Miocene syn-rift sediments in strike-slip basins along the Tanakura Fault were deposited during major strike-slip movement coeval with the opening of the Japan Sea. Interstratified with these sediments is the Nantaisan Volcanic Breccia, an Icelandic-type volcanic rock related to the opening of the Japan Sea. In this study, the genesis and development of a submarine volcano, termed the Nantaisan volcano, has been reconstructed using facies analyses of submarine volcanic rocks of the Nantaisan Volcanic Breccia. The Nantaisan volcano is a composite volcano that consists mainly of the products of effusive eruptions. The orientations of quartz veins in the breccia record NW-SE to NNW-SSE transtension related to strike-slip movement on the Tanakura Fault. The Nantaisan submarine volcano most likely formed due to rapid upwelling of Icelandic-type magma that formed by partial melting of lower crust due to transtension along the Tanakura Fault.
Three experiments were conducted using ice balls to model the effects of the compaction, pressure solution, and cementation of sand grains during lithification. In experiment A, two ice balls were stacked together in a plastic pipe, which was then placed in a freezer at temperatures ranging from −14.5°C to −19.6°C. In Experiment B, the same conditions were used, but stainless-steel nuts and bolts weighing 890 g were placed over the stacked ice balls to simulate compaction. After one month, the ice balls in both experiments were bonded together. Thin section observations of the bonded ice balls from Experiment A showed that the contacts between ice balls resembled the long contacts of sand grains formed during pressure solution. In Experiment C, two ice balls were placed together in a plastic pipe filled with warm air (33.0°C and 46% relative humidity), which was then sealed with a plastic covering and placed in a freezer at temperatures ranging from −17.8°C to −20.0°C. Two months later, these ice balls were cemented together by fine interstitial ice crystals, indicating the formation of an ice cement between ice balls. These results suggest that ice ball experiments can be used to model the processes of clastic lithification by pressure solution and interstitial cementation, thus providing a better understanding of the consolidation of sands.
We used cores drilled up to depths of 600 m from the central Kanto Plain, Japan, to examine the stratigraphy of the Kazusa and Shimosa groups, and correlate marine intervals with marine oxygen isotopic stages (MIS). Both groups contain sediments from the upper Pliocene (Gauss Chronozone), and the lower to upper Pleistocene. We recognized an unconformity in the Kazusa Group at around 1.6 Ma, which represents a 0.5-1.0 Myr hiatus in the sequence. Both groups comprise cyclic deposits of shallow marine and terrestrial sediments, and there was no significant hiatus after 1.6 Ma in either group. From the depths of correlated marine intervals, we infer that this part of the Kanto Plain, including the cities of Kuki and Kasukabe, was a center of tectonic subsidence around 1.6 Ma.
This paper describes a new species of Cyrtospirifer (Late Devonian Brachiopoda) from the Ainosawa Formation of Soma, Abukuma Mountains (South Kitakami Belt), northeast-ern Japan, termed Cyrtospirifer ainosawensis, sp. nov. by Tazawa, Inose and Kaneko. This new species most resembles Cyrtospirifer choanjiensis of Tazawa (2017), described from the Upper Devonian of Choanji, southern Kitakami Mountains (South Kitakami Belt), northeastern Japan.