Pleistocene Tephras (“Loams”) at the Foot of Ashitaka Volcano, Central Japan, with Special Reference to Tokyo-Nagoya Express Way (1969)

At a prospecting stage (1966-1967) preceding the construction of Tokyo-Nagoya Express Way, peculiarities in soil mechanical property of Pleistocene tephras at the foot of Ashitaka Volcano were noticed by some engineers who appealed the necessity for cooperative research of the tephras to members convering several fields of allied science, such as soil science, paleopedology, soil mechanics, clay science, stratigraphy, georgaphy, and archaeology. A research group was soon organized and began its activity. Results obtained are to be published. First, the present article was prepared to deal with stratigraphy of the tephras.The Pleistocene tephras are called here Ashitakea “Loam” Formation, the term “Loam” having been traditionally meant by Pleistocene tephra among Japanese geologists. The formation is divided into three, Upper, Middle, and Lower Loam Members. Upper Loam, 3-6m thick, is characterized by frequent alternation of scoriaceous zones and black bands (buried humose soil horizons). Middle Loam, 4-6m thick, may be distinguished from others by its massive, scoriaceous appearance and by Mishima Pumice Bed interacalated at the middle part. Lower Loam, 3-10m thick, is greyish yellow and clayey in fresh profile but on desiccation it bears a tint of purplish pale grey and numerous cracks develop. Hakone Younger Pumice Flow intervenes, in the upper part of Lower Loam. It is very thin (only a few decimeters thick) but can be traced more than 15km long at a level 50m higher than its base at the site of main deposition. Lower Loam thickens several meters abruptly at middle part of the surveyed area, being 3-4m thick on the west side and about 10m thick on the east side. The transition of thickness is possibly an abut of the member to the basement, tuff breccia (or mud flow) of Ashitaka Volcano.Using Mishima Pumice Bed and Hakone Pumice Flow, as tracers, both erupted from Hokone Volcano eastward of Ashitaka Volcano, Upper, Middle Loams, and the upper part of Lower Loam of Ashitaka Loam Formation are correlated, respectively, to Brown Scoriaceous Loam, CC1 Loam, and to Hakone Young Pumice Flow at Hakone Volcano. At the best of our knowledge on the stratisraphic relation between Kanto Loam Formation and the loams at Hakone Volcano, Upper Loam corresponds with Tachikwawa Loam, while Middle Loam and the upper part of Lower Loam are settled on Musashino Loam. Yasumiba Bed, the uppermost layer of Upper Loam includes microliths and measurement of 14C age of charcoal contained dates the bed 14, 300 years B. P. (Sugihara et al., 1965). Because of an abundance of volcanic glass shard a horizon between first and second scoriaceous zones in the upper part of Upper Loam may be correlated to the upper dark zone, 17, 000 years old, of Tachikawa Loam in the suburbs of Tokyo (Matsui et al., 1968).Sources of tephras of Ashitaka Loam Formation are not yet clear. However, from heavy mineral associations of sand fractions, it is assumed that lower part of Upper Loam and Middle Loam originated from Kofuji or older volcanoes and that the upper half of Upper Loam has a common derivation with Younger Loam in Ina Valley, southern Nagano Prefecture, possibly from one or more volcanoes northwestward thereof.