Transgressive and highstand systems tracts and post-glacial transgression, the East China Sea

Abstract

Abstract The Late Pleistocene–Holocene depositional sequence on the shelf in the East China Sea (ECS) is interpreted on the basis of the analyses of four sediment cores and high-resolution seismic reflection sub-bottom profiler records along a NE–SW across-shelf transect. Sedimentary strata deposited above a lowermost planar erosional surface that was formed during sea-level fall of the last glacial are divided into two units along a NE–SW across-shelf transect (Changjiang–Okinawa). The Lower Unit is characterized by seaward-dipping tangential clinoforms with a thickness of 30–40 m at mid-shelf depths and less than 30 m beneath the outer shelf. Prograding clinoforms are more obvious in mid-shelf environments. The Upper Unit is characterized by an upper transparent layer that is formed into ridge-and-swale topography. The boundary between the Upper and Lower Units is sharp and erosional. Surficial sediments taken by cores from the central ECS shelf are also divided into two facies: a sandy facies consisting of sand or sandy gravel with moderately abundant molluscan shell fragments, and a muddy facies comprising mud intercalated with thin sand layers. The sandy facies is widely distributed on the middle to outer shelf seafloor and has a measured thickness of 30 cm, up to several metres. Radiocarbon ages of molluscan shells in this sediment are less than approximately 10 ka BP except for ages from basal shell lag deposits, which are >10–40 ka BP. The muddy facies underlies the sandy facies and has radiocarbon ages of 2.7–2.9 ka BP (previous work) and 20–38 ka BP (this study) and TL ages of 27–50 ka BP (previous work). The boundary between these lithologic units is an erosional sharp contact. The sandy and muddy facies are correlated with the Upper and Lower Seismic Units, respectively. The Lower Unit is characterized by prograding clinoforms and is interpreted to represent deltaic or nearshore tidal ridge sediments of the paleo-Changjiang River that were deposited during the last glacial lowstand of sea-level as a lowstand systems tract. The underlying erosional surface is interpreted to be a sequence boundary formed as a regressive marine erosional surface during the fall of sea-level. The erosional boundary between the Lower and Upper Units is interpreted as a transgressive surface of erosion formed during the subsequent rise of sea-level. The Upper Unit has a modern sand ridge topography and is interpreted to represent offshore shelf sand ridges of the transgressive to highstand systems tract.