These microbialites consist of 3 components (see Figures 3-5 to 3-23): (1) Filmy laminae are 1-30 micrometers thick and defined by organic inclusions. These laminae were flexible and laterally cohesive during growth and are interpreted as microbial mats that consisted of filamentous microbes with diameters of only a few microns. (2) Supports are 125 micrometers to >300 micrometers wide and are commonly oriented vertically. They are interlocking surfaces in 3-dimensions and branch in some microbialites. They are defined by organic inclusions and also are interpreted as microbial in origin. (3) Dish-shaped to equant primary voids were created by the draping of laminated mat over the supports and by branching of the supports. These voids formed contemporaneously with microbialite growth and contain marine cements* which also precipitated contemporaneously with laminated mat and support development. No known modern analogs are present for these microbialites, and they have not been reported from rocks younger than the Gamohaan and Frisco formations. This may be due to the presence of uniquely Archean depositional environments where very quiet, deep subtidal* seawater was highly supersaturated with respect to calcite*. In addition, low oxygen concentrations may have influenced both the kinetics of carbonate* precipitation and the structure of microbial communities.
The differences in morphology of the supports and laminated mat suggest that they were composed of different microbial communities. Differences in the motility and metabolic needs of microbes may have led to their segregation into the two communities. For example, the supports may have been constructed of microbes with a strong upward phototactic* or chemotactic response, whereas the laminated mats may have been composed of either less mobile microbes or microbes lacking the same taxis* as those in the supports. The interpretation that different microbial communities composed the supports and the laminated mat is supported by differences in the precipitation of early marine calcite on the supports and laminated mat. Herringbone calcite, a fibrous marine cement, precipitated contemporaneously with microbialite growth. It preferentially precipitated on the supports over the laminated mats 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 the laminated mat, and the abutment of herringbone calcite coatings against laminated mat attached to supports (Figures 3-21 to 3-23). These observations suggest that herringbone calcite preferentially nucleated on the supports over the laminated mat. This difference in calcite nucleation could be due to differences in the composition or charge distribution in the extracelluar secretions produced by microbes in the supports versus those in the laminated mat, or it could be due to the release or uptake of different metabolic products by the two communities. Either cause suggests that the supports and laminated mat contained different microbial communities.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616