Lacustrine organic-rich shales are well developed within the Eocene Dongying Depression in the Bohai Bay Basin in eastern China and across Southeast Asia. Understanding the sedimentation of these shales is essential to the study of depositional processes, paleoenvironment, and paleoclimate reconstruction. This study investigates the sedimentary characteristics and formation mechanisms of lacustrine shales in the upper fourth member of the Eocene Shahejie Formation (Es4s) within the Dongying Depression based on thin sections and field emission scanning electron microscope (FESEM) observations of well cores combined with X-ray diffraction and geochemical indicators. Six lithofacies were identified: (1) laminated calcareous mudstone, (2) laminated dolomitic mudstone, (3) laminated clay mudstone, (4) laminated gypsum mudstone, (5) massive mudstone, and (6) siltstone. The organic matter in the Es4s shale is mainly type I and type II kerogens, as well as a small proportion of type III kerogen. On the basis of lithofacies associations, paleosalinity values, redox properties, and terrigenous inputs, the lower Es4s shale can be divided into six intervals from bottom to top, numbered I, II, III, IV, V, and VI. The thickness of each interval ranges from several meters to more than 10 m, reflecting high-frequency oscillations in the environment of the lake basin, markedly different from a relatively stable marine environment.
The laminated mudstones are characterized by fine grain sizes, scarce large terrigenous debris (quartz and feldspar), and compositions that are rich in pyrite and sapropelic organic matter. These features indicate that these lithofacies were deposited out of suspension in a quiet water body characterized by a relatively low rate of deposition. The characteristic laminae of these lithofacies indicate subtle differences in depositional processes. The laminated gypsum mudstone was likely deposited in an evaporative environment, because its formation would have consumed Ca2+ and SO42–, promoting the deposition of a laminated dolomitic mudstone. In contrast, laminated clay mudstone was deposited in a manner that increased the volume of small terrigenous materials. Deposition of this lithofacies was controlled by the nature of the water body, paleoclimate, and terrigenous inputs. Laminated mudstones are dominant in the lower Es4s shale, suggesting that suspension was the main depositional process leading to formation of the lower Es4s shale. In contrast, the massive mudstones were likely rapidly deposited associated with siltstone as the result of fine-grained turbidites. The lower Es4s shale was formed in a depositional environment composed of a saline, medium-depth lake under anoxic conditions, with limited terrigenous inputs. The depositional process included suspension and turbidity currents. The high salinity is suggested to be related to a marine transgression, which may have been facilitated by a rise in sea level caused by global warming in the early Eocene, together with the large-scale tectonic activity of East Asia. Seawater input affected the lithofacies, influenced lake water body conditions, triggered turbidity currents, and prompted the accumulation of organic matter. The deposition of the Es4s shale in the Dongying Depression may help us to understand the deposition of lacustrine shale, paleoclimate reconstructions for the Eocene, and the tectonic activity of East Asia.