Lithostratigraphy and diatom biostratigraphy are established for the Pliocene marine strata in the northwestern area of the Nishikubiki Mountains, Niigata Prefecture, central Japan. Pliocene to lower Pleistocene marine deposits are divided into the Nechi and Nadachi formations, in ascending order. The Nechi Formation is composed of massive sandy mudstone, and correlated to the diatom fossil subzone NPD7Bb. The Nadachi Formation is dominantly composed of massive sandy mudstone with thin sandstone beds and tuffaceous sandstone, and is assigned to the diatom fossil zones NPD8 and NPD9. The Nadachi Formation of this area conformably covers the Nechi Formation. The boundary between the Nechi and Nadachi formations is coincident with the boundary between diatom zones NPD7Bb and NPD8. The Nadachi Formation in this area is correlative with the Kawazume and Nadachi formations in the middle to eastern Nishikubiki Mountains. Our result indicates that the unconformity previously inferred at the base of the Nadachi Formation is absent.
Azuma Volcano, located in Yamagata and Fukushima prefectures, is a large composite Quaternary volcano group. K–Ar ages by the isotope dilution method were determined for 38 lava samples. The volcano consists mainly of lava flows which have mostly similar petrographic characteristics. According to the determined age data, the volcanic edifice was divided into 12 units including Holocene Jododaira Volcano. The volcanic activity of Azuma Volcano began at ca. 1200 ka from the east and migrated to the west, and at ca. 700 ka, reversely migrated to the east. The latest activity is occurring at Jododaira Volcano. There is no long dormancy during the activity of Azuma Volcano from ca. 1200 ka to the present.
Early to Middle Pleistocene volcanic history in the area surrounding Lake Towada was constructed based on stratigraphy, petrology and K–Ar and U–Pb age determinations. The volcanic activity in this area started at 2.5 Ma after the period of inactive volcanic activity during Pliocene. Andesite to dacite magma erupted from several eruptive centers and formed volcanic bodies during 2.5 to 1.6 Ma in the southeastern area. The eruptive style in this period was characterized by formation of lava domes and dome collapse-derived block-and-ash flows. Subsequently, volcanic activity occurred during 1.6 to 0.6 Ma in the northwestern area. In this period, basalt to andesite magma erupted and formed a stratovolcano. Simultaneous dacite magma erupted at the northern and northeastern foot of the stratovolcano and formed several lava domes. Furthermore, andesite magma erupted in the northeastern area and formed an isolated pyroclastic cone just before 0.76 Ma. There was a volcanic repose time of 0.4 million years between 0.6 Ma and 0.22 Ma. The volcanic activity of Towada volcano has started since 0.22 Ma. The long repose time shows that the volcanic activity before 0.6 Ma was different from that of Towada volcano.
Contamination of rock samples from laboratory equipment during crushing and pulverizing was investigated. Pulverization using iron and tungsten carbide mortars, and agate and tungsten carbide mills produced three suites of powders. Whole-rock data from the analyses of the three suites of samples enabled assessment of the degree of contamination of samples by coarse crushing and pulverization. The samples analyzed for major and trace elements compositionally represent basalt, basaltic andesite, dacite, and rhyolite. The results from the analyses of major elements exhibit insignificant contamination from the mortars and the mills. The heterogeneity of rock chips explains the compositional differences observed. The tungsten carbide mill contaminated samples with tungsten and cobalt. The significant scatter of other trace elements data is attributed to sample heterogeneity, with no systematic effect from contamination.