DEPOSITIONAL SEQUENCES OF YITONG GRABEN (EASTERN CHINA) AND ITS RESPONSE TO TRANSFORM-EXTENSIONAL PROCESSES

The Yitong Graben is located in the north eastern segment of the Tertiary rift system in East China.The graben is bounded by a steep dipping transform fault on the northwestern side and subparallel normal faults on the southeastern side, with very thick marginal fan complexes indicating syntectonic sedimentation. The graben is infilled with Tertiary sediments up to 6000 m thick, in which a series of different scaled hiatuses and tectonic inversion events have been discerned during hydrocarbon exploration. According to regional and local conformities, the basin infill may be divided into three tectonic sequences, among which the major sequence TS2 is further subdivided into three sequences sets and fourteen subsequences. Based on the sequence stratigraphic framework, depositional system tracts are reconstructed. Integrative analysis of the depositional structural features showed that the regional tectonic stress system experienced a transformation from dextral transfersional-shear through thermal subsidence to sinistral compressional-shear with a general extensional setting. Extension and wrenching dominated the graben evolution and affected the depositional style in different evolutionary stages and the spatial configuration of depositional system. The conclusions have been applied with some success to prospection of reservoir and hydrocarbon source-rock distribution and lithologic-stratigraphic traps in this basin.