Analysis On Sedimentary Facies Of Mesoproterozoic Ruyang Group In Lushan Area,western Henan

The Mesoproterozoic strata are widely and completely distributed in Lushan Area of Western Henan which is one of the best area in the North China Platform where Mesoproterozoic strata are well developed. Much achievement has been obtained by many scholars on trace fossils and and sedimentology of this area, With the detailed study recently, the trace fossils found before are microbially induced sedimentary structures(MISS)which are the interaction products of microbial mat or biofilm formed by benthic cyanobacteria with physical process. MISS are one of the geological record and important marker of early life activities and the existence of microbial mat on earth.Based on Mail's lithofacies classification on river deposits, with lithology and sedimentary structures, the Mesoproterozoic Ruyang Group can be divided into 40 lithofacies which constitute 7 sedimentary facies and 14 sedimentary subfacies. Combined with the features of lithofacies and sedimentary facies, 14 kinds of despositional successions can be recognized from Ruyang Group. The alternating occurrence of forshore deposits and tidal flat deposits is the main depositional framework at Mesoproterozoic era in Lushan Area of Western Henan Province.Discussion on the relation between MISS and sedimentary environment means that mesh-like and fusiform syneresis cracks are dominant. MISS in study area. The mesh-like syneresis cracks are widely preserved in foreshore deposits, but the varies kinds of fusiform syneresis cracks largely occur in tidal flat deposits. The size of MISS gradually become smaller from Yunmengshan Formation to Beidajian Formation. The diameter of polygonal MISS and the thickness of microbial mat also have the same tendency with the size of MISS. Features above show that the evolution of MISS has much to do with to sedimentary environments. The morphological features of MISS are controlled by the growth rate, thickness of microbial mat and their exposure.