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

Abstract

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• ) before tilt correction, and D/I = 0.7• /61.7• ) 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, α 95 = 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• ) 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.

New dinosaur tracks from Korea, Ornithopodichnus masanensis ichnogen. et ichnosp. nov. (Jindong Formation, Lower Cretaceous): implications for polarities in ornithopod foot morphology

“…2). Various lines of evidence, including the aforementioned molluscs, ostracods, and palynomorphs suggest that the Hayang Group is Aptian to early Albian in age (Choi, 1985(Choi, ,1989Yi et al, 1994 Ar (Doh and Kim, 1994).…”

Paleomagnetic results of the Late Permian Gobangsan Formation, Korean Peninsula: Remagnetization in the southeastern periphery of the Bagjisan Syncline

“…The stability of the observed paleomagnetic directions for the sites 3 and 6 (Fig. 4a) further ascertains that the secondary magnetization is not attributed to a Recent weathering, as suggested by Doh et al (1999).…”

“…However, it is also considered to be unlikely because the secondary magnetization has been observed from both hematite and magnetite: Doh et al (1999) claimed that a weathering process does not form hematite and magnetite at the same time. The stability of the observed paleomagnetic directions for the sites 3 and 6 (Fig.…”

Paleomagnetism and U-Pb geochronology of the late Cretaceous Chisulryoung Volcanic Formation, Korea: tectonic evolution of the Korean Peninsula

“…However, earlier paleomagnetic studies on Mesozoic to Tertiary rocks from the Korean Peninsula showed vertical rotations with respect to China (e.g., Lee et al 1987;Kim and Van der Voo 1990;Doh and Piper 1994;Zhao et al 1994Zhao et al , 1999Doh et al 1999Doh et al , 2002Lee et al 1999;Zhao et al 1999;Uno 2000;Park et al 2003Park et al , 2005Park et al , 2007Kim et al 2009). Then, viable solutions to explain the various degrees of vertical rotations observed in previous paleomagnetic investigations required that individual geologic terranes of the Korean Peninsula formed in Mesozoic to Tertiary experienced different degrees of vertical rotations.…”

“…A robust reconstruction of Laurasia is possible only when Mesozoic paleogeography of the Korean Peninsula is well established, bridging the two sides of the Chinese cratons (e.g., Lee et al 1987;Kim and Van der Voo 1990;Zhao et al 1994Zhao et al , 1999Doh et al 1999Doh et al , 2002Park et al 2003Park et al , 2005Park et al , 2007Kim et al 2009). The primary goal of paleomagnetism is to uncover information about the history of deformation and vertical rotations in geologic units.…”