BIOSEDIMENTARY DYNAMICS OF SILICICLASTIC DEPOSITIONAL SYSTEMS

In contrast to carbonate marine environments, siliciclastic depositional systems are governed exclusively by the physical processes of erosion, deposition and deformation. Early mineral precipitation plays only a minor role. In such settings, epibenthic cyanobacteria do not form stromatolites, but construct dense, planar microbial mats that can cover large areas of sedimentary surfaces. With changing topographic level, a lateral succession of different types of microbial mats establishes.

Bacterial activities interfere with the prevailing sedimentary dynamics, leading to the formation of characteristic 'microbially induced sedimentary structures - MISS'. Relevant biosedimentary processes include (i) biostabilization and (ii) baffling, trapping and binding. Biostabilization changes the response of microbial mat-overgrown sediments to erosion and deformation, documented by sediment surface structures such as 'erosional remnants and pockets'. The sediment surface represents the equilibrium between erosion and counteracting biostabilization; deposition of mineral particles does not occur. Bacterial filaments can influence patterns of sediment accumulation by means of baffling, trapping and binding. This is documented by sedimentologically and petrographically distinctive laminae (biolaminites) visible in vertical section through the sediments. Bacterial activity enhances deposition. Erosion plays no role in structure formation.

By biostabilization (counteracting erosion) and by baffling, trapping and binding (enhancing deposition), microbial mats influence the sedimentary dynamics of Recent tidal flats. Because process and pattern can be linked in Recent environments, MISS provide valuable geobiological tools for investigations of ancient sedimentary rocks. Lower Arenigian sediments from the Montagne Noire, France, provide an example of mat influences on Paleozoic sediments.