| Deep-water deposits is the key field of the oil exploration,and it is also one of the hot issues in the sedimentology.With regard to the development of gravity flow deposit,after more than 60 years' development,the Bouma Sequence and various fan models are represented,and people have made very rich research results for gravity flow deposit.However,the fan model which was popular were abandoned by their own in the early 90 s of the last generation,to Shanmugam as the representative of the scholars to propose the concept of Sandy Debris Flow,and established the corresponding slope filling mode,which brings a new understanding to the research and development of gravity flow deposit.Ordos Basin is located in the five provinces and autonomous regions of Shaanxi,Gansu,Ningxia,Shanxi and Inner Mongolia with an area of 320000km2.It is the second largest oil and gas basin in China and is a typical multi-cycle superimposed oil and gas basin.To Pingliang Formation of Ordos Basin,a lot of research has been done from the aspects of paleontology stratigraphy,gravity flow deposit characteristics,deep-water traction flow deposit and zircon chronology,and many research results have been obtained,but there are still some problems in the following aspects: Sedimentary characteristics has been studied enough,but the application of trace elements in the sedimentary environment of the study area is relatively small.The understanding of the characteristics of gravity flow deposit is different,and thus to some extent,the plane distribution of gravity flow in the study area has different understanding.The gravity flow deposit is still not close to the tectonic background.In order to solve the above problems,the main contents of this paper are as follows.First,analysis of sedimentary environment of Pingliang Formation in the study area: Based on paleontology,lithology and trace elements,the sedimentary environment of Pingliang Formation in the study area is analyzed.Based on the study of the selected two main cross sections,combined with other outcrop profiles and drilling data,the plane distribution characteristics of gravity flow deposit in the Pingliang Formation of the study area are roughly drawn.Thirdly,the main control of gravity flow deposit in Pingliang Formation factor are analylized: Based on the field observation and measured profile,the influence of gravity flow deposit was analyzed from four aspects: tectonic movement,the change of the relative sea-level,paleoclimate and provenance supply.By working out the task above,we get the following understanding:(1)The clastic rocks in the Pingliang Formation are dominated by dark shale sandstone and siltstone,and the carbonate rocks are thin layer of limestone.Develop graptolites,radiolarians and Nereites.The analysis of the trace elements in the section of the Duanjiaxia section of Longxian area shows that it has a large salinity,an overall oxygen-depleted local oxygen-rich and hot drought geochemical characteristics.All these characteristics shows that the Pingliang Formation were in deep water environment.(2)By analyzing the trace element data of the section of the Duanjiaxia in Longxian,we know their province came from the island arc area.The results reflects in the late Ordovician this area were in the position of back-arc basin.(3)The gravity flow deposit of the Pingliang Formation in the Duanjiaxia section of Longxian has the characteristics of deep-water channel deposit.The content of sandstone in the vertical section of the measured profile is reduced,the shale content is increased,and it has the tendency of thinning and thinning in the vertical,which shows the process of erosion,migration and extinction of the deep-water channel in transgressive background.In the section of Fuping,the Zhaolaoyu Formation has the characteristics of Mass transport Sediments.Along the slope the slump deposit,debris flow deposit and turbidite deposit are carried out.(4)The development process of Upper Ordovician Gravity flow deposit in study area can be divided into three stages: First,in Early Pingliang Age,in study area existed carbonate gravity deposit at the edge of the carbonate platform,which the slump deposit was particularly developed.From west to east the scale of gravity flow deposit gradually reduced.At the end of early Pingliang Age,the Liquan area was the dividing line,and the western part of the study area was mainly composed of the terrigenous classic gravity flow sediments,with a small amount of carbonate rock gravity flow deposit.The eastern part of the study area was characterized by gravity deposit,but the scale was lower than that of the former.Second,in Late Pingliang Age the Liquan area formed the submarine fan.Thirdly,in Beiguoshan Age,re-development of the carbonate basin in the southern part of the southern margin of Ordos Basin.After the transgression,the scale further expanded,extending westward to the junction with the Qishan area developed a larger scale of submarine fan deposit.(5)Analyzing its effect on gravity flow deposit from four aspects: tectonic movement,the change of the relative sea level,paleoclimate and provenance supply.Tectonic movement,palaeoclimate and the change of the relative sea level are the indirect factors,and the provenance supply is the main factor.The first three factors will not only affect the development of gravity flow deposit,but also act on the direct influence of the source supply,which will affect the development of gravity flow deposit in Pingliang Formation.However,Tectonic movement and palaeoclimate are the main factors among the indirect factors. |
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