Paleoenvironment Feature For Microbially Induced Sedimentary Structures From The Lower Triassic Liujiagou Formation On Yiyang County Of Henan Province

The Lower Triassic Liujiagou Formation in Western Henan province, lies in the south margin of the North China platform and consists mainly of purple-fine grained quartz sandstone sedimentary succession. Microbially Induced Sedimentary Structures(MISS) occur in purple or red 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. In this paper, MISS of Liujiagou Formation were researched including its type, macroscopic characteristics, microscopic characteristics etc. Furthermore, the paleoenviroment of Liujiagou Formation was analyzed by its petrography character and clastic rock geochemical contents. The following conclusions were obtained:1. In the whole Liujiagou Formation, MISS were well preserved in the midpiece and hypomere of the Liujiagou Formation. MISS were divided into microbial mat growth related structure, microbial mat destruction structure and microbial mat decay related structure. Especially, various dehydration cracks, the spindle-shaped gradually to give priority to with the reticular cracks from the bottom to the top of Liujiagou Formation. It showed that sedimentary environment was the shallow water facies and this area was the littoral or coastal and shallow lake. Moreover, the area from the late Permian in the southern of north China had entered a predominantly continental sedimentary, so the environment of the Liujiagou Formation on Yi Yang county was shore-shallow lacustrine deposits.2. In vertical section, we can found millimetre-sized(dark) dark and bright(light) laminations alternately, forming typical microbial mat biolaminites. Dark layer contains a lot of fine silt and particles, also iron oxides and black residues; light layer is relatively pure quartz grains. Through microscope observation of the sample from the Liujiagou Formation, the microstructure of the MISS can be divided into five types: wavy crinkled laminae, network fabrics, mat layer bound small grains, mat layer bound iron minerals and oriented grains.3. Through the lithologic change of the sandstore, the data analysis of the XRF(SiO2/ Al2O3、MgO/ Al2O3) and ICP-MS(V/(V+Ni)、Ni/Co、V/Cr、Sr/Ba、Sr/Cu), these reflect that the paleoclimate is hot and drought, and Liujiagou Formation were developed in the shore and shallow lake with brackish-water.4. MISS of the Liujiagou Formation shows the prosperity of the microbial mat.We also didn’t found body fossils and obvious trace fossils were few. The extinction of the metazoan and the flourish of the microbial mat appeared negative correlation. Moreover a large amount of trace fossils were found in the Heshanggou Formation on the top of the Liujiagou Formation. It showed that metazoan began to emerge, and that period had entered the biological recovery.