Ancient hyperpycnites in deep-lacustrine basins: a comparison between the Jiyang Rift Basin and the Ordos Cratonic Basin, China

Abstract

Recognizing hyperpycnites from ancient rock records in lacustrine basins is critical for understanding the characteristics and formation mechanisms of sediment gravity flow deposits, as hyperpycnites in lacustrine basins may exhibit unique characteristics that differ from well-studied marine hyperpycnites, but these distinctions are not yet fully understood. A comparative study of hyperpycnites in rift (Jiyang Depression) and cratonic (Ordos Basin) lacustrine basins allows to discuss the characteristic features, evolution processes, and distribution patterns of ancient hyperpycnites in two contrasting tectonically-controlled lacustrine basins. Hyperpycnites are composed of clastic sediments, with grain sizes ranging from pebbles to mud. The presence of mixed mud clasts with different colours, the common occurrence of plant fragments with different shapes, and the sedimentary structures such as crude stratification, climbing ripples, and bioturbation serve as recognition criteria of hyperpycnites in lacustrine basins. In addition to inverse then normal grading intervals, hyperpycnites can also be characterized by partial Bouma sequences, hybrid event beds. Depending on the characteristics of the parent flow, the evolution processes of hyperpycnal flows can include cohesive debris flows, hyperconcentrated flows, concentrated flows, and quasi-steady sediment-laden turbulent flows with occasional hybrid gravity flows. Hyperpycnal flows in rift basins range from very highly-efficient systems on ramp margins (hanging-wall slopes) to poorly-efficient systems on border fault margins (footwall), while those in cratonic basins are dominated by very highly-efficient systems. Hyperpycnites in rift basins are characterised by various distribution patterns, including apron, fan-shaped, elongated, and stepped-stacking sandbodies. In contrast, in cratonic basins, distribution patterns are dominated by fan-shaped sandbodies. Along border fault margin of rift basins, hyperpycnites are dominated by coarse-grained debrites and hyperconcentrated flow deposits, while channels and levees are uncommon. In contrast, rift basins and cratonic basins characterized by a ramp margin are dominated by channel, levee, and lobe elements. Hyperpycnites in the early stage of basin filling are easily manifested as elongated sandbodies or stepped-stacking sandbodies from margin areas to basin plain, while fan-shaped sandbodies are more common in the late stage of basin filling in both rift and cratonic lacustrine basins. This research can serve as a reference for studying hyperpycnites in rift and cratonic lacustrine basins.