Abstract
ABSTRACTThe 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.
Highlights
Late Ediacaran macrofossils (~ 574–539 Ma) offer critical information about the early evolutionary history of large and complex multicellular organisms (Linnemann et al 2019; Matthews et al 2020)
We demonstrate that all observed fossil-bearing facies of the Ediacara Member fall within the marine shoreface complex—the seaward-sloping ramp extending from the 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
The Rawnsley Quartzite in the Central Flinders Ranges is interpreted as an estuarine, shoreface, and coastal succession deposited exclusively above effective wave base
Summary
Late Ediacaran macrofossils (~ 574–539 Ma) offer critical information about the early evolutionary history of large and complex multicellular organisms (Linnemann et al 2019; Matthews et al 2020). We demonstrate that all observed fossil-bearing facies of the Ediacara Member fall within the marine shoreface complex—the seaward-sloping ramp extending from the 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; Reid et al 2020).
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