Research On The Lithofacies Paleogeography Of The Lower Paleozoic In The Southwestrern Margin Of Ordos

Ordos, located in the southwest of the North China platform, is a part of the North China Craton. During the Early Paleozoic, there formed a set of marine carbonate sedimentary interbedded with clastic rock, especially in the southwestern margin of Ordos, the sediment thickness is huge with various sedimentary type, and the strata develop completely with the only Upper Ordovician strata within the North China. Because of the development of natural gas exploration, the middle of Ordos is well researched, comparatively, the southwestern margin of Ordos has complex geological conditions and the research degree is poor. Based on previous researches, through detailed observations and analysis of over thirty field profiles and more than ten wells of the Lower Paleozoic, studies on petrology, stratigraphy, geochemistry, sedimentology and lithofacies paleogeography were carried out in the southwest margin of ordos and its adjacent areas.According to the regional comparison and analysis, five different sedimentary subareas can be divided in Ordos, including western margin, southern margin, central-eastern part of Ordos, Helan Mountain and Hexi Corridor, and the lithostratigraphic units of the Lower Paleozoic in this sedimentary subareas were redefined. Associating with paleontologic data, the correlation of lithostratigraphic units, biostratigraphic units and chronostratigraphic units were analyzed, and then established a uniform stratigraphic division scheme.The Lower Paleozoic in Ordos is characterized by extensive marine carbonates, followed by clastic rocks, but also developed silica rocks, tuffs, evaporite rocks and so on, and their petrologic characteristics, sedimentary environment, distribution horizon and plane distribution characteristics were also analyzed. Then the carbon and oxygen isotopes of the Ordovician carbonate were carried out in the southwestern margin of Ordos, the results show that its carbon isotope compositions are basically consistent with that of the global Ordovician, the carbon isotopic values are gradually increased from the Early to Late Ordovician with a significant positive excursion between the Middle and Late Ordovician.According to sedimentary characteristics of the Lower Paleozoic, four depositional systems can be distinguished in the southwestern margin of Ordos, including coast, carbonate platform, platform margin and deep-water slope-trough, for each sedimentary facies, its characteristics, spatial and temporal distribution were analyzed, and then established a sedimentary facies patterns. Our studies suggest that:in Zhuozishan region in the western margin, abundant edgewise limestones developed in the Middle and Upper Cambrian are not shallow water storm deposits, but the debris flow deposits; In Qishan region in the southern margin, a number of carbonate gravity flow deposits were discovered in the Majiagou Formation, they all represent the deep-water sedimentary environment of platform-foreslope facies. In the Middle and Late Ordovician, there developed banded reef and shoal deposits around the "L" shaped platform margin in the southwestern of Ordos, and the western margin is mainly developed of grain shoal deposits in the Middle Ordovician while the southern margin is developed of reef in the Late Ordovician in Longxian-Fuping area. In the Late Ordovician, there maybe developed a series of transmeridional island arc chain in the south of Weibei area in the southern margin, which provided material sources to the north side, the southern margin of North China Platform has been translated to active continental margin with typical trench arc basin system. The sedimentary systems of both sides of the Helan aulacogen in the western margin of Ordos were obviously asymmetric in the Ordovician. The east side developed carbonate slope facies sediments provided by east Ordos platform, and the west side developed siliciclastic turbidite provided by northwest Alashan paleo-land, the filling materials were migrated downwards and deposited along the Helan aulacogen from NE to SW.Based on these studies and synthesized of various sedimentary characters, the lithofacies paleogeography maps of the Lower Paleozoic in Ordos were composed by each age, and finally discussed the sedimentary evolution history of the Early Paleozoic in Ordos.