The Poronai Formation in the Middle Part of the Yubari Coal-field : Subdivision of the Poronai Formation based on a Quantitative Analysis of its Megafossil Fauna

Abstract

The Poronai formation, predominantly a mudstone more than 1, 000 meters thick, is the thickest and most extensively distributed formation in the southern half of the Ishikari coal-field, which is known as the Yubari coal-field., The formation contains abundant fossils of molluscs, crustaceaus, simple corals, and rarely plant leaves., From the mode of occurrence of the mentioned fossils, the writer concludes that they were preserved in situ, in other words, they comprise a thanatocoenosis, which represents an original biotope., Qualitatively considered, the fauna is similar throughout the entire thickness of the formation, but quantitatively, such uniformity is not recognized and the fauna differs in composition from horizon., Each horizon or phase shows wide distribution throughout the entire Yubari coal-field., Hitherto subdivision of the Poronai formation has been considered to be extremely difficult by the megafossils., However, the writer is inclined to believe in its possiblity by a quantitative treatment of the megafossils, since he has been successfull in subdividing the Poronai formation into 9 (A-I) sequential parts.,