山形県北西部中新統上部のBiofaciesの研究

抄録

Studies on the biofacies of the sediments distributed in the Shonai-, Shinjo-, Nikabo-, and Yajima areas in the northwestern part of Yamagata Prefecture were undertaken parallel with stratigraphical, paleontological and structural analyses. This project was planned to understand the differences in both lateral and vertical distributions of the different lithofacies according to horizon and stratigraphical unit because of the intimate relationship with oil-producing and carrier beds.
For the purpose of the study the Funakawa and Kitaura formations of upper Miocene age and of extensive distribution in the oil-fields of northwestern Japan and important as oil carriers were selected. The Funakawa which is subjacent with conformity to the Kitaura in all of the areas is studied in this work.
Lithofacies change in the middle and lower parts of the Kitaura and Funakawa stages is particularly noteworthy in the Funakawa south of the line combining Shinjo and Kuraoka where sand intercalations abruptly increase and the facies becomes tuffaceous, whereas in the Kitaura which is dominantly of sandy siltstone but in the Nikabo and Shinjo areas tuffaceous sandstone intercalations increase and in the latter mentioned area an alternation is developed and lignite becomes intercalated. By the increase in sandstone there is developed in the south of Shinjo a peninsular deposit.
Details of lithofacies changes were recorded over the whole areas studied and their relationship with thickness of the respective stratigraphic units, faunal yield and their contents were analysed, by which the paleobathymetrical conditions, paleosedimentary basin bottom configuration and changes in the structure of the basin could be worked out.
Of the Foraminifera washed out from numerous samples taken both vertically and laterally within the Funakawa and Kitaura formations, studies were progressed as to their quantity-analyses with regard to lithofacies and horizon as well as position. From the characteristic features of the different Foraminifera distinguished and the assemblages recognized there could be discriminated arenaceous and calcareous assemblages of the neritic zone and calcareous assemblages of deeper water zones. The relationship of these two different assemblages to their environments (lithofacies, horizon, stratigraphic position, etc.) was inferred from biofacies construction.
Also interpreted was the relation between arenaceous planktonic Foraminifera and calcareous benthonic ones, which resulted in when the characteristic features of the above-mentioned were incorporated with the various aspects of them already stated, the general features of the sedimentary basin during the time of the respective horizons which yielded the Foraminifera as related with development and migration was clarified as described in detail in the text.
The various assemblages of the Foraminifera not only served to indicate the features above-mentioned but also were good guides in paleobathymetrical analyses of the respective horizons of the Funakawa and Kitaura formations, the results being supported from the bathymetrical ranges of the Foraminifera on the shallow to deep waters offing Yamagata and Akita Prefecturs.