BEYOND HJULSTROM AND SHIELDS: EXPERIMENTAL MICROBIAL SEDIMENTOLOGY

Interpreting the physical dynamics of ancient environments requires understanding how sedimentary structures, such as ripples and dunes, are created. Traditional interpretations of these structures are based on experimental flume studies of unconsolidated quartz sand, in which stepwise increases in flow velocity yield a suite of sedimentary structures analogous to those found in the rock record. Yet cyanobacteria, which were excluded from these studies, are pervasive in wet sandy environments and secrete sufficient extracellular polysaccharides that they inhibit grain movement and markedly change the conditions under which sedimentary structures form.

Our flume experiments using microbe-inoculated quartz sand demonstrate that bacteria strongly influence the behavior of unconsolidated sand. In freshly inoculated but unbound sediment, ripple production is retarded by nearly 50%, and in weakly developed biofilms ‘no grain movement’ conditions occur at flow velocities under which ripples would normally form in sterile sand. Flip-over and rip-up bedforms form before ripples are produced, and ripples occur only when flow velocities are more than twice as high than would be required in sterile sand. Thus, even thin microbial films can more than double the flow velocity required to produce the traditional sequence of ripple→dune→plane-bed-lamination bedforms and can inhibit the growth of ripples or dunes entirely. Thicker microbial mats mediate terracing of erosional edges, foster transport of multi-grain aggregates, and yield a bedform progression consisting of flip-overs→roll-ups→rip-ups of bound sand. After this bedform progression, the bed then passes directly into the dune phase without passing through production of ripples.

Roll-ups, rip-ups, and flip-overs are common in Precambrian sandstones and in Phanerozoic settings characterized by soft-bodied fossil preservation; ripples, dunes, and plane-bed lamination are ubiquitous in most ancient sandy aquatic environments. This research quantifies how coccoidal and filamentous bacteria influence the formation of these structures. It also suggests that when these primary structures are present in sandstones, ~50-100% higher velocity paleohydraulic conditions may have existed than might be inferred from experiments with sterile sand.