Both the "Ishikari-Kitakami Belt" and "Tomakomai Ridge" are traced from the Kitakami province in northeastern Honshu to the Rebun island off northwestern Hokkaido passing through the Ishikari depression. The former consists of early Cretaceous porphyrites and andesitic volcanic rocks. The basement complex penetrated by Shintotsugawa-1", "Sorachi (MITI)" and "Namporo (MITI)" wells, belongs to the "Ishikari-Kitakami Belt". Both of these tectonic belts correspond to the front of the Northeast Japan Arc, and formed western marginal uplift zone in a geosyncline during the Cretaceous and Palaeogene. From the Umaoi hill to the Yufutsu offshore area (from north to south), in a narrow belt thick volcanic rocks of the Takinoue stage (early Miocene) unconformably covers the Palaeogene sediments pinching out westward. Though during which are roughly grouped the Kawabata age (middle Miocene), many isolated subbasins grew in two large basins, which were separated by an uplift zone running through the Rebun island-Kabato-Namporo-Tomakomai into the Tempoku-Hidaka basin in the east and the Ishikari basin in the west. The Hidaka orogenesis influenced only on the development of the Tempoku-Hidaka basin and the uplift zone were not affected any more by the orogenesis.
In the Sea of Okhotsk off the northern Hokkaido island, two sedimentary basins and two basement plateaus are present. The former are the Esashi-Monbetsu Oki basin and the Abashiri-Kitami Yamato-tai basin, and the latter are the Monbetsu Oki basement plateau and the Okhotsk basement plateau. The Tenpoku sedimentary basin extends over the northwestern Hokkaido nearly to the Rebun island. The author concludes that foldings on the eastern flank of the basin is controled by several lines of thrust fault system almost parallel to the eastern mountainous terrains and also ascribed to diapirism or flow of the middle Miocene shale (Masuporo formation). These faulting system is estimated to be derived from upheaval of the eastern margin of the basin during Neogene time.
This paper represents the toctonic framework of the shelf off Tokai region based on the seismic, gravity and magnetic data. The stable shelf at the northwestern part, the central uplift which may be the extention of the "Akaishi Ressen (rift)", and the outer marginal uplift at the southeastern part are prominent structures in this area. In addition to the eustatic subsidence after the middle Miocene and after Pleistocene and eustatic rising before the Pleistocene, relative upheaval of the outer marginal uplift has successively occurred. The structural movement in this area may be explained with a model that the southern oceanic crust has thrust down under the continental crust.
北陸•山陰地区は盆地の主要部が海域に位置するが,秋田•新潟グリンタフ盆地と非常に類似した盆地の形態,発達様式,層序をもつグリンタフ盆地である。すなわち,各地域とも2度の時期(中新世中期から後期,鮮新世以降)にわたって,陸側から海側へ,つまり現在の日本海側に向かって堆積盆地の中心が移動し,新しい堆積の場を形成している。それと同時に陸側の堆積盆地は隆起帯あるいは褶曲帯に転じ,地質的にそれぞれ固有の特徴をもったI, II, III帯の地質帯が形成された。 本海域の堆積盆地は地層の厚さ,構造の特徴から北陸堆積盆地•丹後沖堆積盆地•山陰堆積盆地に3分される。各盆地ともI, II, III帯の特徴をそなえているが,とくに山陰盆地の鳥取沖盆地から隠岐舟状盆では典型的である。 秋田•新潟における大部分の油•ガス田の分布が第II帯の2～3の構造系列に限られている。本海域にも第II帯に相当する盆地が発達し,なかでも厚い中新世の地層からなり,100kmに及ぶ長大な褶曲帯が東-西に走る鳥取沖盆地は,石油地質的にきわめて重要な地域といえる。各堆積盆地とも褶曲運動あるいは基盤のブロック運動にともなう背斜構造が数多く見られるが,その形態,形成時期は多様で,全般にききめて複雑な様相を呈している。
This article discusses stratigraphy and geological history of the continental shelf area covering offshore San-in to Tsushima Island, the southwest part of the Sea of Japan. Geological data from 5 wells and detailed seismic surveys indicate that sediments in the studied area are devided into 4 stratigraphic groups ranging from Oligocene to Recent in age, namely X, N, K and D groups in ascending order. The oldest X group of Oligocene time comprises paralic sediments including volcanics which deposited in the initial stage of basin-formation. "N" group of mainly lower Miocene time consists of deep marine sediments, representing the highest stage of transgression. Sediments of the K group of middle Miocene time shows distinct off-lapping depositional pattern during the basin-filling stage. The youngest D group covers these older groups unconformably. Strong deformation of sediments prior to the deposition of the D group formed many anticlinal structures. Five exploratory wells were drilled at the selected structures, where only minor gas shows were encountered. The area provides the enough palaeotemperature to mature the source rocks at moderate depth.
The Nansei-Shoto (Ryukyu Islands) is one of the island arcs along the West Pacific continental margins and has typical topographic features as an island arc, where marginal seas, volcanic fronts, island arcs and trenches are regularly and zonally arranged toward the Pacific Ocean. Geology and structures of the Ryukyu Islands are partly discortant with the zonal arrangements, and the northern and southern parts of the Ryukyu Islands were under different tectonic conditions before the latest Neogene time. But finally both of them were combined into a present status by the Quarternary tectonism. Around the Ryukyu island arc, there are several sedimentary basins mainly consisting of the Neogene sediments. The Shimajiri basin, a fore arc basin along the outer side of the island arc, is about 1100km long and contains up to 6000m of sediments deposited during the late Miocene to Pleistocene time, and is forming abyssal plain between the island arc and trench. The Yaeyama basin is also situated in the outer side of the southern Ryukyu arc, but a trend of this fore arc basin is different from that of the himajiri basin. Plio-Pleistocene deposits are unconformably resting on possible early to middle Miocene layers and a total thickness of these sediments attain to 4000 m. Small basins within the older uplift zone such as the Amami trough are collapse basins formed by normal faults and tiltings due to pull-apart. They are filled mostly with Quarternary deposits. The Okinawa trough is a typical back arc basin, which is formed by extensional tectonic forces and supplied with deltaic or submarine fan deposits from the continent to the west. Sediments in this trough will exceed 5000m at maximum. Many step faults and roll-over anticlines caused by them are commonly observed on seismic profiles. Characteristically high heat flows are reported in the trough. The Tokai basin is an older back arc basin compared with the Okinawa trough, and formed between Fukien-Ryeongnam and Goto-Senkaku belts. Presumably, this basin contains early to midde Miocene sand-shale sequences deposited in lagoonal shallow water environments. The presence of large anticlinal structures with gentle dips suggests significant hydrocarbon accumulations in the basin.
Sedimentary basins in the arcs of the NW Pacific Rim have many characteristic features which are ranged in the zones parallel to the arc extension. Two domimant basins are recognized in the arc area. They are outer trench belt and inner sedimentary and volcanic belt intercalated by the intermediate uplifted belt between them. Thick sedimentary basins in the outer belt tend to develope in a few particular areas of the arc. The one is in the contact zone of the arc as is suggested along the contact zone of the Tohoku Arc and the remnant Hidaka Arc. The next is in and around the joint zone of the arcs, as is suggested in that of the Tohoku Arc and the Izu-Ogasawara Arc. Sedimentary basins in the inner belt is thicker than that in the outer belt. Horizontal tentional stress is suggested during the earlier stage of the basin development in the inner belt, which had changed to the horizontal compressional stress in the later stage except in and around the volcanic chain. The horizontal tensional stress which formed thick sedimentary basin in the inner belt might have acted important roll to have dense hydrocarbneferous sediments during the earlier stage.