This paper describes litho- and biostratigraphy of the Upper Cretaceous in the Ashibetsu Lake area, Hokkaido, Japan. The Cretaceous deposits in this area are represented by offshore facies, and are divided into the Takinosawa Formation composed of alternating beds of sandstone and mudstone (upper part of the Middle Yezo Group), and the Kashima Formation composed of mudstone (lower part of the Upper Yezo Group). Each Formation is divided into two units respectively : unit Mh and Mi below, and unit Ua and Ub above. The Cretaceous succession ranges from the Upper Turonian to Santonian, based on age indicative ammonoids and inoceramids. The Turonian/Coniacian boundary is assigned to the middle part of the unit Ua, and the Coniacian/Santonian boundary to the uppermost part of the unit Ub. The species richness and abundance of ammonoids decrease in units Mi and Ua. It is considered that during the late Turonian-early Coniacian time, ammonoid assemblages were affected by the spread of the oxygen-depleted conditions in the bottom water.
Igneous activity of the Middle Miocene Otoge volcanic complex, Shitara district, central Japan, is divided into the cauldron-forming stage with emplacement of the Otoge pyroclastic rocks, and the post-cauldron stage with emplacement of dike and sheet swarm. Crosscutting relationship shows the igneous sequence in the post-cauldron stage : the Kamasawa brecciated intrusions and Kamoyamagawa trachyte dike were formed at the beginning of the post-cauldron stage and the formation of the Otoge cone sheets followed. The Shitara central dike swarm was formed in the final stage. Intrusion structure of these intrusions changed from irregular-shaped peperite to coherent tabular dike reflecting the consolidation of the host Otoge pyroclastic rocks. Most part of the the Otoge volcanic complex consists of alkaline rocks and their compositional change during the post-cauldron stage suggests the repletion of less-fractionated magma into a reservoir beneath the cauldron and progress of crystal fractionation within the reservoir. Input of high-temperature mafic magma to reservoir caused melting of its wall rocks and formed calc-alkaline dacite of the Otoge stocks.
Sidescan sonar and sub-bottom reflection surveys off the town of Shikabe, along the Pacific coast of southwest Hokkaido, show the extent and volume of a submarine debris-avalanche deposit caused by the A. D. 1640 eruption of Hokkaido-Komagatake volcano. The avalanche extends succesively from the subaerial part of the Shikabe lobe, and traveled as far as 20 km seaward from the volcano. Subaqueous deposit reached the 80m-deep seafloor and bifurcated northward and eastward. It covered 126 km2 of the seafloor with 15km width. Hummocks of the subaqueous deposit decrease in height and width with distance, like progressively shattered hummocks of subaerial debris-avalanche deposits. The marginal region of the deposit lacks hummocks and shows a prominent flow front slope, implying the presence of a yield strength of the debris-avalanche deposit. The ratio of height to the distance of travel (H/L) is 0.06, suggesting more mobile origin than many subaerial debris-avalanche deposits. The subaqueous volume of the avalanche, estimated by extrapolating pre-eruptive topography from the surrounding area, ranges from 0.92 to 1.20 km3, which is larger than the previous estimates. This explains the discrepancy of tsunami simulation results which have been underestimating run-up heights.
In the Miocene sediments distributed along the Watarase River, in the northern Kanto Plain, Miogypsina gr. kotoi was discovered from the Miocene marine section of the Yabuzuka Formation at Kanayama Hill, Ota City, Gunma Prefecture, central Japan. Fission track age of 14.9 ± 0.5Ma (1 σ error) was determined for pumice tuff bed interbedded in the marine sequence which represents the age of Miogypsina gr. kotoi.