Carbon isotope and rare earth element composition of Late Quaternary sediment gravity flow deposits on the mid shelf of East China Sea: Implications for provenance and origin of hybrid event beds

Abstract

The East China Sea Shelf has an unusually wide and low gradient shelf, supplied from sediment-charged rivers and large river delta systems, with bottom currents sweeping the sea floor and located in the path of strong typhoons. Sediment gravity flow deposits, including four hybrid event beds and a high density turbidite, are identified in a core from the mid-shelf of the East China Sea. The hybrid event beds typically comprise three or two internal divisions from the base to the top: (i) H1, H3 and H5; or (ii) H3 and H5. Radiocarbon ages of the hybrid event beds were in the range of 3821 to 8526 yr BP. Based on correlation with surrounding cores, the hybrid events may have happened at any time between 1930 yr BP and 3890 yr BP. The d 13 C values in hybrid event beds together with bathymetry data suggest local erosion on the shelf. The average d 13 C value for the H1 division is similar to the H3 division in the hybrid event beds, implying that the organic matter in the H1 and H3 divisions may come from the same source area. Cross-plots of upper continental crust normalized rare earth elements in the five units reveal that the sediment source of the four hybrid event beds and the turbidite was ultimately primarily from Korean rivers. Partial transformation from a moderate-strength debris flow with the additional role of erosional bulking can explain occurrences of hybrid event beds on the East China Sea Shelf. The data indicate that hybrid sediment gravity flow deposits were 1861 sourced from intra-shelf failures and subsequently transformed and deposited as hybrid event beds. The study shows that hybrid sediment gravity flows and turbidity currents may not necessarily indicate proximity to a major fluvial or deltaic system and that intra-shelf sedimentation can be a sediment source. It is unlikely that the debris flows and turbidity currents were triggered by a hyperpycnal flow or tsunami, because both can carry continental and/or coastal signals which have not been recognized in the core. Typhoons are the probable triggering mechanism.

Sole marks reveal deep-marine depositional process and environment: Implications for flow transformation and hybrid-event-bed models

“…Most of the bedload will bypass the top of this solid aggrading surface, but isolated clasts can be incorporated into the aggrading H1 division (Fig. 20), as observed in the hybrid event beds in the study areaeither as randomly distributed clasts, as clasts near the base of the H1 division, or concentrated along horizons and in previous work (Haughton et al 2009;Shan et al 2019aShan et al , 2019b. This gradual aggradation matches the aforementioned sustained nature of the SGFs, inferred from the presence of migrating dunes in the submarine-channel fill.…”

“…Therefore, erosion of both unconsolidated mud and mud clasts likely takes place primarily below the head. Erosion beneath the head of a turbidity current has also been inferred in Late Quaternary hybrid event beds on the East China Sea Shelf where localized erosion is indicated by the presence of distinctive locally sourced mud clasts (with distinct  13 C values) in the resultant H1 division (Shan et al 2019a(Shan et al , 2019b. Sustained erosion below the head then leads to increased flow density and cohesivity, with the latter primarily the result of the incorporation of weak substrate mud.…”

Changjiang coastal mud‐belt deposits in Taiwan Strait: Controls on its distribution and facies

“…The core was logged in detail (centimetre‐scale), recording variations in grain‐size, sorting, bed thickness, sedimentary structures, trace fossils and diagenetic features (for example, iron staining). The grain size of 487 samples with a spacing of 5 cm was measured and analysed using previously reported methods (Shan et al ., 2019).…”

A giant lacustrine flood‐related turbidite system in the Triassic Ordos Basin, China: Sedimentary processes and depositional architecture

“…1D). All samples were pretreated by adding 10% HCl treatment to remove carbonates, and a 30% H 2 O 2 solution to remove organic matter (Shan et al., 2019). After washing the samples, grain size was analyzed using a Malvern Mastersizer 3000 Laser Diffraction Particle Size Analyzer (Malvern Panalytical, Malvern, UK).…”