Abstract
The Miocene Kanomatazawa Formation consists of lower and upper parts. The lower part is mainly composed of volcaniclastic sediments which were formed by the growth of submarine andesitic to dacitic volcano. The upper part is bedded sandstone to siltstone including many marine molluscan fossils (the Shiobara-type fauna) and foraminifera. Zircon fission-track ages of the pumice-lapilli tuff (the uppermost bed of the lower part) are 11.9±0.7 (ED2) and 10.4 ±0.3 Ma (ED1). Considering the geological conditions and experimental reliability, the age of 10.4±0.3 Ma (ED1) is assigned to this sample. The planktonic foraminifera in the upper part indicate the age of the lower to middle part of Zone N.16.
Although the Shiobara-type fauna is known as a cool temperate fossil assemblage, the effect of warm current can be recognized in the upper part of the Kanomatazawa Formation according to the study of foraminifera assemblage. The Shiobara-type fauna in this area occurs under the intertidal part of a marine bay to shallow open marine condition and is prosperous after 10 to 10.5 Ma.
The relationship between the Kanomatazawa and underlying Fukuwata Formations has not been determined to date. However, based on the fission-track age of this study and the age of corresponding formation to the Fukuwata Formation, it is proposed that the relationship of these formations should be an unconformity.
The lithofacies difference between the lower and the upper parts of the Kanomatazawa Formation implies the environmental change related to the regression and ensuing transgression. Continuous activity of submarine volcano should be assigned to one of the reason of regression. Considering the same situation of the paleoenvironmental change in adjacent area at the almost equivalent age, the transgression may reflect relative sea-level change.
