Quaternary Geology and Geomorphology of Sendai and its Environs (2)

Abstract

Three problems are discussed in this paper, they are (1) the double flexured structure of the Aobayama and lower terrace plains; (2) the pollen analysis of the Pleistocene peat layers; (3) the heavy mineral composition of the Pleistocene volcanic ash layers.The Aobayama Plain which is a depositional surface of the Pleistocene Aobayama Formation underlying with unconformity the Medeshima Volcanic Ash (Fig. 2) is subdivided into four berms (Fig. 1). The boundaries between them run in NW-SE direction and parallel to the Kagitori Line which is an axis of flexure of the Tertiary system of the same area. The Aobayama Formation is flexured on these boundary lines as the superficial topograpyh. The berms of the Aobayama Plain are flexured by another axis with NE-SW trend. The second flexure has one syncline and two anticlines which grade into the thrust fault in the eastern part of the Aobayama Plain. The axes of the syncline and anticlines extend northeastwards into Sendai City where the lower terraced plains are gently undulated by the same axes.Pollen analysis have been made of five peat layers, which occur in different terrace deposits, in the environs of Sendai. The pollen floras of the peat of the Aobayama Formation and of the Upper Sendai Terrace deposits show that the forests were influenced by a temperate condition rather cooler than that of the Dainohara Terrace deposits (Fig. 3).Heavy mineral analysis have been made of four volcanic ashlayers. The heavy minerals of the Koeji and Medeshima Volcanic Ashes are mainly composed of magnetite and hornblende with abundant hypersthene and few augite and the Medeshima Volcanic Ash comprises zircon in addition to them. The Hirasawa Volcanic Ash is characterized and distinguished from the others by the proportion of augite. The heavy minerals of the Nagano Volcanic Ash are composed largely of magnetite and hypersthene (Fig. 4).