Study On Sedimentary Facies And Sequence Stratigraphy In Devonian Yangmaba Formation,Beichuan Ganxi,Longmenshan Area

Mixed deposit,as a significant type of deposit,has gradually become an important direction to study deposition.Based on the field measurement of the Ganxi Profile in Beichuan County,the deposit facies of the Devonian Yangmaoba Formation were studied in detail.Combined with the microscopic observation,it is believed that the Devonian Yangmaba Formation has developed a set of mixed deposit which is made up of clastic rocks and carbonate rocks.Storm deposit was an important control effect on mixed deposit.Through field observation and microscopic observation on the Ganxi Profile,the identification marks of mixed deposits were summarized in the Yangmaba Formation.And then,the mixed deposits of that place were systematically studied to distinguish the mixed deposits facies of the Yangmaoba Formation,including carbonate-mixed shelf facies,clastic-mixed shelf facies and mixed shoreline facies.A mixed deposits model of the mixed shoreline-mixed shelf deposits in the Yangmaba Formation of the Longmen Mountains was established.A comprehensive study on the tempestites is carried out after the measurement of Ganxi outcrop section of Devonian Yangmaba Formation in the Longmenshan area of Sichuan Basin and the summary of identification marks of storm deposits.The major features of storm deposits in the Yangmaba Formation are storm erosion structures,like scouring surface and gutter mold structure,and storm wave structures,such as graded bedding,parallel bedding and hummocky cross stratification.The Yangmaba Formation developed six different composites of storm sedimentary units.A complete storm sequence pattern in the area is established with six types of unit sequence of storm deposit.They are made up of the graded bedding section(Sa),the parallel bedding section(Sb),the hummocky cross stratification section(Sc),the wave bedding section(Sd)and the mudstone section(Se).Generally,there are two types of tempestites developed in the study area,the near source storm deposit and far source storm deposits.Based on the profile structure type and sedimentary structure characteristics of storm deposits,the distribution pattern of storm sedimentary sequence in this area is established.Based on the field measurement on profile in Ganxi and data analysis,the sequence interface characteristics of the Yangmaba Formation are summarized,the type and characteristics of the system of the Yangmaba Formation in this area are also summarized as well.The evolution type of sequence structure is: mudstone(LST)? limestone(TST)?sandstone and limestone(HST).Besides,by considering the characteristics of deposits facies,sequence and system domain divided in the area comprehensively,it could come to the conclusion that the general trend of sea level changing of the Yangmaba Formation in this area is “ascend in first and descend at last”.The mixed deposits and storm deposits developed in the Yangmaba Formation in the Ganxi,Longmen Mountains have important indications for the reconstruction of paleogeography in this area,indicating that storm deposit is an important controlling factor for the development of mixed deposit and the inhibition reefs.