Carbonate Rocks of the Tosabae Bank off the Kii Strait, Southwest Japan

Carbonate rocks consisting of calcite, magnesian calcite and dolomite in various proportions, were dredged from the Tosabae bank off the Kii strait. The petrology of these sediments was investigated by optical, X-ray diffraction, differential thermometric and electron microprobe techniques in order to study the lithification of the carbonate sediments in the Tosabae bank. Magnesian calcite is considered to be a post-depositional precipitate, which occurred at water-sediment interface during cessation of sedimentation. Dolomite is non-ideal, i.e., protodolomite, which is restricted to the superficial zone of the carbonate sediments. Dolomite may be formed by replacement of the pre-existing carbonate minerals in very shallow water or hypersaline lagoon environment. If pre-existing carbonate is magnesian calcite, formation of dolomite seems to occur more easily. From these occurrences, the change in the environment of the Tosabae bank is summarized with the related mineralogy of carbonates as follows: 1) Accumulation of hemipelagic sediments on the Tosabae bank during middle or late Pliocene time (calcite) 2) Cessation of accumulation (precipitation of magnesian calcite at the surface layer of the sediments) 3) Regression and development of very shallow-water condition and/or hypersaline lagoon environment (formation of dolomite at the surface layer of the top sediments of the bank) 4) Subsidence of more than 180 m under sea-level

Tsunami Sedimentology and Studies of Tectonics in Geohistory

3rd International Congress of Sedimentology [BRAZIL 2003] June 8-11, Belém - Par

Anthropogenic Sedimetation and Earthquake Disaster

3rd International Congress of Sedimentology [BRAZIL 2003] June 8-11, Belém - Par

Reexamination of the "Phosphate Rocks" collected from the Northwestern Part of the Philippine Sea --with Special Reference to the Samples obtained by GDP Cruises (1972-1977)-- (Part II. Studies on Basement Rocks)

Edited by Tsunemasa Shik

Pyrite spots formed along slaty cleavage texture of pelitic rocks of the Tamba Belt, Ujitawara Town, Kyoto Prefecture, Southwest Japan.

Pyrite spots were identified in pelitic rocks from the Ujitawara area, southernmost part of Tamba belt. XMA analysis shows that the spots are composed of pyrite. The spots occur along slaty cleavage plane and form spotted shape of diameter of about 5mm

An oceanic fragment in the Upper Cretaceous Miyama Formation of the Shimanto Belt, Kii Peninsula, Japan

An oceanic fragment comprising a complete pelagic sequence from basalt through chert to red shale was found in the Miyama Formation of the Shimanto Belt which is a typical accretion complex in Japan. The conformable sequence, from base, consists of MORB-type basalt, micritic limestone, CCH-type chert, TCH-type chert, MCH-type chert and red shale. CCH chert is alternating beds of calcareous chert and opaque-tuffaceous layers. TCH chert is non-calcareous interbedded chert with opaque-tuffaceous parting. MCH chert is bedded chert intercalated with clay parting. Radiolarian biostratigraphy confirmed that the sequence was continuous from Hauterivian-Barremian to Turonian-early Coniacian time. There also are systematic changes in geochemistry such as decreasing of negative Ce anomaly. These continuous changes of lithology might be due to the changes of depositional regime, that is, from the mid-oceanic ridge, ridge proximal regime above CCD, a slightly distant regime from the ridge, distant regime from ridge and approaching to a contiment, and continental margin regime where hemipelagic sediments accumulated. The depositional regime had moved away from a mid-oceanic ridge to near a trench by ocean spreading in Paleopacific ocean. The complete sequence provides kcy information to reconstruct the original situations of chert and accretion complex in the Shimanto Belt. MORB型の玄武岩から, ミクライト質石灰岩・層状チャートを経て, 半遠洋性の赤色泥岩にいたる連続的な遠洋性堆積物の重なりが, 紀伊半島の上部白亜系美山層中に見いだされた。チャートは, 下位からCCH, TCH, MCHの3つのタイプに岩質上細分される。CCH型は, 石灰質チャートと不透明鉱物に富む凝灰質層との互層であり, TCH型は非石灰質のチャートと不透明鉱物に富む凝灰質層と互層である。MCH型は非石灰質チャートと粘土鉱物を主体とする泥質薄層との繰り返しである。これらの重なりは, 放散虫化石の検討により, Hauterivian-BarremianからTuronian-early Coniacianにかけての連続した堆積物であることが確認された。また, 化学組成の検討から, Ceの負異常が系統的に減少していくことも認められた。このような岩質および化学組成の系統的な変化は, もともと中央海嶺付近にあった堆積場が, 古太平洋の海洋拡大によって徐々に中央海嶺から離れ, 水深を増加させてCCDよりも深くなり, かつ, 徐々に陸域に近づいていった過程を反映したものと解釈される。この遠洋性の重なりをもった海洋地殻の断片は, 四万十帯堆積物の起源や付加過程の解明に重要な情報をもたらすものである。Article地質学論集. 48:100-109 (1997)journal articl

Some Geological Results of the Bottom Sampling from the Sea off Kwanto District Western Margin of the Northern Pacific (Report of the Cruise GDP-1, 1972)

The first cruise of the Japanese Geodynamics Project (GDP-1), which aimed at a combined geophysical and geological survey, was carried out in August, 1972. There were five corings with heat flow measuring at the position along the line of 33°30′N, across the Izu-Ogasawara Trench. One of the sediment columns was olive-brown clay, and one collected on the east slope of the Izu-Ogasawara Trench was sandy deposit containing many glasses and scoria fragments. Angular sandstone gravels were obtained at a station on the wall of a submarine canyon of the Ogasawara Ridge. The age of the rocks is most probably Middle Pliocene from the evidence of foraminifers and nannofossils. Bottom materials were collected at the top plane of the Takuyo Seamount II. Nannofossils were found from the pieces of phosphate rocks coated with ferro-manganese compounds in the materials, and were correlated to Late Cretaceous (Turonian~Maastrichtian) in age. Many spines and tubercles of sea-urchins, which indicate the Mesozoic age, were collected from the materials. The occurrence of these fossils offers additional data for the study of the history and movement of the seamounts and ocean floor around the seamounts of northwestern Pacific

A Preliminary Report on the Landslides and Other Ground Surface Movements Induced by 1999 Chichi Earthquake, Taiwan

Preliminary discussion on the geomorphological conditions of characteristic ground surface movements induced by the 1999 Chichi Earthquake is presented on the basis of the field reconnaissance study in December, 1999.The displacement of the soft ground by the earthquake‐inducing fault was dominated by the horizontal movement and induced damage of structures. Two gigantic landslides visited in this study suggest that the dip of the intercalated mudstone and sandstone of the Neogene and the groundwater conditions determined the occurrence of large‐scale landslides.Extensive occurrence of surficial landslides suggests the importance of the weathering characteristics of coarse sedimentary formation of weak cohesion.Preliminary discussion on the geomorphological conditions of characteristic ground surface movements induced by the 1999 Chichi Earthquake is presented on the basis of the field reconnaissance study in December, 1999.The displacement of the soft ground by the earthquake‐inducing fault was dominated by the horizontal movement and induced damage of structures. Two gigantic landslides visited in this study suggest that the dip of the intercalated mudstone and sandstone of the Neogene and the groundwater conditions determined the occurrence of large‐scale landslides.Extensive occurrence of surficial landslides suggests the importance of the weathering characteristics of coarse sedimentary formation of weak cohesion

Annotated record of the detailed examination of Mn deposits from R/V Tokaidaigaku Maru II Cruise GDP-1 stations

The present paper, deals with the results of the bottom sampling. Sampling devices were a gravity corer for heat flow measurement and dredges of bucket type