The palaeoenvironmental context of the Trezona anomaly in South Australia: Do carbon isotope values record a global or regional signal?

Abstract

The composite δ13C record of Neoproterozoic carbonates is characterized by large magnitude (<18‰ Vienna Pee Dee Belemnite) swings that have been interpreted to record (a) changes in dissolved inorganic carbon in sea water driven by a carbon cycle with different boundary conditions, (b) pervasive diagenetic alteration or (c) local controls similar to those active in Cenozoic platformal successions. A sedimentological study of the Trezona Formation in the Flinders Ranges of South Australia was conducted to determine the palaeoenvironmental context of the Trezona anomaly, one of the most widely recognized examples of a Neoproterozoic mega-excursion that occurs beneath glaciogenic sediments of the Marinoan ice age (ca 635 Ma). Stratigraphic mapping identified an unconformity at its base separating it from the shelf deposits of the underlying Enorama Shale. The lower Trezona Formation is dominantly calcareous mudstone with mudcracks, channels, and mud chip and algal flake breccias interpreted to record exposure and desiccation on mudflats, shallow pools and lagoons. A marked increase in microbial carbonate and oolites in the upper Trezona Formation records a relative decline of mud input that terminates with karst. The Trezona Formation is thus interpreted to have been deposited in shallow, localized, salt withdrawal basins on the exposed shelf creating a mosaic of poorly connected lagoons and (alkaline) lakes that were intermittently restricted or isolated from the sea. Variations in δ13C and δ18O correspond with facies changes and reflect locally evolving depositional conditions influenced by sills and channels that shifted the sea water to freshwater balance. This restricted setting and range of δ13C values is more similar to Cenozoic platformal deposits (ca 12‰) than the muted (ca 3‰) global sea water variation recorded in pelagic sediments. The anomalous δ13C values in the Trezona Formation may instead provide constraints on the changing terrestrial biosphere directly preceding the evolution and rapid expansion of metazoan life.