2003 Volume 54 Issue 9-10 Pages 341-350
Marker tephras TB-8 and Ky3 of Middle Pleistocene age are redescribed in this paper. TB-8 and Ky3 are conspicuous key beds indicating the middle part of the Tama Loam and the lower part of the Kiyokawa Formation of the Shimosa Group, respectively. These tephras have been thought to be correlated with each other as they both include orthopyroxene crystal grains exhibiting high refractive indices. However, it has been also pointed out that they are largely different in the range of the refractive indices. Detailed observation of these tephras revealed that TB-8 is composed of six fall units which are different with each other in the refractive indices of orthopyroxene crystal grains. Ky3 is considered to be a mixed layer of some units of TB-8. Moreover, marker tephras TB-9 and Ky3.5,which are settled above TB-8 and Ky3, respectively, both include hornblende crystal grains exhibiting the same refractive indices. Therefore, the combination of TB-8 and TB-9 is correlated with that of Ky3 and Ky3.5.TB-8 (Ky3) widely occurs in the southern and central parts of the Kanto Plain including a metropolitan area. Only a few widespread marker tephras are known in the subsurface of the middle part of the Kanto Plain. This tephra is, therefore, important as a key bed for a study of a subsurface geologic structure in such an urban area.
