Kibi Plateau Research Articles

Estimating the amount of uplift from the current elevation of strata at the Middle Miocene Climatic Optimum: A case study of Kibi Plateau in Southwest Japan

AbstractThe Middle Miocene Climatic Optimum (MMCO) formed shallow marine strata on continents and island arcs at ~15 Ma, and some of the MMCO strata later experienced uplift. The amount of uplift corresponded to sea level rise after the MMCO period, but ancient sea level could not be observed. This study uses a deductive approach to estimate the amount of uplift. The elevation of the youngest marine sedimentary layer of the MMCO strata approximates the sea level at the MMCO. The current elevation was reconstructed by adding the amount of denudation from the MMCO to the current elevation of the denuded MMCO strata at the outcrops, considering isostatic compensation. The amount of denudation since the MMCO period was estimated to be 180 m based on the global average denudation rate (Willenbring et al., 2013), and the estimated isostatic rebound was 153 m. By comparing the current, albeit virtual, elevation of the youngest marine sedimentary layer in the MMCO strata with sea level at the MMCO, the amount of uplift was estimated. For the MMCO sea level, a 66 m ice‐free line was adopted. This method was applied to the Kibi Plateau in Southwest Japan, where the MMCO strata are distributed. The reliability of the denudation amount of 180 m for the MMCO strata was confirmed by paleo‐water depths estimated using benthic foraminifera and mollusk fossils. The largest uplift of 504 ± 27 m occurred at the Jinseki‐Kogen since 15 Ma. A positive correlation was observed between the amount of uplift and the current altitude of the Kibi Plateau. The Kibi Plateau's topography most likely evolved after the MMCO. It is concluded that a new perspective on estimating the uplift was obtained by taking denudation into account.

Kibi Plateau A stable location within active Japan

Kibi Plateau A stable location within active Japan

Kibi Plateau: A stable-coherent tectonic unit in the active Japanese Islands

The Kibi Plateau in the active Japanese Islands consists of mainly Permian to Cretaceous rocks that have been deeply weathered into a red soil, comprising a peneplain with U-shaped valley. Systematic geological analyses of the Eocene fluvial deposits revealed the paleo-rivers that existed in the eastern Asian continent and streamed out to the paleo-Pacific Ocean. Each paleo-river is traced in a flow line shape without any significant vertical and horizontal displacement. The Eocene shallow marine sediments in a possible coastal region have no relevant inclination. These geological data strongly suggest that the Kibi Plateau has been a stable-coherent tectonic unit since the Eocene through the opening of the Japan Sea and the associated quick rotation of SW Japan in the Middle Miocene. The Kibi Plateau region with a thick crust over 30 km existed as a stable eastern segment of the Asian continent in the Eocene. The Kibi Plateau tectonic unit drifted to the south without any destruction due to the peripheral successive tectonic events such as the Philippine Sea plate subduction and the reactivation of Median Tectonic Line. No subduction related arc volcanism since the Eocene has also influenced to preserve the stable tectonic unit.

吉備高原と"山砂利問題"(フォト)

吉備高原と"山砂利問題"(フォト)

Paleogene “mountain gravels” and marine Miocene sediments on the Kibi Plateau in the north of Okayama City, southwest Japan

吉備層群（いわゆる「山砂利層」）は，ほとんど中～大礫サイズの亜円礫からなる，谷埋め成の地層である．時代決定に有効な化石は得られず，更新統とされていたが，稀に挟まれる凝灰岩層を対象とするフィッション・トラック年代測定によって，地層の定義や対比が行えるようになってきた．いくつかの堆積期があることが分かってきたが，岩相では区別しがたく，地層区分は高密度の踏査による地層の追跡が必要である．各層の基底は，地層を構成する礫を運んだ当時の河の谷地形を示す．この復元された古地形は，底からの比高が150m以上に達する深い谷地形である．これは一般的な沈降を続ける堆積盆に形成された地形より，むしろ後背地側の地形である．すなわち吉備層群には，一般的な沈降する堆積盆の地層に対する区分や定義の方法とは異なる，新しい取り組みが必要であり，堆積の要因についても考えなくてはならない．これらは案内者一同を悩ませ続けている課題であり，見学旅行を通じて議論をいただきたい．

吉備高原に分布する第三系のフィッション・トラック年代

Seven tuff samples, 5 from the Kibi Group (so-called mountain gravel) and 2 from the Katsuta Group were dated by using the fission-track technique. These samples were collected from the Kibi Plateau area in Okayama Prefecture. Fission-track ages of 34.3 ± 1.8 Ma (Yoshii Formation), 35.8 ± 1.3 Ma and 34.0 ± 2.7 Ma (Tomiyosi Formation), 27.4 ± 1.9 Ma and 27.1 ± 1.5 Ma (Tsudaka Formation) were obtained from the Kibi Group. Until now, the Kibi Group was considered to be deposited during the Quaternary period due to its unconsolidated nature. These dating results indicate that the Kibi Group is of Eocene to Oligocene in age. From the Katsuta Group, fission track ages of 17.9 ± 2.1 Ma (Yoshino Formation) and 16.2 ± 2.1 Ma (Takakura Formation) were obtained. These data agree with microfossil dating.

Sustainability of Rice Terraces

Paddy fields made on slopes in the form of a series of shelves are called “rice terraces”, or “tanada” in Japanese. These rice terraces are distributed over the whole country except Tokyo, Saitama and Okinawa, and they are particularly concentrated in the Kubiki Hills of Niigata Prefecture, on the Kibi Plateau in Okayama Prefecture and at the foot of the Aso-Kuju Volcanoes in Oita Prefecture. However, farming on these rice terraces has low labor productivity, and with advancement in the age of the farmers in the mountainous areas, it has become difficult for them to continue farming on the rice terraces, and the abandonment of them is now noticeable.Recently, however, the rice terraces have come to be viewed in a different light in their ability to control soil erosion and floods, along with the beautiful landscape which gives amenity and nostalgia to the people. For this report, three settlements endeavoring to sustain the rice terraces-namely, Shirayone, Wazima City in Ishikawa Prefecture, Maruyama, Kiwa-cho in Mie Prefecture, and Ohaka, Chuo-cho in Okayama Prefecture-are selected as the study areas.In Shirayone, Wazima City, the rice terraces are located along the tourist route. Thus, with support from the tourist industry, other economic organizations and the cultivation subsidies from the local government, the rice terraces has long been preserved. In Maruyama, Kiwa-cho, the local government has promulgated the regulations for the sustenance of the rice terraces on the one hand and has employed the farmers to restore the devastated rice terraces on the other hand together with their maintenance and management. In Ohaka, Chuo-cho, the size of operational holdings is relatively large and the agricultural co-operative association is encouraging the farmers towards sustenance of the rice terraces through organic farming and by farming without agricultural chemicals, thus adding value to the rice produced.Now, comparing the measures developed in these three settlements for the sustenance of rice terraces with one another, the following has been found.(1) In the case of a farm household of a small size of operational holdings, it has become difficult to maintain farming the rice terraces unless support of urban inhabitants is given or the government grants income compensation directly to the farm households.(2) In the case of a relatively large size of operational holdings, it is necessary for the sustaining of the rice terraces to increase the value added to the rice produced commensurate with the labor force invested, and one of such ways is the organic farming of paddy rice without agricultural chemicals.(3) As aged farmers retire from farming the rice terraces, the difficulty in securing people who will engage in farming will become a big problem in promoting the sustainability of the rice terraces.To solve this problem, it will be necessary to seriously face the problem of securing the labor force by recruiting farmers from among retired people living on pensions, weekend farmers and voluntary urban inhabitants.

The investigation of Lake Narutaki Dam

Lake Narutaki dam is situated in the Kibi Plateau, middle of the smooth hilly regions of Okayama prefecture. This dam has special water quality problems as it supplies city water. These problems are; (1) Coloring is caused by dissolved Mn and Fe from the bottom of the lake where oxygen is almost decreased to zero, (2) A nasty smell and bad taste of city water is originated from vegatative planktons. Therefore, the investigation into the actual condition of this lake was carried out from June to December of 1991. Mn and Fe were always found to be dissolved at the bottom of the lake in high concentration. In the middle of the lake, Fe concentration decreased but Mn had almost the same concentration as that of the bottom. At the surface of the lake, Fe concentration was 0.1-0.3 mg/L while Mn concentration was 0.02-0.4 mg/L. The vertical profile of the vegetative plankton in the water made no great difference until 4 m depth, but it radically decreased in deeper places more than 6 m depth. The following treatment methods are proposed from these data; (1) In order to avoid the influence and transport of Fe and Mn in the filter and subsequent back washing, the income pipe position should be varied in depth according to the season, (2) Longer contact with manganese powder for removing the color and prolonged contact with charcoal for removing nasty smell and bad taste, (3) Introducing advanced treatment for example Ozone treatment for upgrading the taste. With the use of above treatment methods, the problems are thought to alleviate. However, the measure of the origin of pollution is desirable to protect the water quality of this lake, because water is scarece in this region and the only source of water supply is this dam.

花こう岩地域における地下水流動解析 吉備高原地域をモデルとして

Though flow nets offer a valuable means of visualizing groundwater flow fields, accurate drawing of flow nets are generally difficult in complicated hydrogeological structure. A numerical method was developed for automatically generating a flow net, using the finite element method to calculate the stream function and piezometric head. In this paper, a availability of the method is demonstrated in three hypothetical test models and verticalplane models through the granite region of the Kibi Plateau, Okayama Prefecture. Results of the numerical simulation show that a high-permeable zone can converge flow lines, though low-permeable zone can not effectively dam up stream lines. Groundwater flow in the granite region of the Kibi Plateau is approximated by a porous media flow. Flow nets of six cross sections in the region are constructed using the numerical method. Several areas converging flow lines in the basement are detected, but available quantity of water is small. The greater part of groundwater flows in the high-permeable weathering crust (‘Masa’ zone). Flow net analyses can be useful to identify groundwater exploration zones in the granite region.

28. The tractional crystallizaion of Cenozoic alkaline basalts in Kibi plateau district.(Abstracts of Papers Presented at the Fall Meeting of the Society)

28. The tractional crystallizaion of Cenozoic alkaline basalts in Kibi plateau district.(Abstracts of Papers Presented at the Fall Meeting of the Society)

INCISED MEANDERS IN THE CHUGOKU MOUNTAINS, SOUTHWEST JAPAN

In the Chugoku Mountains, Southwest Honshu, there are many incised meanders, but few free meanders. The purpose of this paper is to inquire into the distribution and the origin of incised meanders in the Chugoku Mountains, and to consider the relationship between their wave-lengths and drainage areas and also their relation to rock properties. As is shown in Fig. 1, the distribution of the incised meanders in the Chugoku Mountains coincides with the distribution of planation surfaces which are presented as “gentle slopes” in the 1:500, 000 geomorphological map (Economic Planning Agency, 1968). A half of all the incised meanders cuts into the Kibi Plateau Surface, and they are concentrated in the areas of the Kibi and Iwami Plateaus. While one third of all the meanders cut conse-quently the boundary slope between the Kibi and the Backbone Ridge Surface. The in-cised meanders which cut into the erosion surfaces are either entrenched meanders and ingrown intrenched meanders. They may be inherited from free meanders. The incised meanders that develop on the boundary between erosion surfaces are of the type of ingrown meanders, and have no erosion surface that might have produced free meanders, because they can develop during an initial cycle of erosion. In comparison with incised meanders in Paleozoic rocks, those in granitic rocks are greater in the width of the meander belt and of the valley. According to the histogram of sinuosity of incised meanders (Fig. 5), incised meanders in granitic rocks have greater sinuosity and variance, the mean sinuosity being 2.10 and the variance 0.75, than those in Paleozoic rocks, of which the corresponding values are 1.96 and 0.59 respectively. Many incised meanders in granitic rocks are structurally controlled by joints (Fig. 4), and have an irregular and zigzag pattern of stream courses. On the contrary, those in Paleozoic rocks generally form regular and smooth curve. Although geologic structure and lithology affect the shape and scale of the incised meanders, the incised meanders in the Chugoku Mountains develop in all sorts of bedrocks, and have no preference to particular rocks. Figure 6 shows the relation between the wave-length and the drainage area for the incised and free meanders in the Chugoku Mountains as well as for those in Europe and North America. According to this, there is no substantial difference between the relations for the Chugoku Mountains and for. Europe and North America. However, the. streams in the Chugoku Mountains have not manifestly underfitted due to climatic change, as in the case of Europe and North America. From the above mentioned relations, the former drainage area before stream capture can be estimated for three rivers, as shown in Fig. 6.

Petrological studies of the alkali basalts from eastern San-in, Japan (3)

Basalts of Kibi plateau are exposed as many domes of small scale and a few lava flows as well which were erupted in Pleistocene age. They all belong to the alkali rock series and are classified into olivine or titanaugite olivine trachybasalt, titanaugite olivine trachydolerite and titanaugite olivine trachyandesite. Eleven of them are chemically analysed. The distinct characteristics of the rocks of this province are as follows : 1) They are chemically richer in MgO and poorer in Al2O3 because of the abundance of olivines, titanaugites and xenocrystic clinopyroxenes. 2) Some have properties which closely resemble those of limburgite in both mineral and chemical constituents. 3) Olivine and pyroxene rich inclusions are contained in the rocks of this province, and especially in Arato-yama and Yunodan-yama are found the largest occurrence of the inclusions in Chugoku area. 4) Zeolite like mineral occurs frequently in the marginal part of the inclusions.

On the Regional Differences of the Structure of Agricultural Productive Powers

Agricultural productive powers vary with districts. The degree of mechanization and the yield per unit area may be used as the measures for indicating difference of productivity. In the case of Okayama prefecture, I got the following results:I. one part is remarkably different from another in its development. The prefecture may de divided into four regions according to the degree of their development: (I) The southern plain region. (II) The southern edge of Kibi plateau region. (III) The Kibi plateau region. (IV) The Chugoku mountain region.2. The regional difference in agricaltural productive powers has close relationship to the difference in the process of modernization of village life. The more remains of feudalism, the less agricultural productive powers.3. The degree in agricultural productive powers can be considered as a scale to measure the degree of agricultural modernization.4. Low productive powers is useful to keep feudalism and feudal customs.It is very important to promote social and economic modernization to develop agricultural productive powers.

吉備高原の散居村落の形態と散居の成立

吉備高原の散居村落の形態と散居の成立

The Distribution of Degree of Paddy and Upland-field in the Kibi Plateau

The Distribution of Degree of Paddy and Upland-field in the Kibi Plateau

Outline of the Decken-Structure of the Kibi Plateau in the Inner Side of Western Japan

Outline of the Decken-Structure of the Kibi Plateau in the Inner Side of Western Japan

Summit Level and River Level, of the Kibi Plateau

Summit Level and River Level, of the Kibi Plateau