This paper reviews the tephrochronological studies carried out in the north Kanto and south Tohoku regions, Northeast Japan Arc. It concludes that the stratigraphy, distributions, ages, and source volcanoes of major tephras erupted during the last 400kyr have been mostly clarified, and a revised time-space diagram showing the chronology of tephra, volcanic activity, terrace surfaces, hills, and Quaternary sediments during the last 1.5 my is shown. Here, the author considers several Quaternary problems based on the revised chronology, and discusses unsolved issues in Quaternary studies in these regions.
By revising the eruptive histories of ten volcanoes including two calderas during the last 300,000 years, the author reexamined the frequency of explosive eruptions (plinian and sub-plinian eruptions) and the eruption rate of fallout tephras of each volcano. Studies on the stratigraphy of ignimbrites derived from Lower Pleistocene calderas in the Aizu area and correlation of associated co-ignimbrite ash-fall deposits in distal areas were reviewed with preliminary estimation of the eruption rate. Moreover, the author reviewed chronological studies on the Middle Pleistocene terraces and hills in inland areas and pointed out that buried valleys have been formed during different lowstands of sea level in coastal areas. Additionally, it is revealed that there are some disagreements among researchers over recognition of source volcanoes of tephras and determination of ages of hills and buried valleys.

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A palynological study has been carried out using samples of the Sakasagawa member, which is a part of the fan deposit from the Late Pleistocene to the Holocene, overlying the Togakushi Highlands (alt. 1,200m). The chronology of the sediment was based on three ¹⁴C dates. Between 18,000 and 15,000yrsBP, large areas of the Togakushi Highlands were covered by humidiherbosas, which were mainly composed of Cyperaceae, Gramineae with Polemonium, and Selaginella selaginoides growing in open sites. The forests surrounding the humidiherbosa were composed predominantly of Picea, Tsuga, Pinus subg. Haploxylon, and Abies, and they were situated in the upper subalpine zone. This period is the latter half of the last glacial maximum. At ca. 15,000yrsBP, this area began to warm. This period is equivalent to the beginning of the late glacial. Between 15,000 and 13,000yrsBP, there had been the humidiherbosa with Polemonium and Selaginella selaginoides growing continuously on the Togakushi Highlands. But subalpine coniferous forests such as Picea, and Pinus subg. Haploxylon with Betula became abundant. Cool-temperate deciduous broad-leaved trees such as Quercus subg. Lepidobalanus began to spread gradually in the area at altitudes lower than the Togakushi Highlands.

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Evolution and aggradation rates of floodplains on a centennial to millennial-scale were investigated by analyzing landform features and borehole core sediments. This study was performed at the floodplain along the former course of the Ibi River flowing through the Nobi Plain, central Japan, because the microtopography in the floodplain can be easily identified. The sedimentary environment changed from a delta to a floodplain after 3,200calBP, and a floodbasin environment continued for nearly 2,000 years. However, an avulsion of the Ibi River that happened approximately 400 years ago caused channel erosion at the point that became a river course and a rapid aggradation of natural levee deposits along the channel. Additionally, it was suggested that a part of the artificial ringed levee (“Wajyu” in Japanese) distributed along the channel originated from the natural levee which was formed in relation to the avulsion. Although the millennial-scale floodplain aggradation rate was about 3mm/yr, the aggradation rate shows a large variation depending on the sedimentary environment. The rate is estimated to be more than 10mm/yr on the natural levee and 1 to 2mm/yr in the floodbasin.

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²¹⁰Pb and ¹³⁷Cs age dating, total organic carbon (TOC) and total nitrogen (TN) contents analysis, and diatom analysis were conducted in sediment samples from two short cores collected from Sakoshi Bay (core HNA) and off Aioi City (core HNB) , northern Harima-Nada, southwest Japan. The results of ²¹⁰Pb and ¹³⁷Cs dating shows that ages at the bottom of core HNA and that of core HNB were estimated at 1883AD and 1786AD, respectively. Spatio-temporal changes of diatom assemblages and TOC and TN contents were as follows : In coastal areas of northern Harima-Nada, eutrophication occurred earlier than the initiation of the high economic growth period, and diatoms flourished during the 1940s-1960s. Deterioration of marine environments peaked in the 1960s-1970s. In the 1980s, Sakoshi Bay and a site off Aioi City underwent different paleoenvironmental changes. Since the 1990s, water and bottom environments in Sakoshi Bay were deteriorated by oyster farming, while water environments off Aioi City were influenced by the improvement of water quality in northern Harima-Nada.

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This article describes the characteristic bedforms of the tsunami deposit formed on the Kujukuri coast, East Japan, during the 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0). Two days later, on March 13, we observed tsunami deposits showing various bedforms reflecting their formative processes before the disturbances by weather. Similar to many previous studies, the tsunami deposits show a gross landward thinning and fining trend on a hundreds-of-meters scale. Our observations suggest that the trend is principally controlled by the sequential segregation and settlements of coarser grains from the inundation flow corresponding to the decrease of flow velocity and depth. Bedforms characterizing the tsunami deposits show a wide morphological variety. Current ripples with a wavelength ranging from several cm to 10cm are the most prevalent bedform. Barchan ripples are also commonly observed on the tsunami deposit. These bedforms suggest a flow condition dominated by a uni-directional current. In the vertical section of the tsunami deposit, the fine alternation of sand sheets and mud drapes recorded the repeated arrival of tsunami waves. Each set of a sand sheet and overlying mud drape represents a tsunami flooding and subsequent slack water period. These sedimentological observations will contribute to understanding the tsunami sedimentation process, and also will improve our ability to differentiate tsunami deposits from storm deposits, which is indispensable knowledge in paleo-tsunami research.

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