Microbially induced sedimentary structures - A new category within the classification of primary sedimentary structures

Cyanobacterial films and mats syndepositonally influence erosion, depositio n, and deformation of sediments. The biomass levels surface morphologies, a nd microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing. Growing microbial mats dredge grains from the ir substrate upwards, whereas cyanobacterial filaments that are oriented pe rpendicular to the mat surface reach into the supernatant water and baffle, trap, and bind suspended particles. These and similar biotic-physical inte ractions are reflected in syndepositional formation of microbially induced sedimentary structures. We distinguish structures on bedding planes (levele d bedding surfaces, wrinkle structures, microbial mat chips, erosional remn ants and pockets, multidirectional ripple marks, and mat curls) and interna l bedding structures (sponge pore fabrics, gas domes, fenestrae structures, sinoidal laminae, oriented grains, benthic ooids, bio-laminites, mat-layer -bound grain sizes). We propose to place this group of microbially mediated structures as a fift h category (bedding modified by microbial mats and biofilms) in Pettijohn a nd Potter's (1964) existing classification of primary sedimentary structure s.