Recognition of muddy hyperpycnites in deep-lacustrine settings: Depositional model for muddy hyperpycnal flows in lacustrine basins

During the past 15 years, the exploration and development of shale reservoirs worldwide promoted a significant progress in the understanding of shale depositional processes. Despite the fact that the suspension settling paradigm was challenged in the case of marine shale preserved in the rock record, deep-lacustrine shales are still commonly interpreted as the products of background sedimentation during periods of quiescence that promote the continuous settling of discrete particles out from suspension. In this paper, the integration of high-resolution sedimentology with organic geochemistry and cyclostratigraphy of deep-lacustrine shales belonging to the Chang 73 sub-member of the Yanchang Formation (Triassic), Ordos Basin, China, allows us to analyze the dynamic sedimentary processes responsible for the transport, distribution, and deposition of the fine-grained sediments in deep-lacustrine basins. Terrigenous organic matter and micro-sedimentary features indicative of dynamic depositional conditions were widely recognized in the deep-lacustrine shales, calling for a critical reappraisal of the prevailing depositional model. The evidence found in this study indicates that muddy hyperpycnal flows were important processes for the delivery of mud and silt to the deep-lacustrine setting. In the proximal regions, silt-grade feldspar and quartz quickly settled out of the decelerating turbulent hyperpycnal flow and formed the fining-upward silt-grade felsic lamina (SSFL) with erosional base. As the flow velocity decreased down the basin, and the flow was progressively depleted in silty materials, mud flocs formed that were transported as bedload along flow direction, and developed the clayey sediments with low-angle foresets, indicating the transformation from SSFL to tuff-rich lamina (TRL). The decreased sediment supply farther offshore induced the transition of clay-mineral−dominated layers to lens-like clay mineral aggregates, along with the increase of organic matter, promoting the replacement of TRL by organic-rich lamina. Shoreline changes caused by lake-level fluctuation further affect the basinward transport of the fine-grained sediments, thus controlling the vertical superposition of different types of laminae within the deep-lacustrine shales. Given the common occurrence of hyperpycnal flow in lake basins, the new depositional model proposed here may provide insights into the origin of deep-lacustrine shales and the process of organic matter enrichment in other lake basins around the world.