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

Paleomagnetic results are obtained from 41 sites from the Chopyeong Formation within the Eumsung basin, located along the northern boundary of the Ogcheon Belt, Korea. The Chopyeong Formation, deposited in early Cretaceous, yields the mean direction of D/I = 347.8°/57.3° (k = 92.8, α₉₅ = 2.5°) before tilt correction, and D/I = 0.7°/61.7° (k = 19.6, α₉₅ = 5.5°) after tilt correction. The parameter 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°/57.9° (k = 177.9, α⁹⁵ = 1.8°) at 20% untilting was acquired during or after tilting of the strata. The comparison of the paleomagnetic pole from the Chopyeong Formation with those from the Youngdong basin and the Euiseong area in the Gyeongsang basin indicates that the remanence was acquired during late Cretaceous to early Tertiary. Electron microscope observations and rock magnetic experiments show that secondary hematite and magnetite grains of single domain to pseudo-single domain size were authigenically formed under the influence 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°/47.7°, k = 40.2, α⁹⁵ = 3.6°) is similar to the Jurassic direction of Korea Peninsula. The age of the granite in the west, however, is left undetermined whether it is Cretaceous or Jurassic because of the weak intensity and instability of the remanence of the granite during demagnetization treatments.