Microbial Mats in Terminal Proterozoic Siliciclastics: Ediacaran Death Masks

A variety of sedimentary structures and patterns in Proterozoic siliciclastic sedimentary rocks cannot be explained by known inorganic processes. In particular, certain bedsurface textures, and domed and disrupted sand lamination, are demonstrably the mechanical products of microbially bound sediment and microbial mats. In all but the most wave and current active marine environments of the terminal Proterozoic, the absence of effective grazing and burrowing allowed mat-communities of cyanobacteria to colonize sedimentary surfaces. The resultant microbial mats inhibited sediment erosion, formed partings when buried between sand beds, and restricted vertical migration of pore fluid and gas in both exposed and subaqueous environments. Distinctive ‘petee’ laminations, known from modern mat-bound, tidal-flat sediments, are recorded for the first time in the rock record from the terminal Proterozoic Rawnsley Quartzite of South Australia. The preservation of external molds of soft-bodied Ediacaran organisms is interpreted as a function of the early diagenesis of a sole veneer. A form of ‘death mask’ resulted from bacterial precipitation of iron minerals in the sand that smothered decaying microbial mats and megascopic benthic organisms. The appearance of three-dimensional trace fossils in Early Cambrian strata signaled a behavioral revolution; the evolution of efficient grazing reduced the development of benthic mat communities in all but the most extreme environments, while bioturbation disrupted buried mats and closed a taphonomic window of preservation for soft-bodied organisms.