Contourites Of Upper Ordovician In Southern Margin Of Ordos Basin

Contourite is the study hotspot of sedimentology.At present,compared with the study of modern contourite,the study of ancient contourite is weak,especially the identification characteristics of the ancient contourite.This research will be helpful to establish the identification method of ancient contourites and improve the theory of deep-water deposits.Contourite system contains important information in paleoceanographic,paleotectonic and paleoclimate,this research will contribute to analysis the information of paleoceanography,paleotectonic and paleoclimate.In addition,contourites can form good oil and gas reservoirs,so it has potential value in exploration and it is a promising field for deep-water oil and gas exploration in domestic and foreign.The thesis stems from National Nature Science Foundation of China(No.41472096)“ contour current deposits and controlling factors of Middle Ordovician in western and southern margin of Ordos Basin ”.The Upper Ordovician in southern margin of Ordos Basin is taken for the research.Major studies involve sedimentary environment,characteristics of contour current deposits,vertical distribution,lateral distribution and influence factors.On the basis of investigation in literature,collection of relative data,geological sections observation,thin sections observation and research on geochemistry,the sedimentary environment of Late Ordovician in southern margin of Ordos Basin has been analyzed,the characteristics of contour current deposits has been summarized.Based on that,the vertical distribution and lateral distribution of contour current deposits have been studied.At last,combined with the characteristics of geochemistry,the controlling factors on the development of contourites have been discussed.In the process of research,more than 200 references were collected.7 field geological sections were observed,3 sections were measured,220 rocks were sampled and 500 photos were taken.These 220 samples were analyzed.There are 200 normal petrographic thin sections,3 large size petrographic thin sections and 3 smooth surface rocks.Major element,trace element geochemical analyses were performed on223,223 samples respectively.Besides,14 figures and 2 tables were drawed.As a result of comprehensive research,the main achievements are as follows.(1)According to lithology,paleontology and other information,the sedimentary environment has been analysed and the characteristics of contourites have been summarized.The sedimentary environment of the Late Ordovician in southern marginof Ordos Basin is deep-water slope to the margin of deep-water basin,and three types of contourites were developed,i.e.the calcarenitic contourite,the calcisiltitic contourite and the calcilutitic contourite.Cross bedding,ripple bedding and current ripple developed in calcarenitic contourite and calcisiltitic contourite indicate the direction of palaeo flow parallel to the slope.Bioturbation structures are well developed in calcilutitic contourite.Five types of contourite sequences were identified,complete contourite sequence,calcisiltitic-calcilutitic contourite sequence,calcarenitic-calcilutitic contourite sequence,calcarenitic contourite sequence and calcilutitic contourite sequence.(2)Based on single section,the vertical distribution of contourites has been studied.In 1-27 layers of Zhaolaoyu section,deep-water autochthonous deposits,debris deposits,turbidity deposit were developed,contour current deposits developed in 28-35 layers,especially in 29 and 32 layers.Contourites were widely distributed in Pingliang Guanzhuang section,1-8 layers mainly developed deep-water autochthonous deposits and gravity flow deposits,contour current deposits well developed in 9-24 layers.On the whole,contourites are mainly developed in the middle and upper part of the Upper Ordovician in southern margin of Ordos Basin,while in the lower part,contourites are not or less developed.Besides,the main development layers of contourites are higher than that of the gravity flow deposits.(3)Combined with two sections,the lateral distribution of contourites has been studied.Comparad with Fuping Zhaolaoyu section and Pingliang Guanzhuang section,contourites developed in Fuping Zhaolaoyu section have less types,and are finer-grained and less development compared with other deep-water deposits.However,contourites developed in Pingliang Guanzhuang section have more types,and are large-grained and more development compared with other deep-water deposits.In addition,according to the direction of contour current in Fuping and Pingliang,speculate that contour currents from east to west occur in Late Ordovician in the southern margin of Ordos Basin,so that contourites may develope in southern margin of Ordos Basin between Fuping and Pingliang.(4)Combined with geochemical analysis,the controlling factors on the development of contourites have been discussed.Geochemical analysis were carried out on Fuping Zhaolaoyu section and the smooth surface rock of contourites,conclusions are as follows: with the temperature of the contour current water mass getting lower and the salinity of the contour current water mass getting higher,the contour current getting more strength;The temperature of the contour current water mass changing from high tolow,then high and the salinity of the contour current water mass changing from low to high,then low may cause the contourites sequences changing from fine to coarse and then fine;Deeper paleobathymetry,drier and hotter paleoclimate,and lower paleosalinity were favorable for the development of contour current and contourites.In addition,compared the palaeo-geomorphology of Fuping and Pingliang,the more limited depositional environment of Pingliang lead to a stronger contour current.