The Segmented Multi-Source Sediment Routing System on the Hangingwall Dipslope of the Xihu Depression, East China Sea Shelf Basin: Insights From Palaeogeomorphology, U–Pb Ages and Heavy Minerals

Abstract

Coeval input systems in rift basins may interact with each other to form a segmented multi-source sediment routing system. Importantly, its division into proximal zones, where a single source dominates, and interaction zones, where multiple sources mix, enables the interactions between input systems to be characterised. Here, we exploit this conceptual framework to revisit the middle Eocene–early Oligocene hangingwall dipslope of the Xihu depression in the East China Sea Shelf Basin, where extensive 3D seismic data, detrital zircon U–Pb ages and heavy mineral compositions are available. We first combined palaeogeomorphological and sedimentological features with age signatures to distinguish three areas: the northwestern area was identified for its proximity to the Haijiao uplift and invariably high proportions of Palaeoproterozoic ages (41%–54%); the southwestern area adjacent to the Yushan uplift was distinct for enriched Cretaceous-aged zircons (36%) and the transition area in between was characterised by its remoteness to both uplifts, an embayed geometry and mixed age signatures that are not identical to any individual input. These spatial variations support the segmented framework for the multi-source system, with the northwestern and southwestern areas representing two palaeo-input systems and the transition area as their interaction zone. In this context, we then used mixture models to determine spatio-temporal variations in the mixing proportions of the two palaeo-input systems. The zircon-based results indicate that the mixing proportion sustained from the middle to the late Eocene, during which the basin was in the late syn-rift stage and marine environments. This is corroborated by heavy mineral composition that shows only minor changes. We interpret the roughly sustained mixing proportions as reflecting both the spatially uniform nature of broad subsidence and the strong tidal processes that ‘erased’ the effects of avulsions. In contrast, a clear provenance shift in both zircon ages and heavy minerals occurred from the late Eocene to the early Oligocene, coinciding with a transition to the tectonic inversion stage and a shift towards non-marine environments. The provenance shift, together with the southward expansion of the axial drainage, likely represents the sedimentary response to the southward decreasing inversion magnitude of the Yuquan Event. In addition, we hypothesize that in the absence of strong tides, avulsions might have controlled the mixing proportion, particularly over short timescales. Ultimately, this study demonstrates the segmented multi-source framework, if properly incorporated, can provide key insights into dipslope sedimentation.