Research On Permian-Carboniferous Depositional Systems And The Sequence Stratigraphy Of Ordos Area

Ordos which located in the western of Huabei area and intersection point of west and east structual domain is an important gas and coal sinks in china. There are many kinds of sedimentary context of permo-carboniferous in Ordos under the influence of the allocyclicity factor and the autocyclicity factor. Based on analysing of outcrops, core, log data, seismic information, geo chemical information, etc, nine types sedimentary context which include barrier coast facies, neritic shelf facies, carbonate plateau facies, fan, fluvial, lacustrine, fan delta, braided delte and meander delta faciss be distinguished. Each sedimentary context characteristic and their genetic be analysed carefully.There is a big difference between west sedimentary province and east sedimentary province, which be divided by central uplift belt from Benxi group of upper carboniferous to Taiyuan group of lower permian. Chasmic bay facies maily developed in the western under the influence of Qilian sea. The eastern of study region be developed epicontinental neritic facies. Seawater coming from the western and the eastern began junction and became a part of Huabei epicontinental in early permian, which be developed tidal flae, neritic shelf, barrier, lagoonal and shoaly delta deposit where coal be often found. The epicontinental slowly began dying away from the late period of early permian to middle permian and then formed paralic inland sag deposition which consists of fan, fluvial, delta and lacustrine facies. The sedimentary deposit evolved to inland sag deposition which include meander and lacustrine.Based on analysing all kinds of sedimentary deposit and using the theory and methods of sequence stratigraphy, 26 sequence boundaries be recognized in the west margin. 15 sequences be divided except the west edge, among them, sequences boundary 1-15 are a pecuiar type I sequence boundary, and sequences boundary 16 is a type II sequence boundary according to the marked bed including plane of deflation, scour, the plane of lithological saltation, structural transformation, down-cutting channel sheet sund and so on. Sequence 1-6 have two-component systems structure which consists of transgressive systems tract and highstand systems tract. Sequence 7-15 have three-component systems structure which consists of lowstand systems tract, transgressive systems tract and highstand systems tract.Sedimentary deposition and sequence of strata be mainly influenced by many factors including structure active, climate and so on. Climate changed from humid, to arid and crust had always elevated and subsided freguently in permo-carboniferous . Except fracture active and volcanic outbreak of later permian in the margin of study region, the structural active show the characteristic of helicism elevation and subsidence changing from strength to weakness. Climate is a main factor of influencing sequence 1-6 forming. Climate change correlated closely with glacier sheet change in geographical pole. During that time, the study region belongs to tropical climate and subtropical climate area which have the characteristic of hot, humid living beings prosperous, and strong chemical weathering. Because climate change leads to elevation and subsidence of sea-level and form the kind sedimentary cycle of sand, mud and coal. Bauxite and rhombohedraliron ore be often found in the bed which be in acidic medium environment besides them. The forming of sequence 7 and sequence 8 be mainly controlled by structural active that shows elevation of the northern in Yimeng region and seawater receding from the area, so that the scour and down-cutting channel often be sight in the bottom of the sequence. Down-catting channel is very developed in the bottom of sequences because of elevation and coarse sedimentbecome more and the plant fossil become less because of hot and arid climate. Autocyclicity is main factor that influence sequence 11 and sequence 12 forming. Down-cutting channel deposit and sand body is very less in the lowstand systems tract, and argillaceous sediment becom...