Geochemistry Of Sedimentary Rocks Of Ordovician In SW Ordos And Its Paleoceanographic And Palaeoclimatologic Implications

The Ordovician is an important stage of the Early Paleozoic tectonic-climate and biological evolution,and is also a typical period for studying the interaction of the Earth's multi-circle layers.It has received extensive attention in recent decades.Ordos is one of the areas where Ordovician is well developed and well exposed in the north of China.Previous studies have focused on Ordovician tectonic-sedimentary environment and Ordovician oil and gas geology.Less research has been done on climate,environment and biological evolution.This paper selects the Ordovician in Qishan section of the southwestern margin of Ordos.Through the study of stratigraphic sedimentary characteristics and geochemical measurements of rock elements,the characteristics of the Ordovician rocks and the climatic environment reflected by them are discussed.Some conclusions as following:Measurements of elements indicate that the contents of elements in carbonate rocks in the lower section are obviously different from those in clastic rocks in the upper section.The samples have higher Ca and Mg contents,and the relative average shale is rich and Al is relatively average shale loss.Trace elements are lower in carbonates than in clastic rocks.The total rare earth(?REE)is lower than the average shale ?REE value,the ratio of Y/Ho ratio in limestone is 55.36×10-6,which is the same as the Y / Ho value of the sea water in the South Pacific Ocean.and the other lithologic Y/Ho ratios are within the range of Y/Ho ratio of PAAS.The results show that the carbonate samples are less affected by terrestrial detritus.The rare earth element shows negative anomaly of Ce,Eu is weakly positive anomaly in early-middle Ordovician,and there is no significant correlation between ?Ce and ?REE and ?Ce and ?Eu.The sample can be a good environmental indicator.Through the analysis of the sensitive elements and element ratios of paleoenvironment,Yeli Formation to lower part of Majiagou Formation in the dolomite sedimentary section,the paleo-ocean salinity is large,and the sediment is in a saline water environment with lean oxygen-anoxia,and the paleoclimatic indication is dry-hot climate environment;In the upper limestone of Majiagou Formation,the paleo-ocean salinity increased,the sedimentary environment was partial oxidation,and the paleo-climate was hotter and dry.At the top of the Majiagou Formation,the climate dryness reached the maximum;in the Pingliang Formation,the paleo-ocean environment fluctuated.In an oxidized-poor brackish-brackish water environment,the climate is humidified relative to the Majiagou period,but it is still in a dry and hot climate.The element content and related ratio fluctuate at the section thickness of 950-1050 m and 2300-2400 m.The paleo-ocean and paleoclimatic significance of the two are different.The former is the result of the deep fresh water addition due to storm deposition,while the latter is the result.The sea level is reduced due to the dry climate and the sea water is salty.The paleo-ocean and paleoclimate information revealed by the geochemical parameters selected in this paper is consistent with the information revealed by the changes of stratigraphic rocks and sedimentary environments,indicating that the conclusions obtained in this paper are reasonable.