Microbial signatures in peritidal siliciclastic sediments: a catalogue

A catalogue of microbial structural signatures is presented, based upon the coupling of fundamental biogeochemical-microbial processes and local morph ogenetic determinants, it summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones (temperate humid: Mellum Island, southern North Sea; subtro pical arid: coast of southern Tunisia). Textural geometries reveal a high s tructural diversity, but their determinants are primarily based upon six ma jor parameters: (1) intrinsic biofactors: structural diversification of sed imentary microbial films and mats inherent in the organisms, i.e. their con struction morphology, growth, taxis and behaviour, and local abundance of s pecific morphotypes. Most prominent are the ensheathed filamentous cyanobac teria Microcoleus chthonoplastes and Lyngbya aestuarii, and the sheathless filamentous cyanobacterium Oscillatoria limosa. (2) Biological response to physical. disturbances: sediment supply, erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and mi crobial mats. (3) Trapping/binding effects: physicobiological. processes gi ve rise to grain orientations and wavy to lenticular lamina, lamina-specifi c grain arrangements and 'sucrose' calcium carbonate accumulations. (4) Sec ondary physical deformation of biogenic build-ups: mechanical stresses acti ng upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures. (5) Post-burial processes: textural fa brics that evolve from mechanical effects of gas formation from decaying ma te, and features related to the formation of authigenic minerals (calcium c arbonates, calcium sulphates, pyrite). (6) Bioturbation and grazing: post-d epositional structures, such as Skolithos-type dwellings, traces of burrowi ng insects, gastropod grazing traces and faecal pellets. In synopsis, the c atalogue firstly comprises a sound set of ubiquitous signatures. This unifo rmity in architectural characteristics is attributed to the presence and lo cal dominance of certain microbes throughout the different settings. The ca talogue secondly documents signatures that are extremely sensitive to tidal position, hydrodynamic regime and overall climatic conditions. These. kind s of signature indicate narrow facies zones, which often coincide with the activity or dominance zones of certain organisms. An overview of structures of microbial origin from the fossil record underlines the potential of man y of the signatures included in this catalogue to become fossilized and pro vide strong indicators of former siliciclastic tidal settings.