Local tephra as an age-determination tool; Example of 2.3 ka Yakedake volcano tephra in Nagano Prefecture, central Japan

Kojima, Satoru*; Kagami, Saya; Yokoyama, Tatsunori; Kariya, Yoshihiko*; Katayama, Yoshikazu*; Nishio, Gaku*

Proceedings of 5th International Workshop on Rock Mechanics and Engineering Geology in Volcanic Fields (RMEGV 2021) (Internet), 6 Pages, 2021/09

A local tephra embedded in a hand-auger boring core drilled at near-shore of Kinugasanoike Pond about 4.6 km NE of Mt.Yakedake, one of the most active volcanoes in central Japan, is composed mainly of several kinds of volcanic glass shards (microlite-bearing, blocky, fluted and micro-vesicular types) with minor amounts of crystal minerals including quartz, plagioclase, hornblende, biotite, and pyroxene. Plant remains recovered from the horizon 10 cm below the tephra layer yield C ages of 2,331-2,295 (19.2% probability distribution) and 2,270-2,155 (76.2% probability distribution) cal yrs BP. We measured major element compositions of 241 individual glass shards using Electron Probe Micro Analyzer. They are plotted on the SiO-KO, SiO-NaO+KO, and FeO-KO diagrams in a region different from those of major regional tephras distributed in central Japan. The clast and chemical compositions coincide with those of a tephra embedded in the Nakao pyroclastic flow deposits distributed about 2 km NNW of Mt. Yakedake dated as around 2,300 cal yrs BP. The tephra could be used as a local marker of 2,300 cal yrs BP in the southern part of Northern Japan Alps.

Mudstone redox conditions at the Horonobe Underground Research Laboratory, Hokkaido, Japan; Effects of drift excavation

Mochizuki, Akihito; Ishii, Eiichi; Miyakawa, Kazuya; Sasamoto, Hiroshi

Engineering Geology, 267, p.105496_1 - 105496_11, 2020/03

https://doi.org/10.1016/j.enggeo.2020.105496

The mechanical and hydraulic properties of rocks around mine drifts change significantly during the construction and operation of a radioactive-waste repository, with air intrusion causing the oxidation of rock and groundwater in excavation-damaged zones (EDZ). Redox conditions in such zones associated with niches excavated in mudstone at the Horonobe Underground Research Laboratory (URL), which is believed to be generally representative of conditions that could exist in the EDZ of a repository, were studied with the aim of improving our understanding of factors that control redox conditions in such rock-groundwater systems. Groundwater Eh values around the niches have reducing values of less than -150 mV. The SO concentration, regarded as an oxidation indicator, is consistently as low as 1 mol L. Gas occupies more than 50% of zone volumes, including CH and CO with traces of N and O. Cores drilled from host rock around a URL gallery were analyzed, with no pyrite dissolution or precipitation of calcium sulfates being found. It is concluded that oxidizing conditions do not exist in the excavation-damaged zones, which is attributed to the suppression of air intrusion by the release of CH and CO from groundwater as pressures decreased and their accumulation in fractures. The modeling of oxygen diffusion into host rock further indicates that a reducing environment is maintained around the URL drifts.

Protolith identification of bedding-parallel, smectite-bearing shear zones in argillaceous and siliceous marine sediments; Discriminating between tephra-derived shear zones and host-rock-derived fault gouges

Ishii, Eiichi

Engineering Geology, 259, p.105203_1 - 105203_9, 2019/09

AA2018-0459.pdf:0.97MB

https://doi.org/10.1016/j.enggeo.2019.105203

Smectite-bearing shear zones are crucial in a variety of geoscientific and geoengineering fields due to the unique physicochemical properties of smectite. Although such shear zones worldwide occur in marine argillaceous/siliceous sediments, tephra-derived and host rock-derived are possible as their origin, and these shear zones potentially differ in amount of smectite, extent of the spatial distribution, and time of the formation. Nevertheless, it is hard to discriminate these origins by observation. This paper proposes a method utilizing the following two indicators for the discrimination: one is a proportion ratio of smectite to illite, sm/(ill+sm), in the clay fractions of the samples; the other is a concentration ratio of conservative elements, AlO/TiO (wt%/wt%), in bulk chemistry of the samples. Although high sm/(ill+sm) values can be an evidence indicating tephra-derived origin, the values may decrease by mineralogical disturbance/alteration during shear deformation, which make the discrimination difficult. The AlO/TiO values different from those of the host rock can also be an evidence of tephra-derived origin, based on previous studies, and the measurement error is also very small, compared to the analytical error of sm/(ill+sm). But the AlO/TiO values of shear zones may intrinsically be the same as those of the host rock even if those shear zones were derived from tephra layers. Application of these indicators to the protolith identification of bedding-parallel clay-rich shear zones in a folded siliceous mudstone (Hokkaido, Japan) demonstrates that combining sm/(ill+sm) and AlO/TiO is useful as a first diagnosis to identify the protolith more easily and more confidently.

Detection and correlation of tephra-derived smectite-rich shear zones by analyzing glass melt inclusions in mineral grains

Ishii, Eiichi; Furusawa, Akira*

Engineering Geology, 228, p.158 - 166, 2017/10

https://doi.org/10.1016/j.enggeo.2017.08.010

We present detailed mineralogical and geochemical analyses of clay-rich shear zones that are associated with bedding-parallel or bedding-oblique faults in drill cores of a siliceous mudstone from the Horonobe area, northern Hokkaido, Japan. The bedding-parallel shear zones are rich in smectite and some shear zones are characterized by melt inclusions hosted by plagioclase and quartz. The chemical compositions of melt inclusions in six shear zones are very similar, but distinct from those of melt inclusions in other shear zones. These six shear zones consistently occur in horizons 350 m shallower than a known tephra layer. These mineralogical characteristics and stratigraphic positions suggest that the six shear zones occur within the same tephra layer and that the smectite formed as a result of burial diagenesis of volcanic glass. Tephra-derived smectite-rich shear zones can be detected and correlated by analyzing melt inclusions preserved in mineral grains.

Microscopic features of quartz and clay particles from fault gouges and infilled fractures in granite; Discriminating between active and inactive faulting

Niwa, Masakazu; Shimada, Koji; Aoki, Kazuhiro; Ishimaru, Tsuneari

Engineering Geology, 210, p.180 - 196, 2016/08

https://doi.org/10.1016/j.enggeo.2016.06.013

This study focused on microscopic features using particle size analysis, SEM and TEM and identified mineralogical and textural characteristics to improve our ability to discriminate between the active and inactive gouges. Particle size distribution analysis largely shows that fault gouge in an active fault, having the most abundant finer particles, experienced repetitive shearing whereas the inactive gouges did not. SEM examination of quartz particles shows that more particles retain their primary crystal structure in the active gouge than in the inactive gouges. In contrast, observation of very fine clay minerals using TEM shows the minerals in the active gouge are abraded and corroded in equal or higher degree as the aseismic gouges. Repetitive fault activities could result in the addition of new quartz particles from the granite with preservation of primary crystal structure, whereas relatively-soft clay minerals had been affected by abrasion and dissolution.

Formative mechanism of inhomogeneous distribution of fractures, an example of the Toki Granite, Central Japan

Sasao, Eiji; Yuguchi, Takashi*; Ito, Yasuto*; Inoue, Takashi*; Ishibashi, Masayuki

Proceedings of 10th Asian Regional Conference of International Association for Engineering Geology and the Environment (IAEG ARS 2015) (USB Flash Drive), 6 Pages, 2015/09

Understanding of the fracture network is important for disposal of high-level radioactive waste. We present our hypothesis on the formative mechanism of inhomogeneous distribution of fracture in the Toki Granite. In the Toki Granite, low- and high-angle fractures are abundant at the shallower part, while less at the deeper part where high-angle fracture is dominant. Distribution of the high-angle fracture is inhomogeneous. Thermochronological study revealed that the rapid cooling occurred at the early stage of granite formation. Paleomagnetic directions of the intact granite were dispersed. This suggests granite was plastically deformed during rapid cooling period. The rapid cooling might cause inhomogeneous distribution of cooling strain. When the granite reached to brittle deformation field, inhomogeneous fracture distribution was formed by the inhomogeneous strain. If so, recognition of the cooling history is essential to understand the distribution of the fracture network.

Three-dimensional fracture distribution in relation to local cooling in a granitic body; An Example from the Toki granitic pluton, Central Japan

Yuguchi, Takashi; Tagami, Masahiko*; Tsuruta, Tadahiko; Nishiyama, Tadao*

Engineering Geology, 149-150, p.35 - 46, 2012/11

https://doi.org/10.1016/j.enggeo.2012.08.009

This paper describes the spatial distribution of fractures in the Toki granitic pluton, Central Japan and their evaluation with respect to the pluton's cooling behavior. Three-dimensional frequency distributions of entire fractures, based on borehole television data collected from 19 boreholes, shows that (1) the central region of the granite has a high fracture frequency and (2) fracture frequency decreases with decreasing elevation (datum is mean sea level), with the exception of the west side of the body. The development of sub-solidus reaction features was measured at two neighboring positions, and the difference was divided by the distance between the two positions. The resulting quantity is here defined as "local cooling rate". We observed a significant correlation between the distribution patterns of entire fracture frequency and local cooling rate; the region with a high local cooling rate corresponds to the region with a high frequency of entire fractures, and vice versa. This correlation indicates that fracture genesis can be explained by "local cooling rate", combined with the concept of thermal stress. Thus, the three dimensional pattern of local cooling rate is a potential diagnostic tool for quantifying the distribution of fracture frequency.

Spatial distribution and characteristics of fracture zones near a long-lived active fault; A Field-based study for understanding changes in underground environment caused by long-term fault activities

Niwa, Masakazu; Kurosawa, Hideki; Ishimaru, Tsuneari

Engineering Geology, 119(1-2), p.31 - 50, 2011/04

https://doi.org/10.1016/j.enggeo.2011.01.011

Fracture zone development has a substantial impact on long-term changes in the underground environment because fracture zones are leading candidates as groundwater flowpaths as well as a locus for mechanical disturbance by fracturing. For understanding fracture zone development we surveyed the spatial distribution and characteristics of fracture zones in an area 4 km to the north and 6 km to the south from the western part of the Atotsugawa Fault, a long-lived active fault in central Japan. This study suggests that fracture zones along the Atotsugawa Fault act as a major conduit controlling regional groundwater flow. Based on rock features and deformation fabrics at meso- and microscopic scales, once fracture zones had formed, epigenetic deformation was concentrated in the ancient fracture zones.

Mineralogical and Petrological Evidence for the Hydrogeological Characteristics of the Tsukiyoshi Fault,Japan

Metcalf; ; Hama, Katsuhiro; Yoshida, Hidekazu; Iwatsuki, Teruki

Quarterly Journal of Engineering Geology and Hydrogeology, (35), p.189 - 201, 2002/00

None

Methodology development for modeling heterogeneous conductivity fields for a sandstone type uranium deposit, central Japan

Mikake, Shinichiro; Yoshida, Hidekazu; Koide, Kaoru; ; ; Yanagizawa, Koichi

Engineering Geology, 56(2), p.185 - 195, 2000/00

None