Stratigraphy of the Volcanic Ash Deposits in the Vicinity of Lake Nojiri, Nagano Prefecture

Abstract

The volcanic ash deposits distributed in the vicinity of Lake Nojiri can be classified into three units; the Older, the Middle and the Younger volcanic ash. The nearly 10 meters thick Older volcanic ash consists of brown to reddish scoria beds. Characteristically the about 0.6-1.5 meters thick beds of hornblende andesite pumice occur in the lower and upper parts of the Older volcanic ash. These beds are good guides.
The Older volcanic ash interfingers with the main part of the Toyono formation which was deposited in the middle Pleistosene lacustrine basin to which the writers adapted the name “the Toyono Lake”. The surface of the Hanami plane I, from 900 to 1000 meters above sea-level, may be accepted as the deposition surface of the Older volcanic ash. This was subjected to up-warping followed with subaerial denudation and the formation of the Hanami plane II, from 700-800 meters above sea-level, around Lake Nojiri. Although the Hanami plane I' about 600-800 meters above sea-level, is separated by the flexure scarp from the Hanami plane I, it is thought that it is a correlative of the latter.
The nearly 2 meters thick Middle volcanic ash is distributed only on the Hanami planes and is superposed upon the Older volcanic ash with little unconformity. A bed of yellowish, pebble size hornblende pumice occurs at its basal part. It attains about 1 meter in thickness and an excellent stratigraphic marker.
The Younger volcanic ash covers several planes in different hights is classified into three parts, each of which is separated by an unconformity, and designated as I, II and III in upwards sequence. From the horizon II of the Younger volcanic ash, during the excavation of the bottom of Lake Nojiri, the remains of Paleoloxodon namadicus naumanni and Megaceros (Shimomegaceroides) yabei were collected, and the ¹⁴C dates from 16.150±520 to 21.600±900 B.P. years are obtained from two drift woods found together with elephant teeth and bones remains.