Factors that controlled deposition of lacustrine, mixed siliciclastic-carbonate sediments in the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, East China

The Zhanhua Sag is located in the east-central Jiyang Depression, southern Bohai Bay, northeast China, and is a half-graben, lacustrine basin. Mixed siliciclastic-carbonate sediments are quite common in Eocene deposits of the Zhanhua Sag. We established characteristics of the lithofacies, facies associations, and depositional environments of the upper fourth member of the Eocene Shahejie Formation in the Zhanhua Sag, using core descriptions, thin sections and well logging data. Mixed grainstones and mixed wackestones are two lithofacies of mixed sediments identified in the study area. Paleogeomorphology, sediment supply and gravity flows jointly controlled development of the mixed sediments in this study. Subaqueous highs, formed by terrigenous clastic sediments, provided substrates for carbonate production. The Ordovician carbonate strata exposed on both sides of the rise provided the material for carbonate production on the north and south sides. The clastic sediment supply, however, developed mainly on the south side. Less siliciclastic influx in the north was probably associated with weakening of tectonic activity and less topographic difference. Massive mixed sediments were the product of gravity flows developed under low-energy hydrodynamic conditions between the carbonate shoals and a deltaic system. Most of the siliciclastic material in the mixed sediments was transported by gravity flows from the fan delta to lows in a shallow lake.