Chronology of the Tachikawa Loam as Established by Fission Track and Obsidian Hydration Dating

Abstract

Measurements of hydration-rim thickness for about 1, 500 obsidian samples, collected from 30 Preceramic occupation layers, mainly from the Musashino and Sagamino uplands, are correlated with four key fission track dates obtained from burnt obsidian specimens collected from hearths at four different sites.
These key dates are:
4, 700±300 years B. P. for Middle Jomon (Klasori),
7, 400±500 years B. P. for the boundary of Early and Earliest Jomon (Kayama),
11, 100±700 years B. P. for Yasumiba (microblade industry), and
15, 200±1, 000 years B. P. for ICU Loc. 15 (backed-blade industry).
By correlation with these four key dates, tentative hydration rates for obsidians found in South Kanto were obtained.
group rate (micron²/thousand years) for
K₁ 0.28 YS dacite obsidian
K₂ 0.98 D₂ and D₅ dacite obsidian
K₃ 2.69 Asama dacite obsidian
K₄ 5.13 Wadatoge and Kirigamine rhyolite obsidian
K₅ 7.89 Assumed to be Hoshikuso rhyolite obsidian
Working hypotheses for chronometric dating by obsidian hydration rates assumed that the linear correlation of the lapse of time after fracturing of the obsidian and the square of the measured hydration thickness was valid and that the contemporaneity of the time of deposition of the tephra layer and the time of the Preceramic human activity was positive. On these assumptions, a chronology of the Tachikawa Loam was derived as follows:
soft loam III 9, 000-12, 500 years B. P.,
hard loam IV 12, 500-18, 500,
black band I V 18, 500-21, 000,
black band II IX around 25, 000.
Based on these dates, MACHIDA's (1971) hypothesis of the constancy of the deposition rate of the tephra is roughly supported. In this connection, the fission track date for the Tokyo Pumice
(TP), 49, 000±5, 000 years B. P., reported elsewhere (MACHIDA and SUZUKI, 1971), was checkedby obsidian hydration dating using the hydration rate K₁; the calculated date of 46, 000 B. P, is consistent with the fission track date within the error limits of each method.
The many discrepancies between the fission track-obsidian hydration dates and radiocarbon dates are not explained at present because of the scarcity of comparable data for both fission track and radiocarbon dates from the Preceramic cultural layers in South Kanto.a