Climate-driven paleolimnological change controls lacustrine mudstone depositional process and organic matter accumulation: Constraints from lithofacies and geochemical studies in the Zhanhua Depression, eastern China

The influence of paleoclimate on the depositional process of lacustrine mudstone and organic matter accumulation is important to both paleoclimate reconstruction and hydrocarbon exploration. Here we study the lower third Member (Es3L) of the Eocene Shahejie Formation in the Zhanhua Depression, Bohai Bay Basin, eastern China in order to understand how paleoclimate influenced depositional processes and organic matter accumulation of lacustrine organic-rich mudstone. By combining detailed core descriptions and microscopic observations, and high-resolution mineralogical and geochemical analyses, we identified two major lithofacies, including massive calcareous mudstone and laminated calcareous mudstone, from the Luo 69 drilling core. The sedimentologic observations and changes of geochemical proxies, including detritus index, Ln(Al2O3/Na2O), B/Ga, and V/(V + Ni), suggest that the massive calcareous mudstone was deposited in a small, shallow, salt lake that was dysoxic-anoxic, and the paleoclimate in the lake catchment was cool and arid, and the laminated calcareous mudstone was deposited in a large and deep stratified lake, which has anoxic, highly saline bottom and oxic, less saline surface water, and the lake catchment was more humid and warm. The dominant lithofacies changed from laminated calcareous mudstone to massive calcareous mudstone in the studied core, suggesting that the lake became shallower and smaller when the paleoclimate became cooler and drier through time. Such a climate trend may be a response to global cooling during the middle Eocene. The average total organic content (TOC) in both lacustrine highstand and lowstand are comparable even though the lake water chemistry and amount of terrigenous input are different. We infer that the accumulation of organic matter within the lacustrine highstand was controlled by the combination of primary productivity, carbonate production, and preservation in anoxic bottom water, while accumulation within lacustrine lowstand was only controlled by primary productivity.