PROTEROZOIC REEF EVOLUTION

The immense period of Proterozoic reef evolution records a progressive replacement of precipitate-dominated buildups by microbe/biofilm structures, with the modern reef archetype appearing in early Neoproterozoic time. Paleoproterozoic (2.5-1.6Ga) reefs were fractal in nature, constructed by stromatolites in which the influence of synsedimentary carbonate precipitation exceeded that of microbes and/or biofilms. There is little variability in stromatolite form; most are hemispherical, columnar, laminated and conical. Isolated buildups are not abundant, most are integrated into platforms and ramps as biostromes because abiotic precipitation is so extensive and widespread. Mesoproterozoic (1.6-1.0Ga) reefs record a long period of stasis with stromatolites broadly similar to older growth forms. They are, however, more 'muddy' and display increasing 'diversity' in stromatolite form. Synsedimentary abiotic precipitation and microbial influence seem roughly equal in importance. The Neoproterozoic (1.0-0.65Ga) is a period of dramatic global change and quickening reef evolution. The appearance of heavily calcified microbial elements (calcimicrobes; e.g. Girvanella and Renalcis) in the Tonian (1.0-0.85Ga), coincident with the disappearance of conical elements and decline in stromatolites, is a critical event. It would seem that the role of microbes/biofilms became more important than that of synsedimentary precipitation. In deep-water buildups this produced rapid upward accretion and the formation of growth cavities containing internal sediment and synsedimentary cement. The importance of calcimicrobes appears to be less in neritic reefs where they are mostly spar-filled filament molds. Nevertheless, in shallow water reefs diverse stromatolite growth forms across the environmental spectrum locally formed discrete, 'kalyptra-like' domains. While the Cryogenian (0.85-0.69Ga) is conspicuous for the lack of large reef structures, post-glacial cap carbonates contain Lazarus-like, Paleoproterozoic-style, precipitate-dominated reefs. Vendian (0.69-0.54Ga) buildups are formed by abundant thrombolites and calcimicrobes, host the first skeletal metazoans (cf. Cloudina), and contain numerous synsedimentary cavities.