Lithofacies and origin of the Late Triassic muddy gravity-flow deposits in the Ordos Basin, central China

Fine-grained sediments from the Late Triassic Yanchang Fm. in the Ordos Basin (central China) were studied by core analysis and geophysical logging. Part of the mudstones in this formation are stratified, part of them are unstratified; the various mudstones can be subdivided into eight types on the basis of their structures and textures. They represent a variety of environments, ranging from delta fronts and subaqueous fans to deep-water environments. Part of the sediments were reworked and became eventually deposited from subaqueous gravity flows, such as mud flows, turbidity currents and hyperpycnal flows that easily developed on the clay-rich deltaic slopes. The sediments deposited by such gravity flows show abundant soft-sediment deformation structures. Understanding of such structures and recognition of fine-grained sediments as gravity-flow deposits is significant for the exploration of potential hydrocarbon occurrences. Because fine-grained deposits become increasingly important for hydrocarbon exploration, and because the sediments in the lacustrine Yanchang Formation were deposited by exactly the same processes that play a role in the accumulation of deltaic and prodeltaic fine-grained sediments, the sedimentological analysis provided here is not only important for the understanding of deep lacustrine sediments like the Yanchang Formation, but also for a better insight into the accumulation of fine-grained prodeltaic deep-marine sediments and their potential as hydrocarbon source rocks and reservoir rocks.