Marine Beds And Sedimentary Environment Since The Late Mid-pleistocene In The West Coast Of Bohai Bay,China

A wealth of information concerning land-ocean interaction was recorded in thewest coast of Bohai Bay. The depositional characteristics and environmental evolutionare constrained by both regional and global variations, thus the muddy coast in westBohai Bay is considered as an ideal area for global change research.This thesis is mainly focused on two major aspects as follows. First, three marinebeds are indentified within the 80-m-thick strata, revealled by a number of boreholes,and the transgressive stages are determined. Also, the geological significance of suchthree marine beds that indicate sea level change and tectonic movement are discussed.Second, sophisticated subdivision for the Holocene marine bed, preserved muchentire geoenvrionmental information, is carried out in order to distinguish a system ofsubfacies, and reconstruct the Holocene palaeoenvironment in the study area. Impactsof the Holocene transgression and sediment supply on the environmental revolution inthe region are analyzed.The analyses of lithology, sediment composition, sedimentary structure, faunalcontent (assemblages of mollusk, benthic foraminifera and ostracoda) as dating ofAMS14C and OSL as well indicate that:(1) Three typical marine beds are identified in the 80-m-thick strata. Inascending order from the base, they are interpreted as Marine Bed 3 (M3) deposited inthe late Mid-Pleistocene, Marine Bed 2 (M2) in the Late Pleistocene and Marine Bed1 (M1) in the Holocene, respectively. These beds record three major sea levelhighstands, occurred since the late Mid-Pleistocene. M3, M2 and M1, correspond tothe marine facies in MIS 7, MIS 5 and MIS 1, respectively. Therefore, this thesisconcludes that these marine beds recorded the regional sea level trend in the westcoast of Bohai Bay and responded well to the global change.(2) Comparative analysis with other boreholes in the region indicates that thethree marine beds are widely distributed and are comparable well in the west coast ofBohai Bay. Changeable elevation and thickness of the marine beds among differentboreholes suggest that the differential depression movements existed during the LatePleistocene, while the tectonics are relative stablely in the Holocene. (3) Our results differ from the previous studies based on the new OSL datingframework: the M1 and M2 in this thesis correspond to the previously concludedmarine bed I, deposited in MIS 1, and marine bed III deposited in MIS 5, respectively.However, there is not such a so-called marine bed II, deposited in the period of MIS 3.The different result indicates that the marine transgression did not reach the study areaduring MIS 3 due to the relative lowstand of sea level at that time and shallowtopography of the Bohai Strait, which separated the sea water from the Yellow Sea tointrude into the Bohai Sea during that period. The previous dating of the marine bed IIneeds to be proven by further dating effort.(4) In the entire Holocnene, the study area was through various stages ofenvironmental development from swamp, tidal flat, shallow sea, prodelta, delta frontto delta plain. The south part of the study area, however, was exposed withoutdepositon in the Early Holocene. Thus, there is unconformity between the Holoceneand the underlying Pleistocene sediments with a time gap of 27ka. Nevertheless,swamps developed in the mid to north parts in the study area by the beginning of theHolocene. The different environments may be caused by undulated palaeotopographyfrom the south to the north coast. Overall, tidal flat developed in the Early Holocenewith about 1m sediment only which was obviously thiner than the accommodationspace provided by the mean high tidal range, if we consider that the palaeotidal rangeis the same as the present one. This thesis speculates that such a rather thin tidal layerexperienced a relatively short time (several hundred to over a thousand years only).Then, the sedimentary environment was transformed into shallow sea with the EarlyMid–Holocene, and then, changed into delta regime with increased sedimentation rateuntil the Late Holocene.(5) The environmental change in the west coast of Bohai Bay reflects the sealevel change in the Holocene. The Holocene marine transgression reached the studyarea at ca. 8.5ka cal BP with relative sea level at 16.7~ 14.7m. During the period of8.5~5.6ka cal BP, sea level rose with a faster rate of about 0.4~0.6cm/a. Possibly, sucha jump was indicated by MWP 1C event?! The relative sea level was 2.5~ 1.8m atca. 6.0 ka cal BP and basically coincident with the eustatic level (the ice-equivalentsea level) caused by the melting ice at the same time.(6) The sedimentation changed with various rates occurred in different periods inthe Holocene responded to different fluvial activities. Such a variety reveals that the sediment supplies impact the sedimentary environment along the west coast of BohaiBay. From the Early Holocene to Early Mid–Holocene, the low sedimentation ratealong the whole west coast presents less impotant fluvial input at that time. Thesediment supplies of Luanhe River were dumped on the north of the west coast ofBohai Bay in the Mid–Holocene, while the Yellow River downloaded its sedimtnes atthe south part of the area in the late Mid–Holocene Finally, during the Late Holocene,both Yellow River and Haihe River emptied to the mid–part of the area.