Study On Microbially Induced Sedimentary Structure(MISS)from The Mesoproterozoic Yunmengshan Formation In Western Henan Province

TheYunmengshan Formation, belonging to Mesoproterozoic Ruyang Group, lies in the south margin of the North China platform and consists mainly of siliciclastic sedimentary successions. Microbially Induced Sedimentary Structures(MISS) occur in purple or yellow sandstone interbedded with mudstone or siltstone, especially in junction surface between sandstone and mud lamination. They displayed a plenty of morphologies and preserved in acompanying with ripple markers as usual. The well-preserved structures include mat growth features, mat metabolism features, mat destruction features and mat decay features with three new types founded in this paper: binding ripples, intense ornamentations and reticulate irregular pits.MISS were well preserved in beach environment from the shoreface to the foreshore and especially in peritidal siliciclastic environments from the subtidal to the supratidal zones. They display some features in Yunmengshan Formation as follows: (1) Some big polygonal desiccation cracks with microbial sand chips are common in beach environments, and middle-small polygonal desiccation cracks with kinds of other MISS mainly occur in tidal flat environments displaying the MISS diversity; (2) The ancient microbial mat became thinner from the bottom to the top of Yunmengshan Formation and the thicker ancient mat colonized in beach facies to fit for higher hydrodynamic condition; (3) MISS are much more abundant in the Second Member of Yunmengshan Formation than that of other Members; and (4) MISS from Yunmengshan Formation correlate with turning points of regression and transgression and especially well developed and preserved in deposits that mark marine flooding surfaces.In vertical section, the typical stacked mm-level siliciclastic biolaminites consisting of alternations of dark silty mudstones and light quartz sandstone represent repeated growth and burial of mats during repeated depositional events. The dark layers are considered to represent the remains of microbial mats, whereas the light layers consist of physical process induced quartz grains. In the further investigation of thin-sections, five types of microstructures have been found: wavy crinkled laminae, network fabrics, mat layer bound small grains, mat layer bound heavy minerals and oriented grains. These structures can reflect the unique characters of mat growth, mat metabolism, interactions with sediments and can be used to trace the thriving microbial mat colonized in this area and reveal the sedimentary dynamics and paleoenvironmental features in this area.Filament fossils, coccoid fossils, silicified EPS and unknown fossils have been found in dark silty mudstones of MISS. Most of the microbial taxa were cyanobacteria which mainly display the horizontal distribution. These Proterozoic microfossils are valuable for the research of microbial communities and their functions. Comparison the ancient mat with modern microbial mat can help us to well understand the colonized palaeoenvironments, microbial community structures and origins of MISS. From this study, we have built the origin model of gas domes, reticulate irregular pits , reticulate growth ridge etc.