Abstract

Microbialites in the 2521+ or -3 Ma Gamohaan and Frisco formations, South Africa, consist of three components: very thin, filmy laminae that are interpreted as remnants of microbial mats; irregular surfaces that acted as supports over which filmy laminae draped, which also are interpreted as microbial in origin; and cement-filled voids that formed contemporaneously with microbialite growth. The structure of the microbialites varies with the proportions of filmy laminae and supports, disruption of had filmy laminae, and abundance of voids. Seven end-member morphologies have been defined: planar laminae, contorted laminae, tented microbialites, cuspate microbialites, irregular columnar microbialites, plumose structures, and herringbone calcite beds. Similar structures have not been reported in younger rocks, but they may have partial analogs in Yellowstone hot springs, ice covered lakes in Antarctica, and Proterozoic conoform stromatolites. Herringbone calcite, a fibrous marine cement, precipitated contemporaneously with microbial growth. It preferentially precipitated on the supports over the laminated mat as demonstrated by: the concentration of herringbone calcite near supports; growth banding in herringbone calcite, which indicates that calcite nucleated on and grew away from supports but not laminated mat; and the abutment of herringbone calcite coatings against laminated mat attached to supports. These observations suggest that the laminated mat inhibited nucleation of calcite crystals. The differences in the morphology and relationships to herringbone calcite in the supports and laminated mat imply that they were composed of distinct microbial communities that interacted with their environment differently. Also, the complexity of the microbialites demonstrates that a remarkably diverse set of microbe-substrate interactions evolved by late Archean time.

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