The Cambro-Ordovician Formation in the Peri-Arcitic Areas are outlined from Greenland to Northern Europe through Northern Canada, Alaska, Siberia and the Ural Mountains, with special reference to trilobites and cephalopods in order to show their provinciality.
The Chichibu Terrain in the Shima Peninsula is, like elsewhere in Southwest Japan, divided into the northern, middle and southern belts by longitudinal faults. The southern belt consists of the Imaura Group and the Chichibu Complex (Tsuiji Group). The Imaura Group is distributed only along the northern margin of the belt, while the Tsuiji Group occupies the major part. Four lithofacies are tentatively distinguished in the Tsuiji Group, i. e., massive sandstone facies, sandstone and mudstone facies, olistostrome facies and chert-clastics facies. The first three may correspond to the elastic interval of the chert-clastics facies. In chert-clastics facies, the lithology changes upwards from chert at the base, through siliceous mudstone, mudstone to coarse grained clastics such as sandstone, sandstone alternating with mudstone, olistostrome. Radiolarian fossils obtained from several horizons of this sequence show that rocks are younger upwards from the Triassic of chert to the late Jurassic of mudstone. The age of the uppermost coarse grained clastics has not been clarified due to lack of fossils therein. However, occasionally observed conformable facies change from mudstone to sandstone justifies the view that coarse grained clastics are the youngest of the sequence. The chert-clastics sequence is reasonably regarded as representing the original stratigraphic unit. A single unit may be a product of successive sedimentation on the Pacific plate gradually approaching to the cotninent. Gradual upward change of the lithology in a unit represents the landward shift of the plate and the increasing availability of receiving the terrigenous material on the plate. The Imaura Group covers the Tsuiji Group either conformably or unconformably. It consists mainly of mudstone containing sporadically limestones rich in the late Jurassic fossils and has been hitherto correlated to the Upper Jurassic based on these fossils. We obtained radiolarian fossils indicating the early Cretaceous in age from mudstone. This leads to the conclusion that the fossil-bearing limestones are resedimented blocks emplaced into the radiolarian-bearing mudstone in the early Cretaceous period.
Awashima is a small (7 × 2 km) island on the continental shelf along the wetsern coast of northern Honshu, and uplifted with northwestward tilt associated with the 1964 Niigata earthquake (Figs. 1 and 5). NAKAMURA et al. (1964) proposed a cumulative character of the coseismic uplift since the middle Miocene, of the island, based on the comparison of terrace profile and geologic structure of Tertiary sedimentary rocks with the coseismic tilt by the Niigata earthquake. This paper intends to establish such a progressive tilt of the island, on the basis of the detailed investigation of marine terraces. Marine terraces of Awashima are divided into H, M1, M2 and L terraces (Fig. 2). They are abrasion platforms and no tephra and terrace deposits are found, except thick deposits of L terrace at Uchiura area. The most extensive terrace, M1, can be regarded to be formed at the time of the last interglacial maximum (ca. 125 ka). The height of the former shoreline of M1 terrace ranges from ca. 75 in to the northeast to ca. 50 m to the southwest, indicating a notable northwestward tilt, which is similar to that of the 1964 earthquake, but having much steeper gradient. The gradient of the northwestward tilt of M1 terrace (20 × 10-3) is about 67 times of that of the coseismic tilt at the time of 1964 earthquake (0.3 × 10-3). It suggests that the similar coseismic uplift has repeated since the last interglacial maximum with interval of about 1900 years. Estimated resultant coseismic uplift is about 1.1 m at the northeastern part of the island, where M1 terrace is 75 m high, which is approximately same to the height estimated by the amount of coseismic resultant uplift of 67 times. Thus, average recurrence interval of the major earthquake resulting in the coseismic uplift of Awashima is estimated to be about 1900 years from the comparison of amount of both tilt and uplift. However, no significant difference is observed in terrace profiles of Ml, between A-B, parallel to strike of the tilt, and C?D or E?F, normal to it (Fig. 4). Therefore, terrace profile seems to be an unsuitable indicator for the detection of tectonic deformation, unless much steeper tilt has occured. Distribution of H and M2 terraces is too limited to discuss the deformation pattern. L terrace at Uchiura is underlain by thick marine deposits attaining ca 60 m thick, suggesting that it was formed in association with the postglacial sea level rise. No datable material was found despite examination of many borehole data and our excavation works. L terrace is subdivided into L1, L2 and L3 terraces. Episodic or intermittent emergence, probably coseismic, should have occurred at least 3 times since ca. 6000 yBP. It is consistent with the result obtained by Mi terrace. The height of L1 terrace is ca. 11 m above mean sea level, which means average uplift rate is ca. 1.5 m/ka, it is much larger than 0.7 m/ka for Mi terrace. Northwestward tilt of the lower terraces, however, is not confirmed by this study, owing to the limited distribution.
Topographic features around Izu Oshima region were summarized with special references to the characteristics relating to subduction of the Philippine Sea plate (PHS) along the Sagami Trough. Several topographic highs having circular contours around Oshima Island may be the results of parasitic eruption of Oshima Volcano. Their trend is almost parallel to the elongation of the linearity of parasitic cones distributing onshore. Along the eastern trench slope off Oshima Island, large scale collapse structures are seen in the submarine topographic map. These are the result of the falling down mostly of pyroclastic materials (tephras) during historic ages. The Oshima Island shows eastward tilting owing to the bending resulted from the subduction of the PHS. Large scale of the collapse structures may be caused by the oversteepening of the eastern trench slope of Oshima Island and soft sediments consisting mostly of pyroclastics including a large quantity of pore water slide down along the trench slope as far as the Sagami Trough. Radial distribution both of saddles and valleys observed northeastern offshore part of the Oshima Island may be lava flows and/or dike swarms derived from the old Okata Volcano whose eruption center was estimated to exist at the northeastern extension of the Oshima Island. About 4 km south off the Oshima Island, large scale of the topographic breakes consisting of caotic highs and depressions are seen in the submarine topographic map. These may be debris flow deposits along eastern slope of Ohmurodashi which consists mostly of the Quaternary rhyolites geologically and chemically similar to those exposed on the Niijima and Kozushima Islands. These are the results of catastrophic phreatomagmatic eruption of the Ohmurodashi such as that of the Myojinsho. Mixture of pyroclastics and lavas will be found from these caotic areas. The upper steam of the Sagami trough shows notable meandering whose direction is almost parallel to that of the movement of the PHS east off Oshima Island and almost perpendicular east off Ohmurodashi. At the latter case, thrust structures may trace along the subduction zone at the landward slope of the Sagami trough. Just landward side of the thrust, curious round topographic highs can be observed. They may be mud volcanoes. Topographic features observed around Oshima Island may best be understood in term of the normal and oblique subduction of the Philippine Sea plate along the Sagami Trough.