Deglacial Sequence And Sedimentary Evolution In The West Bohai Bay, China

From the northwest Bohai Bay, the western part of Bohai Sea, the seismic analysis in more than 1000km of high-resolution sub-bottom profiles, together with the lithofacies, micropaleontology, chronostratigraphy, and geochemistry elements analysis in sediment cores, reveal the structure, sedimentary environment and provenance evolutions of Deglacial succession in response to fluctuations in sea-level and switching of river discharges. The Deglacial succession in northwest Bohai Bay consists of six seismic units (Ua-Uf, in descending order) bounded by key reflection surfaces (To, R1, R2, R3, T1 and T2), corresponding respectively to six depositional units (DU1-DU6) in 4 sediment cores. The Deglacial sequence in the northwest Bohai Bay consists of three systems tracts of lowland, transgressive and highstand, overlying onto the type I boundary.Lowstand Systems Tract (LST), corresponding to the Unit Uf, consists of the deep fluvial valley and floodplain sediments, formed during the Last Glacial Maximum (LGM) period (ca.22～15 ka BP), when the global sea-level was 120m below present sea-level (bpsl).The Transgressive Systems Tract (TST) is associated to the unit Ue, deposited with retrogradational feature during the Deglacial transgression (ca.15～7 ka BP).In early stage (ca.15～10 ka BP) of TST, the deposition of fluvial carbonaceous swamp had been formed, resulting from the gradually warming climate in early Deglacial period; in its middle stage (ca.10～8 ka BP), several transgression channels were formed, and the sedimentation was mainly recorded in near shore environments, such as estuarine and tidal flat; and in its final stage (ca.8～7 ka BP), the original paralic sediments on the seabed far away from the shoreland were subjected to extensive erosion when submerged by the sea-water.The main sources of Deglacial transgression sedimentation in southern region, were from the Yellow River, but mixed with other source material. While in northern region, with no stable supply or source material, the sediments were mainly brought by the two water systems of Yanshan Mountains and Taihang Mountains.Since ca.7 ka BP, when the sea-level basically stabilized in highstand, the seaward progradational sedimentation, resulting from the different river discharges, made the complex of Highstand Systerms Tract (HST) of the upper four seismic units (Ud-Ua). During ca.7～3.5 ka BP (when unit Ud was formed), Yellow River had brought the sediments to the southwest margin of Bohai Bay, which formed a NE-progradational predelta in south region. While in northern and central regions, Yellow River provenance sediment was deposited under the environment setting of the shelf shallow sea.During ca.3.5～1 ka BP (when unit Uc was formed), two deltas were stacked respectively in northern and central regions, leading to the fast progradation of coastline. In northern region, the delta was formed by two water systems of Yanshan Mountains and Taihang Mountains, but the former was the major source. In central region, however, the delta was formed by the combined water system of Yellow River and Taihang Mountains.During ca.1～0.15 ka BP (when unit Ub was formed), in central region, a delta was formed by the combined water system of Yellow River and Taihang Mountains. In northern region, however, the Yanshan Mountains water system brought the main provenance for the sedimentations under littoral and shallow sea environments.Since last ca.150 years (when unit Ua was formed), the Yellow River discharged back into the south Bohai Sea. Under the control of the Yellow River materials, the surface suspended sediments were newly deposited in modern Bohai Bay.