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Mixed siliciclastic-carbonate sediments were well developed in the BZ-X block as a subunit in the Huanghekou Sag in the southeastern part of the Bohai Bay Basin (China) during the Member 1 of the Shahejie Formation. An integrated approach based on detailed microscopic observation and core descriptions was applied to characterize the lithofacies types, facies associations, and depositional environment of the mixed sediments system. A detailed sedimentary facies analysis suggests that various lithofacies types can be grouped into four facies associations: mixed bioclastic sandy beaches (FA1), mixed clastic bioclastic shoal (FA2), mixed sandy oolitic shoals (FA3), and mixed distal bioclastic sandy beaches (FA4). These facies associations are interpreted to be deposited in varying depositional environments ranging from foreshore to shoreface setting in a shallow lake setting. An interplay of the climate regime, nearshore carbonate productivity, and current activity was proved to control the lithofacies types, dispersion and depositional environment change of mixed sediments in different depositional phases. Meter-scale mixed cycles were found in this shallow lake setting and their features seem to differ from the classical mixed sediment models. It is suggested that these particular cycles in the stratigraphic record might indicate a typical mark of mixed sediment system in shallow areas of the lacustrine basin.