SEDIMENTARY EVOLUTION CHARACTERISTICS AND PALEOENVIRON-MENTAL ANALYSIS OF THE ORDOVICIAN IN THE WESTERN MARGIN OF THE ORDOS BASIN
The research on the sedimentary system of the Middle Late Ordovician in the western margin of the Ordos Basin has been controversial and has not yet reached a widespread consensus,which seriously restricts the exploration and development of the basin.This article is based on field outcrop profiles,drill-ing cores,and thin section identification data,combined with various geochemical methods such as carbon and oxygen isotopes,strontium isotopes,and trace elements,to study the sedimentary environment and characteristics of the Middle Late Ordovician in the western edge of the Ordos Basin.The results indicate that during the Middle to Late Ordovician,the western edge of the Ordos Basin developed platform(un bordered)slope basin sedimentary facies from west to east,with sedimentary facies belts distributed in a north-south belt and characterized by east-west differentiation.During the sedimentation period from Sandaokan to Wulalike,the scale of the plateau gradually decreased,and the sedimentary areas of the con-tinental shelf and basin facies gradually increased.Geochemical indicators show that the overall sea level in the Middle Late Ordovician study area showed an upward trend,including two periods of secondary sea level rise and fall,reaching the maximum flooding level during the Wulalike sedimentation period,the av-erage δ13C value has reached its maximum.During the sedimentation period from Sandaokan to Zhuozis-han,the average Sr/Cu ratio was 250.62,and the average V/(V+Ni)value was 0.31,the average δCe value is 0.10,indicating that the climate in the study area is relatively dry and hot,and is in an oxidizing weakly oxidizing environment;During the Kelimoli to Wulalike sedimentation period,the average Sr/Cu ratio was 2.69,and the average V/(V+Ni)value was 0.61,the average δCe value is 1.93,indicating that the climate was warm and humid during this period,and the sedimentary environment gradually trans-formed into weak oxidation-reduction.The changes in sea level and paleoclimatic conditions have affected the distribution of Middle Late Ordovician sediments and the distribution of sedimentary facies belts in the western edge of the Ordos Basin.
Ordos BasinMiddle Late Ordovicianpaleoenvironmentsedimentary facies pattern
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