Drillholes in coral reef are made to reconstruct the coral-reef ecosystems of the past, because long-term geological data are required to predict the ecological response of reef ecosystems to global and anthropogenic stresses. A decrease in the coverage and species diversity of corals has been observed in many reefs worldwide, but it remains uncertain whether this phenomenon is natural or is caused by human activity. Although recent research has improved our understanding of the number of species and the coral assemblage during the period of Holocene sea-level rise, little is known of ecological strategies and the succession of corals during reef formation. Therefore, accurate reconstructions and predictions of coral-reef ecosystems require future drilling of coral reefs at higher temporal resolution to obtain the ecological scope of coral species.
The Komadegawa-biotite (Kbi) tephra is a crystal-rich coarse tuff layer (9 cm thick) intercalated in the upper part of the Middle Miocene Nanatani Formation in the eastern area of the Niigata sedimentary basin, northeast Japan. The Kbi tephra contains plagioclase (oligoclase to labradorite in composition), quartz, sanidine, biotite, trace amounts of garnet, and reddish and colorless zircons. The Kbi tephra is intercalated in the N.9 planktonic foraminiferal zone and the CN4 calcareous nannofossil zone, corresponding to an age of 14.2–14.7 Ma, which is consistent with a newly determined fission track age of 14.6±0.3 Ma (1σ). On the basis of the petrography, fission track age and biostratigraphy, the Kbi tephra is correlated to the Muro Pyroclastic Flow Deposit in Kii Peninsula and the Kn-1 tephra in Boso Peninsula. These tephras are considered as a widespread tephra layer associated with the formation of the Kumano Acidic Rocks in Kii Peninsula. This tephra is expected to be found across large parts of Japan and provides a stratigraphic marker in various sedimentary basins.
Fusulinoidean-bearing limestone clasts are exposed in a new road-cut exposure of the Lower Cretaceous Wakino Subgroup at Otaraguchi, Miyawaka City, Fukuoka Prefecture. The clasts are sub-angular and occur only in a debris flow deposit (tuffaceous pebbly sandstone) in the basal part of the Nyoraida Formation. Well-preserved Middle Permian fusulinoideans (i.e., primitive Neoschwagerina spp., and Parafusulina sp., Schwagerinidae gen. and sp. indet.) and fragments of crinoids are observed in the clasts, indicating that the non-crystalline limestone was deposited in the Tethyan faunal realm or the Circum-Pacific region during the Middle Permian, and that in the Early Cretaceous it was exposed in the hinterland of the sedimentary basin in which the Nyoraida Formation was deposited. The subjacent Permian Aida Formation, which includes accreted blocks of limestone, is a potential source of the limestone clasts.
Quaternary sediments are about 2,000 m thick in the area of Sanmu City, northern part of Chiba Prefecture, on the Kanto Plain. Although the pre-Neogene basement is considered to be a Jurassic to Early Cretaceous accretionary complex, there are no direct constraints on the age of the complex. In this area, we obtained Triassic radiolarian fossils from bedded chert in drillcore recovered from 1,656 m below mean sea level at the Naruto FA-68 well drilled by INPEX CORPORATION. Because Triassic chert in Japan is limited to Jurassic to Early Cretaceous accretionary complexes, this rock type makes up the pre-Neogene basement in the area. The depth of the surface of pre-Neogene basement in surrounding boreholes is variable due to differential erosion.
There exist problems regarding the Japanese notation for chronostratigraphic and geochronologic units, as proposed by Japanese Industrial Standards (JIS). The first problem is the inappropriate translation into Japanese of the terms “Lower/Early, Middle/Middle, and Upper/Late” applied to Series/Epoch units in the International Stratigraphic Chart. The second problem is the inappropriate word order of System/Period units and Series/Epoch units. These problems can be overcome by using logical notation in which the terms “Lower/Early, Middle/Middle, and Upper/Late” for Series/Epoch units are translated as “kabutou/kose, chubutou/chuse, and joubutou/shinse” in Japanese. Although the Geological Society of Japan recommends the use of the JIS-style notation, the revised notation (as summarized here) is more logical and appropriate.