Holocene sedimentary sequence, foraminifera and ostracoda in west coastal lowland of Bohai Sea, China

Abstract

The Holocene sedimentary sequences in the west coastal lowland of the Bohai Sea consists in ascending order of: pre-transgression river and lake sediments, transgressive tidal flat and estuarine sediments, shelf sediments, and prograding delta and river deposits. The delta deposits are the thickest and are typically composed of two imbricating Yellow River delta superlobes that acted at different periods. There are three marker horizons in the sequence at: (1) the bottom of marine sediments; (2) the bottom of shelf deposits; and (3) the boundary between two imbricating delta superlobes. Environmental changes were mainly controlled by a rising sea level prior to 6000 BP and by delta progradation after 6000 BP. Stratigraphic distribution of foraminifera and ostracoda vary according to variations in salinity and sedimentation rate. There is no evidence that the Yellow River delta developed during the Holocene transgression but rather during a later prograding phase. An understanding of the shelly ridges in the coastal lowland indicates that the ridges were formed at a similar level to the present and there is no evidence for Holocene sea levels 1–2 m higher than the present sea level.