白亜紀の放射年代 : とくに地質年代尺度に関連して

概要

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Glauconite ages amount to about half of the data used and play an important part in constructing a Cretaceous time scale. However, quite different time scales are proposed as a result of critical evaluation of glauconite ages. Recently proposed time scales also reveal the difference in duration of the ages; there by further refinement in biostratigraphy will be needed. A Rb-Sr whole-rock age of 125±3 Ma and K-Ar mineral ages of 116±4 Ma for the Miyako Granite, which is assigned to the Upper Neocomian, are important data for the calibration of the Early Cretaceous time scale, and support the time scale of HARLAND et al. (1982). Biotites from the Lower Turonian and Santonian acidic tuffs from the Obira and Manji areas in Hokkaido, respectively, give concordant K-Ar and Rb-Sr ages, but these ages are slightly discordant with the scale of HARLAND et al. (1982). New K-Ar and Rb-Sr age determinations on igneous rocks from the Nemuro Groups give the following results: dolerite and basalt from the Middle〜Upper Campanian Nokkamappu Formation: 75.0 Ma and 72.0 Ma, 73.9 Ma, respectively; monzonite from the Uppermost Campanian〜Lowermost Maastrichtian Otamura Formation: 70.3 Ma and 69.5 Ma; dolerite from the Maastrichtian Hamanaka Formation: 66.O Ma and 67.6 Ma. These age results provide valuable calibration points for the Late Cretaceous time scale, and are generally consistent with the scale of HARLAND et al. (1982).
日本地質学会の論文
1985-03-30