Microbiostratigraphy of the Sagara Group, Shizuoka Prefecture, Japan

The Tertiary deposits distributed in the Sagara district, Shizuoka Prefecture, are classified into the Megami Formation and the Sagara and Kakegawa groups in the named order from the older to the younger. The stratigraphical classification and biostratigraphic characteristics of the stratigraphic units are shown in Table 2, and briefly outlined below. The Megami Formation, the oldest of the recognized units, consists of hard mudstone, sandstone and limestone and is in fault contact with the Sagara Group. The Megami Formation corresponds to the major part of the Saigo Formation of the Mikasa Group distributed in the northern part of Kakegawa City and is correlated with the planktonic foraminiferal Zone N. 8 (Globigerinoides sicanus/Globigerinatella insueta Partial range zone) of Blow (1969). According to Blow (1969), the geological age is Early Miocene (Burdigalian). The Sagara Group comprises two formations each consisting of alternations of siltstone and sandstone, intercalated with conglomerate. The Kakegawa Group is composed of alternations of sandy siltstone and sandstone, intercalated with conglomerate and massive mudstone and lies with conformity upon the Sagara Group. The Sagara and Kakegawa groups have yielded many species of Foraminifera. In this district, zonation by means of the planktonic Foraminifera was previously made by Saito (1963). From the confirmation of a difference of the extinction level between Globigerina nepenthes and Sphaeroidinellopsis seminulina, Saito referred the Sagara Group and the lower part of the Kakegawa Group to the Globorotalia menardii menardiilGlobigerina nepenthes zone of Blow (1959) and assigned the lower to middle part of the Uchida Formation of the Kakegawa Group to the Sphaeroidinellopsis seminulina zone of Blow; he considered that these zones were correlative of the zones of the same name established in the Pozbn Formation, Venezuela by Blow (1959). Blow (1959) used the horizon of extinction of Globigerina nepenthes and Sphaeroidinellopsis seminulina as an index for the uppermost part of the Upper Miocene in the Pozbn Formation. Saito (1960 a, b, 1963) was in his opinion that the lower part of the Kakegawa Group was Miocene and that the Miocene-Pliocene boundary fell within the upper part of the Uchida Formation which was the middle part of the Kakegawa Group. The biostratigraphy of the Upper Miocene to Quaternary sequence was studied later in detail by Bolli, Blow and other investigators in the warm water region, and zonations by means of the planktonic Foraminifera were proposed (Banner and Blow, 1965; Bolli and Bermudez, 1965; Bolli, 1966; Blow, 1969; etc.). Blow (1969) showed that the stratigraphic range of Globigerina nepenthes and Sphaeroidinellopsis seminulina in the Pozbn Formation, Venezuela did not represent the total stratigraphic range but only a part of it and was thus the local range. Also, the first appearance of Sphaeroidinella dehiscens has referred as an reliable criteria to mark the Miocene-Pliocene boun-dary (Bandy, Parker, Blow, etc.). The first appearance of Sphaeroidinella dehiscens is recognized at basal part of the Kakegawa Group. From the reinvestigation of the planktonic Foraminifera of the Sagara Group and the lower part of the Kakegawa Group, it was confirmed that the biostratigraphic classification by those microfossils corresponded well to the zones established originally in the tropical to subtropical region by Blow (1969). The stratigraphic distribution of the identified planktonic Foraminifera is shown in Table 3, and the diagnostic faunal elements of each zone are briefly noted below. In the Sagara Group, three planktonic foraminiferal zones are recognized, namely, Globorotalia (T.) acostaensis acostaensis-Globorotalia (G.) merotumida Partial range zone, Globorotalia (G.) tumida plesiotumida Consecutive-range zone, and Globorotalia (G.) tumida tumida-Sphaeroidinellopsis subdehiscens subdehiscens Partial-range zone. They are respectively correlated with Zone N. 16, N. 17 and N. 18 of Blow (1969). The lowest part of the Kakegawa Group corresponds with Blow\u27s Zone N