MICROBIALLY INDUCED SEDIMENTARY STRUCTURES: A KEY TO UNDERSTANDING THE DYNAMIC INTERACTION OF EPIBENTHIC CYANOBACTERIAL COMMUNITIES AND PHYSICAL PARAMETERS IN SILICICLASTIC SEDIMENTARY SYSTEMS

Microorganisms interact with the ambient physical environment to produce a range of distinctive sedimentary structures capable of preservation in the rock record. The most conspicious examples of microbially mediated structures are stromatolites, found in carbonate and other precipitated lithologies, but microbial mats can also impart a diagnostic signature to siliciclastic sediments. Because mineral precipitation and penecontemporaneous cementation is rare or absent in siliciclastic regimes, mat structures are developed on bedding surfaces, with little accretion of successive laminae.

Observations of modern siliciclastic environments provide an actualistic framework for the interpretation of microbially induced structures in ancient successions. In particular, microbially induced sedimentary structures in sandstones of the terminal Proterozoic Nama Group, Namibia, and the Lower Arenigian of the Montagne Noire, France, can be understood in terms of the syndepositional interplay between physical dynamics that control sediment influx and distribution, and the biological responses of epibenthic microbial communities.

Wrinkle structures record microbially levelled depositional surfaces that were subsequently dewatered and deformed by sedimentary loading. Biostabilization grounds, wavy rock beds or petees result from bacterial communities that interweave and stabilize sediment to resist erosion. Vertical patterns of sediment-rich and organic-rich lamination can originate by microbial baffling, trapping and binding, contributing to sediment accumulation. The physically sculptured surface relief of antecedent depositional surfaces may be accentuated during weathering by former biofilm coverings that gave rise to distinct textures within the otherwise homogeneous sediment.

Such bacterially mediated structures provide insights into ancient biology, environment, and climate. Moreover, these structures are not easily mimicked by purely physical processes. Thus, a search for microbially induced structures should be included in the astrobiological exploration of Mars and, potentially, other extraterrestrial surfaces.

Funded by Deutsche Forschungsgemeinschaft DFG-Proj. NO 380/1-1 and NO 380/1-2 (NN), Deutsche Akademie der Naturforschenden Gesellschaft Leopoldina BMFT LDP 9901/8-2 (NN), the NASA Astrobiology Institute NCC2-1053 (AHK, NN).