The Geological Survey of Japan has been conducting a study on submarine hydrothermal activity in the Izu-Ogasawara (or Bonin) Arc and the northernmost part of the Mariana Arc since 1984. Hydrothermal products, both unequivocal and inferred, are divided into three categories: 1) manganese or iron enriched sediments, and Mn-Fe hydroxide and silica deposits (chimneys) in the back-arc basin area; 2) manganese and/or iron oxide deposits occurring in the mountainous areas of both the back-arc and the volcanic front areas; and 3) a veinlet of iron-copper-zinc sulfides in hydothermally altered andesite on a wall of a submarine caldera along the volcanic front. This paper outlines these products as well as related geological, geophysical and geochemical data in the area.

抄録全体を表示

The outcrop which illustrates the occurrence and horizontal extension of chalk-flint cycles on the Pacific seamount revealed as driling results of the Deep Sea Drilling Project (DSDP) has been found in the land area; in Chirai Quarry, northeastern part of Hokkaido. The Jurassic cyclic sequence of micritic limestone and nodular flints is of pelagic origin, as being recognized by geochemical characteristics. The cycle is unconformably overlain by volcanic debris which is also overlain by bedded red cherts. The stratigraphic sequence outcropped in the quarry can be correlated with the DSDP sequence on the summit of the Hess Rise, which shows the subsidence history of the summit below the calcite-compensation depth. The most important mechanism for the formation of the cycles of flint nodules in the micritic limestone, among others, can be interpreted to be dissolution cycles on seamount which could be caused by the cyclic, climbing movement of deep waters undersaturated in calcium carbonate below the calcite-compensation depth, as results of climatic catastrophes.

抄録全体を表示

Coastal and shallow marine environments were widely distributed over the modern Kanto Plain, central Japan during highstand in sea level of the last interglacial period. The sea area is called Paleo-Tokyo Bay. Two paleocurrent maps of the “Barrier period” and “Period of bird's foot delta” in the last interglacial are presented based on the sedimentary structures, such as cross-bedding, ripples and parallel-bedding. Directions of waves, longshore current, rip current, overwash, ebb and flood currents, fluvial currents and others are distinguished.

抄録全体を表示

The lateral changing pattern of sedimentary structure from sediment gravity flows in the Izumi Group, Awaji Island, southwest Japan are studied on the basis of the detailed field observation. Double-layered turbidite, that has not been categorized into the former proposed evolutional model of sedimentary structure, is composed of massive sandstone in the lower part and normal graded sandstone in the upper part. Each type of four hypothetic sediment gravity flows, high density debris flow, low density debris flow, high density turbidity current and low density turbidity current, deposits and forms conglomerates, massive sandstone or sandy mudstone, normal graded sandstone and mudstone which corresponds to Bouma-E sequence, respectively. Double-layered turbidite is the indicator of boundary part between low density debris flow deposits and high density turbidity current deposits in the continuous structural changing system of sediment gravity flow deposits.

抄録全体を表示

The Earth's orbital variations, reflected in Plio-Pleistocene ice volume (sea-level), were recorded in the turbidite sequence of the Kazusa Group in the Boso Peninsula, Japan. Vertical changes in layer thickness reflect the orbital eccentricity, obliquity, and precessional cycles, so-called “Milankovitch cycles.” Turbudite deposition in submarine fans activated during the low-stand in sea level, the glacial period, and this period corresponds to low obliquity, conversely, a mud dominant part formed in the period of high sea level correspond to the increasing stage of eccentricity.

抄録全体を表示

A data-sheet format for the sedimentary data base is proposed here for clastic rocks. This format is applied not only to sandstone but also to conglomerate and mudstone, though some amendments of the format may be necessary for conglomerate.

抄録全体を表示