Tide to river-dominated changes in the early Miocene mixed-energy paralic western shelf of the Pearl River Mouth Basin, northern South China Sea

Abstract

Many modern paralic depositional systems are characterized by complicated morphologies mixed with the river, tide, and wave processes. However, the prediction of hydrodynamic processes within their ancient counterparts is challenging from the subsurface data due to the limitation of resolution and coverage. This study illustrates an integrated work on the mixed-energy paralic deposits of the lower Miocene Zhujiang Formation in the western shelf of the Pearl River Mouth Basin, northern South China Sea. Through a synthesis of grain size and heavy mineral analysis, well-based facies interpretation, and seismic stratigraphic study, it generally shows a vertical change from the dominance of tidal strait deltas and tide-influenced deltas to a river-dominated delta with the wave-dominated shelf throughout three members, in response to the evolution from semi-closed to open marine settings. Tidal, river, and wave signals were recognized from the mixed-energy paralic deposits, albeit with alternative interpretations and non-negligible limitations. Tidal processes, which were interpreted from grain size distribution unmixing and statistical heavy mineral comparison, intensified towards the distal reach in the Member 2, and they generally declined after the drowning of paleo-Highs in the Member 1–2. Fluvial processes, which were reflected by heavy mineral evidence and sedimentary response, significantly enhanced from Member 2 to Member 1-1 with more extensive drainages and increased sediment supply, despite the long-term transgression. The presence of large-scale shoreline-parallel shelf sand ridges in the Member 1-1 was mostly controlled by the wind-driven Guangdong Coast Currents and intrusion of the South China Sea Branch of Kuroshio Current, which were coupled with the maximum East Asian Monson intensity and the Indonesian Seaway shoaling before the final closure. We demonstrated that a multidisciplinary approach presented can be effectively used to assess the changes in hydrodynamic processes of mixed-energy depositional systems, which unravel more paleogeographic, paleoclimatic, and paleoceanographic information from depositional records.