The Miocene Tanabe Group, which consists of shallow-marine siliciclastic sediments, is distributed in the southwestern part of the Kii Peninsula, Japan. The group is about 1,500m thick, and is divided into the Asso and the Shirahama Formations in ascending order. The Asso Formation is mainly composed of mudstone and conglomerate, and the Shirahama Formation of sandstone, mudstone and alternating beds of them. A thick conglomeratic sedimentary wedge, which was deposited in a fan-delta system (the Shimomisu Fan-delta), is developed in the northern area of the distribution of the group. To clarify the sedimentary history of the fan-delta, at first, sedimentary facies were recognized and classified based mainly on grain-size and sedimentary structures. Consequently, the facies associations and the sedimentary bodies were recognized and defined based on the occurrence and combinations of sedimentary facies. The spatial and stratigraphic interrelationship among sedimentary facies, facies associations and sedimentary bodies were analyzed in detail. The following results are obtained. The sediments of the Shimomisu Fan-delta consist of 3 sedimentary bodies, which are in turn consist of 6 facies associations. The sedimentary history of the Shimomisu Fan-delta is summarized as follows: STAGE OF GENESIS: The drowned valleys facing to the ocean were formed as the sea-level rise began. The debris flow deposits and marsh muds, and then sands and rounded gravels in the neritic environments were deposited in the valley. STAGE OF PROGRADATION: After the burial of valleys, the rate of the sea-level rise decreased. Gravely deposits supplied from the north formed a fan-delta system which prograded toward the south. Gravely deposits were deposited as foreset beds, resting on a downlap surface, which were underlain by the bottomset muds. The fan-delta divided the sedimentary basin into embayed sea in the northern margin and open shelf in the resting area. STAGE OF AGGRADATION: The rate of the sea-level rise again increased, and the supply of sediments were continued. Therefore, the rate of the progradation decreased, and sediments were accreted vertically in the neritic area, forming the topset beds of the fan-delta. STAGE OF ABANDONMENT: The rapid increase in the rate of the sea-level rise took place in the later half of Blow's N 8 stage, and the fan-delta system was abandoned and the shelf mud facies covered throughout the basin.

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The Yachi landslide located in the south-eastern part of Akita Prefecture is an active and large-scale one which covers approximately 100ha or more. However, it occupies only a part of the extensive old landslide area in which many scarps, indentations and other irregularities in the topography remain. The study inquestion is underlain by Miocene strata which are mainly composed of hard shale. The strata are complexly folded and the geologic structure of the area has been interpreted until now as a synclinorium. However, it has now been clarified that some parts of the structure were more or less formed by the primary landslide. In this paper, on the basis of the field investigation, the issue of the relationship between the geologic structure and the primary landslide around the Yachi landslide are discussed. The view that primary landslides mainly formed the complicated structures of this area has not necessarily been proven. However, there is evidence that some geologic phenomena and topography which are observed in this area were formed by old rock glides. One example: the strata observed in Nishikozawa which is the type-locality of the Nishikozawa Formation are composed of detached segments of bedrock. Another example: although it was believed that the Narusegawa Fault is a big one and passes through the point at which two adjacent outcrops of strata show completely different dip and strike each other, this abrupt change between them is clearly due to the displacement of an old rock glide. On the basis of this field evidence, the interpretation of the mechanisms of geologic structure and primary landslide in this area can be summarized as follows; 1. Composite and/or other complicated deformations are concentrated on the anticlinal axis zone, and short extended faults which are parallel to the axial trace of fold are recognized. These structures are fundamentally due to gravitation, that is, they are due to the primary landslide, and stress release resulting from river erosion might have been the main cause. 2. The Narusegawa Fault is intercepted by some younger faults (oblique or transverse faults). Therefore, it is not directly related to the formation of geologic structure in the final stage. 3. Some oblique or transverse faults control the slide direction of primary landslides, and each block surrounded by these faults shows respectively different inner structure. 4. It is inferred that the sliding surface of the primary landslide originates from bedding fault and its maximum depth reaches 200m or more. 5. The primary landslide presumably occurred during the period between the formation of Middle Terrace and Lower Terrace. However, there was no over-all reactivation excepting a few subsequent secondary landslides.

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Stratigraphical studies on the Miocene to Lower Pleistocene strata in the northern part of the Uonuma hill, Niigata Prefecture, were carried out and geologic ages of these strata were determined by the use of the fission track method, and decided by the biostratigraphical study of planktonic foraminiferal and radioralian fossils. In the studied area, the Miocene to Lower Pleistocene show serious lateral changes of facies from east to west direction. Stratigraphical units of rock facies in the eastern area are different from the units in the western area, therefore Stratigraphical divisions are established in each area. The Miocene to Lower Pleistocene sequence in the eastern area are divided into the Nishimyo, Tsunagi, Toyagamine, Shiroiwa, Wanazu and Uonuma Formations, and in the western part are into the Hanzogane, Sarukuradake, Araya, Kawaguchi, Ushigakubi, Shiroiwa, Wanazu and Uonuma Formations in ascending order. The Shiroiwa and Wanazu Formations unconformably cover the Nishimyo and Tsunagi Formations in the eastern area, and the Toyagamine Formation in the south-eastern area. The Kawaguchi Formationin the western area laterally interfinger with the Ushigakubi Formation according to the results of Stratigraphical study of tuff layers. These formations have tendency to be coarser sediments toward Mt. Toyagamine in the south-eastern area. In the studied area, Mein fold structures are distributed in NE-SW trends. The results of the fission track age determination and the biostratigraphical study are as follow. The planktonic foraminiferal zone (Maiya, 1978) of the Araya Formation is assigned to Globorotalia ikebei/Orbulina uinversa zone, and that of the Kawaguchi and Ushigakubi Formation is to Globigerina pachyderma (dextral)/Globorotalia orientalis zone. The radioralian zone (Nakaseko and Sugano, 1973) of the Tsunagi Formation is assigned to Cyrtocapsella tetrapera and Lychnocanium nipponicum zones. The fission track age of Tzp tuff from the upper most horizon of the Araya Formation is 5.23±0.26Ma. OK-2 tuff from the lower horizon of the Shiroiwa Formation is 3.95±0.19 Ma. Ftp tuff from the lower horizon of the Ushigakubi Formation is 3.78±0.17Ma. According to the determination of fission track ages and the investigation of planktonic foraminiferal and radioralian fossils, the Nishimyo and Tsunagi Formations in the eastern area, and the Hanzogane and lower part of the Sarukuradake Formation in the western area are Middle Miocene in age. The Toyagamine Formation in the eastern area, the upper part of the Sarukuradake formation and the Araya Formation in the western area are Late Miocene in age. The Shiroiwa, Wanazu and lower part of the Uonuma Formations in the eastern area, and the Kawaguchi, Ushigakubi, Shiroiwa, Wanazu and lower part of the Uonuma Formations in the western part are Pliocene in age. The Upper part of the Uonuma Formation is Early Pleistocene in age.

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本研究は1970年代にはじめられたもので,本論では,中国東南部の揚子江下流域の地質構造発達史を説明するために提唱された,スラストーナップ構造における広域的な滑動システムについて概説する.本地域には,原生代晩期の張八嶺(Zhanerbaling)層群からジュラ紀後期の象山(Xianshan)層群にいたる地層が,広域的な六つの滑動システムによって区切られて分布する.滑りを生じている地層の岩石物性の特徴を系統的に検討すると,その岩石強度・ヤング率・内部摩擦角・電気抵抗・磁化率などのパラメーターが,周囲の岩石に較べて有意に低いことが判明した.また,スラスト-ナップ帯では脆性変形の前段として延性変形が進行した証拠があり,そこではマイロナイト・ストレス鉱物が認められる.揚子江下流域では,フレーク・テクトニクスの観点に基づいて,三種類のナップが識別されている.それらは,地層が逆転し大規模な横臥褶曲が形成されて褶曲ナップに発達するような圧縮型ナップ(P型),一般に断層によって形成された地境・スラブ・シートによって形成されるような重力滑動ナップ(G型),および,P型とG型の混在する複合型ナップ(C型)である.ナップの運動学や力学はプレート運動と密接な関係にあることを示しており,その発展は四つの段階を経ている.原生代晩期の雪峰輪廻(Xuefenian Cycle),中生代初期のインドシニアン輪廻,中生代の燕山輪廻(Yanshanian Cycle),新生代のヒマラヤ輪廻である.本地域の前縁盆地におけるスラスト-ナップ構造の形成過程は,石油-ガス貯溜層の構造的枠組とその発達を決定する重要な鍵となる.

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Recent progress in biostratigraphic, sedimetologic and geochemical investigations on bedded cherts of the Tanba-Mino Terrane of Southwest Japan are reviewed, and Oceanic Anoxic Event (OAE) recorded in the chert sequences is also presented. This may have two significant meanings i. e. to elucidate tectonics of Jurassic terranes and to evaluate an environmental change of system and condition of oceans during the Phanerozoic time. Radiolarians and spongy spicules are important composition of bedded cherts, of which Late Permian and Triassic ones indicate apparent alignment of tests deposited under the influence of bottom currents. The presumed current direction is from west to east, and this direction is approximately equal among the examined Tanba rocks, although the bedded cherts are embedded in the Jurassic melange as clasts or blocks. This points that bedded cherts could be remained their original attitude and may help to understand the mechanism of accretion of oceanic plate stratigraphy. The OAE was appeared in the P/T boundary and in lower Toarcian (Lower Jurassic), both of which are clearly marked by black organic cherts or mudstones, and in underlying sequence, bedded cherts gradually change their color from red to grey/black via greenish grey, and vice versa in overlying sequence. Late Permian shifting of sedimentary environment from oxic to oxygen depleted (or anoxic), can be linked to decreased rate of plate genesis, and the OAE at terminal Permian recorded in chert sequence shows strong coincidence to extinction of radiolarians. The Toarcian OAE is well known in shallow marine sediments in the world, and the Toyora Group, Yamaoku Formation and Kuruma Group are important examples in Southwest and Central Japan. Black organic mudstones in both bedded chert sequence and these shallow marine sediments characteristically produced acritarchs known as a cause of a "red tide". This plankton can act as an important member of primary production, and at the same time, its intence blooming might be a trigger of the OAE.

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