Features and sedimentary evolution of high-frequency sequence in continental lacustrine rift basin:Example of the lower Shahejie member 3 in Jiyang Depression,Bohai Bay Basin
Lithofacies along with its assemblages is distributed regularly within the sequence stratigraphic framework.Based on T(transgression)-R(regression) sequence division features,variations,architectures and distribution patterns of the key sequential interfaces have been analyzed systematically in fault continental lacustrine basin in the lower part of the third member of the Paleogene Shahejie Formation(Es~(3L)) in Jiyang Depression,Bohai Bay Basin,by core observation and sample test data.And the evolution pattern of sedimentary sequences in Es~(3L) has been summarized furtherly.The results show that ① the major types of key sequential interfaces in Es~(3L) are the regional maximum flooding surface(MFS) and sequence interface.② Three variations of sequences are identified,namely the shallowing-upward R-shaped semi-cycle,deepening-upward T-shaped semi-cycle and stable cycle.While the sequence units can be reduced to three types,namely the type characterized by fast rise but slow drop,the type characterized by slow rise but fast drop,and the type characterized by constant variation.③ As an integrated third-order sequence,the Es~(3L) can be sub-divided into four forth-order sequences,that is,the early lacustrine transgressive system tract(early LTST),late lacustrine transgressive system tract(late LTST),early lacustrine regressive system tract(early LRST) and late lacustrine regressive system tract(late LRST).These forth-order sequences together with the sub-divided fifth-order sequence units feature comparability and integrity.④ The lake level of the Es~(3L) in Jiyang Depression generally fluctuates in a fast-rise-but-slow-fall pattern,and the fluctuation can be classified into four stages:the fast-rise stage during the early LTST,the slow-rise stage during the late LTST,the slow-fall stage during the early LRST,and the fall stage during the late LRST.In the process of sequence evolution,factors,such as tectonic subsidence,sediment supply and climate conditions,play specific roles in different stages,resulting in differential distribution of lithofacies and their assemblages within the sequence stratigraphic framework.