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In 1955, Professor Kiyoshi Asano of the Tohoku University for the purpose to establish the stratigraphic sequence of the Neogene Foraminifera of Japan, organized a research group and the central and eastern parts of the Boso Peninsula, Chiba Prefecture, were selected as the standard area for the project. The writer, a member of the research group, has been engaged in the work on the Neogene and Quaternary biostratigraphy in the Southern Kanto region by the smaller Foraminifera. In 1957, the writer was appointed the position of micropaleontologist of the Teikoku Oil Company where he has continued his study. Especially he has endeavored to work out the Miocene, Pliocene and Pleistocene biostratigraphy by the smaller Foraminifera obtained from the drilled well-cores distributed in the southern part of the Kanto region. The Cenozoic formations of the Miura Peninsula, Kanagawa Prefecture are classified, as shown in Table 1 and divided into two zonules by the benthonic Foraminifera collected therefrom. As to the subsurface geology in the Hodogaya area, the formations shown in Figure 2 were distinguished from the micropaleontological data of the drilled wells and correlated with the standard stratigraphic sequence of the Miura Peninsula. Concerning the Miocene formations no difference was recognized in the foraminiferal associations between the Miocene surface samples of the Miura Peninsula and of the subsurface materials from the Hodogaya area. However, in the Kazusa Group which ranges from Pliocene to Pleistocene, the foraminiferal assemblages indicating a shallow water habitat are found in the southern part of the Peninsula whereas the deep water assemblages in its northern part. Abundant planktonic Foraminifera were recognized in the samples from the base of Nojima formation in the Miura Peninsula. The same species of planktonic Foraminifera were found in abundance from the cores of depths shallower than 760 meters in the Hodogaya well R-1. This paleontological evidence shows that the subsurface formation in the Hodogaya area is correlative with the Nojima formation. On the north side of Tokyo City there are two wells, namely Soka R-1 and Kasukabe R-1, as shown in Figure 3. The subsurface formations which yielded the Miocene plank-tonic Foraminifera are correlated with the Zushi and Ikego formations in the Miura Peninsula. However these subsurface Miocene formations are composed of lithological and faunal characters different from those of the Miura Peninsula, obviously being rich in shallow water Foraminifera. A similar phenomenon is found in the foraminiferal fauna of the Kazusa Group between both areas. This fact shows, as suggested in Figure 4, that toward the north of Tokyo there probably existed undulatory movement of the base rock, and a shallow water fauna became dominant during the after the Miocene. On the other hand, thick Miocene formations are distributed on the east side of Mt. Nokogiri in the Boso Peninsula and they are called, in ascending order, Kinone, Amatsu, Senhata, Inakozawa and Hagyu formations. In the Kinone formation, Lepidocylina or Miogypsina is common and occurs in association with planktonic Foraminifera of the Globigerinatella insueta zone (Saito, 1963). However it is also presumed that the upper part of the formation belongs to the Globorotalia fohsi zone. In the Amatsu formation planktonic Foraminifera are scarce and therefore it is difficult to correlate the formation with any planktonic foraminiferal zone. The Inakozawa formation is correlative with the Globorotalia mayeri-Globigerina nepenthes zone. The Globorotalia fohsi zone and Globorotalia bykova zone of Saito have not yet been confirmed in this area. On the other hand, the benthonic fauna of arenaceous Foraminifera which is abundant in the Kinone formation is considered to have been deposited under an anaerobic condition. The calcareous foraminiferal fauna are generally rich in formations other than the Kinone as shown in Figure 5. The formations of the Kazusa Group are well developed along the Yoro River of the Boso Peninsula. The formations taken together attain 2500 meters in thickness. They can be classified into eight zonules and two subzonules based upon the smaller Foraminifera as shown in Tables 2 and 3. From the foraminiferal fauna of these zonules it is clear that a shallow water fauna is dominant in the upper part of the group and a deep water fauna characterizes the lower part. The boundary between the two faunas is at the base of the Kokumoto formation. As to the planktonic Foraminifera, three characteristic horizons-the base of the Kiwada formation, the upper part of the Umegase formation and the base of the Kokumoto formation-can be discriminated. The planktonic Foraminifera from the samples at the base of the Kiwada formation can be easily correlated with that of the Nojima formation in the Miura Peninsula. The fauna is composed of : Pulleniatina obliquiloculata, Globorotalia inflata, Globorotalia menardii-tumida, Globigerinoides ruber cyclostomus, Globigerinoides trilobus and Sphaeroidinella dehiscens. These species comprise a fauna which is characterized and influenced by the warm current. This kind of fauna is rarely found in the upper part of the Umegase formation. Therefore it may be considered that the warm water current must have become weakened during the middle part of the Umegase. This was accepted by Prof. Asano to be correlated with the boundary between the Pliocene and Pleistocene as indicated by the International Geological Congress of London. In Yachimata R-1 and Naruto R-1 wells drilled in the northern part of Chiba Prefecture, some Miocene formations have been found based upon some characteristic Foraminifera as shown in Figure 6. The Kazusa Group as judged from the lithofacies in the just mentioned area becomes much thinner toward the northeast. In these wells, the same biostratigraphic sequence, as in the type locality, can be recognized. Especially the Uvigerina akitaensis zonule which indicates the upper part of the Umegase formation has been found in all of the wells drilled in Chiba Prefecture and therefore it may be considered to be an important zonule in correlation. However, in the Omigawa R-1 well situated in the northeastern part of Chiba Prefecture, shallower water species are generally dominant from this zonule down to the basement rock. This kind of foraminiferal fauna seems to be characteristic of the marginal facies of the sediments that contains the typically deep water species such as Bulimina and Bolivina. The just mentioned view, also as shown in the Isogal map, may be explained by the high gravity anomaly distributed around the Omigawa area which had existed since the beginning of deposition of the Kazusa Group. The oldest Cenozoic formations are exposed in the southern part of the Miura and Boso Peninsulas, and these are bordered by the Tanzawa-Mineoka tectonic line. This tectonic line is also shown in the Isogal map together with another one of northwest-southeast direction (Kanto tectonic line). From the existence of two low gravity anomalies, the with Tokyo Bay situated between them, and also because of the difference of foraminiferal fauna between the Miura and Boso Peninsulas, a tectonic line running through Tokyo Bay is assumed. This tectonic line may be considered a continuation of the Morioka-Shirakawa line which was active during deposition of the Kazusa Group.

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