Paleomagnetic and rock magnetic studies of Cretaceous rocks in the Eumsungbasin, Korea

Paleomagnetic results are obtained from 41 sites from the Chopyeong Formati on within the Eumsung basin, located along the northern boundary of the Ogc heon Belt, Korea. The Chopyeong Formation, deposited in early Cretaceous, y ields the mean direction of D/I = 347.8 degrees/57.3 degrees (k = 92.8, alp ha(95) = 2.5 degrees) before tilt correction, and D/I = 0.7 degrees/61.7 de grees (k = 19.6, alpha(95) = 5.5 degrees) after tilt correction. The parame ter estimating fold test and the stepwise unfolding test of the red bed and greenish mudstone of the Chopyeong Formation yield the maximum value of k at 21.9% and at 20% untilting, respectively, indicating that the remanence whose mean direction of D/I = 350.8 degrees/57.9 degrees (k = 177.9, alpha( 95) = 1.8 degrees) at 20% untilting was acquired during or after tilting of the strata. The comparison of the paleomagnetic pole from the Chopyeong Fo rmation with those from the Youngdong basin and the Euiseong area in the Gy eongsang basin indicates that the remanence was acquired during late Cretac eous to early Tertiary. Electron microscope observations and rock magnetic experiments show that secondary hematite and magnetite grains of single dom ain to pseudo-single domain size were authigenically formed under the influ ence of fluids presumably triggered by the igneous activities, thus confirm the chemical remagnetization. It is revealed that the age of the granite in the east is Jurassic because the mean direction of the east granite (D/I = 347.0 degrees/47.7 degrees, k = 40.2, alpha(95) = 3.6 degrees) is similar to the Jurassic direction of K orea Peninsula. The age of the granite in the west, however, is left undete rmined whether it is Cretaceous or Jurassic because of the weak intensity a nd instability of the remanence of the granite during demagnetization treat ments.