Abstract
Newly determined remanent magnetization directions of both tephras and fine-grained clastic sediments are used to redefine the Gauss-Matuyama boundary (GMB) in the Tokai Group of central Japan. Samples collected from 19 sites within a ~70-m-thick sedimentary sequence were subjected to progressive thermal or alternatingfield demagnetization, and the demagnetization data were statistically analyzed to determine site-mean remanent magnetization directions. Demagnetization results and rock magnetic experiments show that most samples contain magnetite as the dominant magnetic carrier and that most samples also contain hematite. In the northern part of the Kameyama area in Mie Prefecture, the GMB lies about 30 m below the Reiho volcanic ash bed. This finding is clearly different from a previous magnetostratigraphic study that determined the Reiho ash bed to be of normal polarity and that placed the GMB above the bed. A recent tephrostratigraphic study proposed that the Reiho ash bed could be correlated with a tephra bed below the GMB in a sedimentary sequence on the Boso Peninsula, more than 300 km to the east of the present study area, but this idea needs to be reconsidered. The 22 site-mean directions, which include those reported from above the Reiho ash bed, pass a reversals test and yield an overall mean direction that provides a reliable paleomagnetic direction for the latest Pliocene to earliest Pleistocene (around 2.6 Ma).
The mean direction is marked by a slight easterly deflection of declination from the north, and this raises the possibility that during the Quaternary, a small but significant clockwise rotation (7.4 ± 4.6°) occurred in the study area with respect to Earth’s rotational axis.
