| The Late Permian stratum of eastern part of Inner Mongolia and its adjacent areas, containing thick dark shale, is an important iol and gas exploration area in new district and new layer. However, the deposition environment was hotly dispute. The marine fossils, bryozoan, sponges and others in the upper part of Guandi section, discovered by Zhang Yongsheng and his partner, have subverted the conception of continental deposition in Late Permian in Xingan- Inner Mongolia. Based on this research, through careful measuration and observation of the sections, fossils and sample collection of Late Permian in this area, the stratigraphic framework have been established, and the sedimentary facies and the overlayed characteristics in main sections have been studied. Some new evidences of marine deposition have been discovered, such as Lopingoceras cf. guangdeense,the ammonites, bryozoan of Septepora, and acritarchs.The reefs was discovered in the upper parts of Linxi group. The main types are four: algal bryozoans, sponge and calcium alginate framework reefs and algal reef. The main reef building of bryozoans are Fenestella, Septepora, Fistulipora, Polypora, Thamniscu, et al. The sponge of reef building was mainly Amblysiphonella Steinmann, and calcium alginates of reef building are Mo- nostysisyrinx 、Archaeolithoporella. The reef communities indicated cool water environment. The main types of reefs are knoll reef and bank reef, which refer the hydrodynamic condition. Knoll reef represents relatively weaker water power in flat neritic shelf, while bank reef relatively stronger in steep shelf edge. The trace elements and carbon oxygen isotopic composition of limestone and mudstone was analysed, the results indicated marine nearshore zone, and the depositional water may come from three sources, namely normal sea water, river water or atmospheric precipitation and hydrothermal, which supports the marine environment in this area.Based on the comparation of different sections and the distinguishment of sedimentary unconformity, the formation of Late Permian in research area can be divided into three third-order sequences, and then draw the maps of tectonic paleogeographic environment respectively, find out the sea and continental distribution and characteristics of each sequence. Analysis shows that xingmeng trough undergoes the stages of collision closure, rifting into sea and closure into land. In the late of middle Permian, There are depositional hiatus and structure of intermittent exposure in study area, which indicate collision closure between SinoKorean plate and Siberia plate.Rift into sea appeared in transgressive system tract of Sq2 sequence. Slump, rolling spherical accretion, scroll bedding slope facies structure is very common in this area, and at the same time the turbidite, indicating water become deep quickly, basin with rift characterized. At that time, the rift betweem Siberia and the Sino- Korean plate changes into the sea. In the end of the Late Permian, the depth of Xingmeng trough becomes shallow, and widely distribution of volcanic rocks, represention of strong gaps shorten between the two plate and collision environment. At the end of the period, the Madatun Formation, in Jiutai, Jilin provience, contains red conglomerate layer, on behalf of gradually regression of seawater from this region. Sections including Guandi, Linxi country, Sauron, Horqin Right Front Banner are located in the continental marginal area. The organic matter abundance of source rock in the two area in the period of Sq2 and maximum flooding surface during Sq3 is available. The second is dark mud shale formed in deep water zone between Sq2 and Sq3.The study shows that the type of organic matter is mainlyⅡ2. Ⅱ1 andⅢaccout for a little part. Thermal evolution can be up to high-over mature stage and produce gas mainly.Meanwhile,brittle mineral in mud shale accounts for a large part.It’s easy to form fracture and helpful for shale gas occurrence and exploitation. |
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