Origin of deep-water fine-grained sediments as revealed from the Lower Cretaceous rifting basin sequence in the Lingshan Island, Yellow Sea, Eastern China

Understanding the sediment dynamics responsible for the deposition of fine-grained sediments is essential for studying fine-grained sediment origin and its distribution, which is of a great significance for unconventional oil and gas exploration and development. Fine-grained sediments from a Lower Cretaceous rift-basin sequence in the Lingshan Island, the middle of the Dengta section in the Lingshandao Formation were studied. On the basis of detailed outcrop investigation, thin section observation, and laboratory measurements, seven types of lithofacies and three lithofacies associations were identified in the fine-grained sediment sequence. The sediment dynamics were deduced from a comprehensive examination and interpretcation of the sedimentary structures and lithofades associations. The fine-grained sediments in the research area belong to a silt dominated subclass, which is composed mainly of thin-bedded siltstones with normal grading and mudstones with horizontal lamination. Suspension settling is responsible for the development of horizontal lamination in some mudstone. The Boumalike lithofacies associations are developed by downslope fine-grained turbidity currents. The hybrid event bedlike lithofacies associations are formed by hybrid flow, while turbidity currents transformed to muddy debris flows downslope. The interbedded thin sandstone and mudstone banded lithofacies associations may be resulted from a downslope transitional flow between a turbidity current and a muddy debris flow. The vertical facies stacking patterns of the fine-grained sediment sequence indicatec that the fine-grained gravity flow may have been evolved from a transitional flow to a debris flow then to a low-density turbidity flow and finally to a hybrid flow. Lithofacies associations deposited by hybrid flow and transitional flow are potential sweet spots for unconventional oil and gas exploration in such fine-grained sediment formation.