The Miyazaki Plain has the best-developed late Quaternary terraces and deposits in Kyushu. But the landform evolution of this plain has not been investigated in detail in previous works. This study tephrochronologically describes the landform evolution in late Pleistocene. Thick weathered tephra layers overlying the terrace surfaces are subdivided into an older and a younger groups. The younger tephra group, which has been deposited in the last ca. 100, 000 years, contains many marker tephra layers, in ascending order, Ata, K-Tz, FkP and Aso4 of 90, 000-70, 000y.B.P., ; AyP, IwtP, IwP, AwS, HnS II and OtP of 60, 000-30, 000y.B.P.; OsP, ItoPfl, AT and KbP of 22, 000-15, 000y.B.P.; and Ah of 6, 000y.B.P. The older tephra group covers the higher terrace group of middle Pleistocene. The younger group covers the following lower terrace surfaces, in descending order: the Sanzaibaru surface, the Karasebaru surface, the Nyutabaru I, II, III surfaces, the Saitobaru I, II surfaces, the Toyobaru I, II surfaces, the Oyodo surface, the Kunitomi I, II surfaces, the Mikazukibaru I, II surfaces, and the Holocene terrace surface group. Of many terrace surfaces in the Miyazaki Plain, the Sanzaibaru surface is the most extensive one. It is mostly of marine origin, composed of thick transgressive deposits called the Sanzaibaru Formation and lithologically subdivided into three members. The lower member is fluvial gravelous deposit in the regressive stage. The middle member consists of alternating beds of sand and silt with fossils of mollusccs that lived in the embayment of a warm sea in the transgressive stage. The upper member is sandy deposit of deltaic and beach conditions in the maximum stage of the transgression. On the Sanzaibaru surface, the upper member forms sand ridges which are inferred to have been bars, barriers and dunes. The oldest marker tephra layer on the Sanzaibaru surface is the Ata ash which erupted from the Ata caldera ca. 90, 000-80, 000y.B.P.. The Sanzaibaru surface underlying the Ata ash is thought to have emerged ca. 100, 000y.B.P., in the Last Interglacial Stage. It is estimated by subtracting the height of the base of the Sanzaibaru Formation from the height of the shoreline of the surface that sea level rose more than 100m in the Sanzaibaru stage. The Nyutabaru I, II, III surfaces are characterized by gentle gradients and wide distribution. They are of fluvial origin, although the Nyutabaru II and probably III surfaces are partly of marine origin in the northern part of the plain. The Nyutabaru III surface is an accumulation terrace formed during the little transgression. These surfaces are thought to have emerged ca. 90, 000-60, 000y.B.P., when a relatively high sea level was maintained because regression was slow and debris supply from the Kyushu Mountains increased or discharge in rivers decreased. The Saitobaru I and II, Oyodo, and Kunitomi I and II surfaces, which are characterized by steeper and more linear longitudinal profiles, are erosional terraces and are mostly of fluvial origin. Their surfaces came out ca. 50, 000-10, 000y.B.P., in the Last Glacial Stage, when rapid regression occurred and sea level stayed relatively lower. The discharge of the rivers tended to increase gradually. The Karasebaru, Toyobaru I and II and Mikazukibaru I and II surfaces in the northern part of the plain are fluvial fans. They were formed in three regressional stages: the late Last Interglacial Stage, the early Last Glacial Stage and the late last glacial stage. These fans were formed under conditions in which the river bed gradient was steeper than that of the continental shelf when the sea level went down.
Minuma Lowland is an erosional valley that lies in the Oomiya Upland, Saitama Prefecture. The author has investigated the stratigraphy of the Holocene deposit filling the erosional valley. The surface deposit is about 6.5 meters thick and is composed of peat and gray clay. This is underlain by a sand bed over 6-15 meters thick. Moreover, the author has investigated the Holocene Paleoenvironments and the highest sea level based on the diatom assemblages contained in their deposits in the Minuma Lowland. Diatom assemblages from this area showed that the highest level of the sea attained was at least +3.9-+4.1 meters above sea level. The highest stage of the sea level is probably younger than about 5950-5540y.B.P.
Recently, it has been widely recognized that tephras are important key beds for studies of the Quaternary System. In this connection, many methods of correlating tephras and pyroclastic flow deposits have been proposed. The author has examined the assemblage of the ferromagnetic minerals and their lattice parameter contained in the Aso pyroclastic flow deposits, which are the largest pyroclastics in Central Kyushu. They are classified into 4 compound cooling units. Aso-1, Aso-2, Aso-3 and Aso-4. Two boreholes were used for studying the vertical variation of the characteristics of ferromagnetic minerals. In addition, surface samples were collected in various locations around Aso caldera. After the magnetic separation of ferromagnetic minerals from samples, the species and the lattice parameter of each sample were examined by X-ray diffraction method. Titanomagnetite, titanomaghemite and hemoilmenite were identified. The degree of welding and weathering do not seem to affect the characteristics of ferromagnetic minerals. Each unit of the Aso pyroclastic flow deposits is characterized by the assemblage of ferromagnetic minerals and the lattice parameter of titanomagnetite. In particular, Aso-4, which is a largescale erupution of Aso volcano, is characterized as titanomagnetite with the average lattice parameter 8.417Å. Thus ferromagnetic minerals can be used as one correlation criterion.
Fluvial terraces are well developed along the middle reach of the Ara river in Kanto district, Japan. These terraces can be roughly classified into upper, middle, and lower terraces. The authors investigated the lower terraces in order to clarify the influence of tectonic movements and climatic changes in the formation of fluvial terraces during the last glacial age. The lower terraces are subdivided into three levels-filltop terraces, fill-strath terraces, and non-cyclic terraces-based on topographic features and terrace deposits. The development of lower terraces after the last interglacial age is divided into four stages: “formation of buried valley under valley fills, ” “accumulation of valley fills and formation of filltop terrace, ” “slight down cutting which formed fill-strath terraces” and “rapid downcutting” (Fig. 10). The longitudinal profiles of the filltop terrace and fill-strath terraces are characterized by steeper gradient and less curvature than those of the present river bed (Fig. 3). On the other hand, the longitudinal profile of the base of valley fills is similar to the present river bed in its larger curvature. These profiles suggest that the fluvial process when the buried valley under valley fills was formed is similar to that at present, so the age of initiatial deposition of the valley fills may be the last interglacial. The authors estimate that the relative height of these two profiles indicates the amount of uplift since the last interglacial. The profiles of the filltop and fillstrath terraces, which are characterized by steeper gradient and less curvature, may have resulted from changes in fluvial process during the last glacial age, possibly from a decrease in discharge as represented by a decrease in gravel size of the valley fills.
Excavations by drilling, mechanised digger, and peat-corer were carried out in two coastal lowoands near the southern tip of Izu Peninsula on the Pacific coast of central Japan. The Holocene relative sea-level change was determined by sediment facies, 14C dating and environmental assesment of the fauna in Holocene deposits. Izu Peninsula is situated at the northern tip of the Philippine Sea Plate which is being subducted beneath the Japanese island of Honshu at the Suruga Trough along the west coast of the peninsula. Because the tectonic setting is different from other areas facing subduction zones along the Pacific coast, the record at coastal uplift in the Quaternary might also be expected to differ. In the lower reaches of the Okamo River, approximately 5m amsl (above mean sea level), a borehole reached the base of the Holocene marine deposits at-11.3m amsl. Sediment facies analysis and 14C dates of shell and wood samples from the core indicate the transgression reached this location ca. 7, 500yBP, and was followed by a gradual increase in water depth accompanied by deposition of silt and clay, which continued until ca. 4, 000yBP. Water depth shallowed after this time, judged by sandy deposits containing an intertidal molluscan fauna. The upper limit of marine deposits is about 2.0m amsl. Emergence above marine condition is inferred to be younger than ca. 3, 000yBP, because the youngest 14C date of mollusc shells (3, 860yBP) is -5.2m amsl and Kawagodaira Pumice of ca. 2, 800 to 3, 200yBP age is about -1m amsl. In addition, 14C dates of mollusc shells have been obtained from -8.0m amsl (6, 490yBP) and -7.0m amsl (5, 400yBP). The wide-spread Ah tephra of ca. 6, 300yBP occurs at -9.1 to -9.2m amsl. The time of culmination of the Post-glacial (Jomon) transgression is widely believed to have occurred ca. 6, 000yBP but here marine deposits of that age are at -7 to -8m amsl. In most coastal areas of Japan the paleo sea level of ca. 6, 000yBP occurs at an altitude of 2 to 3m amsl or higher, as the highest Holocene marine terrace or emergent shoreline. The upper limit of marine deposits estimated by the sediment facies and molluscan and diatom analysis at three locations beneath the terrace along the Okamo River range from 2-3m amsl. The time of emergence of the terrace inferred from 14C dates of paet, just above marine deposits, and of molluscs within the uppermost marine bed at these locations, is 3, 000-2, 600yBP. At an altitude of less than 5m amsl in the lower reaches of the Aono River, about 4km southwest of the Okamo area, bore hole stratigraphy and 14C dates of wood and mollusc shells show that the transgression reached this location at ca. 14, 000yBP at the earliest. The upper limit of marine deposits in this lowland and the time of its emergence are estimated at about 2m amsl and ca. 2, 000yBP, respectively. Sparse data in this area suggest the upper limit of marine deposits may be slightly lower and younger than those in the Okamo area. The marine deposits of ca. 6, 000yBP are buried about -9m amsl. The presence of uplifted late Pleistocene marine terraces in the southern part of Izu Peninsula indicates a general tendency of uplift at this area through the late Pleistocene period. The present study revealed, however, that the area was subsiding from at least about 6, 000yBP until uplift began at 3, 000-2, 000yBP. The emergence of the two lowlands may have been due to coseismic uplift because the long-term subsidence appear to have reversed and uplift occurred almost simultaneously in both lowland areas.
Tephrostratigraphical and palynological studies have been done for the Ohnuma-aisawa lake deposits, distributed at the eastern foot of Fuji Volcano. The Ohnuma-aisawa lake deposits lie over the Gotemba mud flow deposits, filling in their hollow places, and consist mainly of a lower half of sand and gravel beds and an upper half of silt and peat beds. Nearly twenty tephra beds, dating after about 2200y.B.P, were found in the upper half. The stratigraphy and characteristics of the tephra are described. The condition of the ground in the area of Ohnuma-aisawa Lake became worse after the fallout of Yu-2 tephra about 2200y.B.P. The Cryptomeria forests were dominant until the 9th century in this area. However, the destruction of the forests by human activities began with the increase of Pinus after that time.