Kita-daito-jima is a carbonate island lying on a lithospheric bulge of the Philippine Sea Plate. We provide a formal stratigraphic description of the coral reef deposits on this island. The deposits comprise the Daito Formation (>2.0 Ma) overlain by the Kaigunbo Formation (∼12.3 ka) . The Daito Formation is divisible into three units. Unit 1 is dominated by coral framestone rich in massive corals and crops out in the interior basin. Unit 2, resting unconformably on Unit 1, is subdivided into two subunits; lower Subunit 2a and upper Subunit 2b. The lower subunit comprises reef-core facies that constitutes the main body of peripheral rim and backreef facies exposed at the cliffs lining the interior basin. The reef-core facies is represented by coral framestone associated with coral bafflestone. The lower backreef facies consists mainly of rudstone and the upper backreef facies is composed chiefly of coral framestone/bafflestone frequently containing Halimeda segments. Subunit 2b, cropping out around the peak of this island, is constructed by coral framestone and bioclastic packstone. Unit 3 is exposed sporadically on the east coast and unconformably overlies the reef-core facies of Subunit 2a. This unit consists of cross-bedded bioclastic packstone associated with coral framestone. Stratigraphic succession, configuration of lithofacies, and coral assemblages indicate that Unit 1 formed in a table reef without lagoonal environment and that thick atoll deposits accumulated on karstified limestone during the deposition of Subunit 2a. Then, the atoll may have been progressively degraded and no distinct reefs formed at the deposition of Subunit 2b. Shoal and coral patches were scattered on eastern part of the island during the deposition of Unit 3. The Kaigunbo Formation occurs abutting on the coastal cliff at elevations less than llm and consists of framestone, which indicates limited fringing reef formation at the last interglacial stages.
Moderately well-preserved Devonian radiolarians were recovered from the mudstone portion of alternating tuffaceous sandstone and mudstone cropping out along a tributary of the Kanashirozako Valley in the Fukuji area of the Hida-gaien Terrane, central Japan. Float samples of tuffaceous mudstone, which were collected from a riverbed near the outcrop mentioned above, also contain well-preserved Devonian radiolarians. These radiolarian-bearing rocks most likely belong to the Yoshiki Formation, based on lithology. Two assemblages (I and II) are present and contain abundant species of the family Palaeoscenidiidae along with ceratoikiscids and undetermined spumellarians. The rocks containing the two assemblages correlate with the upper half of the lower to middle Emsian (upper Lower Devonian) Trilonche (?) sp. A Zone defined in the Nakahata Formation of the Kurosegawa Terrane. Furthermore, the Yoshiki Formation in the lower reaches of the Ichinotani Valley that yields the Zadrappolus yoshikiensis Assemblage can be correlated with a part of the Pridoli to Pragian (upper Upper Silurian to middle Lower Devonian) Futobari solidus Zone of the Nakahata Formation. Therefore, the Yoshiki Formation is probably Pridoli to early to middle Emsian in age. Stratigraphic relationships between the Yoshiki and Fukuji formations have been poorly understood, due to structural complexities, poor exposures, and the lack of age-determinable fossils. However, microfossil data from these two formations reveal that they are almost contemporaneous. Some past workers have suggested that the Yoshiki Formation is unconformably overlain by the Fukuji Formation, but the presence of an unconformity is not supported by the present study.
Stratigraphy of the Miocene marine sequence in the Ota area, Gunma Prefecture, central Japan is established. Based on the lithological defference, as well as on the stratigraphic relationship, the volcano-sedimentary sequence in the Ota area is divided into lower non-marine volcaniclastics of the Yabuzuka Formation and overlying marine sediments of the Midorimachi Formation (newly defined). Pumice flow deposit unconformably covers the Midorimachi Formation is suggested to be correlated with the lowest Middle Miocene felsic products such as the Oya Formation, widely distributed along the eastern margin of the Ashio Mountains. Stratigraphic correlation of the volcano-sedimentary successions around the Ashio and Yamizo Mountains suggests that the Early to Middle Miocene felsic products are divided into two stages, that is before and after the Niwaya Unconformity at about 15 Ma. Twelve samples of the Midorimachi Formation yield age diagnostic diatom fossils, which indicate that the sequence falls between the diatom biohorizons of D33 (16.7 Ma) and D35 (16.4 Ma) of the upper Crucidenticula kanayae Zone (NPD 3A).
Hayakawa Tuff Breccia, situated at the northern end of Izu peninsula, is one of the Tertiary basements of Hakone volcano. It was correlated with the Shirahama Group in the southern region of the peninsula based on a peculiar moluscan assemblages (Shirahama fauna). In this study, microfossils found in bore hole samples of southeastern flank of Hakone volcano were examined to ascertain the exact age of the formation. The age of deposition was determined to nearly 4.2Ma, owing to overlapping biostratigraphic zones of foraminifera and calcareous nannofossils. Of additional interest, several index species of CN3-4 (early to middle Miocene) are also found, even though the samples are nearly monolithological. Yugashima Group, early to middle Miocene formation of this area, is strongly altered by geothermal activity and the presence of fossils is rare. The presence of early to middle Miocene fossils in early Pliocene indicate that the strong geothermal activity took place later than 4.2Ma. The onset of hydrothermal activity in this study area is consistent with previous study undertaken in the southern part of Izu peninsula (Matsumoto et al. 1985). These hydrothermal activities can be simultaneous and related with the collision of Izu terrane to the Honshu island.