We propose the extensively studied Chiba section of the Chiba composite section (CbCS) as the Global Boundary Stratotype Section and Point (GSSP) to define the base of the Middle Pleistocene Subseries and Chibanian Stage. The CbCS is a continuous and expanded marine sedimentary succession within the middle of the Kokumoto Formation, Boso Peninsula, Chiba Prefecture. It contains well-preserved pollen, marine micro- and macrofossils, a tightly-defined Matuyama-Brunhes (M-B) paleomagnetic polarity boundary, and numerous tephra beds, allowing the establishment of a robust and precise chronostratigraphic framework across the Lower-Middle Pleistocene boundary. Its deep-marine, open-ocean continental slope setting, coupled with high sedimentation rates with no evidence of abrupt deposition, has resulted in preserving both terrestrial and marine environmental changes in and around Marine Isotope Stage 19 (MIS 19). We have performed high-resolution oxygen isotope analysis, as well as detailed sedimentological, geochemical, and biostratigraphic studies of the CbCS, confirming the completeness of the stratigraphic interval spanning the Lower-Middle Pleistocene boundary. The M-B polarity boundary serves as the agreed primary guide for the Lower-Middle Pleistocene boundary, and the CbCS represents one of the most detailed marine sedimentary records yet obtained for this reversal, yielding an astronomical age of 772.9 ka with a duration of ca. 1.9 kyr. This section therefore offers an exceptional opportunity to calibrate the geological time scale as well as understand the dynamics of the geodynamo. The widespread Byk-E tephra lies close to the reversal, allowing precise regional lithostratigraphic correlation. The CbCS is easy to access from international airports. The section will be permanently preserved and maintained by the local government as a natural monument. Based on these attributes, the base of the Byk-E tephra bed in the Chiba section is the best horizon for establishing the GSSP that will define the Lower-Middle Pleistocene boundary.

The Kazusa Group is widely distributed in the Boso Peninsula and is composed of forearc basin marine sediments with a total thickness of about 3000 m. This group represents the type stratigraphy of the marine Pleistocene of the Japanese Islands and has been extensively investigated in the past. More than 500 tephra layers are intercalated in the Kazusa Group, and a detailed tephra stratigraphy has been established from the lower Kiwada Formation to the upper Kasamori Formation. Correlations of the tephras within the Kazusa Group with many widespread tephra layers have resulted in the construction of a detailed tephrostratigraphic framework for the Pleistocene tephrochronology of the Japanese Islands.

In this study, we compiled the most widespread tephras of the Kazusa Group identified by previous research. More than 20 tephra layers span the period 2.0-0.4 Ma. Our results indicate possible problems regarding the currently accepted tephra stratigraphy of the Kiwada Formation, including the possibility of double-counting and the order reversal of its tephra layers.

We also investigated the less well known tephras of the Katsuura, Namihana, and Ohara Formations of the lower Kazusa Group, resulting in the discovery of new fine glassy tephra layers. Two widespread tephra layers, namely, Fup-KW2 (2.2 Ma) and Bnd2-O1 (2.1 Ma) were identified in the lower Kazusa Group as a result of considering the correlations between new tephra layers and widespread tephras. These new tephra correlations constrain the base of the Kazusa Group to be older than 2.3 Ma.

Widespread tephra layers provide time maker for geological records, including the eruptive history. This paper reviews the recent progress in chronological studies on widespread tephra and volcanic eruptions over the past 100,000 years in and around the Japanese Islands. Tephrochronology is highly useful for assessing the reliability of various age determination methods. Outcrops and core samples recording stratigraphic relationships are very important. Radiocarbon ages obtained by accelerator mass spectrometer have been calibrated to calendar ages throughout the past 50,000 years by using the Lake Suigetsu dataset to establish the IntCal13 calibration curve. However, because varve chronology always includes some counting errors, it remains difficult to implement with one-year accuracy. Recently, the discovery of AD 775 ¹⁴C spike revealed the eruption age of B-Tm tephra to be AD 946. By using ¹⁴C spike and oxygen isotope dendrochronology, it will be possible to extend the application range of annual tree rings to establish a calibration curve. For volcanic eruptions older than 50,000 years, K-Ar, luminescence and fission-track age determinations have been reported. Unspiked K-Ar, ⁴⁰Ar/³⁹Ar, single-aliquot regeneration-dose red thermoluminescence, and isothermal red thermoluminescence dating can also provide useful ages. Varve chronology, dendrochronology, marine isotope stage, and magnetostratigraphy can link results to environmental changes.

New formation mechanisms of post-LGM incised-valley fill have been inferred on the basis of the sequence stratigraphy and paleogeography of strata beneath the Tokyo and Nakagawa lowlands. The new findings are as follows: (1) Channel sands consisting of meandering river sediments show anastomosed geometry and vertical accretion during rapid sea-level rise, whereas during slow sea-level rise they show sheet-like geometry and lateral accretion; (2) some transgressive estuary systems should be classified as river-dominated estuaries, and these systems comprise fining-upward sand lobes in the subtidal zone of the bayhead portion; and (3) a tide-dominated bay such as a drowned valley can be naturally filled by lateral accreting mud discharged from outside of the bay, and such bays show a fining-upward lithological succession. The first finding, in particular, implies that the change in the rate of sea-level rise is an important control on the formation of fluvial strata in coastal lowlands, as it is also on the formation of shallow-marine strata.

The Nobi Plain, north of Ise Bay, Japan, is a Holocene fluvial-coastal lowland formed mainly by the Kiso, Nagara, and Ibi rivers. This paper introduces the late Pleistocene to Holocene sedimentary sequence and landforms of the plain. The landforms are classified into low-gradient alluvial fans, floodplains, and deltas using aerial photograph interpretation. The stratigraphy of the plain since around the Last Glacial Maximum (LGM) is divided from bottom to top into First Gravel, the Nobi Formation, and the Nan'yo Formation mainly on the basis of lithofacies by analyzing borehole columns. Many valuable data have been accumulated by detailed sediment facies analyses and radiocarbon dating of borehole core sediments, especially during the last 20 years. These data have enabled us to understand the evolution of the sedimentary sequence in response to sea-level change during the last 10,000 years at a millennial timescale. However, information for the interval between the LGM and the Younger Dryas is very limited. Reconstruction of the three-dimensional stratigraphic architecture of incised-valley fills and the estimation of sediment storage of the deltaic deposits were performed by analyzing existing borehole columns and radiocarbon ages and using a geographic information system. Sediment storage increased considerably after 1000 cal BP probably as a result of the increase in sediment production accompanying human activity in the drainage basin. Recent human activities in the drainage basin have caused marked changes in its aqueous and sedimentary environments. The changes are visible in the form of features such as land subsidence, local riverbed scouring, and ecological changes.

Through the comparison of lithofacies fossils, marker tephras, and radiocarbon ages of different subsurface strata, this review aims to characterize the geologic features of the major coastal plains of Shikoku, southwestern Japan, thus promoting a better understanding of the coastal plain formation process. In the Tokushima Plain, the subsurface strata are divided into the alluvial Tokushima and the Pleistocene Kitajima formations. The Tokushima Formation is 30-50 m thick and comprises, from bottom to top, gravel, sand, mud, and sand beds, with marine molluscan fossils in the middle mud beds. This formation is latest Pleistocene to Holocene in age and unconformably overlies the Kitajima Formation. Although a similar sequence is identified under most of the coastal plains in Shikoku, the presence of gravel and sand in the lower parts has been confirmed only in the Takamatsu Lowland. The thickness of the alluvial deposits varies from plain to plain, with this variation probably being due to the long-term subsidence of the basin and the differential erosion of former incised valleys. The Kitajima Formation is characterized by the repletion of gravel and mud beds, with each attaining a thickness of several decameters. Marine fossils are found in the mud beds, suggesting that they were formed during high sea-level periods, namely, interglacial intervals. The Kitajima Formation can be correlated with the strata underlying the alluvial sediments of the coastal plains of Shikoku by means of widespread tephra marker beds. Based on the correlation results of this study, a similarity of lithofacies among the upper Pleistocene and Holocene deposits is identified. In many cases, the bottom part of the alluvial deposits comprises gravel covered with thin marine muds, deltaic muds, and/or sands. Sandy gravels cover the plains, often as fan deposits. The thickness of the alluvial deposits tends to be greater in areas of subsidence compared with that in areas of uplift.

Two adjacent but contrasting Cretaceous granitoids, the O'hara and Ohgi plutons, crop out west of Lake Biwa, southwest Japan. The O'hara pluton (4×1 km) consists mainly of a fine to medium-grained hornblende-biotite tonalite, with minor quartz diorite and granodiorite. The main facies of the pluton comprises magnetite-series grani-toids with adakitic compositions that yield a mineral K-Ar age of ca. 100 Ma. The O'hara pluton is located at the eastern end of a series of adakitic plutons, that are distributed throughout the central Tamba Belt. In contrast, the Ohgi pluton (5×0.5-1.0 km) consists of fine- to medium-grained hornblende-biotite granodiorite, containing conspicuous phenocrysts of hornblende. Most of the rocks are ilmenite-series granitoids that lack adakitic features and were emplaced at ca. 70 Ma. From the mode of occurrence, the Ohgi pluton might belong to the ring complex, the intrusion of which was related to formation of the cauldron, south of Lake Biwa.