Petrological And Lithofacies-paleogeographical Analysis Of Mid-eastern Helan Aulacogen Basin, Middle Ordovician Pingliang Stage

The paper is based on the theory of Astronomical Cycle combined with sequence stratigraphy, sedimentology and other theories. The main study object is outcrop sections field. The author made analysis of the primary section of sequence stratigraphy by measuring meter-scale cycles with the combination of micro-facies and got the conclusion that: 1, the author marked off a total of 52 meters-scale cycles that is divided into five third-order sequences in Shaanxi yuche mountain section, the maximum flooding exists in the fouth sequence; 2, the author marked off a total of 60 meters-level cycles that is divided into five third-order sequences in Gansu Pingliang, the maximum flooding exists in the third sequence; 3, then the author marked off a total of 11 meters-scale cycles in Ningxia Qinglongshan section.The types of meter-scale cyclic are division exhaustively, The author constitute the distribution pattern of meter-scale cycles:the area of Yucheshan main develop tidal flat carbonate meter-scale cycles, subtidal carbonate meter-scale cycles , the area of Qing longshan and Pingliang main develop L-M type meter-scale cycles.A new method of stratigraphic correlation which is based on the maximum flooding events was adopted in this paper. Under the influence of the world of sea-level change, the comparision of the maximum flooding surface has isochronism, making comparison of sequence more precise and helping us prepare isochronism lithofacies palaeogeography.The study area consists of three tectonic units: 1, Zhongwei strong down-faulted zone is mainly concentric to the submarine fan deposition, that is Xiangshan - Luoshan submarine fan; 2, deep-water-face dominates the central and southern edge of passive margin zone. Contour current develops in the southern region, The main development is in Gansu Pingliang. This section of sequence3,4,5, developed into Contour current that can be identified at least three types: shale - limestone– shale type , shale - massive limestone– shale type; silty shale - siltstone clay - silty shale type.; 3,The edge of stability platform sediment a semi-deep to shallow water deposition, the representative is Shanxi yuche mountain section. The author finished two isochronism lithofacies palaeogeography in the Middle Ordovician period Pingliang Stage from the detailed field work and laboratory work, Compared to previous researches, isochronism lithofacies palaeogeography is better and more accurate reflection of the real time to the history of ancient geography. With isochronism Palaeogeography ,The author consider that contour current northward flow through the Qinglongshan which is to in the same deep water face.The development of hydrocarbon source rocks is closely related to sedimentary environment. The study area is mainly in deep-water-face sedimentary which a favorable place for good source rocks is. Therefore the more sophisticated study with restoration of the ancient geography is more useful to explore and develop the study of oil and gas resources.