HR: 1340h
AN: GP33A-0097    [Abstracts]
TI: A Paleomagnetic Study of the Late Cretaceous Sedimentary and Volcanic Rocks in the Tongri Basin, Korea: Origin of Magnetization and its Implications
AU: * Kim, S
EM: sjkim98@korea.ac.kr
AF: Korea University, Dept. of Earth and Environmental Sciences, Seoul, 136-713 Korea, Republic of
AU: Doh, S
EM: sjdoh@korea.ac.kr
AF: Korea University, Dept. of Earth and Environmental Sciences, Seoul, 136-713 Korea, Republic of
AU: Park, Y
EM: aegis@korea.ac.kr
AF: Korea University, Dept. of Earth and Environmental Sciences, Seoul, 136-713 Korea, Republic of
AU: Kim, W
EM: wmkim@korea.ac.kr
AF: Korea University, Dept. of Earth and Environmental Sciences, Seoul, 136-713 Korea, Republic of
AB: Tongri Basin has been traditionally regarded as the Cretaceous basin which consists of the late Early Cretaceous sedimentary rocks (i.e., lower unit) and the Late Cretaceous volcanic rocks (i.e., upper unit) based on the correlation with the lithology of the Cretaceous Gyeongsang Basin. However, several authors have recently argued that the age of development of the Tongri Basin was extended from the Late Cretaceous to the Early Tertiary on the basis of the isotopic age dating data of volcanic rocks. The original aim of this study was to provide paleomagnetic constraints for the age of the Tongri Basin. A total of 297 samples were collected from 25 sites of sedimentary rocks (red beds and tuffaceous sandstone) and from four layers of volcanic rocks (rhyolite). The characteristic remanent magnetization (ChRM) directions of volcanic rocks pass a regional fold test, while those of sedimentary rocks show a maximum clustering at 40% untilting. These results indicate that the volcanic rocks have retained a primary magnetization whereas the sedimentary rocks acquired a syn-tectonic magnetization during tilting of the strata. Paleomagnetic poles calculated from the primary component of volcanic rocks and from the mean directions at 40% untilting of sedimentary rocks are compared with previously reported paleopoles of the Korean Peninsula since the Cretaceous. The paleomagnetic pole position (85.7°N, 221.7°E, A₉₅=3.3°) of the sedimentary rocks is statistically different from the Cretaceous paleopoles but it is close to the Miocene paleopole. On the other hand, the paleomagnetic pole position (68.2°N, 230.9°E, dp/dm=3.0°/4.5°) of the volcanic rocks is close to the Late Cretaceous paleopole. It can be interpreted that the ChRM of the sedimentary rocks is a remagnetized component acquired during the Tertiary times, independent of the Late Cretaceous~Early Tertiary extrusion of the volcanic rocks, because the paleopoles of the sedimentary and volcanic rocks are statistically different from each other. Therefore, the thermo-viscous remanent magnetization can be excluded from the process of remagnetization of the sedimentary rocks. It is reported that the sedimentary rocks in the Tongri Basin have not been buried deeply (e.g., less than tens of metres) and, therefore, the period for lithification of sediments might be very long (e.g., more than several million years). It is possible that the sedimentary rocks might remain unlithified at the time of acquisition of the ChRM. We assume that the selective Tertiary remagnetization of the sedimentary rocks was associated with either a fluid-mediated chemical process due to much more pore space of sedimentary rocks relative to rhyolites, or a re-arrangement of magnetic minerals due to influx of fluids into unlithified sediments. The test result for these possibilities will be presented based on the optical/electron microscope observations, rock magnetic studies and sedimentary petrology.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1533 Remagnetization
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005