Biostratigraphy of the Isizawagawa Group in Honjo and its Environs, Akita Prefecture

From lithological characteristics the Isizawagawa group, the principal oil-bearing strata in Honjo and its environs in Akita Prefecture, is classified into four formations as shown in Figure 1., Biostratigraphical studies were concentrated on these formations based upon the smaller Foraminifera., About 250 species of benthonic Foraminifera were discriminated in the 288 samples taken from the area., From the foraminiferal assemblages and stratigraphic distribution of some of the characteristic forms, a total of four zonules and 11 subzonules (Figure 3) were distinguished within the formations recognized., These zonules and subzonules have uniform distribution within the area studied (Figure 5)., The foraminiferal fauna of the Isizawagawa group indicates a cool thermal condition contrary to the underlying Sugota formation, which contains a fauna of warm thermal waters., From the faunal evidence and geology of the area, it is considered that the major transgression which resulted in the deposition of the Isizawagawa group, commenced with the deposition of the Toridame formation; this formation occupies the lowest position in the said group., From the sequence of the foraminiferas it is inferred that the transgression reached an inundation phase during the Tate formation., A gradual regressive phase was initiated towards the latter part of Tate time., The foraminiferal assemblages in the basal part of the Minaminomata and Osawa formations include a few reworked, shallow water species of Foraminifera, which have distorted and water worn shells., From such evidence it is inferred that there were two orogenic movements within the geosyncline during the regressive phase, at least in the area studied.,