The Kakegawa District is the type area of the Japanese Neogene. The Neogene/Quaternary boundary has been known at the top of Flysch type alternation of Horinouchi facies belonging to lower and middle Kakegawa Group. The upper part of Kakegawa Group consists of sandy shallow water deposits of the Soga Formation in the north and massive mud of the Hijikata Formation in the south area. The Ogasa Group unconformably overlying the Kakegawa Group is composed mainly of the gravels of the Ooigawa River Basin. In the north part, it intercalates many mud beds of marsh, lagoon and intertidal zone and several beach pebble beds. The normal paleomagnetic polarities of the top of the Hijikata mud in the south are correlated to the Jaramillo event. The lower part of Ogasa Group has reversed polarities and contains Metasequoia cones. The normal polarities of the upper part of Ogasa Group are correlated to the Brunhes Normal. The uppermost mud bed of Ogasa Group, the Okazaki mud yields warm-water and inlet shell fossils, such as Dendostraea paulucciae and Anadara granosa. This bed may be correlated to Ma 8, the uppermost warm-marine clay of Osaka Group or to Ma 10 in the Higher Terrace sediments.
The area around Ise Bay has been broken into many blocks by faultings. As subsiding blocks, Ise Bay and Nobi Plain are surrounded by 30 to 200 meters high hilly blocks and by upheaved mountain blocks ranging from 500 to 1200 meters high. The mountain blocks are made up of pre-Tertiary basement rocks. The hilly blocks are composed mainly of the Tokai Group; deposited in“Lake Tokai”, a sedimentary basin from Pliocene to Early Pleistocene time. The hilly blocks are extensively dissected by rivers and only a series of hill tops at approximately the same height is left as a remnant of the erosional surface in early stage. Older fluvial deposits, “the highest gravel”, unconformably overlie some of the hill tops. Younger fluvial deposits, the higher and lower terrace sediments, are developed along major valleys. Since these fluvial deposits consist mainly of gravelly sediments and the relative altitudes of surfaces of the terraces are modified by the movements of local fault blocks, detailed stratigraphic positions of these were not clear. Cenozoic sediments underlying the Nobi Plain attain 1500 meters in thickness. In the sequence of the sediments, a remarkable unconformity is recognized. The upper sediments, more than 350 meters thick, consist of alternating marine and fluvial sediments, suggesting that they were deposited under the influence of the glacial sea level oscillations. The lower sediments corresponding to the Tokai Group are unconformably overlain by the upper sediments. Microfossil analyses of a large number of drilling cores have clarified the stratigraphic succession of these sediments. The upper sediments are divided into four units: The lower Middle Pleistocene Yatomi Formation, the upper Middle Pleistocene Ama Formation, Upper Pleistocene formations, and Holocene sediments. The biostratigraphic information about the upper sediments furnishes valuable clue to the stratigraphic position of the fluvial deposits on land. Thus, the distribution pattern and altitude of these upper Quaternary sediments reveal the differential movements of each fault block surrounding Ise Bay. The Nobi Plain block has rapidly subsided since early Middle Pleistocene. Subsequently the gentle uplift in the hilly blocks occurred in the later Middle Pleistocene time, the stage of“the highest gravel”, and followed by the dissection of the hilly area. The subsidence of Ise Bay block may have started later than that of the Nobi Plain block, though the subsidence rates of both blocks have been of the same order since late Pleistocene.
Quaternary sediments are widely developed under the Nobi Plain, and attain more than 350 meters in thickness. Pollen analytical studies of these sediments are carried out on the core samples from Saya and Tsushima in the Nobi Plain. The results are summerized as follows. 1) The pollen assemblages of the Holocene Nanyo Formation are characterized by Cyclobalanopsis in the upper part, and Lepidobalanus and Celtis-Aphananthe in the lower part of the formation. 2) The Nobi Formation, the uppermost Pleistocene, is dominated by Lepidobalanus and herb pollen, including Gramineae and Cyperaceae. 3) The pollen assemblages of the Lower Atsuta Formation, Late Pleistocene in age, are characterized by the dominance of coniferous forests, Fagus and Cyclobalanopsis in the upper half, and coniferous forests and Lepidobalanus in the lower half. The Lower Atsuta Formation contains a small amount of Lagerstroemia. 4) The pollen assemblages of the Middle Pleistocene Ama Formation are mostly composed of the Fagus and Ulmus-Zelkova. Am 1, the lowest marine clay bed of the Ama Formation, is predominant in Cyclobalanopsis. 5) The pollen assemblages of the upper and the lower parts of the Nanyo Formation are correlated with the pollen assemblages of the RIIb and RIIa pollen zones (ONISHI, 1977), respectively. The Lower Atsuta Formation is correlated with the Ma 12 marine clay bed of the Osaka Group and pollen assemblages of the Am 1 marine clay bed of the Ama Fomation is correlated with Fagus-Cyclobalanopsis pollen zone (NISHIMURA, 1980).
The author has carried out the diatom analysis on the middle Pleistocene sediments underlying the Nobi Plain, called the Ama and the Yatomi Formations, and has discussed the changes in sedimentary environments estimated from the diatom flora. The two Formations are composed of alternating beds of fluvial gravels and lacustrine deposits interbedded with a marine deposit. Four marine deposits have been found in the Formations, and this shows that four transgressions, at least, took place at the Nobi Plain Basin in Middle Pleistocene Age. The sedimentary environments of these deposits are summarized as follows; G2 gravel bed Ama Formation Lacustrine bed-Brackish water to marine bed, being three to five meters thick, and yielding abundant diatoms such as Cyclotella striata, Actinocyclus spp. etc. -Lacustrine bed Ag2 gravel bed Lacustrine bed-Marine bed, being about five meters thick, and yielding abundant diatoms such as Cyclotella striata, Melosira sulcata etc. -Lacustrine bed Ag1 gravel bed Lacustrine bed-Brackish water to marine bed, being 5-13 meters thick, and yielding abundant diatoms such as Cyclotella striata, Melosira octogona etc. -Lacustrine bed G3 gravel bed Yatomi Formation Lacustrine bed-Brackish water to marine bed, being five or more thick, and yielding abundant diatoms such as Actinocyclus spp. etc. -Lacustrine bed
Drilling core samples of Lake Biwa sediments of 200m and 1000m thick obtained from the lake bottom and the east coast are correlated to the Pleistocene strata distributed in the Kinki district, using volcanic ash layers and paleomagnetic polarity data. The 200m core sample composed of clay sediments with some volcanic ashes has provided various data of tephrostratigraphy, paleomagnetism, biostratigraphy and geochemistry covering the last 0.5m.y. These records are suggested to be usefull as a reference data to investigate the Pleisotcene in the Kinki district. A significant key for the correlation is a volcanic ash layer which has a fission track age of 0.17m.y. and remanent magnetization of reversed polarity. This volcanic ash layer occurs in the horizon of the Biwa I event of the 200m core sample. The layer is also found in the Kobiwako Group on the northwest coast of Lake Biwa, and between the marine clays Ma 10 and Ma 11 of the Osaka Group in the Akashi area. According to this correlation, type of topographic surfaces in the Kinki district can be divided into three fundamental shapes (Fig. 13). It is implied that the marine facies and topographic flat planes were formed in accordance with the sea level change by the global climatic oscillation.
The Middle Pleistocene formations of the South Kanto district attract many geologists' attention as one of the important Quaternary type successions in Japan, because of wide distribution, extremely thick sequence of aqueous and aeolian deposits and containing a large number of fossils. The geologic structure of the formations in this district, however, was complicatedly deformed during Quaternary, and detail stratigraphy were veiled till quite recently. In this paper, we proposed a preliminary stratigraphic chronology on the Middle to Upper Pleistocene in the district (Table 2), which has been compiled in some recent fruits by several authors (KANTO QUATERNARY RESEARCH GROUP, 1974; MACHIDA et al., 1974; UESUGI, 1970; KIKUCHI, 1977; NISHIMURA, 1980; etc). Some geologic characteristics of the Middle Pleistocene formations are enumerated, too. (1) Some volcanoes situated in the western region produced the enormous volume of pyroclastic fall materials during Quaternary. An almost continuous columnar section of aeolian pyroclastic materials deposited during the past 400, 000 years is compiled from several sections of the Oiso Hill adjacent to the Fuji-Hakone volcanic region, which is more than 250 meters in thickness. (2) Distribution of the Middle Pleistocene aqueous and aeolian deposits in the district is very complicated. Lithofacies of these deposits are extremely variable in a narrow area. Many characteristic pyroclastic key beds are most useful to correlate with these various deposits. (3) The marine deposits of the Middle Pleistocene are 300 meters thick in the Oiso Hill and 600 meters in the Boso Peninsula. They are divided into 8 to 10 stratigraphic units by disconformable boundaries caused by lowerings of the sea level. Formation of each unit corresponds to a stage of glacial eustatic sea level rising. (4) It is generally believed that the South Kanto is one of the rapid upheaval districts in Japan and there are several marine terraces recording the Middle to Late Pleistocene sea levels in the district. We must indicate, however, that there is no graded feature formed during the Middle Pleistocene at least except the Tama Hill area along the west coast of Tokyo Bay. The contemporaneous marine deposits accumulate one after another or intermittently with disconformities at the great part of the coastal area and do not constitute any terraces.
Stratigraphic changes of fossil assemblages of macroscopic plant fragments and pollen during the middle Pleistocene in Southwest Japan are briefly enumerated as follows. 1. Most of relict elements of the Pliocene flora as Metasequoia, Glyptostrobus, Juglans megacinerea, Liquidambar etc. already disappeared during the early Pleistocene. But some species as Cunninghamia lanceolata, Picea koribai, Ilex cornuta and Paliurus nipponicus still remained, and they disappeared during the middle Pleistocene. 2. There is a large possibility that Liquidambar revived temporally its distribution in the southern coastal region of Southwest Japan during an interglacial age in the middle Pleistocene. 3. Such northern elements as Larix gmelini, Abies veitchii, Picea jezoensis var. hondoensis, Oxycoccus palustris etc. have their first appearance during a glacial age in the middle Pleistocene. 4. The specialization of Larix leptolepis, one of the endemic species of Japan, may have occurred during the middle Pleistocene in age. And its ancestral species may be L. gmelini inferring from their present ecological and phytogeographical relations.
Magnetostratigraphic studies on the Middle and Late Pleistocene deposits in Japan are summarized. It is emphasized that the identified records of geomagnetic excursion as well as reversal in sedimentary sections are useful evidence for time-correlation and chronology of the Pleistocene sections. Three VGP paths during the Matuyama-Brunhes transition defined in Japan are compared with each other. These paths are laid in the sector of longitude between 70°E and 120°E, and it is considered that the main dipole field decayed and standing part of the nondipole field predominated during the polarity transition. The comparison of several VGP paths recorded in the Neogene and Pleistocene sections in the southern Tohoku indicates that it is possible to distinguish the individual excursion or reversal from others by its characteristic features of VGP path.
The middle Pleistocene time range is defined here the period before the last interglacial in the Brunhes epoch. In Japan, the Shimosa-Kazusa Groups around the Tokyo Bay in Kanto and the upper part of the Osaka Group in Kinki both represent the standard middle Pleistocene sequences and have been studied in detail. These groups are characterized by the cyclic sedimentation caused by transgression and regression corresponding to climatic changes. The purpose of this paper is to attempt the correlation of the Kazusa Group and the Osaka Group by identification of widespread marker-tephras. Accurate determinations of the refractive indices of volcanic glass, orthopyroxene and hornblende, together with other determinations, have enabled successful characterization for correlation to be made for several tephra layers in southern Kanto and Kinki. The following marker-tephras are found over two districts, resulting in the establishment of several important datum planes in the middle Pleistocene sequences. The vitric tephra called Ks 11, which is sandwiched in the Kasamori Formation and its correlatives in southern Kanto, can be identified as a marker tephra of the Osaka Group called Sakura ash, in the vicinity of Osaka and Kyoto. The estimated age of this tephra, about 450, 000 years, is based on its stratigraphic relationship with the underlying dated tephra, Kinukawa ash (460, 000-470, 000 years). Stratigraphically, it is included in the deposits immediately below the transgressive horizon (Tama-f and Ma 7) and also occurrs in the biozone of Stegodon orientalis in both districts. The vitric tephra called Ku 1, sandwiched in the Kokumoto Formation, is identified and correlated with a tephra called Imakuma I ash. It is included in the deposits immediately above the Brunhes-Matuyama boundary. The very important dated tephra, Azuki ash (870, 000 years), in the Osaka Group, can be correlated with Ku 6C tephra in the lower part of the Kokumoto Formation by their peculiar petrographic properties. Both are sandwiched in the deposits below the Brunhes-Matuyama boundary. From the relationship between the identified tephras and marine sequences, it is concluded that during the period from 700, 000 years to 450, 000 years at least three interglacial-glacial cycles are recorded, and that after 450, 000 years the following major interglacial episodes are indicated; 450, 000YBP, 370, 000YBP, 300, 000YBP, 230, 000YBP, and 130, 000YBP.
Various opinions on the Quaternary tectonism previously pointed out are discussed and summarized as follows: 1) The tectonic culmination in the middle Quaternary is not at least an extensive phenomenon at this area. 2) The uniform rate of deformation in the Quaternary seems to be accepted in the order of less than 105yrs, but not likely in the order of more than 106yrs. 3) Although the accelerated rates towards the present are often suggested by many recent studies, these may be an apparent feature due to a fluctuation in a relatively short time span within a long-term displacement, or due to a change from a long-wave (or extensive) deformation to a short-wave (or limited) deformation. And also may result from a growing process, as typically expressed with formative advance of fault, even in a same stress condition. 4) The SW Japan has been under the regional stress field of WNW-ESE compression through the Quaternary, being originated from the subduction of oceanic plates. 5) The shallow depression along the median zone of SW Japan had come into existence simultaneously with the subduction along the Nankai Trough from the middle Pliocene. This may suggest the rejuvenation of the Philippine Sea Plate from that time. 6) The leading edge of the underthrusting slab of the Philippine Sea Plate locates at the outer zone in the active segment of the Median Tectonic Line, whearas it extends to the inner zone in its inactive segment (Fig. 4A). These indicate that the outer zone underthrust at very low angle (ca 10°) by the plate behaves as a rigid bloack. As such belts as the outer zone, the longitudinal depression zone and the upwarping mountain of the inner zone have gradually shifted to the northwestward with the times (Fig. 4), these systematic migrations of tectonic province with miscellaneous changes of its process are likely caused by an advance of the underthrusting slab. 7) The landward tilting terraces along the Pacific coast of SW Japan are considered to be caused by reverse faults which have emerged as accretionary prism on the upper part of continental slope from the middle to late Quaternary.