Peritidal Micrite Precipitation within Late Archean Microbial Mats

Dawn Y. Sumner

1998, GSA Annual Meeting, Abstracts

The role of microbial processes in stromatolite formation are not always clear. In some cases, morphology is consistent with abiotic precipitation processes. In others, the presence of cohesive microbial mats is required to explain morphological features. Several aspect of deposition of the 2550-2520 Ma Campbellrand-Malmani carbonate platform, South Africa, are consistent with solely abiotic processes. However, 50-100 cm domal stromatolites and associated structures in peritidal deposits require the presence of microbial mats to explain several morphological features.

The domes contain four styles of lamination: bumpy colloform, smooth colloform, peaked, and smooth deformed laminae. Bumpy colloform lamellae isopachously coat mm-scale irregularities producing composite domes with occasional overhanging sides. Bumpy colloform lamellae also grew downwards from the bottom of platy intraclasts forming 1-8 mm mini-domes, requiring a precipitated origin with or without the presence of a mat. Smooth colloform lamellae are similar, but lack the mm-scale topographic irregularities. Commonly, smooth lamellae are truncated at the edges of domes, and lighter color lamellae occasionally thicken and thin across domes and are intercalated with grainstone lenses. However, lamellae occasionally form overhangs suggesting that the micrite forming them precipitated on the dome surface rather than being trapped-and-bound. Oncolites composed of smooth colloform lamellae are also present. They are deformed and stretched out in the direction of current flow as defined by ripple stratification. The buckling of downstream laminae requires binding of the micrite by a microbial mat whether it was trapped or precipitated in place. Peaked laminae form 5 mm-wide and 10 mm-high ridge-like peaks with high inheritance. Layers of peaked laminae commonly thicken into troughs, usually with an increase in the number of laminae. The ripple-like morphology of the laminae suggests that they consist of microbially trapped-and-bound micrite with a precipitated component possible. Smooth, deformed domes are composed pairs of gray and light orange lamellae that thin on the sides of domes where they are inclined up to 80. Lenses of cross-stratified grainstone are present between flat-lying laminae. Synsedimentary breaking and folding of lamellae was common in the interiors of domes. In many cases, light orange lamellae disaggregated, whereas gray lamellae maintained their integrity, suggesting that the gray lamellae consist of in situ precipitated micrite or more tightly bound detrital micrite than the orange lamellae. The steep inclination of the laminae that disaggregated during deformation requires the presence of a microbial mat.

Although at present it is unclear if microbial processes induced precipitation within the mats forming these structures, the presence of a mat is required. The downward growth of small domes on the undersides of stromatolites demonstrates that a phototactic response is not necessary for stromatolite formation.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616