Radiolarian and foraminiferal biostratigraphy was investigated in the Fukaura district of western Aomori Prefecture, northern Japan, where the Miocene-Pliocene chronostratigraphy is well established. Based on the newly obtained data and reinterpretation of previous studies, we determined that the Odoji Formation ranges in age from the Middle to Late Miocene Epoch, the Akaishi Formation ranges from Late Miocene to Early Pliocene, and the Maido Formation is Pliocene. Our biostratigraphic and chronostratigraphic results indicate that the Miocene/Pliocene boundary exists in the Akaishi Formation. Benthic foraminiferal assemblages of the Odoji Formation contain abundant and diverse calcareous taxa indicative of warm, mesotrophic and oxic marine conditions. In contrast, the upper part of the Odoji Formation yields less diversified assemblages, which are mainly composed of infaunal agglutinated foraminifera, Martinottiella communis. These assemblages dominated by M. communis suggest high organic matter flux to the sea floor closely relating to the high primary production of the surface water, and/or oxygen-depleted conditions. The stratigraphic horizon of these faunal changes is possible to be correlated with the Foram. Sharp Line (FSL). The upper part of the Akaishi Formation contains two assemblages one dominated by M. communis as upper part of the Odoji Formation, and the other dominated by Miliammina echigoensis, which suggests oxic bottom water conditions with high primary production. The lower part of the Maido Formation contains only Globobulimina sp., which also suggests a depositional environment associated with high primary production and/or oxygen depletion. Consequently, condition with high organic matter flux caused by high primary production in surface water is inferred to have persisted from the time of the FSL to Ca. 3 Ma, when a diverse foraminiferal assemblage returned.
Monocline and strike-slip faults are at the Moniwa and Aobayama hills, west of Sendai City area. The monocline trends to the northwest and southeast, and constitutes the Kagitori-Ayashi Line. The monocline is an asymmetric concentric fold formed by northeast-southwest compression, after the deposition of the Pliocene Sendai Group. The north-south trending Aoba Fault Zone crosscuts this monocline. The fault zone contains the Aoba Eastern Fault, Aoba Central Fault, and Aoba Western Fault. The Aoba Central Fault and Aoba Western Fault are dextral faults with a relative uplifting component of their eastern sides, and the Aoba Eastern Fault is a sinistral fault with a relative uplifting component of its western side. The Aoba Fault Zone is a northern extension of the Futaba Fault, and reactivated under the northeast-southwest compression as dextral faults and under later northwest-southeast compression as a sinistral fault.
Mesozoic to Paleozoic systems in the Joetsu region show complex geologic structure caused by crustal movements and sub-sequent Cretaceous to Neogene plutonism and volcanism. This study reconstructed a once continuous low-grade schist zone by tracing sporadically distributed schist bodies with a similar tectonic history. Their representatives are the Kawaba metamorphic rocks and Mizunashigawa metamorphic rocks. Both schists have similar deformation chracteristics such as assemblages, occurrence, fabric and deformation mechanism of deformed minor structures, and inverted shear regimes that form the deformed minor structures. In addition, deformation grade expressed by mean ductility at the formative stage of each deformed minor structure in the both schists shows sequential decrease with time. Earlier and later deformation phases recognized in the Mizunashigawa metamorphic rocks correspond to the earlier phase with high mean ductility and the later phase with low mean ductility in the Kawaba metamorphic rocks, respectively. The Kawaba and Mizunashigawa metamorphic rocks are regarded as remnants of a once continuous N-S trending schist zone before the disturbance of the zonal arrangement of the Joetsu region caused by Cretaceous-Neogene crustal movements, plutonism and volcanism.
A diagram involving quartz crystallinity index, mean diameter of quartz grains and temperature based on conodont color alteration index has been constructed from data obtained from cherts in the accretionary complex, Jurassic Tamba Belt, Tamba Mountains. The diagram can be used to estimate thermal conditions in the range 50-300°C which is ideal for rocks subjected to diagenesis and low grade metamorphism. According to this diagram, the bedded cherts in Type II Suite of the Tamba Group is of lower metamorphic grade than Type I Suite of the same group which occurs in the footwall of the thrust which juxtaposes them. This is an unexpected result as Type II Suite is older than Type I Suite.
We investigated calcareous nannofossil biostratigraphy of the Amatsu Formation, the Awa Group, distributed in the Boso Peninsula, central Japan. Fourteen Miocene nannofossil datums and bioevents were detected and six nannofossil zones (Zones CN5 to CN10) were identified in the formation. On the basis of the analysis, the Amatsu Formation is assigned to the middle Miocene to lower Pliocene, ranging from 13 Ma to 5 Ma. Comparison of stratigraphic levels of nannofossil datums and bioevents in the Amatsu Formation with low-latitude results are also discussed in detail.