Rift marginal coarse-grained sediment gravity flow deposits in the Eocene Dongying Depression, Bohai Bay Basin, China: Balancing tectonic and climatic controls

Abstract

Previous studies have indicated that sediment gravity flow deposits developed in lacustrine active extensional rift basins are primarily influenced by tectonics and to a lesser extent by climate. Our present work reveals that climate can obscure the effect of tectonic subsidence by regulating sediment supply; conversely, tectonics can impede the sedimentary manifestation of climatic impacts. Here a case study has been presented to assess the impact of climate-modulated rapid lake-level rise and tectonic subsidence on the development of coarse-grained gravity flow deposits in the Dongying rift margin of the Bohai Bay Basin, eastern China. The lithofacies analysis reveals frequent bed amalgamation, abundant thick massive coarse-grained deposits, widespread cross bedding and plant fragments, and incomplete composite bed formed by high-energy erosion, indicating that the hyperpycnal flow is an important mechanism driving the deposition of these coarse-grained sediments. Detailed sequence stratigraphic analysis and sediment dispersal pattern suggest that the long-striped nearshore subaqueous fan systems induced by outburst-flood hyperpycnal flow distributed along the border fault, are primarily controlled by long-term tectonics, while the rapid rise of lake level driven by short-term climate change possibly intensifies seasonal flood-generated hyperpycnal flow occurrences and consequently promotes the basinal fan progradation. The maximum scale of these coarse-grained gravity flow deposits of the basinal fan systems are typically attained during the transgressive systems tract, which deviates from the classical sequence stratigraphic model. Furthermore, it presented a continuous transition from the proximal to the distal part, encompassing traction flows and turbidity currents during the periods of relatively stable tectonics. Nevertheless, gravel-rich debris flows appear to predominate the dispersion of coarse-grained sediments during periods characterized by intense tectonic activity. The coarse-grained gravity flow deposits in the lacustrine rift margin reported here, challenge the traditional beliefs: this study suggests that subaqueous deposits abundantly preserved in the transgressive setting.