Magnetostratigraphy of drill hole G2 in the Tianjin coastal area and its tectonic significance

It is of great importance to establish reliable chronologic framework in Binhai area for understanding the late Cenozoic depositional environment change, neotectonic movement and evolution history of paleoenvironment. In this paper, the authors present the magneto-stratigraphic investigation result in drill hole G2 (1226m), which is the longest drill hole in the Tianjin coastal area. On such a basis, the chronostratigraphic subdivision and its tectonic significance were also discussed. The results show that the core of this drill hole began deposition at about 8.5Ma BP. The strata of 1226~658m may be correlated to late Miocene and assigned to upper member of Guantao Formation, the strata of 658~303m belong to Pliocene Minghuazhen Formation, and the upper strata of more than 300m belong to Quaternary, including Mapengkou, Tonglou, Tanggu and Tianjin formations. The basal lower boundary of Holocene is identified at about 19.4m by 14C dating. Given the chronostratigraphic scale, two stages of relatively high sedimentation rate (>200m/Ma) can be revealed from drill hole G2, i.e., 8.5~6.43Ma and 3.58~3.03Ma, respectively. Both epochs have good comparability on the time scale with extension and uplift of the Tibetan Plateau. It is suggested that the uplift of plateau might also have had a significant impact on sedimentary and tectonic movement of the North China plain.