CYCLICAL MIXED-SILICICLASTIC CARBONATE SEDIMENTARY SYSTEM IDENTIFICATION AND CHARACTERIZATION IN LACUSTRINE ENVIRONMENT: AN EXAMPLE FROM HUANGHEKOU SAG, BOHAI BAY BASIN, CHINA

As a critical and unique sedimentary system, mixed deposits derived from the interaction of different processes in the same basinal sector have been mostly studied in continental shelf, carbonate platform, or other marine settings. However, less studies have concerned about lacustrine mixed sediments, which is considered as a key in interpreting paleogeographic setting during the sedimentation period. During the deposition of Eocene Es1 formation in Bohai Bay Basin, a critical petroliferous basin of China, widely distributed mixed deposits have been found in various sub-basins because of the relatively stable tectonic activity and arid climate. A detailed analysis of both lithofacies and stratigraphic architecture of a mixed-siliciclastic carbonate depositional system in the Eocene Es1 formation of Huanghekou sag, Bohai Bay Basin, is carried out. Five different lithofacies are recognized and classified based on core observation, microscopic description and whole-rock X-ray diffraction analysis, including (A) calcite/dolomite-cemented conglomerate, (B) calcite/dolomite-cemented sandstone, (C) grain dolostone/limestone, (D) micrite dolostone/limestone and (E) mud shale. Besides, lithofacies C can be divided into two sub-facies, C1 has abundant inter- and intra-granular porosity, while C2 has most of its pores filled with calcite/dolomite cements. These facies associations are believed to have deposited in various depositional environments with differential-hydrodynamic conditions. Vertical stratigraphic analysis displayed there existed meter-scale cyclically interbedded mixed-siliciclastic carbonate rocks comprising parts or all five lithofacies, indicating a high-frequency rhythmicity of hydrodynamic conditions.