Mizunami Group Research Articles

Miocene palaeogeography of the Mizunami group of the Tono region, central Japan

The Miocene Mizunami Group developed in three basins, the Kani, Mizunami and Iwamura basins of the Tono region, central Honshu, Japan. The Miocene palaeogeography of the Tono region is reconstructed on the basis of data obtained during the last ten years. Six palaeogeographic maps (I-a, b, II-a, b, c and III) covering the time period from 20 to 15 Ma ago show the palaeoenvironmental conditions at each developmental stage: I-a, volcanic activity in a small lake in the Kani Basin; I-b, two lakes in the Kani and Mizunami basins; II-a, marine invasion of the Iwamura and Mizunami basins and persistence of a lake in the Kani Basin; II-b and c, continued spread of the sea in the two basins and disappearance of a lake in the Kani Basin in stage II-c; III, rapid subsidence and formation of a wider and deeper sea. A palaeogeographic map of southwest Japan at ca. 16 Ma is also presented to show the geological setting of the Mizunami Group. The Mizunami Sea was a quasi-cratomic basin in the Tono region which was connected to the Pacific Ocean throughout the Miocene.

Paleoecological Significance of the Miocene Mizunami Group, Central Japan

Paleoecological Significance of the Miocene Mizunami Group, Central Japan

Timing of Rotational Motion of Southwest Japan Inferred from Paleomagnetism of the Setouchi Miocene Series

Paleomagnetic measurements have been made on tuff and mud samples from 25 sites of the Morozaki Group and the Mizunami Group, distributed in the eastern part of Southwest Japan. Stability tests by alternating field and thermal demagnetization reveal the existence of characteristic remanent magnetization, of which directions show significant clockwise deflection; the mean declination shift exceeds 50° in the late Early Miocene members (N6-N8), and is less than 30° in the early Middle Miocene members (N9). This result, compared with the other paleomagnetic data from the Setouchi Miocene Series and the volcanic rocks in the San'in district, suggests that coherent rotation of Southwest Japan began between the late Early Miocene and the early Middle Miocene. Nearly a half of the rotation might have been attained in the stage of Zone N9, which possibly corresponds to the period of the opening and increasing subsidence in the Japan Sea.

岐阜県東濃ウラン鉱床における粘土化と沸石化

The Tono Mine, Gifu Prefecture is one of the richest sedimentary uranium deposits in Japan. This areas is mainly composed of Tertiary system overlying granite basement. Uranium has been deposited in the lower part of the Tertiary sediments, Mizunami Group, and its location is controlled by the socalled channel structure of the surface of the basement granites. The matrixes of these sediments usually contain tuffaceous materials, which have been argillized and zeolitized diagenetically, and uranium was absorbed in clinoptilolite. The clay mineral species and their relative amounts are closely related to the redox potential and hydrogen ion concentration of the deposits, which are controlled by the movement of groundwater. Main authigenic minerals are montmorillonite, kaolinite, halloysite, sericite and clinoptilolite. In an S-N gallery, the amount of montmorillonite regularly decreases with increasing amount of kaolin minerals and uranium contents. Some iron minerals and calcium minerals are closely associated with clay minerals, zeolite and uranium contents, so the stability relation of their minerals and mineral paragenesis suggest the environments of alteration in this area.

On the Swelling Behaviours of the Artificially Crushed Tertiary Mud-stones

In this paper the property of free swelling of artificially crushed Tertiary (Kobe, Tsuyama and Mizunami group) mudstone is discussed. The degrees of free swelling depend on the the content of active clay minerals, such as montmorillonite and vermiculite, and the water contents are decided with pF value which is determined by soil suction, i.e. The results show that the degrees of free swelling are in proportion to the content of montmorillonite minerals in a soil and the water contents at a certain soil suction except Mizunami group mudstones.

Some Miocene boring Shells and their life tracks ("Lebens-spuren") from the Mizunami group, Gifu Prefecture, Japan

Some Miocene boring Shells and their life tracks ("Lebens-spuren") from the Mizunami group, Gifu Prefecture, Japan

A GEOMORPHOLOGICAL STUDY OF THE MIKAWA HIGHLAND AND THE TERRACES OF THE ADJACENT DISTRICT

By analyzing the topography of the Mikawa Highland, which extends about 150 square kilometers in the east of Nagoya, central Japan, and the fluvial terraces of the adjacent area, the geomorphic develop-ment and the geological age of the highland erosional surfaces, the so-called “Mikawa peneplaln”, have been discussed. Pedological observations on the surface soils of some fluvial terraces have also been described additionally. Along the down stream of the Yahagi River, the western margin of the Mikawa Highland, several stepped terraces are extending: The highest one, about 240 meters above the sea-level, can be correlated northwestward with the depositional surface of the Seto Group, which is the lacustrine and fluvial deposits accumulated at Seto and Nagoya region in the Pliocene Epoch. While the lower terraces are fluvial ones built by the Yahagi River during the Pleistocene Epoch. The topography of the Mikawa Highland is composed of four series of low-relief erosional surfaces, i. e. from older to younger, surfaces of 1, 100-1, 000, 900-700, 600-400and 300-100 meters in altitude, respectively (figes. 1 and 2). The lowest one, named the lower Mikawa low-relief surface, seems to be continuous with the basal surface of the Seto Group, which also represents an erosional level of low-relief (fig. 3). The basal surface of the Seto Group is intersecting the structure of the middle Miocene Mizunami Group. It is, therefore, most probable that the lower Mikawa low-relief surface was formed in the early Pliocene. In the catchment area of the Yahagi a basin-forming movement with an axis of NE-SW trend, accom-panying faults of NE-SW direction along its both sides, has been continuing since the pre-Pliocene. Accordingly, the low-relief surfacesin the Mikawa Highland may be said the “Piedmonttreppen” formed in connection with theupheaval of the surrounding mountains, which is a counterpart of the above-mentioned basin-forming movement.

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The Cenozoic Strata in the Iwamura Basin, Gifu Prefecture, Japan

The stratigraphic succession of the Cenozoic strata developed in the Iwamura basin is as follows; Age Terrace deposits unconformity Hirooka gravel Tazawa sand and gravel I2 Seto group Higashihora formation H2 Mizunami group Toyama formation Agi formation F2∼F3 unconformity and fault Granite and Quartz-porphyry The Mizunami group represents a cycle of sedimentation and its lower part or the Agi formation may be freshwater in origin while the upper or Toyama formation, which consists of five members is marine., The Mizunami group is probably middle Miocene (F2∼F3) in age as inferred from the molluscan fossils., It forms a synclinal structure controlled by the fault of NE-WS trend.,

On the Velocity of Sedimentation of the Tukiyosi Tuffaceous Sandstone in the Miocene Mizunami Group

The Miocene Mizunami group is typically developed in the environs of Mizunami city, Gihu Prefecture. In Tukiyosi and Togari regions of the same city, the group is lithologically divided in descending order into the following four formations:4) The Oidawara tuffaceous mudstone3) The Hazama pumiceous sandstone and conglomerate2) The Togari tuffaceous siltstone1) The Tukiyosi tuffaceous sandstone

On Some Fundamental Geological Problems Suggested from the Study on the Mizunami Group (Part 1)

During the writer's study on the Mizunami group developed in the environs of Mizunami-Toki-mati, Toki-gun, Gihu-prefecture, he entertained some doubts concerning the fundamental geological conception hitherto been generally accepted by Japanese geologists., In order to solve a part of the doubts, the writer has considered in this paper the following three problems:(1) Significance of fossils as an indicater of sedimentary environment of strata, (2) Significance of fossils as an indicater of geological age of strata, (3) Meaning of fossil-facies as one of factors defining lithofacies., These are all interesting and important problems concerning fossil-stratigraphy and biochronology laid in the recent Japanese geologists., The fundamental geological conception hitherto been adopted is standing on a point of view that fossil-stratigraphy is synonymous with biochronology, while in the writer's present opinion these two are to be separated distinctly from each other., Then, the term biostratigraphy hitherto generally been used by almost all geologists should be abandoned., It must be done, however, under the world-wide discussion in future., The writer expects that this paper is a reference on the occasion of such a discussion.,

On the Mode of Occurrence of Fossils in the Mizunami Group in the eastern Mino, Japan

On the Mode of Occurrence of Fossils in the Mizunami Group in the eastern Mino, Japan