Sequence stratigraphic control on the variations of organic matter in Eocene lacustrine shales within the Dongying Depression, Eastern China

Abstract

• A third-order sequence is identified in Eocene shales in the Dongying Depression. • The abundance, type and preservation of organic matter (OM) are intimately related to the sequence development. • Sequence development affects OM accumulation, resulting in differential OM distribution within the Eocene sequence stratigraphic framework. Eocene organic-rich shales in the Dongying Depression, Eastern China, are well preserved and ideal for examining stratigraphic controls on the variations of organic matter (OM). This study concerns the Eocene sedimentary succession in the Dongying Depression and recognizes a third-order sequence based on the stacking patterns of lithofacies, mineralogical changes, and geochemical proxies. The total organic carbon content, Rock-Eval parameters S1 and S2 values, and the chloroform bitumen “A” content are low in the lowstand systems tracts, reaching their maxima within the transgressive systems tracts and gradually decreasing within the highstand systems tracts. The paleoclimate index and ratios of quartz/feldspar, Sr/Ca, TiO 2 /Al 2 O 3 , Cr/Al, Ni/V, Th/U, Zn/Al, and Ni/Al indicate that OM accumulation was influenced by primary productivity, sedimentation rate (SR), and redox conditions, which are intimately related to the sequence development under the influence of tectonics, climate, sediment supply, and relative lake-level fluctuations. The OM distribution is related to the sequence stratigraphy in three ways: (1) climate and relative lake-level fluctuations affected the primary productivity and OM production via algae blooms and terrestrial OM input; (2) relative lake-level fluctuations and sediment supply were influenced by tectonics and climate, causing a change in SR and OM deposition; and (3) relative lake-level fluctuations under different climatic conditions affected the development of anoxic bottom-water favoring OM preservation.