Continuous occurrence of international planktonic foraminiferal index species was recognized in the Middle to Upper Turonian successions in the Hobetsu area, Hokkaido, Japan. It enables us to assume the stratigraphic position of the upper limit of the Helvetoglobotruncana helvetica Zone, an important planktonic foraminiferal zonal boundary. Helvetoglobotruncana praehelvetica, Marginotruncana pseudolinneiana, and Praeglobotruncana gibba, which are also internationally recognizable species, occur associated with Helvetoglobotruncana helvetica. This association indicates that stratigraphic top of the Kajiyanosawa route should be assigned to close to the top of the total range of Helvetoglobotruncana helvetica. Correlation of the carbon isotope fluctuation through the Kajiyanosawa route with that of well-studied sections supports our chronostratigraphic interpretation from planktonic foraminiferal biostratigraphy Concurrence of genus Archaeoglobigerina with Helvetoglobotruncana helvetica in the Middle Turonian of this area demonstrates that the timing of the first appearance of Archaeoglobigerina is older than previous known.
The Cretaceous Hidakagawa Group of the Shimanto Belt in the western coast of the Kii Peninsula is divided into the Yukawa Unit consisting of only terrigenous deposits and the Miyama Unit consisting of terrigenous deposits with chert and basalt. The Yukawa Unit is subdivided into two sub-units; the Ogui Sub-unit composed largely of age-unknown thick-bedded sandstone and the Nagasaki Sub-unit largely of late Albian to late Cenomanian sandstone-rich alternating beds of sandstone and shale. The Miyama Unit is subdivided into four sub-units from the north to the south; the Karakozaki Sub-unit composed largely of Coniacian shale-rich alternating beds of sandstone and shale, the Chidenohana Sub-unit largely of Santonian to early Campanian thick-bedded sandstone, the Shiofukiiwa Sub-unit largely of Santonian to early Campanian shale-rich alternating beds of sandstone and shale, and the Hinomisaki Sub-unit largely of early Coniacian to early Campanian thick-bedded sandstone. These six sub-units are correlated with the formations of Hidakagawa Group in the central part of the Kii Peninsula in general. The Ogui and Nagasaki Sub-units are correlated to the Y3 and Y1 Members of the Yukawa Formation, respectively. The Karakozaki, Chidenohana, Shiofukiiwa and Hinomisaki Sub-units correspond to the M1, M2b, M2a and M2b Members of the Miyama Formation, respectively.
Anastomosing networks of massive dark-colored veins (Dark-colored silica veins hereafter) ranging from a few millimeters to 8 cm in thickness occur in a fault zone of Tertiary rhyolites (the Iozen formation) from the southern Ishikawa Prefecture, Japan. Optical microscopic observations combined with SEM and XRD studies revealed that the Dark-colored silica veins mainly consist of very fine-grained quartz (10 μm in size) with minor amounts of feldspar and Fe-bearing unidentified phase. Angular to subangular lithic clasts derived from the host rhyolite are frequently embedded in fine-grained quartz matrix of the Dark-colored silica veins. The Dark-colored silica veins are apparently abundant in SiO2 relative to the host rocks. Chondrite-normalized rare earth element (REE) pattern of a Dark-colored silica vein is similar to that of the host rhyolite for middle to heavy REE, whereas light REE (La and Ce) concentrations are higher in the Dark-colored silica vein than in the host rhyolite, resulting in high La/Yb ratio in the Dark-colored silica vein. The Dark-colored silica veins might be originated by extreme crushing of the host rhyolite in fault zones followed by the selective enrichment of SiO2 (+LREE) coupled with diluting of other elements. The outcrop including the Dark-colored silica vein was finally suffered from low-temperature alternation (<100ºC), leading to form clay minerals, particularly along the fault zone.
We carried out fossil diatom analysis and fission-track dating of the upper Koetoi Formation, which is distributed in the western limb of the Sarobetsu anticline in the western part of the Horonobe area, northern Hokkaido. A mudstone sample collected from the upper Koetoi Formation contains abundant autochthonous diatoms which can clearly be correlated with the Neodenticula koizumii Zone. The assemblage includes N. seminae, which has a first appearance age of ~2.4 Ma. A zircon FT age obtained from an interbeded volcanic ash is 2.3±0.1 Ma. This FT result gives a precise depositional age for the upper Koetoi Formation in the area surveyed, and is consistent with the results of the diatom fossil analysis. These results indicate that the depositional age of the upper Koetoi Formation is about 2.3 Ma. Considering previous chronological work and our results, the age of the boundary between the Koetoi and the Yuchi Formations in the western part of the Horonobe area is younger than about 2.3 Ma, which is at least 1 million years younger than that of the same boundary in the eastern part. This implies westward depositional migration of the Koetoi and Yuchi Formations, and a diachronous relationship between the eastern and the western parts. These geochronological constraints from diatom fossils and FT data further suggest that the period of incipient active tectonism, which formed complex anticlinal and synclinal structures in this area is younger than about 2.3 Ma. The volcanic ash layer in the upper Koetoi Formation contains characteristic zircon crystals with unusually high uranium contents (>1000 ppm), and may become a useful marker bed in this area.
Fission-track age of zircon crystals was obtained as 0.18±0.04 (2σ) Ma for the pyroclastic-flow deposit (Shotsugawa pfl) of Tanabu C tephra in the northern Shimokita Peninsula. The Tanabu C tephra lies as a stratigraphic marker between the marine terrace succession, with the constant uplift-based chronology of which the new dating result can match.