Sedimentology of microbial carbonate is a challenging subject with controversial views and complex links to petrography, geochemistry, and microbiology. Pratt (2001) presented an intriguing hypothesis that a considerable part of Paleozoic carbonate mud may have been derived from disintegrated micrite tubes considered to have been formed by calcified cyanobacteria. Such micritic tubes from many Paleozoic and Mesozoic carbonate rocks are known as Girvanella-type microfossils; most Proterozoic examples are less typical—a fact correctly noted and described, e.g., by Turner et al. (2000).


While we agree with Pratt's suggestion that micrite can be derived from loosely calcified cyanobacterial sheaths, we...

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