| The Tarim Basin has a history of complex tectonic evolution,and each stage of tectonic movement has caused the sea level to rise and fall relatively,forming a series of stratum unconformity interfaces.This paper uses field outcrops,carbon and oxygen isotopes,main-trace elements,logging,and seismic data to identify and study the critical unconformity surfaces（T90,T81）of the Cambrian,and analyzes the key unconformity surfaces of the Cambrian.Developmental characteristics and their geological significance.After identification and characteristic analysis of unconformity surface by various identification methods,it is found that on the outcrop,the lithology of the top and bottom of T90 and T81 interfaces changes obviously,which are lithofacies conversion surfaces;on the aspect of carbon and oxygen isotopes,δ13C andδ18O near T90 interface show obvious negative drift phenomenon,in whichδ13C has the synchronization of global negative migration trend,experiences global sea level drop and occurs Mass extinction event.The decrease ofδ13C at the interface of T81indicates that there is a short-term exposure,the average value ofδ13C is significant,andδ18O is significantly reduced,indicating that the sedimentary environment is transformed from a strong oxidation environment to a reduction environment.In the aspect of main and trace elements,the average mass fraction of Ca,Mg,Nd,Pr,Ce,La elements in T90interface is larger than that in T81interface,and the mass fraction of elements in Zr,Sr,Zn,Ba,Mn,S,Si,Ni,Fe,Al,Rb,K,Cl and P interface is smaller than that in T90interface,indicating that there are obvious exposure weathering and leaching at the interface;Ce,La,Al,Fe,Si measured at T81 interface The values of Sr/Ba and Mg/Ca interface higher than those of the interface,indicating that the two key unconformities are exposed to weathering,and the sedimentary environment of the interface has changed greatly.In logging,the mass fraction of U,Th,and K at T90and T81interface increases suddenly,the GR curve value increases obviously,and a large number of clay minerals are formed after weathering at the marking interface.On the seismic section,the upper and lower part of the T90interface can be seen with the characteristics of superelevation and truncation.The T81interface is characterized by truncation,top lap,and progradation.Different types of unconformities are developed at different positions of T90interface in Tarim Basin,and parallel unconformities are mainly distributed in Kuruktag,Keping outcrop area,northern depression of the basin and its vast area to the north;angular unconformities are distributed near the north wing of central uplift belt.The T81interface and sedimentary exposed unconformity are distributed in the platform margin,the platform interior is parallel unconformity,the central uplift area is structural erosion unconformity,and the basin margin is multi-stage superposition unconformity.Theδ13C value of the key unconformities（T90、T81）changes as a whole,which can be compared with theδ13C value of different regions,providing a reliable basis for the division and correlation of the global Cambrian strata.The paleogeomorphology of the T90 unconformity interface restricts the distribution of sedimentary facies of the overlying strata,and then affects the distribution of source rocks of the yuertus formation,and controls the development of karst reservoirs in the underlying strata.The T81 unconformity interface controls the development of gypsum Yunping dolomite reservoir and paleokarst reservoir under the interface. |
