Microbial mats in terminal Proterozoic siliciclastics: Ediacaran death masks

A variety of sedimentary structures and patterns in. Proterozoic siliciclas tic sedimentary rocks cannot be explained by known inorganic processes. In. particular certain bed-surface textures, and domed and disrupted sand lami nation, are demonstrably the mechanical products of microbially bound sedim ent 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 sediment ary surfaces. The resultant microbial mats inhibited sediment erosion, form ed partings when. buried between sand beds, and restricted vertical migrati on of pore fluid and gas in both exposed and subaqueous environments. Disti nctive 'petee' laminations, known from modern mat-bound, tidal-flat sedimen ts, are recorded for the first time in the rock record from the terminal Pr oterozoic Rawnsley Quartzite of South Australia. The preservation of external molds of soft-bodied Ediacaran organisms is in terpreted as a function of the early diagenesis of a sole veneer. A form of 'death mask' resulted fi om bacterial precipitation of iron minerals in th e sand that smothered decaying microbial mats and megascopic benthic organi sms. The appearance of three-dimensional trace fossils in Early Cambrian strata signaled a behavioral revolution; the evolution of efficient grazing reduce d the development of benthic mat communities in all but the most extreme en vironments, while bioturbation disrupted buried mats and closed a taphonomi c window of preservation, for soft-bodied organisms.