Abstract:

Two 80 meters deep boreholes (BT113 and BT114) were carried in the west coast of Bohai Bay in order to clarify the revolution of sedimentary environments. Three marine sedimentary beds were identified based on sediment composition and texture, faunal assemblages (mollusk, benthic foraminifera and ostracoda) as well as AMS ¹⁴C and OSL ages. According to their order, from bottom to top, they are interpreted as Marine Bed 3 (M3) deposited in the Later Mid-Pleistocene, Marine Bed 2 (M2) in the Late Pleistocene and Marine Bed 1 (M1) in the Holocene. These marine beds, from M3 to M1, reveal three major sea level movements since the Late Mid-Pleistocene. M3, M2 and M1, corresponding to marine facies deposited in MIS (marine isotope stage) 7, MIS5, and MIS1, respectively, indicate the repeated highstands during those periods. Therefore, it can be conclude that the development of these marine beds recording the regional sea level trends in the west coast of Bohai Bay, is well accordant with the global change. Different from the previous studies, authors consider that the marine transgression which controlled sea water to intrude into the Bohai Sea did not impact the studied area during MIS3 due to the relative lowstand and shallow topography of Bohai Strait. Contrastive analysis of multi boreholes indicates that the three marine beds are distributed widely and comparable well in the west coast of Bohai Bay. Changeable elevation and thickness of the marine beds within different boreholes imply that there have happened differential depressional movements during the Late Pleistocene period.

Key words: marine sedimentary beds, sea level changes, differential depressions, Late Quaternary, west coast of Bohai Bay, stratigraphy