10:45 AM - 11:00 AM
[SEM17-07] Magneto-biostratigraphy of the Upper Triassic to Lower Jurassic bedded chert succession from Inuyama area, central Japan: an improved method for measurement of red chert
Keywords:magnetostratigraphy, Triassic, Mino Belt, chert, conodont, radiolarian
In total, 477 oriented hand samples (red cherts) were collected through the bedded chert succession at average sampling interval of ~30 cm: 42 horizons from the R section, 244 horizons from the Q section and 191 horizons from the H section, Each hand sample was cored and cut into cylindrical specimens (25 mm in diameter, 22 mm in length). To remove magnetic particles attached the surface of specimens, all specimens were polished with sandpaper and washed in a dilute hydrochloric acid. During demagnetization and measurement, we used a magnetic shielding box, a conductive glove and slipper to avoid the problem of VRM acquisition.
Magnetostratigraphic results delineate 20 substantive normal and reverse magnetozones, defined by measurement of 357 samples. Thermal demagnetization of a three-component IRM shows that the low-coercivity magnetization is unblocked at 580 °C and that the high-coercivity magnetization is demagnetized by 690 °C. Although the magnetostratigraphic data in the vicinity of the early-late Carnian boundary is the first record from the marine section, the magnetostratigraphy of samples in Carnian-Rhaetian interval was well correlatable with that of Tethyan marine sections. This correlation implies that the bedded chert of Inuyama area was deposited in the Northern Hemisphere, assuming that the rocks in the Tethyan marine sections were deposited in the Northern Hemisphere. The mean inclination of the last demagnetized component (primary remanent magnetization) suggests the bedded chert originated in an equatorial area through the Late Triassic. The intercalibrated conodont–radiolarian biostratigraphy and magnetostratigraphy from the studied sections accurately calibrates the radiolarian zones in Japan with standard chronostratigraphic stages and substages.