High-precision radioisotopic ages for the lower Midian (upper Wordian) Stage of the Tethyan time scale, Shigeyasu Quarry, Yamaguchi Prefecture, Japan

摘要

Global correlation of strata to the Guadalupian Series (Permian Period) of the International Geologic Time Scale remains provisional due to a lack of the sufficient biostratigraphically constrained radioisotopic ages. Five new high-precision U-Pb radioisotopic ages were obtained from tuffs in the lower Midian (upper Wordian) volcano-siliciclastic succession of the Akiyoshi Plateau, southwestern Japan. Two undisturbed and continuous tuffs occur within fusulinid-bearing laminated limestone turbidites, with precise biostratigraphic control. The Colania douvillei and Lepidolina shiraiwensis fusulinid Zones in the Akiyoshi succession provide an early Midian age in the Tethyan provincial time scale. The range of ages (267.46 +/- 0.04 Ma to 265.76 +/- 0.03 Ma) over 25 m of section including the limestone turbiditic beds suggest that this lower Midian Stage of the Tethyan time scale is correlated with the upper Wordian Stage of the International Geologic Time Scale. The sedimentologic, bio- and geochronologic data affirm the effectively conformable nature of the siliciclastic-carbonate succession of the Tsunemori Formation at the Shigeyasu Quarry, and the presence of acidic volcanic tuffs are consistent with a forearc basin or trench slope basin depocenter for the Tsunemori Formation.