985 Density and relative abundance of microspheric and megalospheric forms of Nummulites as sedimentologic and taphonomic indicators

抄録

The density and relative abundance of smaller (megalospheric) and larger (microspheric) forms of Nummulites provide excellent indicators of sedimentary condensation, dilution and size-sorting during the formation of fossil assemblages, using an inferred 10 : 1 original ratio of the two forms. Various degrees of current sorting were recognized in the sandy facies of Nankinhama of Hahajima, Ogasawara Islands; storm condensation in low-angle, cross-bedded sandstone, winnowing in sandstone beds with strongly imbricated Nummulites tests, and highly diluted occurrence in debris flow deposits. A poorly sorted, pale green, bedded sandstone is interpreted to represent the least disturbed and perhaps indigenous preservation of Nummulites. In this sandstone, the ratio of the two forms is roughly 10 : 1, as for other published examples. No significant modification of density and relative abundance between the two forms occurs in an apparently allochthonous occurrence of Nummulites in conglomeratic debris flows. Therefore, preservation of the original ratio of the two forms of Nummulites does not necessarily exclude transportation.