Karst terraces are widely distributed in Guizhou Province, China. “Wanfenglin” (forest of 10,000 peaks), located in the Southwest part of Guizhou Province, consists of a group of cone Karst at relative height of around 200 m. This name is an appropriate expression in Chinese, since the array of peaks like trees in the forest. All of this area is composed of a limestone plateau formed in the Triassic period. The dominant geological structure of this area is aligned in the direction of from northwest to southeast (the direction of the plane). The planar parts of Wanfenglin are rift valleys among the Karst terraces, in which several oblique geological grabens are observed. Concentric circles of paddy rice fields are spread out in alluvial planes and typical disc sinkholes (dolines) are distributed along several planes. They are deduced to be corrosion dolines because of their disc shapes.
(Photograph and Explanation: Keiji NAKAIE
Photographed on August 9, 2013)
From May 2016 to April 2017, Chichi-jima in the Ogasawara (Bonin) Islands suffered a severe drought. After an absence of 37 years from 1980, the ratio of effective water storage at four dams supplying drinking water on Chichi-jima dropped below 20%. Atmospheric conditions during the drought are studied and compared to those of severe drought years in 2011 and 1980. In comparison with the long-term mean from 1981 to 2010, a negative precipitation anomaly was apparent from May to July 2016, and from September to December 2016. It was also apparent in March and April 2017. From May to July 2016, the Indo–western Pacific ocean capacitor mode was responsible for less precipitation. From September to October 2016, the P–J (Pacific–Japan) pattern brought about less precipitation. After November 2016, similar atmospheric conditions did not continue for more than one month; however, Chichi-jima was covered with an anti-cyclonic circulation anomaly, except in March 2017 when a winter pressure pattern intensified. Less precipitation was also observed from January to July 2011 and from May to October in 1980. The former was attributable to an intensification of convective activity in the tropical western Pacific, accompanied by a sea surface temperature anomaly greater than +0.5°C. In the tropical western Pacific, a cyclonic circulation anomaly was found, while an anti-cyclonic circulation anomaly prevailed around Chichi-jima. In 1980, precipitation from the Bai-u frontal zone was rarely observed from May to June. From July to September, the Okhotsk High was extremely strong, so the Pacific High remained in the southern part rather than in the usual summer position. Fine weather continued until October, which together brought about the severe drought in 1980.
Shallow marine and non-marine Cretaceous strata of the Amakusa–Mifune area in central Kyushu, SW Japan, represent fore arc sediments of the Late Mesozoic arc-trench system along East Asia. To clarify their provenance and secular changes, U–Pb ages of detrital zircons from the Cretaceous sandstones of these units were measured. The age spectra of detrital zircons obtained from 10 sandstone samples, ranging from the Albian to Maastrichtian, clarify the following four aspects of provenance; i.e., (1) dominant exposure of Cretaceous granitoids throughout the Albian to Maastrichtian associated with minor Triassic and Paleozoic ones, (2) onset of influx of Jurassic and Paleoproterozoic zircons into the fore-arc domain in the Campanian, (3) essential resemblance in age spectra of detrital zircon to other coeval sandstones deposited in the fore-arc and intra-arc domain of East Asia (e.g. Kanmon Group on the continent side and the Monobegawa Group on the trench side), and (4) ubiquitous age spectra shared by coeval sandstones throughout SW Japan from Kanto to Kyushu along the arc. These suggest that the Cretaceous fore-arc of the East Asian margin featured the same provenance as dominant Cretaceous arc granitoids with small quantity of early-middle Mesozoic and Paleozoic granitoids from older orogens. A major change in provenance occurred in the Campanian, which allowed the influx of terrigenous clastics from much older continental sources, such as the Paleo-proterozoic basement. This probably reflects the disappearance of the pre-existing topographic barrier, which was probably composed of the uplifted Cretaceous arc batholith belt.
Because of limitations regarding available statistical data for units of year and enumeration, studies on socio-spatial patterns of Tokyo have not responded sufficiently to the dynamics of the so-called “doughnut phenomenon” period—a combination of urban sprawl and inner city decay—from the late 1960s to the early 1990s. Social maps for 1965 at the cho scale are created using statistics produced separately by the Tokyo Metropolitan Government. These maps are compared to those for 1980, because only the population censuses for 1965 and 1980 enumerate occupation data at this geographical scale. Subsequently, the dynamics of socio-spatial patterns in Tokyo for this period are examined. There were significant changes to names and territories of cho following enactment of the Addressing System Act of 1964. This was a serious problem for creating a social map on the cho scale, which was overcome by allocating either the 1961 cho territory or the current one according to cho name and date of statistics. The results of the analysis demonstrate that, in 1965, white-collar areas were formed as buffers around radial commuting train lines in western Tokyo, and areas between these in outer western Tokyo were not white-collar. Therefore, a star-shaped model, rather than a sector one, is suitable for representing socio-spatial patterns of the inhabitants of Tokyo in 1965. In 1980, the white-collar occupation ratio of these “between” areas rose rapidly, and a sectoral pattern clearly emerged. In eastern Tokyo, almost all areas were blue-collar in 1965, and moderate white-collar areas emerged along radial commuting train lines. The four main reasons for these changes are: 1) housing complex development at the sites of large plants; 2) abolishment of green belt policy; 3) new construction and expansion of radial train lines, especially subway lines; and, 4) expansion of sewage service areas. In western Tokyo, the culvertization of medium and small rivers, around which were former industrial areas, also made an important contribution to transforming blue-collar areas into white-collar areas. These results show the importance of infrastructure development in bringing about socio-spatial changes in Tokyo during this period.
This study classifies composite small-scale fans along the eastern foot of Mt. Ikeda into five categories, and estimates their formation ages to discuss factors controlling fan development. Besides, it describes microtopography of fan surfaces and surface geology (sedimentary units), focusing on one of the typical terraced small-scale fans to discuss small-fan forming processes. Methods are interpretation of aerial photographs for terraces division, field observations with microtopography measurements, and radiocarbon dating for overlying sediments of terrace deposits. The results show that the five terrace surfaces are mainly formed by a few debris flow deposits, and the estimated formation ages obtained with radiocarbon dating fall into the general classification of the Last Glacial Maximum (LGM) and the early Holocene. However, the difference between each formation periods is 1,000-3,000 years because estimated formation ages are about 20,000 years (I surface) and about 17,000 years ago (I surface) in the LGM, about 9,000 years (III surface), and about 8,000 years ago (IV surface) in the early Holocene. Geomorphic development of the study area might be difficult to explain using a simple formation model of river terraces with time scales of 104-105 years, expressed by dynamic river fluctuations under the full influence of global climatic changes. This speculation is highly suggestive for further investigations on the timing of debris flow sedimentations forming alluvial fans in this area.
Along the Sanriku coastal area, discrepancies in crustal movements have been suggested between uplift on a timescale of 105 years and subsidence on a timescale of 101-102 years. Assuming the cause of these discrepancies and details of tectonic history cannot be clarified immediately, knowledge of the Holocene sedimentary succession with extensive radiocarbon dating data may provide a basic and important guideline for feature analyses of crustal movements in this area. Therefore, a study was carried out on a sediment core, YMD1, acquired from the lower reach of the Yamada Plain, central Sanriku coast. Core YMD1 was divided into three units on the basis of lithofacies, grain size distribution, and molluscan assemblages. Unit 1 (10,000 to 9000 cal BP), consisting mainly of peat or peaty silt, was considered to be marsh sediments. Unit 2 (9000 to 6000 cal BP), consisting of pebbles to silt, showing an upward finning succession with inner bay molluscan shells, was considered to be inner bay sediments. Unit 3 (after 6000 cal BP), consisting of silt to coarse sand, showing upward coarsening succession with marine molluscan shells, was considered to have been deposited in upward-shallowing marine environment after 6000 cal BP. Accumulation rate decreased from 3 to 1 mm/yr between 10,000 and 6000 cal BP, which indicates a landward retreat of the river mouth caused by an expansion of the inner bay environment, and increased to 5 mm/yr after 6000 cal BP, which reflects shoreline progradation. These features of core sediments clearly show that the sediment successions were formed in relation to the postglacial transgression and subsequent regression. Although sediment successions include possible intertidal sediments, data obtained from this study cannot allow for the exclusive interpretation that these horizons are of an intertidal origin. As a result, rate of crustal movement cannot be estimated in the Yamada plain at present. Further investigations are needed to estimate the rate of crustal movement on a timescale of 103-104 years based on analyses of a number of all-core sediments within the plain.