Geologic age and depositional environments of the uppermost part of the Middle Miocene Iriomote Formation, which is widely distributed in the Iriomote-jima Island, Okinawa, southwest Japan, were examined. Based on depositional facies analysis, four depositional cycles are recognized in the study interval. Calcareous nannofossil biostratigraphic study was tried to determine geologic age. Well-preserved calcareous nannofossils were found only from the infills of the trace fossil Thalassinoides suevicus piping down from transgressive lag deposits of the third depositional cycles. Calcareous nannofossil biostratigraphic data strongly suggest that the uppermost interval of the Iriomote Formation has deposited during the period between 14.91 Ma and 13.53 Ma, early Middle Miocene (calcareous nannofossil zone: CN4). The following scenario can be considered to explain the fact that well-preserved calcareous nannofossils were found from the T. suevicus fills: (1) Due to selective cementation of the burrow boundaries, infilled nannofossils were also protected from chemical dissolution during diagenesis, (2) The sediments containing calcareous nannofossils in the infills were filled down from the sea-floor through the burrow opening during transgressive to regressive stages, and (3) The burrows penetrating deep below the seafloor were protected from the subsequent physical and biological destructions. Biostratigraphic data obtained from the burrow fills can provide very informative data to determine the geologic age.
This study reports the first optically stimulated luminescence (OSL) dating of an active fault. The examined fault is the Atotsugawa Fault which was triggered the Hietsu earthquake in AD 1858. Ultraviolet thermoluminescence (UV-TL) was also used for an evaluation of an annealing condition of the fault movement. The OSL dating was conducted using quartz grains separated from a fault gouge. An annealing experiment of quartz grains showed that OSL fast component and 270°C UV-TL signals were removed by a temperature of 300°C held for 20 and 12-30 seconds, respectively. These results provide useful information to investigate the temperatures generated during fault activity. The quartz grains analyzed using linear modulated OSL were divided into two types, F and dim-F, with respect to the OSL fast component. The F type emitting the fast component intensely was used for OSL dating as it showed a stable ratio in the dose recovery test. The average OSL age derived from 80 aliquots (each containing about 10 grains) showed an age range of 0.2 ± 0.2 ka. The OSL age agrees well with the age of the Hietsu earthquake of AD 1858 within a range of one standard deviation.
Staples for a stapler can be used as frames for collecting small oriented samples from fragile and rare areas of fault gouge. Slots are produced on the surface of the fault gouge in order to insert staples; these staples are used in groups of 10 or less and comprise three small surfaces at right angles to each other. Pre-oriented staples within the slot are then removed before preparation for Scanning Electron Microscope (SEM) observation and analysis. This approach requires staples, a knife and other simple equipment for use in both the field and laboratory.
We discovered a NNW–SSE-trending, 85-cmwide basalt dike that was intruded into Early Miocene Tomikusa Group sediments in southern Nagano Prefecture, central Japan. K–Ar dating of plagioclase within this dike yielded an age of ca. 13.6 Ma (Middle Miocene), indicative of the timing of emplacement/cooling of the dike. Intrusion of the dike was coeval with late-stage volcanic activity that formed the Shitara igneous rocks a few tens of kilometers to the southwest of the study area.