Figure 2.
Formation of microbially induced sedimentary structures (MISS) versus stromatolites. Top: The microbial mat overgrows the sedimentary surface and incorporates some of the sedimentary grains. The process of individual microbes actively assembling to form a microbial mat fabrics is called “binding.” During episodes of erosion, the microbial mat biostabilizes its substrate; during episodes of deposition of sediment, baffling and trapping accumulates sedimentary particles on the microbial mat surface. Bottom: In stromatolites, another process is added. Here, mineral particles (usually carbonates) form within the extracellular polymeric substance (EPS-mineralization). In the formation of MISS, the processes of binding, biostabilization, baffling, and trapping elevate the mat-covered sedimentary surface somewhat; the amount of precipitated minerals typical for stromatolites contributes significantly to their positive topography. Later, the MISS- or stromatolite-forming microbial mats fossilize—that is, the organic matter is replaced by minerals. In stromatolites, however, the EPS-precipitated mineral particles now recrystallize and contribute to the layering (lamination) characteristic for most stromatolites. MISS commonly lack this lamination. Note that there are also stromatolites predominantly formed by mineral precipitation with no or very minor detrital sediment in them.