AE5 to 8 m thick beds) were not depositedin a basinal setting, but rather in sub-wave basesubmarine channels. Similarly, the ‘micro-turbi-dites’ (Facies Association E) were interpreted aspart of a distal pro-delta environment. Suchshelfal prodelta turbidites are common in depo-sitional systems with high sediment supply andhigh rates of accommodation generation (e.g.McIlroy et al., 2005). We concur with Gehling(2000) that the thick-bedded, massive, and dewa-tered channelized sandstones are not readilycompatible with fluvial facies models and aremore consistent with deposition from submarineflows (e.g. Kneller & Branney, 1995). The pres-ence of hummocky cross-stratified beds (Gehling,2000, fig. 10d) has been confirmed by manyauthors. Such bedforms were not observed bySedimentology (2013) 60, 624–627