Magnetostratigraphy of two deep boreholes in southwestern Bohai Bay: Tectonic implications and constraints on the ages of volcanic layers

Abstract The sediments of Bohai Bay Basin have recorded the processes of basin infilling and structural evolution, reflecting the destruction of the North China Craton during the late Mesozoic and the early Cenozoic. However, the absence of a reliable chronostratigraphic framework for the sedimentary sequences in the basin has precluded a comprehensive understanding of these processes. This is especially the case for southwestern Bohai Bay, the evolution of which has been affected by two Quaternary volcanoes, Dashan and Xiaoshan. In this study, we combine paleomagnetic and sedimentary analyses of the sediments from two boreholes (CK3 and G4) from southwestern Bohai Bay Basin in order to provide new insights into the sedimentary history and regional tectonic processes since the Pliocene. The main findings are as follows. (1) Magnetite and hematite are the main carriers of the characteristic remanent magnetization in these sedimentary sequences, with hematite the main component. (2) The two drill cores record the Brunhes, Olduvai and Gauss normal chrons, and the Matuyama and Gilbert reverse chrons. (3) Subsidence-related differences in the depths of the Matuyama/Brunhes (M/B) and Gauss/Matuyama (G/M) boundaries in boreholes across Bohai Bay, distributed within different structural units, reflect the fact that northern Bohai Bay is the regional subsidence center and that the development of the WNW-orientated structures of the basin has intensified during the Quaternary. (4) The trend of changes in the elevation of three marine layers is similar to that of the M/B and G/M boundaries across Bohai Bay. Based on magnetostratigraphic constraints of the boreholes, the ages of the second and third marine layers are assigned to Marine Oxygen Isotope (MIS) 3 and MIS 5, respectively. The WNW-orientated structures and ‘super interglacials’ resulted in the formation of transgressions in the Huanghua depression. (5) In borehole CK3, the ages of four volcanic layers at the depth intervals of 13.14–16.18 m, 33.37–48.02 m, 145.8–154.05 m and 222.16–233.80 m, are 10–18 ka BP, 80–90 ka BP, 2.2 Ma and 3.1 Ma, respectively. In borehole G4, the age of the volcanic layer at the depth of 46.4–55.8 m is 0.33–0.55 Ma. Comparative analysis of subsidence-related differences between the NEE and NE-orientated uplifts and depressions in western and southern Bohai Bay indicates that structural evolution in the vertical plane during the Quaternary was mainly controlled by the subduction of the Pacific Plate.