[Abstract]

Tamagawa Welded Tuffs comprise the late Pliocene Kurasawayama Welded Tuff and the early Pleistocene Bunamori-bokujo Welded Tuff, which represent the Plio-Pleistocene regional felsic magmatism in NE Japan Arc. They are thought to have issued from the same caldera buried below Akita Yake-Yama volcano, though the multiple drillings into the caldera have recovered no or little materials of the two welded tuffs and a time gap of ca. 1 million years is present between the two welded tuffs. This paper describes the Taihei-Ko and Haruyama pyroclastic deposits that individually occur between the two welded tuffs at a road cut on the northern side of Taihei-Ko (Lake) and at a cutting wall of Haruyama on the eastern side of Tazawa-Ko (Lake). The deposits at the two locations are 30 km apart but contain the same crystal fragments of quartz, plagioclase, orthopyroxene, hornblende, magnetite, and extremely minor biotite. The Taihei-Ko pyroclastic flow deposit and the Haruyama pyroclastic deposit are 1.4+-0.2 Ma and 1.7+-0.2 Ma, respectively in fission-track age for zircon. They are likely to have derived from the same source at the same time or shortly after another and may represent a missing activity in the source area of the Kurasawayama Welded Tuff, to which they are similar in constituents. The Taihei-Ko pyroclastic deposit comprises a pyroclastic surge deposit and overlying pyroclastic flow deposit. The Taihei-Ko pyroclastic surge deposit is 14 m thick and well stratified with rip-ups of the underlying rhyolitic Kurasawayama Welded Tuff. The Taihei-Ko pyroclastic flow deposit is over 6 m thick and entirely welded. Stratification and grading are not obvious. The Haruyama pyroclastic deposit fills an incised valley of the Kurasawayama Welded Tuff, and is vaguely stratified across the thickness of over 20 m. Compared with the Taihei-Ko deposit, the Haruyama deposit is, however, relatively poor in fines and contains many lithic fragments, so is likely to be more proximal to the source. The stratification of the Haruyama deposit inclines outward from Tazawa-Ko. The flow direction of the Taihei-Ko pyroclastic surge is from the south to the north and 30 km to the south, there is Tazawa-Ko. In addition, a deposit potentially correlative to the Haruyama deposit occurs between Tazawa-Ko and Taihei-Ko. These facts collectively suggest that an alternative source of the Taihei-Ko and Haruyama deposits is Tazawa-Ko. Because of the dimension 5.8 km across and 423 m deep and absence of confirmed deposits, it has been open to question for a long time whether the lake is a caldera or an impact crater.

Key words:

Tamagawa Welded Tuffs, Tazawa-Ko, caldera, pyroclastic flow, pyroclastic surge, Early Pleistocene