Takashi Miki

Sedimentologic and palaeoclimatic classification of Cretaceous red beds in East Asia: a general view

Abstract Based on the results of lithostratigraphic, petrographic and geochemical studies, the Cretaceous continental red beds in East Asia are classified into three groups of different origins: red beds formed in hot and dry conditions, and those influenced by metamorphic and igneous source rocks in moderate and humid environments. In order to redden sediments, haematite must be very finely diffused in the matrices of mudstones, or coat detrital grains as a thin film. Haematite and/or its precursors are derived from metamorphic and igneous basement rocks in nearby source areas, or from syngenetic pyroclastic rocks. The pigmenting haematite forms by decomposition of silicate minerals in source rocks during weathering, and is also preserved as neoformed ferric oxides in depositional sites. For these processes, moderate and humid, and hot and dry climates are favourable, respectively. The present study reveals that reddened Cretaceous rocks in East Asia are formed by different processes operating in a range of palaeoclimatic conditions.

A Combination of Organic Carbon/Total Nitrogen Ratios and Pyrite Contents in Sediments as a Paleoenvironmental Indicator: A Preliminary Study in Northern Kyushu, Japan

Organic carbon (Corg) and total nitrogen (N) contents in Tertiary mudstone samples from four basins in northern Kyushu, Japan, were determined by using a CHN analyzer. A good correlation was observed between the stratigraphic distribution of the Corg/N values and geologically inferred depositional environments. Pyrite contents in carbonaceous sediments from the same basins were also obtained employing a reflecting microscope. Stratigraphic changes of pyrite content in car-bonaceous sediments can be correlated with these depositional environments. Pyrite is expected to play a useful role as an index mineral in estimation of depositional environments in future studies.

Petrography and geochemistry of Cretaceous (?) red beds in Hong Kong

Abstract A preliminary investigation was carried out on the petrography and geochemistry of Cretaceous red beds in Hong Kong. The supply of iron minerals from the basement volcanic rocks in the source area to the depositional sites, and a palaeoclimatic condition favourable for the preservation of iron in the ferric state are necessary in causing the colouration of the beds. Red colouration of the sediments was chiefly accomplished before deposition, and was partly supplemented by following transformation of minerals after deposition.

Regional Patterns of Diagenetic Alteration in the Tertiary of the Kyushu-Korea Region: Implications for Plate Tectonics at the Eastern Asian Continental Margin

ABSTRACT Studies of organic and inorganic diagenesis of Tertiary successions in the Kyushu-Korea region was combined with systematic correlation of the onshore geology. The degree of diagenesis can be attributed to two types of alteration: regional burial diagenesis in each locality was accelerated by additional heat supply from a zone of high paleotemperature in the Sea of Japan. This zone is related to the opening of the Sea of Japan in the Eocene.

Tertiary sedimentation and organic maturation in Kyushu, Japan

Abstract In northern Kyushu, Japan, Tertiary formations, which contain coal in complex geological structures, are present in many isolated basins formed as tectonic depressions. The coals are formed in fluvial to marine basins and show marked lateral variation in thickness, lithology and sediment characteristics. The rocks were subjected to high paleo-geothermal conditions, which influenced diagenesis. All these features are related to the geotectonic settings of the basins at active plate margins. A combined organic and inorganic maturation study on these formations has been performed for an evaluation of the oil potential.