Shark Bay stromatolites: Microfabrics and reinterpretation of origins

Detailed analysis of microfabrics in Hamelin Pool stromatolites leads to reinterpretation of the origins of these structures. Previous studies have concluded that Shark Bay stromatolites form primarily as a result of sediment trapping and binding by microorganisms. Our results suggest that microbial precipitation of microcrystalline carbonate (micrite), as both framework and cement in these stromatolites, is also a fundamental, heretofore unrecognized, process in their formation. Microbial trapping and binding is the primary mechanism of stromatolite accretion in the intertidal zone, forming grainy, calcarenite structures. Microbial precipitation is the primary accretionary mechanism in the subtidal zone, forming muddy, micritic stromatolites. Microbial precipitation also lithifies trapped and bound sediment in the calcarenite stromatolites. Recognition of microbially precipitated micrite in Shark Bay stromatolites is important, as many ancient stromatolites are micritic.