New Quaternary magnetostratigraphic results from Beijing, China, and their urban geological implications

A new magnetostratigraphic investigation has been carried out on the terrestrial sedimentary sequences from three boreholes ZK11-2, ZK12-1 and ZK12-2, which were drilled from the alluvial-proluvial fan of Chaobai River, Beijing, China. Seismic data showed that two parallel normal faults may lie beneath. But good dating data are still unavailable to constrain the activity time of both faults till now. So the magnetostratigraphic data were achieved in this study. ZK11-2 and ZK12-1 cores are both 299 m long and ZK12-2 core is 719 m long. Paleomagnetic samples were collected at a 1 m interval for ZK11-2 and ZK12-1 cores and a 0.5 m interval for ZK12-2 core. The samples were cut to cubic specimens by a stainless saw and then were worn to 8 cm3 cube using sandpapers. The K-T curves indicate that the main magnetic grains of the three cores are magnetite. Stepwise thermal demagnetization on 885 specimens was finished using ASC TD-48 furnace in 40-100°C steps up to 580°C. All remanence measurements were finished on the 2G 755-4K superconducting magnetometer in Paleomagnetism and Environmental Magnetism Laboratory, China University of Geosciences (Beijing). All the instruments are placed in the magnetic shield room with a low magnetic field less than ~300 nT. In the vector component diagrams, a low-stability component exists between NRM to 100°C in many samples, which may be the viscous remanence overprinted by the present geomagnetic field. Most specimens could isolate a stable component that decayed toward the origin after being heated above 400°C. This is considered as the characteristic remanent magnetization (ChRM). Based on the results of the ChRM, the Brunhes/Matsuyama (B/M) and Matsuyama/Guass (M/G) boundaries, the Jaramillo subchron and the boundaries of the Olduvai subchron were determined. The B/M boundary is at the depth of 93 m for ZK11-2 core, 118 m for ZK12-1 core and 170 m for ZK12-2 core. The M/G boundary is at the depth of 232 m for ZK12-1 core and 710 m for ZK12-2 core. The Jaramillo subchron is at the depth of ~105 m for ZK11-2 core, ~138 m for ZK12-1 core and ~205 m for ZK12-2 core. The Olduvai subchron is at the interval of 179-200 m for ZK11-2 core, 174-182 m for ZK12-1 core and 466-521 m for ZK12-2 core. On the basis of the B/M, M/G boundaries and the Jaramillo and Olduwai subchrons, successive magnetostratigraphic sequences were established for each borehole. The estimated sedimentary accumulating rate was different between the three boreholes. The sedimentary accumulating rate of ZK12-1 was slowest between 1.95 Ma and 0.78 Ma; while it surpassed that of ZK11-2 during the last 0.78 Ma. This suggested that there might be two normal faults. One is located between the boreholes of ZK11-2 and ZK12-1; the other is situated between the boreholes of ZK12-1 and ZK12-2. Both the faults have developed since 1.95 Ma ago. But the one between the boreholes of ZK11-2 and ZK12-1 has transformed into a reverse fault since ~0.78 Ma.