Sedimentary Characteristics and Model of Storm Deposits in the Xuzhuang Formation at the Western Margin of the Ordos Basin
[Objective]Gravelly limestone is developed in Cambrian strata at the western margin of the Ordos Basin,and typical storm sedimentary structures are developed.However,there is still a lack of systematic research on its sedimentary environment,formation process,and geological significance.[Methods]This detailed study of the development of gravelly limestone and typical storm deposits in the Xuzhuang Formation at the western margin of the Ordos Basin was conducted by means of field observation and thin section observation of four sections:Wuhai Moer-gou,Wuhai Laoshidan,Huanxian Yinshixia and Tongxin Qinglongshan.[Results]The following conclusions have been made:(1)storm sedimentary structures(e.g.,channel mold,bottom scouring surface,mound cross-bedding and parallel bedding)are developed in the study area.(2)There are four gravelly limestone facies are categorized by shape,arrangement and infill characteristics(e.g.,particle-supported gravelly limestone facies;argillaceous filled gravelly limestone facies;radial gravelly limestone facies and matrix-supported gravelly limestone facies).(3)In ad-dition,five typical storm sedimentary sequences of the Xuzhuang Formation of the Middle Cambrian are identified at the western margin of the Ordos Basin from field-measured data,and a storm sedimentary model of the study area is established.(4)The study area was located in an epicontinental sea environment during the Cambrian and combined with previous research results,the genesis of gravel limestone is analyzed and the formation process is illustrated.It is considered that the storm wave and seismic action are the dynamic conditions for the formation of gravel limestone.Storm waves and earthquakes were the dynamic conditions for the formation of gravelly limestone.The ancient plate was located in a tropical storm belt,which was the reason for frequent storms.(5)The development of storm deposits in the study area reveals that the Ordos Basin was located in a low-latitude area near the equator during the Cambrian Xuzhuang Formation,which is consistent with the position of the North China Plate at that time established by paleo-magnetism studies.[Conclusions]Through the study of storm deposits in the study area and the establishment of sedimentary models,it is of great significance for the restoration of paleoenvironment and paleogeography in the Cam-brian of Ordos Basin.In addition,the formation process of gravel limestone in the study area is revealed by establish-ing the genetic diagram of gravel limestone in the study area.
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