The Mid-Pleistocene Marker-tephras in the Southern Central Parts of Miyagl Prefecture, Northeastern Japan

Abstract

This paper describes the characteristics of the marker-tephras in the tephra sequence overlain by the Medeshima Pumice layer and the relationships between the markers and the geomorphic surfaces in the area to the south of Sendai.
Five marker-tephras are found in the tephra sequence overlain by the Medeshima Pumice. They are named Tsubonuma-1 Pumice layer (TbP 1), Tsubonuma Lithicfragments layer (Tblf), Tsubonuma-2 Pumice layer (TbP 2), Tsubonuma-3 Pumice layer (TbP 3), and Tsubonuma-4 Pumice layer (TbP 4), in ascending order. The marker-tephras are identified by both chemical analysis of ferromagnetic minerals and X-ray diffraction data. In chemical composition of ferromagnetic minerals, TbP 1 and Tblf are of andesitic to dacitic, and TbP 2 is of rhyolitic, while TbP 3 and TbP 4 are of dacitic.
Both the isopack maps and petrographic characteristics indicate that source volcanoes of TbP 1 and Tblf are inferred to be around the Aoso and the South Zao volcanoes, while those of TbP 2 and TbP4 to be in the areas extending from the north Zao to the Funagata Ranges. Moreover the relationships between the marker-tephras and geomorphic surfaces make clear that the Aobayama Surfaces are divided at least into the following two groups of different ages: the Aobayama I and II surfaces and the Aobayama III and IV surfaces, and that the hill-top surfaces of the Motoisago Hills and the Medeshima Hills are older than the Aobayama Surfaces.
The five marker-tephras overlying the surfaces as above are considered to be of mid-Pleistocene age.