Ediacaran Life Close to Land: Coastal and Shoreface Habitats of the Ediacaran Macrobiota, the Central Flinders Ranges, South Australia
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Journal of Sedimentary Research, 2020, v. 90, 1463–1499 Research Article DOI: 10.2110/jsr.2020.029 EDIACARAN LIFE CLOSE TO LAND: COASTAL AND SHOREFACE HABITATS OF THE EDIACARAN MACROBIOTA, THE CENTRAL FLINDERS RANGES, SOUTH AUSTRALIA 1 2 2 1 WILLIAM J. MCMAHON,* ALEXANDER G. LIU, BENJAMIN H. TINDAL, AND MAARTEN G. KLEINHANS 1Faculty of Geosciences, Utrecht University, Princetonlaan 8a, 3584 CB, Utrecht, The Netherlands 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. [email protected] ABSTRACT: The Rawnsley Quartzite of South Australia hosts some of the world’s most diverse Ediacaran macrofossil assemblages, with many of the constituent taxa interpreted as early representatives of metazoan clades. Globally, a link has been recognized between the taxonomic composition of individual Ediacaran bedding-plane assemblages and specific sedimentary facies. Thorough characterization of fossil-bearing facies is thus of fundamental importance for reconstructing the precise environments and ecosystems in which early animals thrived and radiated, and distinguishing between environmental and evolutionary controls on taxon distribution. This study refines the paleoenvironmental interpretations of the Rawnsley Quartzite (Ediacara Member and upper Rawnsley Quartzite). Our analysis suggests that previously inferred water depths for fossil-bearing facies are overestimations. In the central regions of the outcrop belt, rather than shelf and submarine canyon environments below maximum (storm-weather) wave base, and offshore environments between effective (fair-weather) and maximum wave base, the succession is interpreted to reflect the vertical superposition and lateral juxtaposition of unfossiliferous non-marine environments with fossil-bearing coastal and shoreface settings. Facies comprise: 1, 2) amalgamated channelized and cross-bedded sandstone (major and minor tidally influenced river and estuarine channels, respectively), 3) ripple cross-laminated heterolithic sandstone (intertidal mixed-flat), 4) silty-sandstone (possible lagoon), 5) planar-stratified sandstone (lower shoreface), 6) oscillation-ripple facies (middle shoreface), 7) multi-directed trough- and planar-cross-stratified sandstone (upper shoreface), 8) ripple cross-laminated, planar-stratified rippled sandstone (foreshore), 9) adhered sandstone (backshore), and 10) planar-stratified and cross-stratified sandstone with ripple cross-lamination (distributary channels). Surface trace fossils in the foreshore facies represent the earliest known evidence of mobile organisms in intermittently emergent environments. All facies containing fossils of the Ediacaran macrobiota remain definitively marine. Our revised shoreface and coastal framework creates greater overlap between this classic ‘‘White Sea’’ biotic assemblage and those of younger, relatively depauperate ‘‘Nama’’-type biotic assemblages located in Namibia. Such overlap lends support to the possibility that the apparent biotic turnover between these assemblages may reflect a genuine evolutionary signal, rather than the environmental exclusion of particular taxa. INTRODUCTION the Flinders Ranges, in which Reginald Sprigg originally discovered Precambrian macrofossils (Sprigg 1947), ultimately lent its name to the Late Ediacaran macrofossils (~ 574–539 Ma) offer critical information Ediacaran System (Knoll et al. 2004, 2006). As an important global focal about the early evolutionary history of large and complex multicellular point for studies of Ediacaran life with a long history of research (e.g., organisms (Linnemann et al. 2019; Matthews et al. 2020). How the Glaessner and Daily 1959), it is essential that the preserved depositional Ediacaran macrobiota relate to extant animals, their life habits, and the environments of the Rawnsley Quartzite are both well studied and robustly conditions under which their fossils were preserved are fundamental understood. questions whose answers require an understanding of the environments inhabited by the organisms, evidence of which is archived in the Detailed accounts of previously proposed facies schemes for the sedimentary record. This study presents revised interpretations of the Rawnsley Quartzite have been provided in a number of recent publications sedimentary facies and stratigraphic architecture of the siliciclastic and will not be repeated here (e.g., Gehling 2000; Tarhan et al. 2017; Reid Ediacaran-age Rawnsley Quartzite of South Australia, whose eponymous et al. 2020). However, it is worth noting that early descriptions of the unit Ediacara Member hosts one of the world’s most taxonomically diverse favored intertidal and lagoonal depositional environments for the fossil- assemblages of the Ediacaran macrobiota (e.g., Droser et al. 2006, 2017; bearing facies (Jenkins et al. 1983) (Table 1). Gehling (2000) provided Gehling and Droser 2013; Droser and Gehling 2015). The Ediacara Hills in detailed descriptions of the sedimentary facies at a large number of previously undocumented Rawnsley Quartzite sections from across the * Present Address: Energy & Environment Institute, University of Hull, Flinders Ranges, and reinterpreted the fossiliferous parts of the succession Cottingham Road, Hull HU6 7RX, U.K. to comprise five facies, four of which were considered to have been Published Online: November 2020 Open Access CC-BY 4.0. Copyright Ó 2020, SEPM (Society for Sedimentary Geology) 1527-1404/20/090-1463 Downloaded from http://pubs.geoscienceworld.org/sepm/jsedres/article-pdf/90/11/1463/5214434/i1527-1404-90-11-1463.pdf by guest on 23 September 2021 1464 W.J. MCMAHON ET AL. JSR deposited below effective (fair-weather) wave base (and three beneath maximum (storm-wave) base) (see Gehling and Droser 2013, their Fig. 1). Since that seminal study, most sedimentological research on the unit has focused solely on fossil-bearing facies, predominantly at one location (Nilpena; Fig. 1; though see Reid et al. 2020). Furthermore, interpretations of Rawnsley Quartzite depositional environments have remained relatively or sandy shoals Shallow subtidal unchanged (Table 1). Our revised environmental framework, presented following fieldwork at five sections in the central and western reaches of the Flinders Ranges (Fig. 1A), considers Gehling’s (2000) inferred water depths to be overestimations at these locations. We demonstrate that all se used in this study. Note that observed fossil-bearing facies of the Ediacara Member fall within the marine shoreface complex—the seaward-sloping ramp extending from the Not described low-tide mark to the lower limit of the fair-weather wave base (e.g., Reinson 1984; Pemberton et al. 2012) (Fig. 2), in addition to a number of distinct coastal environments. This finding contrasts with previous studies, which considered the marine shoreface complex to be only scarcely fossiliferous (Gehling and Droser 2013 (their Table 1); Tarhan et al. 2015; Not described Not described Not described Not described Not described Not described Reid et al. 2020). GEOLOGICAL SETTING The Ediacaran Rawnsley Quartzite, presently divided in ascending order into the Chace Quartzite Member, The Ediacara Member, and The Upper 2 Not described Not described Not described Not described Not described Not described Not described Not described Rawnsley Quartzite, crops out over an outcrop belt of 20,000 km in the vicinity of the Flinders Ranges of South Australia (Fig. 1). Following formalization of the Chace Quartzite and Ediacara Member (Jenkins 1975; Reid and Preiss 1999), Gehling (1982, 2000) demonstrated that: 1) the 678910 Chace Quartzite Member, for which a sandflat-to-supratidal depositional FACIES environment was assigned, is separated from overlying strata by a distinct combined flow incised valley (Fig. 1B), and 2) the fossiliferous Ediacara Member Upper wave-base Shoreface Oscillatory and comprised all deposits from the base of this valley-contact to the topmost fossiliferous facies. The upper Rawnsley Quartzite was proposed to extend from the first unfossiliferous facies overlying the Ediacara Member to the disconformable contact with overlying Cambrian-age deposits (Fig. 1). Gehling (2000) demonstrated the variable stratigraphic thickness (10–300 m) of the Ediacara Member across the Flinders Ranges (though see flows canyon fill discussion in Sequence Stratigraphic Evolution, below) and identified Sediment gravity Subwave-base upper Intertidal Intertidal valley-shaped incisions that are occasionally tractable at outcrop (Fig. 1B). Considering its occurrence disconformably beneath a notable erosional facies, in this study interpreted as the product of seismic activity and represented in a number of facies. ’’ hiatus of unknown duration (i.e., an incised sequence boundary), the Chace Quartzite Member should be more appropriately treated as a distinct formation with respect to the rest of the Rawnsley Quartzite. In this study prodelta prodelta shelf we do not explicitly address the Chace Quartzite Member, and instead mass-flow Delta-front or Delta-front or ‘‘ focus on the rest of the Rawnsley Quartzite (the Ediacara Member and the Upper Rawnsley Quartzite) at four locations across its central outcrop belt: 1) Brachina Gorge, 2) Bunyeroo Gorge, 3) Moralana, and 4) Wilpena Pound (Figs. 1, 3, 4). prodelta prodelta Mixed-flat Lagoon? Lower shoreface Middle shoreface Upper shoreface Foreshore Backshore Distributary channels Delta-front or Delta-front or Delta-front Delta-front Sheet-flow Wave-base Shoreface Intertidal Lagoon or neritic FACIES ANALYSIS Methodology