Study On Sequence And Depositional System Of Mixed Clastics-Carbonate Sediments In Epicontinental Sea Basin ——A Case On Bachu Formation Of Carboniferous In Tarim Basin

The research on sequence development and deposition of hybrid sediment is not only of theoretical significance,but also has important practical application value.On the one hand,compared with simple terrigenous clastic rocks or endogenous rock,the influential and controlling factors of hybrid sediment are more complicated.The research on hybrid sediment can lead to further understand the hydrodynamic condition,climate,sedimentary supply and sea level change of the sedimentary environment and the complicated relationship between each other,which can provide new ideas and theoretical basis for the research of palaeogeographical environment,sedimentary evolution and sequence stratigraphy.On the other hand,hybrid depositional system has favorable combined condition of generation,reservoir and overlying strata of hydrocarbon.The discussion of the relationship between hybrid sediment and petroleum geological significance has become an important question in the field of sedimentary petrology and petroleum geology.Based on the principle of sedimentology and sequence stratigraphy,this paper take an example of Bachu formation in Tarim basin and discuss deeply the sequence stratigraphic pattern and sedimentary evolution of hybrid depositional system which developed in the ancient epicontinental sea basin under the background of general transgression.Based on the analyses of mud logging,well logging and seismic data,the sequence stratigraphic framework of Bachu formation was established and sedimentary evolution in the basin was studied under the control of sequence stratigraphic framework.The results of the research show that the palegeomorphology is relatively flat during depositional period of Bachu formation,and there is no continental shelf slope break.The result of analyses on sea level fluctuation,which based on the evolution of the sedimentary facies,shows that Bachu formation can be divided into two sequences:SQb1 and SQb2.Three sequence boundaries and two maximum flooding surfaces can be recognized.The composing of sequence is different from sequence stratigraphic styles of classical sequence stratigraphic theory.The two sequences don't development lowstand system tract(LST).Each sequence merely consists of transgression systems tract(TST)and highstand systems tract(HST).Stratigraphic distribution range of both sequences extends further layer by layer and individual sequence overlaps the former.Depositional system include terrigenous clastic littoral-neritic sea and hybrid littoral-neritic sea.The sedimentary environment includes mainly shallow sea,shoreface,foreshore,backshore,coastal dune and relevant hybrid sedimentary facies.From early to late stage of Bachu formation deposition,terrigenous detritus mixed with authigenic deposition continuously.In the mass,sedimentary facies evolution tendency turned from major terrigenous detrital depositional system into major carbonate platform.The palaeogeomorphology of Epicontinental Sea and the background of general transgression controlled the sequence development and sedimentary evolution of Bachu formation.