Ichnological study of deep marine hyperpycnites: A case study from the Shenhu continental slope (northern South China Sea)

Abstract

Ichnological analysis plays a pivotal role in the study of sedimentary basins as it records the evolution of paleoenvironments and the associated changes, providing crucial support for sedimentological and stratigraphic interpretations. Yet, rare ichnological studies on deep-marine hyperpycnites have been conducted, resulting in less conclusive ichnological characterization of deep-marine hyperpycnites, compared to pelagites/hemipelagites, turbidites, and contourites. This study conducted detailed investigation of trace fossils distribution within deep-marine hyperpycnites, using non-destructive X-ray computed tomography scanning. The sediment core S19 on the upper Shenhu slope in the northern South China Sea documented deep-marine hyperpycnites facies S3, S2L, and L, possibly representing the proximal to distal hyperpycnal lobe. In this study, hydrodynamic energy and salinity are suggested to be the dominant stress factors within proximal lobes, resulting in less bioturbation and scattered distribution of vertical trace fossils such as Skolithos and Siphonichnus. In the middle lobe, the environment is predominantly characterized by moderate to high hydrodynamic energy, a rich abundance of organic material, and occasional slight salinity fluctuations. These environmental factors have led to the prevalence of trace fossils such as Arenicolites, Conichnus, Nereites, Phycosiphon, Rosselia, Scolicia, and Thalassinoides, which are well-represented in this region of the lobe. The organic material concentrated within the distal lobe to fringe acts as the main stress factor promoting opportunistic organism colonization and the formation of deposit-feeding traces. Therefore, the represented traces within distal hyperpycnites are mainly Nereites and Phycosiphon. This study provides a detailed investigation into trace fossil distribution within deep-marine hyperpycnal lobes, enriching our knowledge of the ichnology of deep-marine hyperpycnites.