新潟県下のローム層について そのI

The Loam formations, weathered volcanic ash deposits, is ditributed on the terraces and the hills along the River Shinano-gawa from Tsunan to Nagaoka in Niigata prefecture, central Japan. The formations have been subdivided into three formations according to the respective terraces that are overlain by each formations, namely Taniage, Maibara, and Kaisaka Loam formations in ascending order. The present authers propose here to name these formations comprehensively as the Shinano-gawa Loam formations. They have surveyed these formations, described it in detail, traced the intercalated nineteen key beds widely to the adjacent area, analyzed the mineral assemblages contained, and estimated roughly origin of materials of several key beds. The results are as follows: 1) The Taniage Loam formation is subdivided into three parts. The lower part is brown colored clayey loam that contains grains of biotite and high quartz, and are intercalated with three key beds of scoria and pumice. The middle part is brown colored viscid and clayey loam with two key beds. The upper part consists of yellowish brown colored clayey loam sporadically including orange colored scoria, and are intercalated with four key beds. The Maibara Loam formation is subdivided into two parts. The lower part is brown colored viscid and clayey loam and are intercalated with two key beds. The upper part is brown colored clayey loam that stippled with scoria and are intercalated with three key beds. The Kaisaka Loam formation with intercalations of three key beds is brown colored soft loam with fragments of volcanic glass in the upper part. Some archaeological remains have been found in the upper half of the formation. 2) On the bases of several key beds and contained mineral assemblages, the Kaisaka Loam formation is correlated with the Nojiri Loam formation in the northern part of Nagano prefecture, likewise, the Maibara and Kaisaka Loam formations are correlated with the Kubiki Loam formations in the southwestern part of Niigata prefecture. 3) Isopach maps and mineral assemblages of some key beds show the following facts which suggest the origins of main components of the Shinano-gawa Loam formations: Volcanic ejecta inferred to be derived from the Iiji and, Iizuna or Kurohime volcanos are found in the lower and upper part of the Taniage Loam formation, respectively. The thicknesses of some scoria beds in the lower part of Maibara Loam formation tend to increase toward the Kurohime volcano. In the upper part of the Maibara Loam formation and the lower part of the Kaisaka Loam formation, volcanic ejecta inferred to be derived from both the Kurohime and Myoko volcanos are recognized, respectively, the latter ejecta are also found in the upper part of Kaisaka Loam formation.