Impact of deep‐time palaeoclimate on the sedimentary records and morphology of lacustrine shoal‐water deltas, Upper Eocene Dongying Depression, Bohai Bay Basin, China

Abstract

AbstractShoal‐water deltaic systems are predominant during the deposition of the second member of the Eocene Shahejie Formation (Es2) in the Dongying Depression, Bohai Bay Basin, China, and their presence is ascribed to low slopes, shallow water bodies and extremely slow tectonic subsidence. Integrated analyses of sedimentological, seismic geomorphological, palaeohydraulic, palynological and element‐geochemical data reveal the influence of palaeoclimate on the morphology and depositional processes of distributary networks in shoal‐water deltaic strata. Two contrasting palaeoclimatic stages were identified based on palynological and geochemical evidence, namely the earlier warm and humid climatic stage, and the later arid climatic stage. Compared with the earlier humid stage, the arid stage features much more abundant xerophytic species (Labiatae and Ephedra) as well as much higher Mg/Ca, Sr/Ba, and Th/U ratios and B content, indicating extensive evaporation. A third‐order sequence stratigraphic unit was recognized, consisting of a transgressive systems tract with landward stepping parasequence sets, a poorly developed highstand systems tract, and a falling stage systems tract with basinward stepping parasequence sets, which were products of the humid and arid climatic stages, respectively. A series of distributary channel complexes exhibiting reticular patterns was identified in the strata corresponding to the humid climatic stage, with amalgamated, multi‐storey distributary channel sandbodies showing a sinuosity range of 1.1 to 1.3. In contrast, the strata corresponding to the arid climatic stage are dominantly composed of distributary channel systems exhibiting dendritic forms, with prevalent single‐storey channel fills and a low sinuosity range of 1.0 to 1.2. The dendritic system is more stable than the reticular counterpart due to the effects of frequent incision and limited vertical aggradation. Typical modern shoal‐water deltas formed in different climatic conditions (i.e. Poyang Lake delta and Neales River delta) serve as analogues to the proposed conceptual depositional models of distributary networks related to palaeoclimate.