HCG26-P01

Japan Geoscience Union Meeting 2016

Discriminant functions for formative conditions of bedforms in open-channel flows*Koji Ohata1, Hajime Naruse1, Miwa Yokokawa2
1.Kyoto University, 2.Osaka Institute of Technology
Fluvial bedforms such as ripples and plane bed are formed by interactions between flows andprocesses of sediment transport. Sedimentary structures formed by bedforms, hence, are clues toreconstruct paleo-flow conditions. For analysis of sedimentary structures, bedform existence
diagrams have been proposed on the basis of laboratory and field observations, and have been widelyused. To utilize the bedform existence diagrams for analysis of sedimentary structures, boundarylines of bedform stability regions are significant.
This study provides boundary surfaces of bedform stability regions defined as discriminantfunctions of dimensionless parameters. In previous studies, the lines were described manually
without quantitative examination. Thus, previous studies that apply these diagrams to sedimentary
structures lack sufficient statistic basis. To this end, this study obtained boundary lines aspolynomial equations of dimensionless parameters.
First of all, we defined a new bedform existence diagram based on 3272 existing laboratory andfield data. We used three parameters in dimensionless form. Some of diagrams in previous studies
were not sufficient in the number or kinds of parameters. This study produces a new bedformstability diagram, which defines bedform stability regions by three dimensionless parameters:
dimensionless grain size D*, Shields mobility parameter τ* and Froude number Fr.
On the basis of the diagram described above, discriminant functions are derived by following
procedures. At first, a polynomial function with arbitrary coefficients which divides parametricspace is supposed as a candidate of a discriminant function. Then, the ratio of data points ofbedform experiments that was judged incorrectly by the polynomial function is calculated. At last,in order to minimize the ratio, coefficients of polynomial function is optimized.
The discriminant functions of bedforms obtained in this study can serve quantitative paleo-flow
analysis of sedimentary structures. Their form is Fr = ƒ(D*, τ*). This function can be recast in
the form U = ƒ(D, h), where U = flow velocity; D = sediment particle size; h = flow depth. Thus,
when we obtain the data of sediment particle size and flow depth from sedimentary structures, we
can estimate paleo-flow velocity. The discriminant functions are useful and objective for
estimation of paleo-flow conditions.
Keywords: Bedform, Discriminant analysis©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P01 -HCG26-P02

Japan Geoscience Union Meeting 2016

Quantitative analysis of erosional marks at the bases of sediment gravity flow deposits*Yoshiro Ishihara1, Mayuko Yumi3, Yuri ONISHI2, Ayako Okuma3
1.Department of Earth System Science Faculty of Science, Fukuoka University, 2.Graduate School of
Fukuoka University, 3.Aero Asahi Corporation
Erosional marks, such as flute and groove marks, are frequently observed at the base of sedimentgravity flow deposits, especially for turbidites. It is suggested that the flow velocity and theduration of the erosional flow affect the distributions and sizes of the marks. Additionally, thesubstrate materials and the characteristics of the erosional flow have an impact on the marks. A
large olistolith in the Nichinan Group is distributed at the Izaki Cape, Nichinan City, Kyushu
Island, Japan. The olistolith is a block of a turbidite succession deposited in a deep sea
environment. Many turbidites deposited on the channel levee and the frontal splay in the block havevarious types of erosional marks. The distributions and sizes of the erosional marks on theturbidites in many parts of the block suggest differences in the erosional flow duration indifferent areas, even if it is over an individual turbidite bed (Yumi and Ishihara, 2012). The
results suggest that the flow velocity and the duration of the erosional flow deduced from theerosional marks provide additional information about the depositional settings and thepaleotopographies of deep sea environments. Yumi and Ishihara (2012) analyzed the distribution andsizes of the erosional marks on a two-dimensional horizontal surface; i.e., they analyzed theplanar projections of the erosional marks. However, because most of the erosional marks areobserved as a cross section on an outcrop surface, three-dimensional topographical analyses should
be carried out. In the present study, we show results of a three-dimensional analysis of outcrop
examples of the erosional marks in the Izaki Cape and those formed by flume experiments.
Yumi and Ishihara, 2012, Jour. Sedim. Soc. Japan, 71, 173-190
Keywords: turbidite, flute mark, geomorphic analysis©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P02 -HCG26-P03

Japan Geoscience Union Meeting 2016

Investigation of numerical forward model toward inverse analysis of ancient turbidities:
Comparison between results of numerical simulation and grain-size analysis of the ancientturbidite bed in the Pliocene Kiyosumi Formation, Boso Peninsula, Japan*Kento Nakao1, Hajime Naruse1
1.Department of Geology and Mineralogy, Graduate School of Science, Kyoto University
Turbidity currents are considered as a main mechanism of sediment-transport toward deep-seas, anddeposits of turbidity currents, i.e. turbidite sandstones, can be reservoir rocks of petroleum.
Therefore it is important to understand the behavior of turbidity currents not only from theperspective of earth sciences, such as sedimentology and stratigraphy, but also resource geology.
However, it is quite difficult to observe turbidity currents directly because of theirhigh-velocity and intermittency of occurrences, so that the detailed mode of sediment transport anddepositional processes remain unclear. To this end, the methods for inverse analysis to reconstruct
flow conditions of turbidity currents from thickness and grain-size distribution of ancientdeposits have been developed. However, previous studies have difficulty in feasibility of
application to natural examples because of calculating costs of their forward model (2D DNS model).
Here we examine that applicability of the forward model of turbidity currents based on the
non-steady 1D shallow-water equation for inverse analysis of ancient turbidites. The 1D
shallow-water equation model of turbidity currents is superior to the DNS model from the viewpointof calculation costs, but it is not fully tested for flows that contain sediments of multiple
grain-size classes.
First, this study investigated thickness and grain-size distribution of the ancient turbidite bed
in the Mio-Pliocene Kiyosumi Formation, Boso Peninsula, Japan. The Formation is composed ofsand-rich alternating beds of sandstone and mudstone, which have been interpreted as deposits ofthe submarine fan. This study focused on the turbidite sandstone G1, which is sandwiched between
two characteristic tephra and therefore it can be traced over 40 km. We measured thickness of thesandstone bed and collected samples for grain size analysis by using the settling-tube method. As aresult, it was revealed that (1) bed thickness decrease downcurrent non-linearly. There are points
where volume per unit area of each grain-size class decrease remarkably. (2) Locations of thepoints where sediment volume per unit area decrease vary depending on grain size classes. Then, we
conducted numerical experiments to reproduce geometrical features of the turbidite bed describedabove. We employed 1D shallow water equation model with sediment conservation equations of eachgrain-size class and active-layer approximation of grain-size distribution of the basal surface. In
this study, grain-size distribution is approximated to the two grain-size classes: medium sand (2phi) and silt (8 phi). As a result, distribution pattern of sediment volume per unit width was well
reproduced by our model. Sand-sized sediments pinched out downcurrent, whereas silt-sized sediments
show continuous thickness distribution downcurrent. This result imply that the inversion modelusing the forward model based on shallow-water equation could be applicable for ancient turbidites
that have 10s km in spatial scale.
Keywords: Kiyosumi Formation, numerical simulation, turbidity currents©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P03 -HCG26-P04

Japan Geoscience Union Meeting 2016

Changes in shapes and microtextures on quartz grains of fluvial sediments at Hime Kawa.
*Hiromi Itamiya1,2, Toshihiko Sugai2
1.National Research Institute of Police Science, 2.The Graduate School of Frontier Science, The
University of Tokyo[Introduction]
Quartz is a highly resistant and ubiquitous mineral in nearly every environment. Microtextures suchas small pits and conchoidal fractures can be observed on quartz grain surface. Many researchers
have observed quartz surface by scanning electron microscope (SEM) and studied to link
microtextures to the sedimental environments and transport mechanisms (Krinsley and Doornkamp,
1973; Mahaney 2002). Quartz microtextures have been mainly examined in continental regions such as
Europe. It is supposed that geological and geomorphological features relate to the frequency
distribution of microtextures. However, little is known on the microtextures of quartz deposited inorogenic belt and monsoon region like Japan.
In this study, we focused on the fluvial sediments in Japan. The changes in the surface textures ofquartz grains were determined from upstream to downstream areas of a steep river, Hime Kawa. The
shapes of the quartz grains were also examined.
[Methods]
The studied area was situated in Hime Kawa in Nagano and Niigata Prefectures. Seven fluvial
sediment samples from the bed of Hime Kawa and one nearshore sample near the mouth of Hime Kawa
were collected. Carbonates, iron oxides and organic matters were removed from samples by chemicals.
The quartz grains in the size range of 0.1mm up to 1mm were sputter coated for 60s with a currentof 40 mA in a palladium coating. Fifteen grains per sample were observed by SEM (JEOL, JSM-6610LV)
in high vacuum mode and the voltage was 25kV.
[Result and Discussion]
Most of the quartz grains in fluvial samples show angular outline and there are no clear changes
along Hime Kawa. The nearshore sample also contains angular grains which resemble the fluvial
samples. Most of the grains in upstream area have high relief, and the relief gets slightly lower
along Hime Kawa. These changes may be caused by the collision of the grains in subaqueous
environment. Some grains show characteristics shapes, namely twinned quartz. Both penetration twinand contact twin are found in fluvial samples.
On the quartz surface, conchoidal fractures, V-shaped percussion cracks and straight steps arefound in both fluvial and nearshore samples. It has been proposed that they are produced by apowerful impact or pressure on the grain surface in high-energetic subaqueous environment (Vos,2014). In our study, frequency distribution of each microtexture has not drastically changed except
small pits.
Small pits (< 5μm), which derives from small inclusions in quartz, can be abundantly observed in
the samples along upstream area of Hime Kawa. They are observed sparsely along downstream area. It
means the amount of inclusions in quartz is quite different between upstream and downstream areasat Hime Kawa. The tributaries such as Tsuchitani Gawa and Nakatani Gawa flow into the middle
reaches of Hime Kawa. Quartz particles we observed in this study may have different derivations dueto the sediment supply from tributaries in the middle reaches.
Keywords: quartz, surface texture, SEM, twins, provenance study©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P04 -HCG26-P05

Japan Geoscience Union Meeting 2016

Use of statistical information to characterize sedimentary facies of alternating
successions: an example using gamma rays and graphic logs
*Yuri ONISHI1, Hana Sasaki1, Yoshiro Ishihara2, Osamu Takano3
1.Graduate School of Science, Fukuoka University, 2.Faculty of Science, Fukuoka University, 3.JAPEX
Alternating successions consisting of monotonously rhythmic alternating beds occur in various
depositional environments. Because quantitative data can be obtained from these alternating
successions, they sometimes have been quantitatively analyzed sedimentary facies, in addition to
traditional qualitative sedimentary facies analysis work conducted by geologists. Especially,
well-log data of turbidite successions are frequently and quantitatively analyzed because turbidite
successions can be important reservoirs for oil and gas (Rider and Kennedy, 2011). However,integration of well-log analysis data and traditional sedimentary facies analysis data from graphic
logs is often not achieved because quantitative studies of sedimentary facies are rare. In thisstudy, we examined common statistical indices that can be used to characterize sedimentary facies
from well-log data and graphic logs collected on the same turbidite succession.
In regard to the well-log analyses, spectroscopy gamma ray (SGR) data were mainly used because they
are the most sensitive parameter for lithofacies. The SGR data commonly have a poor resolution for
individual beds because the vertical resolution is only 15 cm. Thus, graphic logs were interpreted
for equal interval data corresponding to the SGR data. The lithology, ratio of sandstone to
mudstone, and vertical grain size variation were used in the sedimentary facies indices to identify
the lithofacies of turbidite successions. These parameters were grouped into clusters indicative ofthe different sedimentary facies through a cluster analysis technique.
Lithologies from SGR data were identified by the lamina identifying method proposed by Sasaki etal. (2015). Ratios of mudstone to sandstone in analysis windows were then estimated by using theidentified lithology. Stratigraphic grain size variations were estimated by using differences in
the SGR data of upper and lower parts in the analysis windows. In the case of graphic logs, ratios
of mudstone to sandstone and stratigraphic grain size variations were estimated from the equalinterval lithological data.
As a result of the cluster analysis, 12 clusters were derived. These clusters contained information
about the lithology, ratio of mudstone to sandstone, and stratigraphic grain size variation. For
example, the cluster information of sedimentary facies was useful for distinguishing between
weathered sandstone or mudstone, sandy or muddy horizons, and upward fining or coarsening in thefacies. We show the results for the identified sedimentary facies by considering the verticaltransitions in these types of information.
References
Rider, M. and Kennedy, M., 2011, The Geological Interpretation of Well Logs. 432p.
Sasaki et al., 2015, Journal of the Sedimentological Society of Japan, 74, 31-43.
Keywords: sedimentary facies analysis, turbidite succession, gamma-ray log, cluster analysis©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P05 -HCG26-P06

Japan Geoscience Union Meeting 2016

Record of thickness and fluorescence intensity of annual layers in a stalagmite of Shiraho
Saonetabaru Cave, Ishigaki Island*Hana Sasaki1, Yuri ONISHI1, Yoshiro Ishihara2, Kazuhisa Yoshimura3
1.Graduate School of Science, Fukuoka University, 2.Department of Earth System Science, Fukuoka
University, 3.Faculty of Sciences, Kyushu University
Various paleoenvironmental proxies, such as fluctuations in oxygen isotope ratio and Mg/Ca, havebeen extracted from stalagmites using geochemical analyses. In these studies, fluorescent annual
layers observed in stalagmites are often used as a proxy for time. Although the thickness of theselayers has been interpreted as a proxy for precipitation in some paleoenvironmental studies (e.g.
Burns et al., 2002), some items, particularly concerning the formation process of the annual
layers, remain unclarified. Furthermore, implications regarding the fluorescence intensity ofannual layers have not yet been examined. In this study, a lamina analyzing method, developed by
Sasaki et al. (2015), was applied to a stalagmite obtained from the Shiraho Saonetabaru Cave,
Ishigaki Island. Accumulative changes in thickness and fluorescence intensity of annual layers are
examined and these time series are then used to clarify the paleoenvironment.
The study cave is located near New Ishigaki Airport on the east coast of Ishigaki Island. In thisarea, some caves, including the Shiraho Saonetabaru Cave, developed in the Ryukyu Limestone
Formation, which extends from west to east. Numerous fossils, including human and animal bones,
were discovered from cave deposits of the Shiraho Saonetabaru Cave (e.g. Nakagawa et al., 2010).
Using the lamina analyzing method, annual layers representing approximately 600 years (2015 1415) have been identified. The average thickness of annual layers in the stalagmite is about 0.35mm, slightly thicker than those of caves on Honshu Island. In time series of annual layer
thickness, remarkable thin layers are observed during 1540-1580 and 1790-1810. Commonly, stalagmite
growth rate is influenced by dripping water intervals, cave air temperature, and supersaturation
degrees with respect to calcite in dripping waters depending on the partial pressure of CO2 in the
cave air. The partial pressure of CO2 is predominantly affected by the degree of cave air
circulation, because cave temperature is almost constant throughout the year. Therefore,
significant decreases in layer thickness can be related to periods of decreasing intervals of rainwater precipitation or cave air circulation. Relative fluorescence intensity decreases noticeably
during 1480-1500, 1530-1540, 1880-1890, and 1960-1990. The fluorescence intensity is thought to bemainly influenced by the growth rate of annual layers and the flux of fulvic acid in dripping
waters. Namely, fluorescence intensity is weaker when the growth rate of annual layers is higher
and/or the flux of fulvic acid in dripping water is lower. However, both annual layer thickness andfluorescence intensity decreased during the period 1480-1500, suggesting that the flux of fulvic
acid in that period was significantly low. We discuss the potential for paleoenvironmental proxies
suggested by these results.
References
Burns et al., 2002, Journal of Geophysical Research, 107, 4434-4442;
Nakagawa et al., 2010, Anthropological Science, 118, 173-183;
Sasaki et al., 2015, Journal of the Sedimentological Society of Japan, 74, 31-43
Keywords: annual layer, stalagmite, time series analysis©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P06 -HCG26-P06©2016. Japan Geoscience Union. All Right Reserved.

Japan Geoscience Union Meeting 2016

- HCG26-P06 -HCG26-P07

Japan Geoscience Union Meeting 2016

Variation in depositional conditions across the Toarcian (Early Jurassic) OAE: Mudstonelithofacies analysis of the Nishinakayama Formation*Kentaro Izumi1, David B. Kemp2
1.Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental
Studies, 2.The University of Aberdeen
The early Toarcian (Early Jurassic) oceanic anoxic event (T-OAE) was a significant
palaeoenvironmental perturbation that led to marked changes in ocean and atmospheric chemistry.
This event is characterized by the widespread occurrence of a ~3–7‰ negative excursion in the
carbon-isotope (δ13C) composition of marine organic and inorganic matter and terrestrial plantmaterial. In addition, one of the distinct phenomena during the early Toarcian is the abrupt riseof pCO2 and consequent global warming, which led to tropical to subtropical storm intensification.
Although such storms are predicted to have been intensified globally at low to mid latitudes during
the Early Jurassic, tropical storm intensification outside the Tethys realm has not been
demonstrated. To address this issue, we investigated the Nishinakayama Formation of the Toyora
area, southwest Japan, which represents an organic-rich silty mudstone-dominated succession
deposited at the shallow margin of the northwestern Panthalassa Ocean. First, we established ahigh-resolution carbon-isotope chemostratigraphy. As a result, the characteristic T-OAE negative δ13C excursion was recognized around the middle part of the Nishinakayama Formation, making accurateinternational correlation possible. Then, we carried out a mudstone microfabric analysis toreconstruct hydrological and sedimentological changes. Our results indicate that the Nishinakayama
black silty mudstones exhibit a variety of microfabrics, sedimentary structures, and textures.
These features indicate a dynamic range of depositional conditions. In particular, we note evidence
for mudstone deposition by bottom currents, and silty mudstones exhibiting evidence of energetic
conditions are concentrated during the carbon-isotope negative excursion interval. Mudstones fromthe pre- and post-excursion intervals generally show parallel-laminated features that suggest
settling from suspension. These sedimentological changes, in combination with carbon isotope
stratigraphy, provide the first evidence for storm intensification during the T-OAE interval fromthe mid latitude Panthalassic margin. Our results are consistent with previously published papers
documenting sedimentological changes in Tethyan T-OAE successions.
Keywords: Toarcian oceanic anoxic event, Panthalassa, Nishinakayama Formation©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P07 -HCG26-P08

Japan Geoscience Union Meeting 2016

A rationale of shoreline autoretreat provided by the grade index model*Tetsuji Muto1, Hajime Naruse2
1.Department of Environmental Science, Nagasaki University, 2.Graduate School of Science, Kyoto
University
The dynamics of delta distributary channels can be intensely affected by basin water depth in frontof the delta, particularly in terms of a long time scale. The previous experiments conducted in use
of tank facilities suggest that with deeper basin water, delta distributary channels have lower
rates of alluvial aggradation and lateral migration. If the delta faces very deep water, thechannels attain alluvial grade and are stabilized in a particular position in the delta plain. Thiseffect of basin water depth can be numerically expressed with grade index (Gindex), which ranges
between 0 (perfect alluvial aggradation) and 1 (alluvial grade). Given basin water depth (h),delta plain radius (x) and alluvial slope (Sa), we define dimensionless basin water depth as h*=h/S
ax.

Assuming a set of particular geometrical conditions that (1) the basin floor is flat andhorizontal, (2) the delta is always attached with a vertical wall on the back, and (3) base levelis stationary, grade index is given as Gindex=(1+2h*+Sa*h*2)-1, where Sa* is alluvial slope normalizedwith the delta’s foreset slope Sf (i.e. Sa*=Sa/Sf).
In a hypothetical setting where (1) base level rises at a constant rate (i.e. h also increases inproportion to time), (2) the entre sediment supplied from the outside of the system is constant(rate Qs) and accumulates as part of the delta, and (3) the delta’s angle parameters are always
retained constant, time derivative of the delta’s volume V(x,h) is equal to Qs. Based on thisrelation, easy calculation leads to a dimensionless progradation rate (Rpro*) of the delta: Rpro*=(1-AB*)Gindex, where AB* is the delta’s bottom surface area that is made dimensionless with
autostratigraphic 3D length scale L3D. It follows that shoreline autoretreat starts when AB* exceeds
unity. By a similar procedure, we find that the dimensionless rate of alluvial aggradation (Ragg*)
is given by: Ragg*=A*+(1-AB*)Gindex, where AB* is the delta plain’s horizontal cross section area thatis made dimensionless with autostratigraphic 3D length scale (L3D). When the retreating shoreline
arrives at the back wall, alluvial plain disappears and the entire depositional system is drowned(autodrowning). At this critical moment, Gindex=0 and A*=0, thus Ragg*=0.
The argument above brings a proposition that the shoreline autoretreat-autodrowning sequence, as a
non-equilibrium response of the delta to steady sea level rise, is closely related to grade index.
This sequence is due to the delta’s progressive expansion and increasing basin water depth (i.e.
sea level rise), and thus clearly related to grade index. The grade index model provides a novel
rationale for the occurrence of the autoretreat-autodrowning sequence.
Keywords: deltas, shoreline autoretreat, grade index, sea level rise, nonequilibrium response©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P08 -HCG26-P09

Japan Geoscience Union Meeting 2016

Verification of the method to reconstruct histories of crustal uplift rate from river
longitudinal profiles: Application to the central part of Boso Peninsula, Japan*Yuika Nakajima1, Hajime Naruse1
1.Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University
A method for estimating spatial and temporal histories of crustal uplift rate from longitudinalriver profiles was proposed by Roberts and White (2010). When bedrock is uplifted, an abrupt change
in gradient (i.e., knickpoint) occurs along the river bed, and then subsequent erosional processes
cause upstream-migration of knickpoints. Therefore, river longitudinal gradient change reflects thehistory of uplift rate of bedrock, where older history is recorded in the upstream region. For
analyzing river longitudinal profiles, a forward model that calculates incisional processes ofriver profiles from uplift rate histories with four erosional parameters was developed. Then, theinverse algorithm that minimizes the misfit between calculated and observed river profiles by
optimizing uplift rates and erosional parameters was produced to obtain spatial and temporal
patterns of regional uplift rate histories. This method is superior to other existing methods for
estimating spatial variation of bedrock uplifting rate, and has been applied on profiles of rivers
in stable continental regions (e.g., Africa and Australia). However, it has not been tested totectonically active regions such as the Japan Islands.
Here, we aim to verify the method for analyzing river profiles in tectonically active regions where
the uplifting rates of bedrock varies in smaller spatial and temporal scales. We investigateduplift rate histories of the central part of the Boso Peninsula in Japan, and examined theeffectiveness of the method, comparing results of multiple rivers that locate in same region. As aresult, we found that the uplift rate histories estimated from multiple rivers were inconsistenteach other although they locate on the same tectonic terrain. We infer that cause of thisinconsistency in estimated uplifting histories can be attributed to the heterogeneity inerodibility of the bedrock in the study area. The forward model employed in this study assumes thatthe erodibility of bedrock is constant in space; however, several knickpoints actually locate at
the lithological boundaries, suggesting the possibility that variation in erodibility should betaken account in the model. Thus, we suggest that parameterization of physical controls such aslithologic variation is significant in order to improve the method to estimate uplift rate
histories from longitudinal river profiles in tectonically active regions.
Keywords: river longitudinal profile, inverse analysis, erosion©2016. Japan Geoscience Union. All Right Reserved.

- HCG26-P09 -HCG26-P10

Japan Geoscience Union Meeting 2016

Deep-seated Rockslide Avalanches of The Eastern Sekita Mountains, Niigata Prefecture,
Central Japan; Pleistocene tectonic events*Toshihiro Asahina1
1.PASCO CORPORATION
1. Introduction
Sekita Mountains, which locate near the border on Niigata and Nagano Prefecture, is mainly composedof Quaternary system. It is known that there are a number of huge collapses of the terrain in thisarea. This report is intended to describe the huge collapse of the Pleistocene that develops in thenorth side of Sekita Mountains.
2. Topographic Setting
East of Sekita Mountains continues from Mt. Hishigadake (1,129.2m) Mt. Sanpousan to Mt. Amamizuyama(1,088m), forms to continue flat-top peak and is reducing the gradually advanced to the east. The
Nonomi pond is distributed widest small undulating flat surface in Sekita Mountains. Northern sideof Sekita Mountains develops continuous with the steep cliff from 150 meters to 400 meters in
relative elevation. Following on the north side of the steep cliff, small undulating flat spreads
widely.
3. Geologic Setting
Volcanic rocks of Uonuma Group that is from the late Pliocene to the late Pleistocene in age are
distributed widely in the southern slopes of ridgeline of Sekita Mountains. However, in the northside of this ridgeline sedimentary rocks of Uonuma Group are distributed widely, the lower strata
are distributed more as go to the north.
4. Topography and geology of the huge collapse slopes
Nonomi Collapse has scarp of about 150 meters in relative elevation and sliding width of about
2,000m is ascertained. Sediment deposition surface formed by collapse is ascertained Syoubu
plateau. The horizontal travel distance from the scarp reaches about 2,700 meters. The thickness ofthe deposit will be less than 50 meters. Tensui Collapse has the sliding height to reach from 200
meters to about 400 meters in relative elevation and the sliding width of about 2,600 meters.
Sediment deposition surface can be ascertained as a small relief flat surface with sliding block inastringent the Shibumi River. The horizontal travel distance reaches about 4,500 meters. The
thickness of the deposit will be less than 50 meters and it is estimated to be from 15 meters to 30
meters on average.
Most of the sediment is andesitic one, rarely there is deposit which is mainly composed ofsiltstone. Features of andesitic deposits have no sedimentary structure with or in water, composedof gravel, such as andesite, siltstone, sandstone, tuff and “kusare” gravel, matrix be composed ofsandy silt, tuffaceous silt. The collapse big event is estimated as follows; collapse consisting
mainly of siltstone as the initial event occurred, after which the collapsed mainly composed ofvolcanic rocks occurred at many times. Equal coefficient of friction (H/L) are Nonomi Collapse
0.18, Tensui Collapse 0.14.
Hinterland of huge collapse, namely the south side of the mountain slopes from the ridgeline iswhere Kubota et.al (2015) has claimed the Sagging slope. It can be seen the depression terrain oflarge and small like valley and many cliffs in the hinterland around the huge collapse. It is acharacteristic that many small cliffs show the displacement of the southern inclination.
5. Conclusions
It is estimated that huge collapse, which has occurred in Uonuma Group distribution areas aroundthe north of Sekita Mountains, had not intervening water as debris flow. Both large-scale collapse©2016. Japan Geoscience Union. All Right Reserved.

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Japan Geoscience Union Meeting 2016

and edifice deformation developed in the eastern mountains of this area and the Sagging terrain
formation in the south of the ridgeline are important big event related to the edifice formation.
6. References
Takeuchi, K., et al. (2000) Geology of the Matsunoyama Onsen District. With Geological Sheet Map
at 1:50,000, Geol. Surv. Japan, 76p. (in Japanese with English abstract 5p.).
Kubota, Y. et al. (2014) Sagging landform and its formation factor in the southern foot of the
Sekita mountains, Niigata-Nagano border area, central Japan. AGCJ, Monograph 60, pp143-160.
Keywords: rockslide avalanche, giant collapse, Sekita Mountains, Uonuma Group©2016. Japan Geoscience Union. All Right Reserved.

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