충남탄전에 분포하는 대동층군에 대한 고지자기학적 연구

Paleomagnetic study of the Daedong group in the Choongnam coal field

Paleomagnetic study on the sedimentary rocks in the Choongnam Coal Field has been carried out to determine the direction of declination and inclination of NRM and position of paleomagnetic pole, and to investigate the geotectonism and geomagnetic stratigraphy of the sedimentary rocks in the Daedong Group. As a result of paleomagnetic study, the study area can be divided tectonically into two blocks by Baegunsa fault, namely northwestern and southeastern blocks. Site mean declination and inclination of Baegunsa and Seoungjuri Formations in the northwestern block are 23.2° and 54.9°, respectively. Those of Amisan, Jogyeri, Baegunsa and Seoungjuri Formations in the southeastern block show normal direction with declination and inclination of -22.1° and 11.2°, and reversed direction with those of 158.5° and -12.6°, respectively. Average paleomagnetic pole position in the northwestern block is located at 212.9°E and 71.1°N, and that in the southeastern block at 345.7°E and 53.3°N. This difference suggests relative rotation of about 45° between two blocks. The paleolatitude of Daedong Group at the time of sedimentation is 5.6°N much lower than present latitude of 37.7°N. Compared with worldwide Mesozoic paleomagnetic polarity stratigraphy, Amisan Formation is correlated with the lower boundary of Nuanetsi reversal zone in Graham interval, and Baegunsa and Seoungjuri Formations are correlated with just upper part of the upper boundary of Nuanetsi reversal zone, and their geologic ages are Late Triassic to Early Jurassic. The position of paleomagnetic pole acquired from Daedong Group in the study area is different from those in other places. This may be attributed to the different tectonic movement by Daebo Orogeny occurred after the deposition of Daedong Group.