北上山地北縁の新生界-1-

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概要

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• Cenozoic formations, found along the northern border of the Kitakami Mountains, are marginal deposits of the so-called Uétsu geosyncline. A detailed study of stratigraphy and geologic structure of Neogene Shiratorigawa group which is the lower portion of the above Cenozoic formations, are reported.<BR>1) The Shiratorigawa group overlies the basement Paleozoic and plutonic rocks with a remarkable clino-unconformity, and is divided into the following three formations in descending order:<BR>Suénomatuyama formation (100-150 m) : mostly composed of tuffaceous sandstone, with two-pyroxene andesites and their pyroclastics in the upper horizon. Pectinid and other molluscan remains are abundant.<BR>Kadonosawa formation (50-100) : mainly composed of siltstone accompanied by thin conglomerate and sandstone beds in its basal horizon. The molluscan remains are abundant in its upper and basal horizons.<BR>Yotuyaku formation (150-250 m) : chiefly composed of siltstone and sandstone with lenticular conglomerate beds. An intercalation of the andesitic tuff breccia is found in its middle horizon. Plant and molluscan remains are present.<BR>These three formations are subdivided into several members.<BR>2) The basement rocks are exposed in two masses, at the eastern margin and the southwestern corner of this district, and the depression between the two masses is filled up by the Neogene formations. Geologic structure of this district is best considered as consisting of the following three subdivisions :<BR>1. Southern area or the Namiuchi-toge syncline ; the axis of the syncline dips to the northwest direction.<BR>2. Central area or the Maisawa basin structure ; a semicircular shallow basin with a westward opening.<BR>3. Northern area ; gentle monoclinal structure, dipping in west to northwest direction.<BR>In addition to the areal structures mentioned above, weak undulations and small faultings are observable. In general, structural trend of N 30°W in the Neogene formations prevails in this district. This is due to the structural trends of the basement rocks.<BR>3) Shapes of the sedimentary bodies of each formation or member are illustrated in the isopachous contours.<BR>4) The relationship between the Kadonosawa and the Suénomatuyama formations is conformable in general. A partial unconformity, however, can be recognized between the two. Near the eastern margin of the basin, crests of the undulation of the Kadonosawa formation are eroded out and covered by the basal conglomerate of the Suénomatuyama formation with unconformity.<BR>5) The Nisatai dacite extruded probably during or after the deposition of the upper members of the Suénomatuyama formation.

• 社団法人 東京地学協会の論文

著者

• 鎮西 清高

  東京大学理学部地質学教室

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