A Proposal to Focalize on the Cenozoic Deposits along the Sanriku Coast, the Pacific Side of Northeast Japan

Abstract

Field works and the bibliographical studies on the coastal geomorphology along the southern Sanriku Coast need to re-examine the Pleistocene formations including terrace deposits hitherto so estimated.
Near Kesennuma City, Miyagi Prefecture, it has been said that an unconformity separates the Senganda Formation (the uppermost Pliocene) from the Matsuzaki Formation (the lower Pleistocene). The author's examination suggests that the two are transitional, although partial and slight breaks of sedimentation exist between them as well as within the Matsuzaki Formation itself.
It has also been said that the Matsuzaki Formation distributes from near the present sea level to about 100m a. s. l., and that coastal terraces of various heights cover the same altitudinal extent. The morpho-stratigraphical relation between the two formations and the terraces has not been strictly elucidated.
Along the southern Sanriku Coast, such younger deposits develop widely but sporadically. The area is chiefly composed of the Mesozoic and the Paleozoic strata, then the Cenozoic research on the area has not been intensely done.
Detailed studies on such deposits in the southern Sanriku and its comparative studies with the landforms and the Cenozoic geology of the northern Sanriku seem to be useful for the interpretation of terraces, ria topography, and the identity of younger deposits of the southern Sanriku as well as of the whole Sanriku.
Based on the field data and published ones, the author believes that the equid fossils used to determine the age (as the Würm glacial age) of such younger deposits in the Ofunato Bay district, to the north of Kesennuma, were included in the secondary material derived from the deposits, and that accordingly the fossils there do not indicate the true age of such younger deposits.