Magnetostratigraphy Research Of ZK4 In The Beiqijia Area Of Beijing Plain

Magnetostratigraphy is an important method for classifying and correlating sedimentary stratigraphy, as well as for establishing the time frame of sedimentary sequences, and it has played a very important role in Quaternary geochronology. The Quaternary sediments are widely distributed in the Beijing area. To investigate and monitor active faults in the Beijing Plain, it is necessary to date the Quaternary sedimentary sequence covering the basement. Based on a detailed paleomagnetic and rock-magnetic study, a magnetostratigraphic time frame has been constructed for the ZK4 core drilled in the town of Beiqijia, Beijing. The main results are summarized as follows:(1) Magnetic susceptibility anisotropy(AMS) results show that the maximum and intermediate inclinations of the AMS axes are nearly horizontal and the minimum inclinations are nearly vertical, which, combined with that plots of shape parameter(T) versus anisotropy degree(P) and lineation(L) versus foliation(F) show a dominance of oblate forms, confirms that the studied sediments still preserve a primary sedimentary fabric.(2) Isothermal remanent magnetization(IRM) curves and thermomagnetic analyses indicate that characteristic remanent magnetization is mainly carried by magnetite and hematite.(3) Stepwise thermal demagnetization shows that two well-defined magnetization components can be isolated from most samples. A low-temperature component isolated between 100oC and 200-250oC presents a downward inclination, and a high temperature component decaying toward the origin can be isolated between 200-250oC and 680-690oC and includes downward and upward inclinations.(4) A magnetostratigraphic sequence has been constructed for the ZK4. It confirms that the Matuyama-Brunhes(M-B) and Gauss-Matuyama(G-M) boundaries occur at the depth of 170.8 m and 550.6m, respectively, and the upper(O-M) and lower(M-O) boundaries of the Olduvai subchronozone are located at the depth of 384.5 m and 420.4 m, respectively. Combined with deposition rate, this new magnetostratigraphic result dates the ZK4 bottom age at 2.94 Ma.