Abstract
In the northeast Japan arc, a narrow linear-lowland runs along volcanic front. The Kamikita Plain occupies the northern end of this tectonic linear-lowland; and mainly consists of shielding marine-terraces and thick marine-sediments. Using the tephrochronology, the author identifies four marine terraces hereafter referred to as the Fukuromachi surface, Shichihyaku surface, Tengutai surface, and Takadate surface from higher (older) to lower (younger) in this plain. The terrace deposit of Fukuromachi surface is composed of the Ohira Formation and Fukuromachi Formation in ascending order. The Ohira Formation is subdivided into two parts; bay-environment sandy-fill (lower Ohira Formation) and shoreface-environment sandy- to gravelly-layer (upper Ohira Formation). Moreover, the Fukuromachi Formation is subdivided into two parts; bay-environment sandy-fill (lower Fukuromachi Formation) and shoreface-environment sandy-layer (upper Fukuromachi Formation). The above succession of sedimentary environments show that relative sea-level changed from regression to transgression to regression before the construction of Fukuromachi surface. The terrace deposits of Shichihyaku, Tengutai, and Takadate surfaces are defined as the Shichihyaku Formation, Tengutai Formation, and Takadate Formation, respectively. The Shichihyaku Formation is composed only of shoreface-environment sandy-layer; but the Tengutai and Takadate Formations are subdivided into two parts; bay-environment sandy- to muddy-fills (lower Tengutai Formation and lower Takadate Formation, respectively) and shoreface-environment sandy-layers (upper Tengutai Formation and upper Takadate Formation, respectively). Especially, both bay-environment lower-parts bury the drowned valleys. Each sedimentary environment of Tengutai and Takadate Formations therefore show that relative sea-level changed from transgression to regression. The relative transgression of Takadate Formation is correlated with the marine isotope stage 5e high-stand. Therefore, the relative high sea-levels of Tengutai and Fukuromachi Formations are correlated with the ages older than the marine isotope stage 5e, probably the middle Pleistocene.
