Research On Sequence Stratigraphy Of The Ordovician Outcrop In The Southern Part Of Ordos Basin

The Ordovician outcrops of the southern margin of Ordos Basin developed well. Certain achievements were obtained in recent years for the sequence stratigraphic research of Ordovician in southern basin, while the fine sequence stratigraphic research of outcrop section has never been carried out yet. The earliest observation of sea-level change cycle origined from earthquake seismic stratigraphy. People can indentify third grade sequence at most with the help of high seismic resolution. Logging can be used for high-frequency sequence analysis, but in regard to the recognition of sedimentary sequence and important sequence stratigraphic boundary, the fine measurement of outcrops and detailed analysis of sedimentary environment are still needed.According to particular measurement of Ordovician profile of HeJin region in the southern fringe of Ordos Basin, the author indentified sequence stratigraphic boundary and important marine-flooding surface and in turn identified third grade sequence and system tract. Moreover, the sedimentary environment was analyzed for the purpose of identifying the parasequence sets and parasequences, analyzing the law of the eustasy. Throughout the sequence stratigraphic correlation of Ordovician profiles in Qisha,Chunhua, the autor studied the characteristic of the eustasy during Ordovician Period in the southern margin of Ordos Basin.The author measured the Ordovician section in Hejinkou and the low of Ordovician section in Qishan and divided the lower Ordovician section of Xiweikou in Hejinkou region into 8 third grade sequence, identified 24 parasequence sets,68 parasequence, the Pingliang formation and Beiguoshan formation of middle and upper Ordovician into 5 third grade sequence.The Ordovician sedimentary system of Ordos Basin could be devided into five types:The carbonate ramp sedimentary system, carbonate platform sedimentary system, continental shelf sedimentary system, slope sedimentary system and basin sedimentary system, correlated to different part of the south margin in the basin and its various evolution step. In according with the regression after the Cambrian transgression, Yeli formation and Liangjiashan formation got some degree of erosion, which is the first cycle of transgression and regression between Cambrian and Ordovician in the south margin of the basin. Majiagou formation developed well in the south margin of the basin. Corresponding with the transgression happened after the Huaiyuan Movement, there existed two small size regression, named Ma 3rd age and the Ma 5rd age. Ping Age and Beiguoshan Age was the largest size of regression in according with the the regression after the general transgression of Majiagou age. The transgression of Majiagou age and the regression during the Pingliang age and Beiguoshan age composed a completed cycle of transgression-regression.