With the progress of activities by the Deep Sea Drilling Project, multiple stratigraphic techniques have been developed. The application of planktonic microbiostratigraphy, by using the ranges of taxa of several different taxonomic groups in conjunction, has brought about a marked refinement in stratigraphic resolution and improvement on the accuracy of global correlation of marine sedimentary sequences. Differences in the degree of biostratigraphic resolution may have principally risen from changes in the rate of evolution and in the diversity of planktonic biota, both of which are deeply affected by paleoceanographic conditions. The need of different biostratigraphic zonal schemes for different latitudinal regions indicates the existence of floral and faunal provincialism in the geologic past. By examining the patterns of temporal fluctuation of boundaries between these biologic provinces, it is possible to reconstruct paleoceanographic history. Difficulties in the application of the standard scheme of chronostratigraphy based on stratotypes distributed mostly in the northwestern European province to other regions have worked as a basic force toward the development of biochronology based on deep-sea sequences. Future plans for the ocean drilling project promises further improvement in stratigraphic resolution and provides a better coverage of oceanic provinces toward achieving better understandings of ocean paleoenvironment.
This paper presents a review of two recent and important approaches to interregional stratigraphic correlation and chronostratigraphy. These approaches are 1) the use of oxygen isotopic variations for stratigraphic correlation in marine sequences containing calcareous microfossils and 2) the stratigraphic interpretation of seismic reflection patterns to deduce time-synchronous unconformities and their correlative conformities. The method of seismic stratigraphy developed by Vail and others (1977, 1980) is primarily based on studies of shallow continental margin seismic data and correlates the reflection boundaries with sea-level changes. Because the melting and freezing of high-latitude ice sheets are the primary factor controlling 18O/16O ratio variations in the calcareous tests of foraminifera, the record of isotopic changes in deep open ocean sediments is globally synchronous. This approach is particularly useful to stratigraphic studies of the late Cenozoic when large ice sheets occurred in the polar regions and fluctuated in volume. Since the ice volume fluctuations are also responsible for eustatic sea-level changes, the oxygen isotope study can be used to reconstruct past changes in sea-level. A sharp change in oxygen-isotopic values near the beginning of the Middle Miocene at about 14Ma brought about an influx of siliceous microfossils in Japanese Miocene sediments by establishing a region of upwelling over much of the northeastern Japan owing to the development of a cool, southward-flowing current. Another marked change in the oxygen isotope record at about 3.2Ma has been interpreted to have caused a sea-level lowering of as much as 40m. A paraconformity separating the non-marine Mukaiyama Formation from the subjacent marine Tatsunokuchi Formation in the environs of Sendai City is considered to have resulted from this large sea level drop. On the other hand, the method of seismic stratigraphy has so far received little attention in Japan.
Recent progresses in the Neogene bio- and chronostratigraphy in Japan have been achieved by the establishment of microplanktonic biostratigraphy in land-based marine sequences and recognizing of datum levels for the precise interregional correlation, within the framework of radiometric time scale. The microplanktonic time scale has been accomplished by incorporating various methods of radiometric dating, magnetostratigraphy, geomagnetic polarity time scale, and detailed biostratigraphies of deep sea cores and typical land-exposed sections. The Neogene marine sequences in the Pacific side of southern Japan yields much tropical planktonic foraminifers throughout the entire succession, which are directly correlative with those characterizing the world's tropical region. In northern Japan and also on the coast of the Sea of Japan, however, warm water elements are restricted to a lower part of the Neogene, and cold-water inhabitants dominate younger stratigraphic horizons, where correlations can be made mainly by diatoms and additionally by radiolarians and calcareous nannoplankton. Some planktonic foraminifers characterizing middle latitude areas are also useful for the correlation of sequences in the Pacific side of southern Japan with those in the Sea of Japan side. The resultant correlation by means of planktonic microfossils made clear its relation to the biostratigraphy of benthic marine molluscs and larger foraminifers, and some refinements in the Neogene history of Japan became necessary. To achieve precise interregional correlation of the Japanese Neogene sequences, most important problems to be solved in future are further refinements of radiometric calibrations in the microplanktonic time scale and the stratigraphic relationship between datum levels of planktonic foraminifers and those based upon other fossil groups. The bio- and chronostratigraphic correlation. within the framework of the newly accomplished microplanktonic time scale provides a basis for achieving global correlations of the Neogene events.
Biostratigraphic studies of microfossils, especially of diatoms, recently carried out on the Neogene strata of Hokkaido have revealed that traditional chronostratigraphic subdivisions of those strata must be partially revised, although there still remains several problems to be solved. In this article, such recent results available mainly on the Neogene System developed in the Southwestern Part of the Kushiro Coal Field Area are presented, and epoch boundaries in the Neogene of several other areas of Hokkaido are also suggested.
Biostratigraphic correlation between the Neogene sections from the Noto peninsula and the offshore Kanazawa 1x well is described in the present paper based on planktonic microfossils. The main parts of the Taniguchi Formation are considered to be late Early Miocene in age, which is characterized by abundance of Discoaster deflandrei. The Nakanami Formation and the lower part of the Sugata Formation assigned to early Middle Miocene based on calcareous microfossils. The middle to upper part of the Sugata Formation in the Nadaura route is placed in Middle Miocene (KOIZUMI, 1979).The Pliocene Yabuta Formation unconformably covers the Sugata Formation with the absence of the Upper Miocene section. Showing some local variations, the unconformity penetrated by offshore Kanazawa 1x well is widely traceable in the offshore Hokuriku to San-in sedimentary basins by using the seismic profiles (TANAKA and OGUSA, 1981). From these facts, the main structural movements which formed oil-traps terminated during the latest Miocene to early Pliocene in age. Moreover the early Pliocene strata are also unconformably overlain by the late Pliocene to Pleistocene strata which are characterized by onlapping patterns in the offshore basins. These patterns suggest that the late Pliocene to Pleistocene strata consist of the sediments deposited during the rise of the sea level. On the Pacific side, the Kurotaki Formation in the Boso Peninsula and the Nobori Formation in the Shikoku district were formed by a transgression in late Pliocene (IKEBE, 1978; ODA, 1977; TAKAYAMA, 1980). These facts suggest that the influence of the late Pliocene transgression extended all over the Japanese islands.
Based on recent studies on various plankton microfossils as planktonic foraminifera, calcareous nannoplankton, radiolarians and diatoms, an integrated scheme of biostratigraphy is constructed to be applicable to the middle and upper Miocene onshore sections in the central and northeast Honshu, Japan. Diatoms, diverse and abundant in the higher latitudes, have revealed usefullness for the Miocene biostratigraphy of northern Japan. On the other hand, the remaining three microfossils biostratigraphic zonations are principally established on the basis of warm water species. Because of their geographic limitation in distribution, these zonations are often difficult to be applied to the higher latitude sequences. Through cross-checking of seccessions of datum levels of diatom and other planktonic microfossils within several sections at the Sannohe, Ichinoseki, Sendai, Takasaki, Boso, Kakegawa areas and DSDP site 438A, a composite sequence of datum levels consisting of more than 20 paleontological events is established. It will provide the necessary framework for reconstruction of the Miocene paleoceanography and paleoenvironment of the northwestern Pacific margin.
Recent progress on Chrono-stratigraphy revised time scale in Pliocene and Pleistcene in Niigata sedimentary basin, Japan. It changed the burial history of Miocene source sediments in the area, but did not change the absolute geologic time of the sediments. Computer simulation model has been used to estimate the effect of geologic time interval on oil generation and expulsion. Two cases of model studies (one for old time scale, the other for new time scale) for the Miocene sediments have been conducted. It is concluded that, regardless of burial history, there is no significant difference of depths of oil generation nor expulsion between the two cases. Oil generation occurrs in the 1800-2500m interval and oil expulsion occurrs in the 2500-3700m interval in both cases. Another model study on the effect of absolute time interval reveals that depth interval of oil generation and expulsion would be shallowered by 1000m if the geologic time of the Miocene sediments (8 MYBP) changes to early Paleocene (65MYBP).