Paleomagnetism of Triassic bedded chert from Japan for determining the age of an impact ejecta layer deposited on peri-equatorial latitudes of the paleo-Pacific Ocean: A preliminary analysis

Abstract

Bedded chert samples from the Norian (Upper Triassic) Sakahogi section of the Mino Terrane in the Inuyama area, southwest Japan, have been collected for paleomagnetic study in order to determine the age of an impact ejecta layer interbedded within it and the paleolatitude of its deposition. Thermal demagnetization of the bedded chert samples revealed four distinct remanent magnetization components. The last demagnetized component with both polarities is interpreted to be the primary magnetization, which produced the first magnetostratigraphic data of the middle to upper Norian from Panthalassa, consistent with Tethyan magnetostratigraphic sections. Tie points were derived from paleomagnetic and paleontological data, from which the stratigraphic position of the ejecta layer was compared with the astronomically tuned geomagnetic polarity time scale (APTS). The age of ejecta layer is estimated to be about 212Ma. This magnetostratigraphic age is consistent with the radiometric age of the Manicouagan impact that formed the second largest known crater in the Phanerozoic at Quebec, Canada. The results of this study, as well as previous studies, suggest that the chert section, which includes the ejecta layer, was deposited within a paleolatitude range of 0.3–14.4°N. This indicates that the material ejected by the Manicouagan impact event reached near the equatorial region of the paleo-Pacific Ocean.