MICROBIAL MAT SANDWICHES AND OTHER ANACTUALISTIC SEDIMENTARY FEATURES OF THE EDIACARA MEMBER (RAWNSLEY QUARTZITE, SOUTH AUSTRALIA): IMPLICATIONS FOR INTERPRETATION OF THE EDIACARAN SEDIMENTARY RECORD

Abstract: The Ediacara Member of the Rawnsley Quartzite of South Australia hosts some of the most ecologically and taxonomically diverse fossil assemblages of the eponymous Ediacara Biota—Earth's earliest fossil record of communities comprised of macroscopic, complex, multicellular organisms. At the National Heritage Site, Nilpena, fifteen years of systematic excavation and reassembly of bedding planes has resulted in reconstruction of over 400 square meters of Ediacaran seafloor, permitting detailed and sequential sedimentary, paleoecological and taphonomic assessment of Ediacara fossilized communities and the shallow marine settings in which these ecosystems lived. Sedimentological investigation reveals that the Ediacara Member consists of successions of sandstone event beds and a paucity of other lithologies. Moreover, these Ediacara sandstones are characterized by a suite of sedimentary features and style of stratigraphic packaging uncharacteristic of Phanerozoic sandstone successions considered to have been deposited in analogous shallow marine, storm-dominated environments, including: (1) extremely thin (sub-mm- to mm-scale) bed thickness; (2) lateral discontinuity; (3) textural uniformity, including lack of disparity in grain size, between adjacent beds; (4) lack of amalgamation; (5) lack of erosional bed junctions; (6) doubly rippled bedforms defined by rippled bed tops and bases which crisply cast the tops of underlying rippled beds; (7) ubiquity of textured organic surfaces (TOS); (8) positive correlation between body fossil size and abundance and bed thickness; and (9) texturally immature assemblages of sandstone rip-up clasts along bed tops. We interpret these features to reflect the presence of widespread matgrounds, which facilitated seafloor colonization by and ecological development of Ediacara macroorganisms in high-energy environments. Further, we argue that pervasive matgrounds directly mediated the formation and preservation of non-uniformitarian sedimentary features and stratigraphic packaging in the Ediacara Member and were responsible for the anactualistically complete nature of the Ediacara stratigraphic record.