Paleomagnetism of red cherts: A case study in the Inuyama Area, central Japan

This study is an attempt to isolate and identify the primary magnetization in red cherts. Red chert samples from a middle Triassic sequence in the Inuyama area (35.4°N, 137.0°E), central Japan have a multicomponent magnetization, which is delineated by thermal demagnetization. Two of the components have high coercivity and high blocking temperatures; one is carried by hematite and the other by magnetite. The former was concluded to be the primary magnetization for two reasons: (1) it predates intraformational folding, and (2) it records geomagnetic reversals. The other component, which accounts for the dominant part of natural remanent magnetization, was probably acquired at a time long after the deposition. Two paleomagnetic results were drawn from the Middle Triassic red chert sequence: (1) The mean inclination of the samples in the sequence was 1.4° ± 6.8°; therefore the paleolatitude amounts to 0.7° ± 3.4°, in contrast to the paleolatitude expected from the paleopole of Eurasian continent, which is about 70°, and (2) the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the Middle Triassic are considerably longer (more than several million years) than the average of those in the Cenozoic.