Geochemical characteristic and its application for recognition of sequence boundary of Late Permian mudstone in Zhijin,Guizhou Province

Based on ICP-MS analysis,the measurement of the REEs and trace elements were carried out on Late Per- main mudstone in well J2 area,Zhijin,Guizhou,for the research of its geochemical characteristics and the recognition of sequence boundary. Geochemical analyses reveal that trace elements V,Co,Ni,Cu are slightly enriched,elements Cr and Rb are depleted,and the contents of the remaining elements are close to the averages of that for the upper crust. The total content of REEs is high. The ratios of w( LREE) / w( HREE) and w( La) N / w( Yb) N indicate an apparent differentiation of LREEs and HREEs. The ratios of V / (V+Ni),Sr / Ba,Ce / Ce∗ and Ceanom suggest that mudstone developed in the anoxic conditions of marine-terrigenous facies. ∑REE has a significantly or critically positive correlation with lithophile elements Nb,Ta,V,Ga,Rb,Th,Ba,Sr and a negative or no obvious correlation with elements Co,Ni, Mo,Zn,Cu,indicating that REEs originate mainly from terrigenous minerals. The distribution patterns of REEs and w(∑REE)-w(La) / w(Yb) diagram demonstrate that the source rocks of these samples are dominantly Emeishan ba- salt,as well as a small part of sedimentary rocks and granite. Abnormal surfaces near the sequence boundaries( SB2, SB3,SB4) correspond significantly with the boundaries,identified by relative geochemical indicators of REEs and trace elements,such as ∑REE,w(LREE) / w(HREE),(La / Yb) N ,Eu / Eu∗,Ceanom,(V / V+Ni),and Sr / Ba. In addi- tion,geochemical indicators record the geochemical conditions of water-body in sedimentary process,which can provide a geochemical evidence for the fine stratigraphic correlation in the sedimentary area where marine maker beds were not developed.