Intraplate volcanism initiated shortly after the cessation of Cenozoic seafloor spreading in the South China Sea (SCS) region, but the full extent of its influence on the Indochina block has not been ...well constrained. Here we present major and trace element data and Sr-Nd-Pb-Hf isotope ratios of late Cenozoic basaltic lavas from the Khorat plateau and some volcanic centers in the Paleozoic Sukhothai arc terrane in Thailand. These volcanic rocks are mainly trachybasalts and basaltic trachyandesites. Trace element patterns and Sr-Nd-Pb-Hf isotopic compositions show that these alkaline volcanic lavas exhibit oceanic island basalt (OIB)-like characteristics with enrichments in both large-ion lithophile elements (LILE) and high field strength elements (HFSEs). Their mantle source is a mixture between a depleted Indian MORB-type mantle and an enriched mantle type 2 (EMII). We suggest that the post-spreading intraplate volcanism in the SCS region was induced by a Hainan mantle plume which spread westwards to the Paleozoic Sukhothai arc terrane.
The Yangtze shoal lies east of the Yangtze (Changjiang) River estuary, China, at water depths of ~25–55 m and is one of the largest subaqueous, sheet-like sand banks in the world. Previous studies ...have not reached a consensus regarding its origin and timing of formation, mainly due to the scarcity of coring and high-resolution seismic profiling in the shoal. We recovered a 70.20-m-long sediment core (CRE-1402) through the sand shoal and acquired 1586 km of high-resolution shallow seismic profiles across the estuary and shoal. We conducted systematic analyses of the core, including its sedimentary characteristics, down-core changes in benthic foraminiferal assemblages, and 14C and optically stimulated luminescence (OSL) dating. We also analyzed the seismic stratigraphy and integrated these results with data from other previously well-studied cores onshore and offshore of the Yangtze River mouth. Our results indicate that the stratigraphic architecture in the paleo-Yangtze estuary (i.e., the area from the present-day delta plain to the subaqueous delta and Yangtze Shoal) was controlled mainly by changes in sea level during the post-glacial transgression. The post-14 ka sedimentary successions in the paleo-estuary rest unconformably on fluvial or coastal deposits that formed mainly before MIS 1, and are separated by three distinctive bounding surfaces corresponding to MWP-1A, MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka shortly after the Younger Dryas cold period, and the 8.2 ka event, respectively, and the maximum marine flooding surface (MFS). The depositional unit of the Yangtze Shoal ranges in thickness from 6 to 32 m. The shoal exhibits large-scale, SW-dipping (generally 0.5°–2°) foresets with local cross-cutting of foresets, which are composed mainly of very fine- to medium-grained sands. The shoal is underlain by muddy intertidal to subtidal flat deposits that formed during the Younger Dryas, with an erosional surface corresponding to MWP-1B (?) or the rapid sea-level rise at ca. 11.5 ka between these and the shoal deposits. The Yangtze Shoal came into being mainly between ca. 11.5 and 7.0 ka in the form of tidal sand ridges actively moving southwestwards under intense and reciprocal tidal currents on a tidal shelf. The tidal sand ridges have evolved into sheet-like tidal sand banks with relatively low relief as a result of tidal remobilization under rotary tidal currents since ca. 7.0 ka when the MFS formed. Therefore, the Yangtze Shoal can be considered to comprise quasi-active tidal sand banks.
•The origin and age of Yangtze Shoal are revealed based on high-resolution seismic profiles and well-dated core data.•The stratigraphic architecture in the paleo-Yangtze estuary was controlled by the stepwise post-glacial sea level rise.•The early to middle Holocene sand ridges in the shoal have been evolving into sheet-like sand banks since ~7.0 ka.
This study focuses on sedimentary research at the subaqueous Changjiang (Yangtze River) delta, based on five high-resolution seismic profiles and seven borehole cores with accurate AMS 14C datings. ...Three distinct seismic units were identified from the seismic profiles according to seismic reflection characteristics, and five sedimentary facies were recognized from borehole cores. These facies constituted a fining upward sedimentary sequence in relation to postglacial sea-level transgression. Three sequence surfaces (sequence boundary (SB), transgressive surface (TS), and maximum flooding surface (MFS)) demarcate the boundaries between early transgressive system tract (E-TST), late transgressive system tract (L-TST), early highstand system tract (E-HST) and late highstand system tract (L-HST), which constitute the sixth order sequence. These system tracts were developed coevally with postglacial sea-level rise. E-TST (~19–12kaBP) corresponds to an incised-valley infilling in the early stages of postglacial transgression whereas L-TST (~12–7.5kaBP) was formed during the last stage of postglacial transgression. The progradational structure of L-TST reflected in seismic profiles is possibly related to the intensification of the East Asian summer monsoon. E-HST (~7.5–2kaBP) was deposited in response to the highstand after maximum postglacial transgression was reached, while L-HST (~2kaBP–present) was initiated by accelerated progradation of the Changjiang delta.
•Sedimentary facies in relation to postglacial sea level change were revealed.•A facies mode of the tidally dominated paleo-Changjiang estuary was proposed.•A sixth order sequence had been recognized in the subaqueous Changjiang delta.
Sedimentary architecture dominated by transgression–regression cycles in the shallow Bohai shelf region contains information about global sea-level, climate and local tectonics. However, previous ...studies of transgression–regression cycles in this region at orbital timescales that extend back to the early Pleistocene are sparse, mainly because of the shortage of well-dated long cores. Although transgression–regression sedimentary cycles in the region have been interpreted in terms of local tectonics, sea-level, and climate change, the detailed structure of marine transgressions and their significance for Quaternary global sea-level variations remains to be examined. In this study, we present an integrated sedimentological, geochemical and paleontological study of a 212.4 m (∼1 Ma) core (BH08) recovered from the Bohai Sea, China, for which an astronomically-based age model is available. Correspondence between marine-terrestrial sedimentary cycles and global sea-level fluctuations suggests that stacking of marine and terrestrial sediments was driven mainly by glacio-eustatic sea-level fluctuations in a context in which tectonic subsidence was largely balanced by sediment supply over the last ∼1 Ma. We report a dominant 100-kyr cycle beginning at ∼650 ka, which reflects the worldwide influence of the mid-Pleistocene transition (MPT) in sea-level records. We find that neritic deposits after the MPT were relatively thicker than before the MPT, which indicates an important control of the MPT on sedimentary architecture through lengthening of the duration of sea-level highstands.
•Marine and terrestrial deposition alternated in the Bohai Sea over the last ∼1 Ma.•Sea-level changes governed the sedimentary architecture in the Bohai Sea.•The MPT influenced the deposition through modulating sea-level changes.
The East China Sea (ECS) is a vast river-dominated depositional basin because of its broad shelf and extensive river sediment supply. This study investigated the mid–late Quaternary stratigraphic ...framework and evolution of the outer ECS shelf based on a borehole core (DH02) and seismic profiles. Seven sedimentary facies were recognized in the core sediments according to lithology, grain-size composition, sediment structure, and microfossil assemblages, while eight unconformity-bounded major stratigraphic units were identified in the seismic profiles. Based on seismic–core correlations, dating data (radiocarbon and optically stimulated luminescence), and comparison between sedimentary cycles and sea-level curves, the 100 m thick strata below the seafloor were divided into three sequences corresponding to glacial–interglacial sea-level cycles of marine isotope stage (MIS) 1–2, MIS 3–6, and MIS 7–8 (Sq1–Sq3 from top to bottom). In particular, Sq1 is developing in the present sea-level highstand and consists mainly of deposits of transgressive systems tract (TST), including tide-influenced river, tidal flat, and tide-dominated estuarine facies. Sq2 and Sq3 consist mainly of prodelta and related inner-shelf facies of forced regressive systems tract (FRST) and transgressive deposits similar to those of Sq1. For all three sequences, a thin-sand sheet layer of open-shelf facies represents deposits of highstand systems tract (HST). Deposits of lowstand systems tract (LST) were typically represented by river facies. In addition, comprehensive analyses of seabed sand ridge distribution, core sediment characteristics, and ridge deposition mechanism suggest that the occurrence of a paleoestuary with transgression determined the development of a tidal sand ridge field. Based on all these results, a conceptual stratigraphic sequence model in association with the deposition of LST, TST, HST, and FRST was constructed. Overall, our stratigraphic framework, along with a comparison of the sequence stratigraphy to that of other shelves, suggested that glacial–interglacial sea-level fluctuations, huge amounts of river sediment supply, active subsidence, gentle shelf topography, and marine erosion processes (transgression and regression) controlled the mid–late Quaternary stratigraphic development of the ECS shelf.
•A mid-late Quaternary stratigraphic framework (since MIS8) was established.•Three sequences were geochronologically corresponded to the recent three glacial/interglacial sea-level cycles.•The paleoestuary retreated with post-LGM transgression, yielding modern seabed tidal sand ridge field.•Comparison of the sequence stratigraphy to other shelves suggested controlling factors of stratigraphic development.
Detrital minerals in sediments can provide insight into sedimentary provenance and depositional environments. Here we report a record of heavy minerals of a 300.1 m long core recovered from the ...western south Yellow Sea, which was dated back to the late Pliocene (~3.5 Ma). Results of detrital mineral analysis show that sediments are dominated mainly by silicate minerals sourced from the Chinese mainland through river inputs. Heavy minerals suggest that marine transgression occurred around 76–84 m depth of the core depth, about at 792–840 ka BP. The upper part of the core (0–76 m) is dominantly marine and the lower part (below 84 m) is terrestrial. From the Miocene to Early Pleistocene, the Yellow Sea area generally subsided; the deposition center began to move from west to east, therefore the content of fine sand increased relatively in the core sediment. Meanwhile, under the influence of depression evolution in the Subei Basin, the paths of rivers were shifted, as a result the contents of augite and garnet changed, sediments in the study area were dominated by proximal sediments sourced from the Subei Basin, possibly of mixed sediments of the Yellow River and Yangtze River. The lower part of the core was characterized by a dramatic increase in augite with hornblende and epidote as dominant minerals. In the Early Pleistocene, under the background of subsidence in the Yellow Sea area, while transgression occurred, rivers shifted southward. Comparison with the mineral composition of adjacent core NHH01 and surface sediments from the region suggests the Yellow River dominated the study area, and the old Yangtze River input being occasionally dominant. The heavy mineral assemblage was dominated by hornblende and epidote, as well as mica, garnet and authigenic pyrite in some sediment layers.
This study investigates Holocene sedimentary evolution and hypoxia development using borehole cores CJK06 and CJK09, in combination with other published core data. Based on lithology and microfossil ...(benthic foraminifera) characteristics, seven types of sedimentary facies were identified from the base upward: river, tidal flat, tide-influenced river, transgressive lag, estuary, inner-shelf, and prodelta. Isochronous correlation among the cores was established relying on accelerator mass spectrometry 14C dates. Facies succession indicates that a weak progradation occurred in coastal environments (12–10 ka BP) due to the Younger Dryas-induced deceleration of sea-level rise; rapid deposition locally occurred in the southern marginal area of tide-dominated estuary environments (10–8 ka BP), likely due to the junction of the Yangtze and Qiantangjiang River currents; and marine current-induced fluid mudflows prevailed in the shallow marine environments (8–1 ka BP), with the cooling climates of 5–1 ka BP. Furthermore, prodelta and shallow marine environments co-occurred with an uneven progradation of the delta within the last 1 ka, while deposition occurred just inside the more southern, delta-influenced area. The occurrence of euryhaline benthic foraminifera suggests that an enhanced freshwater discharge of the mid-Holocene (8–5 ka BP) was followed by a sharp decline in the late Holocene (5–1 ka BP) with climate change. The occurrence of cold-water benthic foraminifera indicates a severe cold-water condition during the mid-Holocene due to the intrusion of upwelling currents triggered by the propelling force of warm currents and summer monsoon winds. In addition, the occurrence of low-oxygen foraminiferal assemblages reveals that hypoxia has developed since 10 ka BP in stages consistent with the sedimentary evolution: localized hypoxia formation coincident with the southern depocenter during the early Holocene (10–8 ka BP); severe hypoxia with enhanced freshwater discharge and upwelling current intrusion during the mid-Holocene; and weakened hypoxia of the late Holocene, mainly due to the sharp decline in freshwater discharge. Within 1 ka BP, freshwater discharge from the approaching river mouth and increased nutrient inputs from enhanced human activities on land have contributed to the prevalence of hypoxia, especially in the southern deltaic area. Overall, it was revealed that the freshwater discharge modulated with climate variations and delta progradation plays a primary role in controlling the sedimentary evolution and hypoxia development during the mid-late Holocene.
•Gravity flows (fluid mudflow) prevailed during the late Holocene (5–1 ka BP).•Holocene freshwater discharge was enhanced (8–5 ka BP) and then sharply declined.•Severe upwelling current intrusion occurred during the mid-Holocene (8–5 ka BP).•Hypoxia boomed during the mid-Holocene (8–5 ka BP).
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 δ13C values in hybrid event beds together with bathymetry data suggest local erosion on the shelf. The average δ13C 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 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.
The lack of suitable indicators of changes in such as sea‐level and circulation has been a major limit to paleoenvironmental and paleoceanographic investigations in continental shelf regions. This ...paper presents an environmental magnetic study by comparing two late‐Quaternary sediment cores (DH02 and DH03) from the outer shelf of the East China Sea (ECS). Late and early Marine Isotope Stage (MIS) 3 sediments were deposited in a prodelta under cold coastal currents and an open‐shelf with the Taiwan Warm Current and upwelling. The dominant iron‐bearing minerals of the late and early MIS 3 sediments are authigenic greigite (Fe3S4) and pyrite (FeS2), respectively, which were assumed to be formed nearly syndepositionally. The overlying sediments, however, are magnetically dominated by detrital magnetite. This pattern corresponds well to the temporal changes in sea‐level over this period. The widespread occurrence of greigite in the late MIS 3 sediments can also be used for future stratigraphic division and correlation in the ECS. Additionally, compared to microfossil assemblages, rock magnetic parameters based on greigite may be more sensitive to environmental changes on continental shelves. Furthermore, the inter‐borehole spatial comparisons imply not only a sedimentary hiatus/erosion of at least 30‐m thickness in core DH02, most probably during the Last Glacial Maximum, but also that core DH02 was in a more reductive environment than core DH03 during late MIS 3. The findings highlight the potential of authigenic greigite as an indicator of spatiotemporal changes in paleoenvironmental and paleoceanographic conditions on the continental shelf at orbital or even suborbital timescales.
Plain Language Summary
Environmental and oceanographic conditions on continental shelves are vital for sedimentation processes and human settlements, but the reconstruction of their evolution is hindered by the lack of effective indicators. Recent studies have indicated that syndepositional formation of greigite, a precursor to pyrite, is highly likely in shallow‐water environments with rapid sedimentation. Here, two sediment cores meridionally traversing the outer shelf of the East China Sea were investigated. The magnetic mineral assemblage and sedimentary environment exhibited almost identical change patterns, with pyrite, greigite, and magnetite as the major magnetic minerals of the early and late Marine Isotope Stage 3, and Last Glacial Maximum/last deglacial deposits, respectively. This agreeably reflects the temporal fluctuations in sea‐level and ocean circulation. By comparing the two cores, the spatial characteristics of stratigraphy and redox conditions could be determined clearly. This study thus provides a good reference for paleoenvironmental and paleoceanographic research based on authigenic greigite in continental shelf sediments.
Key Points
Greigite was further confirmed as the major magnetic mineral of the late Marine Isotope Stage 3 deposits widespread on the outer shelf of the East China Sea
Spatiotemporal changes in environmental and oceanographic conditions on the continental shelf are well revealed by greigite‐based parameters
Authigenic greigite can serve as a sensitive indicator for paleoenvironmental and paleoceanographic studies in continental shelf regions
The internal fabric of mud caps, which are on top of sediment gravity flow deposits, remains unexplored so far. We identified four hybrid event beds on one of the widest and gentlest shelf (East ...China Sea Shelf). Three types of lithofacies, including massive sand, chaotic mud and homogeneous mud, were distinguished. Homogeneous mud represents mud caps of these hybrid event beds. Scanning electron microscopy performed in both homogeneous mud and chaotic mud has revealed the presence of mud aggregates (mud clasts) conforming sand-size clasts. Chemical analyses of the aggregates show the presence of abundant gypsum/anhydrite and clay mineral components as well as relicts of quartz, muscovite and feldspar, suggesting an origin related to erosional events and an intraclast origin. The vertical shortening, deformation together with the pore spaces of mud clasts indicate a shallow burial depth of erosion, where mud sediments were still unconsolidated and water-saturated. No background sedimentation is present in the mud caps, because we did not observe any bioturbated interval in core images and X-radiographs. The lack of laminated muds and synsedimentary trace fossils in the homogeneous muds suggest an origin related to fluid mud flows. As proved on previous experiments, the mud caps could be related to head erosion and shearing of the muddy debris flow/mud flow's upper surface which generated a slower moving fluid mud flow. Our study further confirms that no laminated sand/silt (H4 division) would be present in hybrid event beds with mud-dominated H3 division. Head erosion of muddy debris flow/mud flow would not allow laminated H4 division to form, because the fluid mud flow derived from the parent mud-dominated debris flow was mainly composed of grains smaller than 30 μm which can effectively damp turbulence effectively. Grain size analysis performed on samples from fluid mud flow deposit fall into pelagites/hemipelagites area (T area), suggesting that fluid mud flow deposits are also a candidate for T area, and the area does not necessarily reflect a deposition under un-stressed environment.
•1 Mud caps of hybrid event beds from ECSS are interpreted as fluid mud flow deposits.•2 SEM and chemical analyses of mud caps suggest a shallow burial depth.•3 CM pattern of fluid mud flow deposit fall into pelagite/hemipelagite area (T area).•4 muddy debris flows generate a trailing fluid mud flow.
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