Middle Eocene paleohydrology of the Dongying Depression in eastern China from sedimentological and geochemical signatures of lacustrine mudstone

Previous understanding that stratigraphic variability in lacustrine sequences is primarily caused by depositional processes governed by regional tectonics and climatic change remains to be tested on microscopic scale. Here we study the upper fourth Member (Es4U) of the Eocene Shahejie Formation, the major source rock of the Dongying Depression, eastern China, in order to understand stratigraphic heterogeneity and tectonic and climate controls on depositional processes in lacustrine mudstone. Our study integrates core descriptions, microscopic observations, and studies of mineralogy, TOC and chemical weathering proxies of CIA* and Ln(Al2O3/Na2O) from two full-bore cores and correlation of gamma ray logs from six wells across the depression. We identified four major lithofacies, including the interbedded evaporite and mudstone deposited in a shallow salt lake; the silt-bearing clay-rich mudstone deposited in a pro delta environment within a shallow open lake; the laminated calcareous mudstone deposited in a deep, stratified meromictic lake with frequent earthquake activities; and the massive calcareous mudstone deposited in a shallow and oxygenated lake. The Es4U Member changed from the interbedded evaporite and mudstone-dominated lithofacies association to silt-bearing clay-rich mudstone-dominated lithofacies association, and then laminated calcareous mudstone-dominated lithofacies association, suggesting basin-wide changes in depositional processes, lake level, lake water chemistry, and paleohydrology. We suggest that these changes reflect balance between sediment and water supply and potential accommodation of the lake basin, which were governed by paleoclimate in the lake drainage and extensional tectonics. We infer that during the deposition of the Es4U Member, the lake basin evolved from an underfilled lake basin with salty lake water under arid climate to a shallow balanced-fill lake basin when climate became humid, then to a deep balanced-fill basin with stratified bottom lake water when climate became more humid and the basin was deepened by tectonic activity. This study demonstrates that climate and tectonics both control paleolimnological changes and depositional processes of lacustrine mudstone.