Keywords: 
• hyperpycnal flow /
• hyperpycnite /
• gravity flow deposits /
• Ordos Basin /
• Yanchang Formation

Abstract: As a major focus of both academic and industrial circles, deep-water sandy sedimentation is not only a record of gravity flows transporting a great deal of continental sediments into basin, but also important reservoir of oil and gas with great economic value. Subaqueous sediment density flows are one of the most important processes for moving sediments from provenance to depositional basins, but people still know little about these subaqueous gravity flows such as slump, sandy debris flow, muddy debris flow, granular flow, fluidized flow, turbidite current, and so on. What is more, they are extremely difficult to monitor directly. A new kind of gravity flow sandstone deposits diferent to sandy debris flow and slumping turbidity current was discovered in the sixth and seventh member of Yanchang Formation (for short, YC6 and YC7 members) in the southern part of the deep lacustrine Ordos Basin. Characteristics of the gravity flow deposits dominated by: ① a series of upward coarsening interval (inverse grading) and upward fining interval (normal grading) always exist in pairs; ② changes of relative high clay content (high-low-high) consistent with that of granularity (fine-coarse-fine) in each size-graded couplet; ③ inner micro-erosion surface sometimes separated a couplet of an upper, upward fining interval and a lower, upward-coarsening interval; ④ sandstone interbedded with dark mudstone and grey siltstone; and ⑤ granularity changes in silty mudstone is similar to that of sandstone. It was considered as flood-generated hyperpycnal flow deposit in the late Triassic deep lacustrine Ordos Basin, based on drill core observation and slice identification. A hyperpycnal flow is a kind of sustainable turbidity current occurring at a flooding river mouth when the concentration of suspended sediment is so large that the density of the river water is greater than that of lake (sea) water. It is turbid river plume that can plunge to form turbidity current where it enters a water body with lesser density and flow at basin floor. Associated with high-suspended concentration, hyperpycnal flow can transport considerable volume of sediment to lacustrine basins. Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Turbid river flow must move through transfer belt of a backwater zone, depth-limited plume, and plunging zone before becoming a turbidity current. The transfer belt can extend tens of kilometers offshore and significantly affect the transfer of momentum from river to turbidity current. Sedimentary architecture of deep lacustrine gravity flows in the southern part of the late Triassic Ordos bain consist of sandy debris flow deposits, turbidites and hyperpycnites, interbedded with fine-grained deposits (thin turbidites, hyperpycnites, and deep lacustrine mudstones). Sand and mud rich turbidite systems fed by mountainous “dirty” rivers and slumps at deep angle deltas front. Storm-influenced, hyperpycnal flows generated subaqueous channelized forms at the mouth of the river deltas, which later filled with sand. The typical deposit of hyperpycnal flow in the YC6 and YC7 members in the southern part of the deep lacustrine Ordos Basin is a compound of a basal coarsening-up unit, deposited during the waxing period of discharge, and a top fining-up unit formed during the waning period of discharge. Hyperpycnites differ from other turbidites because of their well-developed inversely graded intervals and intrasequence erosional contacts. Deposits of hyperpycnal flow, hyperpycnite is different to others turbidite as for well developed upward-coarsening interval and inner micro-erosion surface in size-graded couplets. The lower, upward-coarsening interval represents deposition of waxing hyperpycnal flow. The upper, upward-fining interval was generated from waning hyperpycnal flow. The two parts of the size-graded couplet of upward-coarsening interval and upward-fining interval in pairs represent a cycle of event sedimentary of flood-generated hyperpycnal flow. The micro-erosion surface that sometimes divides the two parts of the size-graded couplet resulted from waxing flows of sufficiently high velocity to erode the sediment previously deposited by the same flow. Some bed forms and sediment grading patterns in hyperpycnal- flow deposits can record multiple flow accelerations and decelerations even during a simple single-peaked flood. Because hyperpycnal flow provides one of the most direct connections between terrestrial sediment sources and lacustrine depositional basin, its deposits might preserve an important record across a variety of climatic and tectonic settings. Depositional processes in the late Triassic deep lacustrine in the studied area were dominated by sediment gravity flows originating from gravity induced slumps and mountainous “dirty” river discharged hyperpycnal flow. Gravity flows deposits in the YC6 and YC7 members in the southern part of the deep lacustrine Ordos Basin appear to be primarily controlled by the strong climatic and tectonic forcing parameters. The basin also must be deep enough, in some cases greater than tens of meters, in order for the plume to collapse and form a turbidity current. All in all, controlling factors of hyperpycnal flow include seasonal flood river, deep angle depositional slope, enough water depth and large density difference between basinal water mass and discharged flood river. The discovery of hyperpycnite in Yanchang Formation in the Ordos Basin can not only provide an example to probe hyperpycnal flow deposits in continental lacustrine environment, but also has theoretical and realistic significances to study on genesis of deep water sandbodies, to reservoir forecasting and oil-gas exploration.