--> ABSTRACT: Miocene Reef and Nonreef Carbonate Rocks in Japan, by Kenji Konishi; #91030 (2010)

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Miocene Reef and Nonreef Carbonate Rocks in Japan

Kenji Konishi

Japan's main islands experienced temperate climates throughout the Neogene with a tropical invasion around 16 Ma (early middle Miocene). This climatic warming, accompanied by a eustatic sea level rise, caused the unusual occurrence of reef facies, mangrove deposits, and lateritic beds in Japan. In cooler climates both before and after reef growth, sediments rich in bryozoan and algal material were widespread.

Reef rocks emplaced as penecontemporaneous olistoliths in deep-water clastics at the Pacific coast of central Honshu are characterized by a wide lithologic spectrum, ranging from grainstone to bindstone. These rocks include rudstone and floatstone, which are rich in coralline algae (encrusting forms such as Lithophyllum and Mesophyllum and articulate forms such as Amphiroa) and codiacean algae (Halimeda) with hermatypic corals and large benthic foraminifera (e.g., Nephrolepidina and Miogypsina) being less common.

Two types of dolomite occur: (1) limpid dolomite with O18 = -5.77 and with bipyramidal quartz and (2) microcrystalline dolomite with O18 = 2.00 and with length-slow chalcedony. While microcrystalline dolomite tends to predominate in muddy matrix material, limpid dolomite appears to fill pores, some of which are moldic. Dolomitization is attributed to evaporation during an early stage of diagenesis and succeeding fall of sea level, causing an enlarged mixed-water diagenetic environment.

Younger nonreef carbonate rocks, as occur on the Noto Peninsula of central Honshu, are commonly cross-bedded, contain Bryozoa, mollusks, small foraminifera, and echinoids, and are locally dolomitized. These dolomites are ascribed to a mixed-water origin. A different type of nonreef, yet reservoir-forming, dolostone occurs in the late middle Miocene of northeast Honshu and is interpreted to have formed as a transformation from bathyal opal.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.