A MICROSCOPIC SEDIMENTARY SUCCESSION OF GRADED SAND AND MICROBIAL MATS IN MODERN SILICICLASTIC TIDAL FLATS

Microscopic studies of thin sections from modem siliciclastic tidal fl at sediments in the southern North Sea demonstrate the significant rol e of microbial mats in the buildup of sedimentary sequences. This,is d ocumented by a unit only a few millimetres thick. It starts at the bas e with a fine- to medium-grained quartz sand often containing secondar y pores ('fenestrae type') merging gradually into finer sediments. The lower siliciclastic part is superposed by an upper organically domina ted layer built by microbial mars. Within the organic material, single quartz grains without any contact to each other are oriented with the ir long axes parallel to the bedding planes. Each siliciclastic part i n the lower section of the unit indicates the initial deposition of co arser grains in a stronger flow regime followed by gradually decreasin g flow velocities. Each microbial mat in the upper part essentially re presents a period of low sedimentation rate. During its growth, grains still settle down onto the mat and become bound in the organic matrix . The orientation of these grains with their long axes parallel to the bedding plane points to an energetically suitable position to gravity achieved by the friction reduction of the soft organic matter. Repeat ed depositional events followed by low-rate deposition cause the build up of various units. There is no visible reworking of the former surfa ces, since the microbial mats prevent erosion during periods of increa sed flow. The buildup is characteristic of siliciclastic sediments rep eatedly occupied, stabilized, and fixed by microbial films or mars.