テフロクロノロジーによる前期旧石器時代遺物包含層の検討

仙台平野北部の遺跡を中心に

抄録

A very important problem for Quaternary research is determining when human beings first settled the Japanese islands. Recently many artifacts of the Early Paleolithic age, dating from before 30, 000 years ago, have been discovered in the northern part of Sendai plain, North Japan. The age of these artifacts has been determined mainly by radiometric dating methods.
On the other hand, tephrochronology is an effective technique for establishing Quaternary stratigraphy in the Japanese islands and their surrounding area. The author investigated the age of horizons bearing artifacts on representative sites of that age from the viewpoint of tephrochronology.
The stratigraphy of proximal tephra layers is indicated in Fig. 1. Useful widespread tephra layers for chronological study of this area are Toya (ca. 90, 000-100, 000y.B.P.), On-Pm I (ca. 80, 000y.B.P.), Aso-4 (ca. 70, 000y.B.P.) and AT (ca. 21, 000-22, 000y.B.P.). Toya, On-Pm I, Aso-4 and AT were discovered from the horizons between IcP and KtA, IcP and KtA, N-N and N-Y, and N-Y and NK-U, respectively. At Babadan A site, artifacts of the Early Paleolithic age were excavated from horizons in Ando soil found below IwP, just below and above IcP, between IcP and KtA, just above N-Y, and between N-Y and NK-U. Consequently, the artifacts excavated from the horizons below KtA belong to the Early Paleolithic age. At Zazaragi site, another representative site belonging to the Early Paleolithic age, the artifacts were excavated from the horizons of orange-colored volcanic ash (the lower part of Yasuzawa volcanic ash). During the excavation, this deposit was regarded as an airfall tephra layer. But the author interpreted this deposit as a weathered part of a pyroclastic flow deposit (N-Y₃) on the basis of the following observations:
(1) The boundary of the lower part of Yasuzawa volcanic ash and lower pyroclastic flow deposit is irregular and not clear in some place.
(2) It is massive and not layered. There is no intercalation of soil.
(3) It is not well sorted.
(4) It includes charcoals.
(5) It has segregation pipes. Some of them develop at the boundary of the lower part between the Yasuzawa volcanic ash and the lower pyroclastic flow deposit.
The author believes that the artifacts were incorporated into pyroclastic flow and carried to their present site.
The horizons from which artifacts of the Early Paleolithic age in Japan have been excavated are plotted in Fig. 8. It has been confirmed that some widespread tephra layers originated from gigantic eruptions and covered the area surrounding the Sea of Japan (cf. MACHIDA and ARAI, 1933). The author believes that common chronological studies of Paleolithics in this area are made feasible by tephrochronology.