In Japan trilobites are profuse in the Lower Devonian Fukuji Formation in the Hida plateau, followed by the Middle Devonian ones in the Nakazato Formation in the Kitakami Mountains where they declined in the late Devonian Tobigamori age. These Lower Devonian ones are intimate to the contemporaries in Manchuria and Mongolia, while the later ones bear relationship with those in South China and Southeast Asia, though in the lessened degrees.
Upper Cretaceous strata Izumi Group which is distributed in the Kami' ita' cho-Hiketa' cho area, located in the eastern part of the Asan Mountains (fig. 1) is stratigraphically divided into the following six formations inascending order as shown in fig. 7 : Gosho Formation (900m+ in thickness), Kawamata F. (330-450m), Izumitani F. (150-800), Miyagatani F. (130-350 m), Ohyamahata F. (110 m-420 m) and Osaka F. (1500m+). The total thickness of these formations exceeds 4420 m. The present strata is lithologically subdivided into three units, namely the northern, central (axial part of the sedimentary basin) and southern parts (figs. 2, 3, 4 and 5). Turbidited facies are prevalent in the central part. The turbidites consist mostly of 20-300 cm of stratified sandstone and of 10-50 cm of alternating bedded sandstone and mudstone. On the contrary, the northern and southern parts mainly consist of mudstone which interfingers with turbidites along the boundary between the central part of the sedimentary basin. The sole marks which were measured at 118 points in the present area show mainly an east to west orientation (figs. 8, 9 and 10). Other orientations, e. g. from north to south or from south to north, were hardly found. The orientations of sole marks suggest that turbidity currents run in the direction of the axis of the sedimentary basin. Pyroclastic materials (fig. 6), which are well-stratified in the strata, are rhyolitic in composition. These were transported from the easternareas by the bottom currents and deposited normally in the present area.
By a series of studies to clarify the humus characteristics of volcanic ash soils and soil forming environment, the influence of climate variation was recognized in the eastern foot area of Nantai Volcano, Nikko (Watanabe 1985, 1987). The purpose of this study is to obtain useful information for the reconstruction of global scaled environmental changes from spatial distribution of soil humus characteristics. Volcanic ash soils developed in the southern foot area of Akagi Volcano were examined as a case. The result is summarized as follows. 1. Soils developed over southern foot area of Akagi Volcano show several types of soil profiles regulated by the thickness and rate of tephra deposits and geomorphic surfaces. On mountain slopes formed by pyroclastic flow, brownish volcanic ash bed are to be seen between surface and buried humus layer. Soils distributed on central cone lacustrine surface have thick and dark humus layer called “gleyed Kuroboku”. Soils with light humus layer develop on piedmont surface under the influence of artificial activity. The brownish volcanic ash bed mentioned above is difficult to distinguish from this layer. 2. The chronological sequence of soil formation in the study area was considered on the basis of properties of soil parent materials and radiocarbon dates. Horizon I is the layer which continues to accept humus supply and humification under soil forming environment in the present. Fresh volcanic ash errupted from Asama and Haruna Volcano, about 200-1700 years ago are included in this layer. Horizon II correspondent to the brownish volcanic ash bed has accepted humus substances from the upper layer. Though the source volcano is unknown, this layer is assumed to be deposited about 4, 000 years ago. Horizon III, a buried humus layer intercepted from surface environment, started soil formation about 9, 000 years ago following to the erruption of Asama Itahana Yellowish Pumice (YP). 3. The vertical distribution of humus characteristics (brightness of soil color, carbon content and humus composition) was classified into two types, which reflects the conditions of vegetation and groundwater level regulated by gemorphic surface. 4. The spatial distribution patterns of humus characteristics such as carbon content and RF value of humic acid in each horizon show vertical zonality. The principal areas of humus accumulation in the age of horizon II and III formation were located in higher location than present, about 400m upward. Humic acid classified such as type A+, P+ +, have remarkable absorption spectra near 615, 570, 455nm which are regarded to be the features of soils formed under vegetations such as Fagus, Quercus mongolica and also under cool and wet condition in the study area. The appearance of these humic acid types in each horizon also tend to move its location upward by 300m in the age of horizon III formation.
The analysis of dimensional grain orientation in sediments has traditionally been carried out either to determine the paleocurrent direction or to detect the mechanism of penecontemporaneous deformation. In this report, grain orientation in cross-sets and grain fabric in penecontemporaneous structures are summarized from the past studies. Three types of grain orientation are recognized from cross-strata. 1) The planar and steep cross-bedding of micro-delta origin that have angular contact with the basin floor display a current-normal preferred orientation. 2) The cross-lamina characterized by concave of gentle inclination, together with a tangential basal contact, shows, parallelism between flow direction and the preferred grain orientation. Such parallelism of grain fabric is ascribed to the intergranular encounters while the grains are in suspension. 3) Grain fabric in the planar cross-set that is formed by lateral accretion shows an obliquely downslope preferred orientation. The skewed mode fabric is produced by the resultant force of gravity pull that orients the grains to the dip direction and the lateral traction current of the channel flow which tends to imbricate the grains to the upstream dip. Two types of soft sedimentary deformations, fluidized and liquefied deformations, are discussed in this report. Fluidized deformation is produced by turbulent flow and all primary structures are destroyed within a sediment. Liquefied deformation is caused by laminar flow and continuity of primary structures is likely to be preserved. Because grain separations are commonly involved in turbulent flow, there may be no significant grain alignment in fluidized sedimentary structures. In contrast, sand-sized grain separations are generally small during liquefaction. Hence a significant preferred orientation is expected in these structures. However, silt- or clay-sized sediments are freely rotated between sand-sized grains even in the liquefied condition. Thus, fine-grained materials may not form preferred grain orientation by liquefaction.
The deformation of SW Japan arc, due to the Neogene clockwise rotation of the arc and resultant back arc spreading of Japan Sea, is interpteted dynamically using a horizontal buckle model. The Neogene rotational drift of SW Japan arc induced various strain features within the arc. The megakink bands are one of the actual manifestation of such crustal deformations (YANAI 1986). The megakink bands are discrete narrow zones of kilometric width, which are similar in geometry with meso-and microscopic kink bands. The megakink bands are tectonic, because the Butsuzo Tectonic Line in Kyushu and Shikoku and slaty cleavage in Shikoku are bent by them. The zigzag distribution of strata in the Shimanto belt is well explained by the occurrence of megakink bands. The typical megakink bands are Oritate megakink bands (MKB) and Komatagawa MKB in Kii Peninsula, and Sakihama MKB and Muroto MKB in eastern Shikoku, and Kubokawa MKB and Oyu MKB in western Shikoku, and Hokusatsu-Nojiri MKB in southern Kyushu. Conjugated megakink bands are outstanding. The hinge zone (megakink band boundary) is angular or rounded. The orientation of megakink band changes systematically from Kii Peninsula to southern Kyushu ; the dextral megakink bands trend NNE in Kii Peninsula and N-S to NNW in southern Kyushu, and sinistral ones trend NW in Kii and WNW to E-W in Kyushu. The famous Hokusatsu Bend is actually just the northern megakink band boundary of the biggest Hokusatsu-Nojiri Nojiri MKB. The kinematic rotation axis of megakink bands is almost vertical everywhere. The timing of megakink band development is constrained to 20 to 14 Ma by geological data; the megakink deformation involved early Miocene (20 Ma) strata, while the Omine acid dikes (14 Ma) are not rotated by megakink bands. The megakink bands are indicators of regional stress field (axial orientation) and shortening magnitude. The δ1 axial trajectory map using conjugated pairs of megakinkbands is delineated ; the al trajectory is horizontal and parallel to the arc trend, i.e., N 80° in central Kii, N55° in western Shikoku and N25° to 50° in Kyushu, and δ2 axis is vertical. The shortening magnitude due to the megakink band development, along the reference lines from Kii Peninsula to Kyushu, increases southwards up to 22% from the line just to north of the Butsuzo Tectonic Line. The stress and strain pictures of the megakink bands implicate that the rotation mode of SW Japan arc is approximated by buckling with subvertical kinematic rotation axis. In this gigantic buckle fold model, the SW Japan wing was rotated with respect to the almost-fixed Ryukyu wing, under the operation of N30°-oriented horizontal push. The fold would have two hinge zones, i.e., the Shibisan-Nojiri hinge and Bungo hinge. The megakink shortening is interpreted to be resulted from the inner overcompression of Ryuky-SW Japan buckling.
Tanna basin is one of tectonic depressions caused by the downwarping of the western slopes of Taga and Yugawara volcanoes in the northern part of Izu peninsula. Owing to this movement, the thick accumulation of sediment has filled up this basin during the late Quaternary. The Tanna fault that caused the 1930 Kita-Izu Earthquake (M 7.3) traverses this basin in N-S direction and has displaced the sediment with left lateral slip. The subsurface information of this basin gives us data on the geological history of the basin and the tectonic features of the fault since the late Pleistocene. In December 1980, Geological Survey of Japan carried out the drilling investigation in the northern part of Tanna basin to get the stratigraphical and tectonic information of the basin fill sediment. Four borings of 20 m depth were made at intervals of 20m along the nearly E-W direction so as to cross the fault trace. Obtained core samples were investigated thoroughly from the stratigraphical, chronological and paleoecological view point. As a result, it is revealed that the sediment to the depth of 20 m below the basin surface accumulated during the last 20, 000 years. And it is also found that the sediment has suffered from the small scale deformation of pressure ridge shape, as shown in Fig. 6, by the movement of the Tanna fault. However, such deformation was not observed from the geological section along the railway tunnel which penetrates into the volcanic rocks just below Tanna basin. It is thought that the small scale deformation is only a local disturbance limited to the upper horizon of the basin fill sediment and has no relation to the structure of the basin. Throughout these studies, it is revealed that the deformation in Tanna basin is composed of two different scale deformations associated with the fault movement during late Quaternary period. The larger one is the downwarping of a few kilometers in extent and the smaller one is the pressure ridge of a few tens meters in width in the upper part of the basin fill sediment.