Tanabe and Ishihara Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00411-0(2021) 8:20

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Formation of undulating topography andgravel beds at the bases of incised valleys:
Last Glacial Maximum examples beneaththe lowlands facing Tokyo Bay
Susumu Tanabe1*

and Yoshiro Ishihara2

Abstract
Recent studies using well density distributed borehole logs have revealed undulating topography at the bases ofincised valleys formed during the Last Glacial Maximum (LGM). In this study, from analysis of 4702 borehole logs,
undulating topography forming a series of pits 1–2 km long, < 1 km wide, 5–10 m deep, and spaced at 1–2-km
intervals was discovered at the bases of LGM incised valleys beneath the Tama River Lowland on the west coast ofTokyo Bay. This undulating topography can be attributed to scouring at braided river channel confluences. In the
study area, single borehole logs are available within each 187 m × 187 m grid cell, and the logs sample both thebottom and marginal portions of the scouring, which suggests that this undulating topography is not an artifact oferroneous values arising from mathematical interpolation. The morphologies and incision depths of two incisedvalleys in the study area show a cover effect of the gravel bed at the base of the post-LGM incised-valley fills. The
basal age of this basal gravel bed (BG) is confirmed at < 30 ka because the LGM incised valleys dissect the MIS 3
Tachikawa buried terrace overlain by the AT tephra dated 30.0 ka. This means that the BG, which represents
braided-river sediments, is interpreted as resulting from the LGM sea-level lowstand after 30 ka.
Keywords: Basal gravel bed (BG), Braided river, Scour, Cover effect, Sea-level change, AT tephra

1 Introduction
An incised valley is a topographic feature formed byriver incision related to allogenic forcing such as sealevel lowering (Dalrymple et al. 1994, 2006). Incised valleys were formed in the Last Glacial Maximum (LGM)
in many places in the world because of the global lowering of sea level. LGM incised valleys are filled by riverine
sediments deposited in association with the subsequentdeglacial sea-level rise and stillstand; these sediments,
which can comprise fluvial, estuarine, and deltaic sediments from the bottom to the top of the sequence, aretermed post-LGM incised-valley fills (e.g., Zaitlin et al.
* Correspondence: s.tanabe@aist.go.jp
1
Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba 305-8567,
Japan
Full list of author information is available at the end of the article

1994; Tanabe et al. 2015). Modern riverine coastal plains
are situated on these post-LGM incised-valley fills. In
these kinds of settings, the post-LGM incised-valley fills
in riverine coastal plains form the present-day groundsurface. The post-LGM incised-valley fills have a highwater content and form unconsolidated soft strata, contributing to various geological hazards such as subsidence caused by groundwater pumping and amplificationof earthquake ground motion (e.g., Kaizuka and Matsuda
1982; Endo et al. 2001). The LGM incised valleys constitute a physical boundary between the basement and thepost-LGM incised-valley fills. Therefore, the study of themorphology of LGM incised valleys and of the depositional architecture of the post-LGM incised-valley fills
are important for mitigation of geological hazards as well

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Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

as for unraveling spatial and temporal patterns of coastal
landscape development and its controlling factors.
The morphology of incised valleys has been drawn assmooth depth or isopach contours in modern and geological analogues on the basis of human interpretation(e.g., Dalrymple et al. 1994, 2006; Anderson and Rodriguez 2008; Milli et al. 2016; Walsh et al. 2016). However,recently, several studies from Japan have suggested thatundulating topography prevails at the bases of LGM incised valleys on the basis of several thousand well density distributed borehole logs obtained for geotechnicalpurposes. Undulating topography has been reported
from the Kanto Plain, including the Tokyo Lowland,
central Japan (Tanabe et al. 2008, 2014; Ishihara et al.
2012; Ishihara and Sugai 2017; Tanabe and Ishihara2020); the Nobi Plain, central Japan (Yamaguchi et al.
2006; Hasada and Hori 2016); and the Osaka Plain, western Japan (Mitamura and Hashimoto 2004; Ito et al.
2018). The undulating topography detected by thesestudies consists of a series of scours that are 1–10 kmlong and wide and 3–10 m deep, and are spaced at 1–
10-km intervals. Most of these studies reconstructed valley topography on the basis of the mathematicallyinterpolation of the depth values of the valley’s bases,
and the undulating topography is clearly illustrated intheir figures. These undulating topographies might reflect abnormal values calculated by mathematicalinterpolation; however, they could also be attributed toacceptance of values erroneously interpreted to be false.
Similar undulating topography has been reported fromthe Charente, Lay-Sèvre, and Seude incised valleys on
the Atlantic continental shelf off France on the basis ofhigh-density seismic surveys (Chaumillon and Weber
2006; Chaumillon et al. 2008) and even from significantly older geological records of the Lower Cretaceous
McMurray Formation in western Canada (Horner et al.
2019) and the upper Pliocene Santa Barbara coalfield incentral Italy (Ielpi and Ghinassi 2014) on the basis ofabundant well logs.
Tanabe et al. (2008) reconstructed several buried terraces formed after Marine Isotope Stage (MIS) 5e and a
70-m-deep LGM incised valley in a 700 km2 wide areain the Tokyo Lowland on the basis of correlation among
7000 borehole logs. They discovered undulating topography consisting of a series of scours, each 1 km long
and wide and 5 m deep, which occur at 1-km intervals
in the incised valley. Tanabe et al. (2014) preliminarily
interpreted this topography as representing pools andriffles in the braided river system. However, those studies regarded the top of the basal gravel bed (BG) of the
post-LGM incised-valley fill as the base of the incisedvalley because few borehole logs entirely penetrate theBG, which constitutes a firm ground for the foundationsof buildings. The BG in the Tokyo Lowland has a

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uniform thickness of 5 m; thus, the undulating topography of its top surface is assumed to occur also at thebase. Furthermore, there was an area of sparse data in
the northern Tokyo Lowland, so some of the undulating
topography might reflect abnormal values calculated bymathematical interpolation.
Since 2014, the Geological Survey of Japan (GSJ) has
carried out surveys of the Tama River Lowland on thewest coast of Tokyo Bay to mitigate geological hazards
such as amplification of earthquake ground motion. The
Tama River Lowland is a small delta with an area of 120
km2. The area contains the Tokyo Metropolitan area;
therefore, a high-density distribution of borehole logs isavailable. In the Tama River Lowland, one borehole log isavailable for each 187 m × 187 m grid cell, and the data
density is higher than that of the Tokyo Lowland (316 m
× 316 m grid cell). In the Tama River Lowland, consolidated Quaternary strata underlie the post-LGM incisedvalley fills, so many borehole logs penetrate the BG.
In this study, we reveal detailed topography of the late
Pleistocene buried terraces and of the LGM incised valley on the basis of more than 4700 borehole logs, 12radiocarbon-dated sediment cores, and six geologicalcross sections. As a result, the presence of undulating
topography at the base of the LGM incised valley, which
is not a result of mathematical interpolation, is confirmed, and the basal age of the BG is clarified. Furthermore, we discuss the formation of buried terraces andthe LGM incised valleys on the basis of comparison withsea-level changes in and around the LGM. And then, finally, we examine the incision processes of the undulating topography by comparing the modern analogues ofscouring at river beds.

2 Regional setting
Tokyo Bay, which has an area of 922 km2, has a maximum water depth of 70 m but a mean water depth of
only 17 m (Fig. 1a, b). The mean wave height in the bay
is 0.3 m and the tidal range is 1.8 m. According to theclassification scheme of Davis and Hayes (1984), the bay
is a tide-dominated coastal environment. Before theseventeenth century, the Tone River, which has the largest catchment in Japan, flowed into Tokyo Bay (Fig.
1b). The course of Tone River was diverted to the Pacificto prevent flooding in the Tokyo urban area (Okuma
1981). The present Tone River drains an area of 16,840
km2 and has a water discharge of 8.7 km3/year (276 m3/s) and a sediment discharge of 3 Mt/year (95 kg/s; Milliman and Farnsworth 2011). At present, the largest river
flowing into Tokyo Bay is the Tama River, which drains
an area of 1240 km2 and has a water discharge of 40 m3/
s (Fig. 1c). The Tsurumi River, which was the major
tributary of the Tama River during the LGM (e.g.,
Matsushima 1987), drains an area of 235 km2 and has aTanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

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Fig. 1 Geological map of the Tama River system. a Japanese Islands. b Tokyo Bay. The gray shading indicates the distribution of lowland. The blue dashed line
marks the course of the Tone River, which flowed into Tokyo Bay before being diverted to the Pacific during the 17th century. c Geological map and AA’ crosssection of the Tama River system based on Geological Survey of Japan, AIST (2019)

water discharge of 10 m3/s. The total water discharge of
the Tama and Tsurumi rivers is less than one-fifth ofthat of the Tone River.
The Tama River originates in the Kanto Mountains,1953 m above the Tokyo Peil datum (TP; mean sea-levelat Tokyo Bay), and flows east for 138 km to Tokyo Bay(Fig. 1c). Most of the mountain area consists of a preTertiary accretionary prism, and most of the hills anduplands in the lower reaches of the Tama River areformed of Quaternary sedimentary rocks (Geological
Survey of Japan, AIST 2019). The Quaternary strata of
the hills and uplands become younger from west to east,and consist of pre-MIS 12 deposits, MIS 12–6 deposits,
and MIS 5 deposits (Fig. 1c). The pre-MIS 12 and MIS

12–5 deposits are called the Kazusa and Shimosa groups,
respectively.
The MIS 5e deposits of the Shimosa group and the
MIS 5a deposits constitute the Shimosueyoshi andMusashino terraces, respectively (Fig. 2). The Shimosueyoshi Terrace is a marine terrace, and occurs on the
Shimosueyoshi Upland and part of the Musashino Upland (Fig. 2). The Musashino Terrace is a fluvial strath
terrace. The Shimosueyoshi Terrace on the Musashino
Upland is a remnant of erosion that provided sediments
for the alluvial fans forming the Musashino Terrace (Fig.
1, section AA’). The Shimosa group filling the MIS 6 incised valley forms a relatively soft layer, which extends
beneath the Musashino Upland (Fig. 1, section AA’, Fig.

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Fig. 2 Geological map of the area in and around the Tama River Lowland. Red squares indicate the locations of stratotype cores used in thisstudy. Black dots indicate the locations of borehole logs used to reconstruct the morphology of the LGM incised valley. The blue lines and lettersdenote the cross sections shown on Fig. 6. Red lines indicate the position of the MIS 6 incised valley filled with the relatively soft Shimosa Group(Nakazawa et al. 2019). The MIS 6 incised valley flowed down in the direction of the red arrow. The geological map is based on Geological Surveyof Japan, AIST (2019)2; Oka et al. 1984; Nakazawa et al. 2019). The Musashino Terrace is dissected by further lower alluvial fansof the MIS 3 Tachikawa Terrace (Figs. 1c and 2). The
Shimosueyoshi, Musashino, and Tachikawa terraces areoverlain by aeolian sediments called the Kanto Loam,which is composed mainly of Pleistocene tephra (Fig. 1,
section AA’). The Kanto Loam is divided into the Shimosueyoshi, Musashino, and Tachikawa loams, which
determine the chronology of the terraces (Fig. 3; Kaizuka
et al. 1977). The Shimosueyoshi Terrace has an elevationof 20–40 m TP, and is higher in the southwest (Koike
and Machida 2001). The relative sea-level during MIS 5e
was 14 m TP in Tokyo Bay (Okuno et al. 2014). Fromthese values, the tectonic uplift rate in this region can becalculated as 0.04–0.21 m/kyr.
The apex of the Tama River Lowland is at 13 m TP at
Mizonokuchi (Fig. 2). The gradient of the Tama River becomes gentle at Mizonokuchi, changing from 1.5–4.0/
1000 to 0.5–1.0/1000 (Fig. 4). Because of this change, theriver style alters from braided to meandering (Kadomura
1961). The lowland is mostly lower than 5 m TP and iscomposed of floodplain, natural levee, and sand bar sediments. The natural levees and sand bars rise 2 m above
the floodplain. The coast of the lowland has been
reclaimed since the 1910s. The Tsurumi, Owoka, andKatabira river lowlands are west of the Tama River

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

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Fig. 3 Correlation of eustatic sea-level changes since MIS 6 and ages of upland terraces around the Tama River Lowland and buried terraces andLGM incised valleys beneath the Tama River Lowland. The sea-level curve is based on Grant et al. (2012). RSL relative sea-level, T1 buried terrace 1,T2 buried terrace 2, T3 buried terrace 3, LGM valley LGM incised valley

Lowland (Matsuda 1973; Sugimoto and Umehara 1994),
while to the east the lowland around the Ara River mouthis called the Tokyo Lowland (Kaizuka et al. 1977; Fig. 2).
The post-LGM incised-valley fills beneath the Tama,
Tsurumi, Owoka, and Katabira river lowlands and theTokyo Lowland unconformably overlie the Kazusa andShimosa groups (Fig. 5; Matsuda 1973, 1974; Matsushima 1973, 1987; Kaizuka et al. 1977; Oka et al. 1984;
Pollution Research Institute, Yokohama City 1988;

Sugimoto and Umehara 1994; Port of Tokyo GeologicalResearch Group 2000; Bureau of Port and Harbour,Tokyo Metropolitan Government 2001; Tanabe et al.
2008). The post-LGM incised-valley fills consist of theBG, lower sand bed (LS), lower clay bed (LC), middle
sand bed (MS), upper clay bed (UC), upper sand bed(US), and uppermost alluvium bed (UA; Matsuda 1973,
1974; Kaizuka et al. 1977; Oka et al. 1984; Matsushima1987). The Paleo-Tokyo River Valley, which is filled by

Fig. 4 Thalweg of the Tama (black line) and Tsurumi (red line) river beds. The knickpoint of the Tama River is at Mizonokuchi. The graph is basedon Kanto Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism (2017a, b)

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

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Fig. 5 Correlation of stratotype columns in the Tama, Tsurumi,
Owoka, and Katabira river lowlands and the Tokyo Lowland.
Lithostratigraphy, sedimentary systems, and bounding surfaces ofsequence stratigraphy are correlated as stratotype columns ineach lowland

80-m-thick post-LGM sediments beneath the Tokyo
Lowland, is the largest LGM valley in the study area,
and was formed by the Tone River.

3 Materials and methods
In this study, 8745 borehole logs from Tokyo Metropolis(north of the Tama River) and Kawasaki and Yokohama
cities, Kanagawa Prefecture (south of the river) wereused (Fig. 2). Of these logs, 3549 were from the Civil Engineering Support and Training Center, Bureau of Construction, Tokyo Metropolitan Government (2019); 2333
were from Kawasaki City (2019); 2330 were from Yokohama City (2019); and 533 were from the Public Works
Research Institute (2019).
The borehole logs used for this study were obtainedby standard penetration tests for building construction,
and all data was prepared as JACIC-formatted XML data
(Ministry of Land, Infrastructure, Transport and Tourism 2016). In this format, a borehole log is ruled to contain information on the location (latitude, longitude, andelevation measured by leveling or GPS) and the lithology
and N value observed/measured every meter. Details of
the lithological description differ among borehole
drillers; however, lithologies can be roughly classified
into gravel, sand, muddy sand, sandy mud, mud, peat,loam, artificial soil, and bedrock. The lithological description also contains information on color and thepresence of molluscan shells, burrows, and plant material. The N value, which is used in Japan to test groundstiffness, refers to the number of times a 63.5 kg weightmust be dropped from a height of 75 cm to penetrate
the soil to a depth of 30 cm (Editorial Committee of“Story of N value,” 1998). N values are usually high incoarse sediments and low in fine sediments, and increase
with depth because of the effect of compaction. Verticalchanges in N value are indicative of fining and coarsening trends.
The XML dataset of the borehole logs was converted
into a voxel model with 50 m × 50 m × 1 m grid cells byusing the interpolation method of Ishihara et al. (2013).
Lithology and N value cross sections were constructedusing this voxel model.
In this study, data from 12 previously reported sedimentcores (stratotype cores; Table S1) with 29 radiocarbon
dates of plant and shell fragments (Table S2) obtained
from the sediment cores were used. Five cores (coded“GS”) were recently obtained by the GSJ; these cores
yielded detailed facies information including clastic

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

contents coarser than 4 phi (Fig. S1; Tanabe and Nakashima 2016). However, detailed lithological information
was not available for seven cores (cores 1–7 in Table S1;
Matsushima, 1987; Matsushima and Yamaguchi, 1987;
Port of Tokyo Geological Research Group, 2000); therefore, only the age data were used for these cores. Calibrated 14C ages were calculated with CALIB 7.1 software(Stuiver et al., 2019) using the IntCal13 and Marine13
dataset of Reimer et al. (2013). For calculation of ages
from molluscan shells, ΔR (the difference between regional and global marine 14C ages; Stuiver and Braziunas
1993) was regarded as 0 year, and the marine carbon content as 100%. All ages reported here are cal BP (calibrated
14
C ages) unless otherwise noted as yr BP (conventional
14
C ages).
The stratigraphy of six lithological and N value crosssections was interpreted on the basis of correlation withthe 12 stratotype cores and previous studies. These crosssections were selected because they represent typical facies succession and architecture of the Tama, Tsurumi,
Owoka, Katabira, and Paleo-Tokyo river valley systems
in the proximal and distal portions. The base of the
post-LGM incised-valley fill was identified on each borehole log by applying the stratigraphic sequence of the six
cross sections. Stratigraphic interpretation of the borehole logs was approximately based on Oka et al. (1984)
and Matsushima (1987). The Kazusa group consists ofsand and mud beds with an N value of 50. The N valuesof the mud, sand, and gravel beds of the Shimosa groupare 5–20, 20–50, and > 50, respectively, and these sediments are lighter in color than the post-LGM incisedvalley fills. The N values of the mud, sand, and gravel
beds of the post-LGM incised-valley fills are 0–20, 5–30,
and 30–50, respectively. The Kazusa and Shimosagroups are lithologically laterally continuous comparedto the post-LGM incised-valley fills.
The basal depths of the post-LGM incised-valley
fills, the presence of the Kanto Loam on the buried
terraces, the presence of a gravel bed in the LGM incised valleys and buried terraces, and the thickness ofthe gravel bed in each borehole log were extracted
manually. The base of the BG was identified as thebase of the post-LGM incised-valley fill. The spatial
distribution of the LGM incised valley was reconstructed by mathematically interpolating the depth
values of the base of the post-LGM incised-valley fill.
An interpolation method of ArcGIS 10.6, 3D Analyst
tools (Kriging) was applied. It is known that when thedensity of borehole data points is high enough, thedifference in interpolation methods does not affectconsiderably on the reconstruction of the spatial distribution of the incised valley (e.g., Tanabe et al.
2008; Tanabe and Ishihara 2020). A total of 4702
borehole logs, which penetrate the post-LGM incised-

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valley fill, were used to reconstruct the spatial distribution of the LGM incised valleys. In 2822 boreholelogs, the gravel bed was absent, and the drilling
reached to the bedrock of the Kazusa and Shimosagroups. In 1880 borehole logs, the gravel bed was present;
those boreholes penetrated the gravel bed and reached the
bedrock of the Kazusa and Shimosa groups. The wholestudy area including the Tama River Lowland has an areaof 164 km2; thus, the density of borehole logs is calculated
as one per 187 m × 187 m pixel grid although it is partly
inhomogeneous.

4 Results
4.1 Geological cross sections

This section contains a description of the lithologies andN values of the Kazusa and Shimosa groups and thepost-LGM incised-valley fills in six cross sections extracted from the voxel model (Figs. 2 and 6). We also
describe the sedimentary system and the sequence stratigraphic bounding surfaces of the post-LGM incisedvalley fills on the basis of the sedimentary facies andradiocarbon dates obtained from the stratotype coresand previous studies (Fig. S1). Based on these data, thedepth distributions of the bases of the post-LGM
incised-valley fills are described.
4.1.1 Section AA’

Section AA’ extends across the Tama River Valley (Fig.
2), which is filled by post-LGM sediments that unconformably overlie the Kazusa and Shimosa groups (Fig. 6;
Matsuda 1973; Kaizuka et al. 1977; Oka et al. 1984; Matsushima 1987; Tanabe and Ishihara 2015).
The Kazusa and Shimosa groups consist of sand andmud beds with N values > 50. The post-LGM incisedvalley fill contains the BG (N value = 50), the LC (Nvalue = 5–20), the MS (N value = 5–30), the UC (Nvalue < 5), the US (N value = 5–20), and the UA (Nvalue < 5), in ascending order (Fig. 5). The BG is correlated to lowstand braided river sediments, the LC, MS,
and the lower portion of the UC are correlated to transgressive estuary sediments, and the upper portion of UC,
the US, and the UA are correlated to regressive delta
sediments (Fig. 5). The braided river sediments mainly
consist of clast-supported gravel bed, which is typical in
braided river channel environments (Miall 1992). The
estuary and delta systems are composed of upwardfinning (deepening) and -coarsening (shallowing) sandand mud beds, respectively (Boyd et al. 1992). In the estuary sediments, amount of plant and wood fragments
decrease upward, likely recording the river mouth retrogradation, while in the delta sediments, those increase
upward, representing the river mouth progradation (Fig.
S1; Tanabe and Nakashima 2016). The unconformity between the Kazusa and Shimosa groups and the overlying

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Fig. 6 Cross sections of lithology (upper section in each figure part) and N value (lower section in each figure part) in the Tama River Lowland(sections AA’, BB’, and CC’), the Tsurumi River Lowland (section DD’), the Owoka and Katabira river lowlands (section EE’), and the Tokyo Lowland(section FF’). The traces of cross sections are reported on Fig. 2. Drilling of most borehole logs ceased at the firm surface of the Kazusa andShimosa groups; therefore, the lithology and N value of the pre-LGM deposits are not known for most areas

Tanabe and Ishihara Progress in Earth and Planetary ScienceFig. 6 (continued.)(2021) 8:20

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Fig. 6 (continued.)

post-LGM incised-valley fills is regarded as a sequenceboundary (SB). The boundary between the braided
river/estuary sediments represents an initial flooding
surface (IFS; a surface of marine flooding onto fluvial
sediments; Zaitlin et al. 1994) and the boundary
between the estuary/delta sediments represents a
maximum flooding surface (MxFS; a surface that separates a retrogradational transgressive system from a
progradational highstand system; van Wagoner et al.
1988). Although the LC in the Tokyo Lowland isregarded as part of the transgressive meanderingriver sediments (Fig. 5; Tanabe et al. 2010), the LC
in the Tama River Lowland consists of mud withrootlets and burrows that yield a mixture of marine,
brackish, and freshwater diatoms (Fig. S1; Tanabe2020). Therefore, the LC in the Tama River Lowland
is interpreted as salt marsh sediments (Tanabe andNakashima 2016). These salt marsh sediments aremarine sediments, and form part of the estuary
sediments.
A radiocarbon date of 35.7 kyr BP shows that the baseof the GS-KKW-1 core contains pre-LGM deposits (Figs.
S1, 6; Tanabe and Nakashima 2016). Radiocarbon dates
from sediments overlying the Kazusa and Shimosagroups and the BG from west of the GS-KKW-1 core
range from 12.1 to 8.5 cal kyr BP (ka), and become
younger to the west, into the upper reaches of the TamaRiver Valley (Fig. 6). These dates demonstrate that theestuary sediments onlap the Kazusa and Shimosa groups
and the braided river sediments. The MxFS is dated at
7.8 ka in the Tama River Lowland (Fig. 6; Tanabe andNakashima 2016).
The base of the LGM incised valley can be identified
between the Kazusa and Shimosa groups and the braided
river sediments or the estuary sediments. The valley has
a depth of − 60 m TP in the east and 0 m TP in thewest. Undulating topography can be identified at thebase of the LGM incised valley (Fig. 6).
4.1.2 Section BB’

Section BB’ is a transverse cross section of the TamaRiver Valley in the southeast of the Tama River Lowland(Fig. 2). In this section, the post-LGM incised-valley
fill unconformably overlies the Kazusa group at thevalley axis at a depth of − 50 m TP, the Tachikawa
buried terrace at − 20 m TP, and the Shimosa groupat − 5 m TP (Fig. 6; Matsuda 1973; Oka et al. 1984;
Matsushima 1987).
The Kazusa and Shimosa groups (N values > 50) consist of sand and mud beds. The Tachikawa buried terrace contains a gravel bed (N value > 50), and is overlain
by the Kanto Loam and peat bed (N values = 5–40), in
ascending order. In the valley axis, the post-LGM
incised-valley fill consists of the same succession as inTanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

section AA’. The UC, US, and UA overlie the Tachikawa
buried terrace, and the US and UA overlie the platform
at − 5 m TP (Fig. 6). The UC, US, and UA constitutedelta sediments. In Core 4, molluscan shells from −12.0 and − 11.5 m TP are dated to 8.1 and 7.7 ka, respectively; therefore, the MxFS is at the base of the
UC (Fig. 6).
A peat bed at the base of Core 5 is dated to 22.7 ka(Matsushima 1987). This age suggests that the Tachikawa buried terrace had formed before the lowest sealevel of the LGM (20.5 ka; Yokoyama et al. 2018), andmay be correlated to MIS 3. The platform at − 5 m TP isregarded as an abrasion platform that formed during the
middle Holocene sea-level highstand (Matsuda 1973;
Kaizuka et al. 1977). The − 40 m TP platform in this section might have formed between MIS 3 and the LGM(Fig. 6). Details of the − 5 m TP and − 40 m TP platforms are provided in Section 4.2.
4.1.3 Section CC’

Section CC’ is a transverse cross section of the TamaRiver Valley in the northwest of the Tama River Lowland (Fig. 2). In this section, the post-LGM incisedvalley fill unconformably overlies the Kazusa group at
the valley axis at − 20 m TP and the Tachikawa buried
terrace at a platform at − 5 m TP (Fig. 6; Matsuda 1973;
Oka et al. 1984).
The Kazusa group and the Tachikawa buried terrace
consist of mud and gravel beds (N values > 50), respectively. The post-LGM incised-valley fill consists of the
BG, LC, US, and UA at the valley axis, and the UA on
the Tachikawa buried terrace (Fig. 6). The LC is correlated to the estuary sediments, and the US and UA are
correlated to the delta sediments. In the GS-KNH-2
core, plant fragments from − 6.8 and − 0.2 m TP aredated to 8.4 and 7.4 ka, respectively; therefore, the MxFS
is at the base of the US (Figs. S1, 6).
This section is located in the extension of the MIS 6
incised valley (Fig. 2; Nakazawa et al. 2019). However,the MIS 6 incised-valley fill, the Shimosa group thatconsists of 20-m-thick massive mud (N value < 5; Nakazawa et al. 2019), is not present in this section, which indicates that the MIS 6 incised-valley fill was eroded
during formation of the MIS 3 Tachikawa terrace or the
LGM incised valley.
4.1.4 Section DD’

Section DD’ extends across the Tsurumi River Valley(Fig. 2), which is filled by post-LGM sediments unconformably overlying the Kazusa group (Fig. 6; Matsuda
1973; Oka et al. 1984; Matsushima 1987).
The Kazusa group consists of sand and mud beds (Nvalue > 50). The post-LGM incised-valley fill consists of
the UC (N value < 5), the US (N value = 5–20), and thePage 11 of 21

UA (N value < 5), in ascending order. The lower portionof the UC is correlated to the estuary sediments, and theupper portion of the UC, the US, and the UA are
correlated to the delta sediments (Fig. 5). To thewest, the sand bed of the US changes laterally into asandy mud/muddy sand bed, and the N value decreases as a result of this lithological change (Fig. 6).
A 7.8-ka isochron (MxFS) is located at – 10 m TP in
Core 1 and – 15 m TP in Core 2, and dips downward
toward the west (Fig. 6). The detection of this surface
can be explained as a result of the progradation of aflood-tidal delta to the bay head portion of the PaleoTsurumi Bay during the middle Holocene highstand(Matsushima 1987). The unconformity between the
Kazusa group and the post-LGM incised-valley fills isregarded as the SB.
In this section, the BG is absent at the base of the
post-LGM incised-valley fill, which is at depths of − 40
to 0 m TP, but undulating topography prevails at thebase (Fig. 6).
4.1.5 Section EE’

In section EE’, the Owoka and Katabira river valleys (Fig.
2) are filled by post-LGM sediments that unconformably
overlie the Kazusa group (Fig. 6; Matsushima 1973; Sugimoto and Umehara 1994).
The Kazusa group consists of sand and mud beds (Nvalues > 50). The valley fills consist of Holocene marine
mud and sand, which is composed of the UC, US, and UA(Fig. 5). The UC has N values < 5; the US has N values 10−50; and the UA has N values < 10. The lower portion of
the UC and the upper portion of the UC, the US, and the
UA correspond to estuary and delta sediments, respectively. The unconformity between the Kazusa Group andthe post-LGM incised-valley fills are regarded as the SB.
The modern Owoka and Katabira rivers have low sediment discharge. Tanabe et al. (2015) reported that the ageof the MxFS corresponds with the middle Holocene sealevel highstand of 7 ka in an embayment with low
sediment discharge in the Tokyo Lowland. Sugimoto andUmehara (1994) reported radiocarbon dates of 7.2 ka (−33 m TP) and 6.6 ka (− 18 m TP) in the vicinity of Core 6;
therefore, the MxFS must be at ca. − 30 m TP in the
Owoka River Lowland (Fig. 6). The US in the Owoka
River Lowland is interpreted as spit sediments (Matsushima 1973); this unit is absent in the Katabira River Lowland. The spit sediments are local marine sand bed
representing upward-coarsening (shallowing) lithologicalsuccession associated with upward increase in reworked
shell fragments (Tanabe et al. 2015).
The buried terrace and BG are absent from this crosssection. The bases of the LGM incised valleys are above− 45 m TP (Fig. 6).

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

4.1.6 Section FF’

In section FF’ across the Tokyo Lowland (Fig. 2), thepost-LGM incised-valley fill unconformably overlies the
Shimosa Group (Fig. 6; Matsuda 1974; Kaizuka et al.
1977; Port of Tokyo Geological Research Group 2000;
Bureau of Port and Harbour, Tokyo Metropolitan Government 2001).
In the east and west, the Shimosa group is dissected by
the Paleo-Tokyo and Kanda river valleys, respectively (Fig.
6). The lower half of the Shimosa group consists of a sandbed (N value > 50); the upper half contains a sand bed (Nvalue = 20–50) and a mud bed (N value = 5–10). The
strata of the Shimosa group dip 0.2–0.3° to the east (Fig.
6). The post-LGM Paleo-Tokyo River Valley fill consistsof the BG (N value = 50), the LS (N value = 20–50), the
LC (N value = 10–20), the MS (N value = 5–10), the UC(N value < 5), the US (N value = 5–10), and the UA (Nvalue < 5), in ascending order (Fig. 5; Matsuda 1974; Kaizuka et al. 1977). The BG is correlated to the braided river
sediments, the LS and LC are correlated to the meandering river sediments, the MS and the lower portion of the
UC are correlated to the estuary sediments, and the upper
portion of the UC, the US, and the UA are correlated tothe delta sediments. The meandering river sediments consist of alternation of fluvial channel sand and floodplain
mud (Tanabe et al. 2015). The SB is at the base of the BG.
A transgressive surface (TS; a surface that separates a progradational or aggradational lowstand system from a retrogradational transgressive system; van Wagoner et al.
1988; Catuneanu 2006) is at the braided/meandering river
sediments boundary (Fig. 5). The base of the estuary sediments corresponds to the IFS, and the MxFS separates theestuary and delta sediments (Fig. 5; Tanabe et al. 2010).
The post-LGM Kanda River Valley fill consists of the BG(N value > 50), the UC (N value < 5), and the UA (N value= 5), in ascending order (Fig. 6).
There is a platform at − 10 m TP between the PaleoTokyo and Kanda river valleys (Fig. 6). This platform isregarded as an abrasion platform formed during the middle
Holocene sea-level highstand (Matsuda 1974; Kaizuka et al.
1977; Bureau of Port and Harbour, Tokyo Metropolitan
Government 2001). However, the basal peat of Core 7 isdated to 24.3 ka, and the Hk-TP tephra (MIS 4; Machida
and Arai 2003) has been obtained from the Kanto Loambeneath the basal peat. These ages suggest that this platform can be correlated with the MIS 5a Musashino terrace(Port of Tokyo Geological Research Group 2000) or a certain terrace formed between MIS 5a and MIS 4 (Fig. 3).
The Paleo-Tokyo and Kanda river valleys are at levels shallower than − 75 m TP and − 50 m TP, respectively (Fig. 6).
4.2 Distribution of incised valleys and buried terraces

The depth distribution of the bases of the LGM incisedvalleys and the buried terraces in the Tama, Tsurumi,

Page 12 of 21

Owoka, and Katabira river lowlands and the Tokyo Lowland are illustrated in Fig. 7.
The Tama River Valley lies to the south of the present
Tama River, and the valley’s deepest point is at − 70 m TP in
the vicinity of the Tama River mouth. The Tsurumi River
Valley merges with the Tama River Valley, and the elevation
at the convergence point is − 40 m TP. The deepest points
in the Owoka and Katabira, Paleo-Tokyo, and Kanda Rivervalleys are at − 45, − 80, and − 50 m TP, respectively. The
Haneda River Valley is a small incised valley (maximum
depth − 40 m TP) between the Kanda and Tama river valleys(Fig. 7; Bureau of Port and Harbour, Tokyo Metropolitan
Government 2001). The topography of these valleys does notdiffer very much from that obtained in previous studies(Matsuda 1973, 1974; Kaizuka et al. 1977; Oka et al. 1984;
Matsushima 1987; Pollution Research Institute, Yokohama
City 1988; Port of Tokyo Geological Research Group 2000;
Bureau of Port and Harbour, Tokyo Metropolitan Government 2001; Tanabe et al. 2008); however, the resolution usedin this study (187 m × 187 m grid cells) is higher than that of
all previous studies.
A platform at − 15 to − 5 m TP lies between the
Paleo-Tokyo and Tama river valleys, and a gentle slope
at − 20 to 5 m TP and a platform at − 40 m TP lie in thenorth of the Tama River Valley (Figs. 7 and 8). These
platforms are considered as buried terraces. The platform at − 15 to − 5 m TP, the gentle slope at − 20 to 5
m TP, and the platform at − 40 m TP are named buried
terrace 1 (T1), buried terrace 2 (T2), and buried terrace
3 (T3), respectively (Fig. 8). The T1 is an abrasion platform formed during the middle Holocene sea-level highstand, superimposed on and reshaping the older MIS
5a–4 terrace, with the latter having on its top the Hk-TP
tephra of the Kanto Loam (Fig. 3; Matsuda 1974;
Kaizuka et al. 1977; Port of Tokyo Geological ResearchGroup, 2000; Bureau of Port and Harbour, Tokyo
Metropolitan Government 2001). The locations at which
the Kanto Loam is identified in borehole logs aremarked with white dots in Fig. 7. The Kanto Loam iscommon in the northern part of the T1 between the
Paleo-Tokyo and Kanda river valleys, which supports thecorrelation of the platform with the MIS 5a Musashino
Terrace by the Port of Tokyo Geological ResearchGroup (2000). The T2 can be correlated with the MIS 3
Tachikawa Terrace that crops out northwest of Mizonokuchi (Figs. 1c, 2, and 3; Matsuda 1973; Kaizuka et al.
1977; Oka et al. 1984). The gradient of the Tachikawa
Terrace is consistent (3/1000–4/1000) from the edge ofthe buried terrace at − 20 m TP to the apex of the alluvial
fan at 180 m TP. The AT tephra, which is dated to 30.0
ka (Smith et al. 2013), is found in the Kanto Loam of theoutcrop of the Tachikawa Terrace. Therefore, the gentle
slope (i.e., the buried terrace) must have formed before
30.0 ka, and represents the continuation of an alluvial fan

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Page 13 of 21

Fig. 7 Elevation map of post-MIS 5a buried terraces and LGM incised valleys in and around the Tama River Lowland. The white dots show the
distribution of the Kanto Loam on buried terraces. The contour interval is 2 m. The base of the BG is the base of the post-LGM incised-valley fillor a braided river from the upper reaches. The Kanto
Loam does not overlie the T3 and the LGM incised valley, meaning that these surfaces formed between 30.0 ka
and the time of lowest sea-level during the LGM (20.5 ka;
Fig. 3). The Kanto Loam is more common at the edges orin the hollows of the buried terrace than on the buried
terrace platform (Fig. 7). This suggests that the top platform of the buried terrace was slightly eroded by waves
during the last deglacial sea-level rise.
4.3 Presence and thickness of gravel beds

The distribution of gravel beds at the bases of post-LGM
incised-valley fills is illustrated in Fig. 9. Gravel beds arewidely present in the Tama River Lowland, and locallyin the Tsurumi, Owoka, and Katabira river lowlands andthe Tokyo Lowland. Gravel beds are widespread in theTama River Lowland because of the presence of the T2
and the BG. The gravels forming the T2 and the BG
were discharged from the Tama River. Today, 70% of
the gravel in the Tama River bed is the product of sandstone beds of the accretionary prism in the Kanto
Mountains (Fig. 1c; Nakayama 1954). In contrast, thegravel beds of the buried terrace and the BG are absent
from the Tsurumi, Owoka, and Katabira river lowlands,
where the post-LGM incised-valley fills directly overlie
the Kazusa Group. The gravel beds are absent because
the river catchments of the Tsurumi, Owoka, and Katabira Rivers consist of hills and uplands formed of sandand mud of the Kazusa and Shimosa groups, and there
is no source of coarse-grained materials originating fromhard rock (Fig. 1c). In the Tokyo Lowland, the gravel
bed is absent on the T1, but is locally present on theTanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Page 14 of 21

Fig. 8 Classification of upland and buried terraces and LGM incised valleys, all correlated with marine isotope stages. T1 buried terrace 1, T2
buried terrace 2, T3 buried terrace 3, LGM valley LGM incised valley. MIS marine isotope stage. The T1 partly contains abrasion platforms formed
during the middle Holocene sea-level highstand; therefore, it is correlated to MIS 5a–4 and MIS 1

slope and at the base of the Kanda River Valley (Fig. 9).
This is because the rivers that formed the T1 did not deposit coarse-grained materials, and the gravel beds on
the slope and at the base of the Kanda River Valley areoutcrops of the Shimosa Group (Fig. 6, section FF’). The
Musashino Upland is within the catchment of the modern Kanda River; however, the modern Kanda River does
not have sufficient power to erode and transport thegravels forming the Musashino Terrace. The gravels at
the base of the Paleo-Tokyo River Valley were discharged from the Tone River.
The thickness variations of the gravel beds at the basesof the post-LGM incised-valley fills are illustrated in Fig.
10. The thickness of the gravel bed was divided into four
categories (0–2, 2–5, 5–10, and 10–22 m) on the basisof the depth of the modern Tama River channel (ca. 5m; Fig. 4); 2 m and 10 m are approximately half anddouble the depth of the modern channel, respectively.
The gravel bed of the T2 is 2–5 m thick and the BG at
the base of the Tama River Valley is 10–22 m thick. Thisdifference arose because the Tama River Valley is narrower (0.8–0.9 km) than the T2 (width 1.9–3.1 km), thus
with less accommodation available for alluvial sediments.
The BG is thin (5–10 m) in the lower reaches of theTama River Valley because the valley widens there. BGs
in the Tsurumi, Owoka, and Katabira river valleys are locally present and are only several decimeters thick.
Lithological description of the borehole logs indicatesthat the gravel clasts in these BGs are mudstone fromthe Kazusa Group. The gravel beds on the slope andat the base of the Kanda River Valley are 5–10 m
thick, similar to those of the Shimosa Group (Fig. 6,
section FF’).

5 Discussion
5.1 Formation of buried terraces and incised valleys

Little is known about sea-level changes during the LGM
because few sea-level index points are available for thisperiod. Until now, the sea-level fall to the lowest sealevel during the LGM has been considered to be gentle(Siddall et al. 2003; Lambeck et al. 2014) or step-wise(Yokoyama et al. 2018; Ishiwa et al. 2019; Fig. 11). In thisstudy, the existence of the T2 at − 20 m TP, the T3 at −40 m TP, and the base of the incised valley at − 50 m TP
is documented (Fig. 6, section BB’). Based on the presence of the AT tephra on the surfaces and the assumption that the incised valley was formed at the period oflowest sea-level during the LGM, the ages of the T2, T3,

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Page 15 of 21

Fig. 9 Occurrences of gravel beds at the bases of the post-LGM incised-valley fills. Black and white dots indicate presence and absence of a gravel
bed, respectively, and are plotted on the elevation map of Fig. 7. Gravel beds are frequent along the Tama River. a–d Show × 2 enlargements of theundulating topography at the bases of the LGM incised valleys. Borehole logs are available for the bottoms and margins of pits in (a), (b), and (c), but
are not available for the pits in (d)and incised valleys can be estimated to be > 30.0, 30.0–
20.5, and ca. 20.5 ka, respectively. A step-wise fluvial terrace that responds sensitively to sea-level changes will
form in a tectonically stable area (Kaizuka 1977). The
uplift rate in the Tama River Lowland has been very low

since MIS 5e (0.04 m/kyr; Koike and Machida 2001;
Okuno et al. 2014). Therefore, the step-wise fluvial terraces consisting of the buried terraces and the incisedvalley in the Tama River Lowland might reflect two
steps of sea-level lowering prior to the lowest sea-level

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Page 16 of 21

Fig. 10 Distribution of the thickness of the gravel bed at the base of the post-LGM incised-valley fills, plotted on the elevation map of Fig. 7. Thickgravel beds are found along the Tama Riverduring the LGM. Lambeck et al. (2014) inferred a sealevel plateau before 31 ka, and the T2 formed duringformation of this plateau (Fig. 11). Based on Yokoyama
et al. (2018), the second sea-level plateau is dated to 29–
21.5 ka; the T3 formed during this plateau. The T3 is locally present in the Tama River Valley (Fig. 8); therefore,
its continuation in the lower reaches must be confirmedin the future. The formation of the incised valley is estimated to have occurred at the lowest sea-level during
the LGM at 21.5–20.5 ka (Yokoyama et al. 2018). The
basal age of the BG is regarded as < 30 ka because theLGM incised valleys comprising the BG at the bases dissect the T2. The AT tephra does not overlie the BG anywhere in the Tama River Valley (Fig. 7). In otherriverine coastal plains, the LGM incised valleys and T3might be difficult to discriminate because the

distribution of the T3 is presumably local; therefore, thebasal age of the BG there can be also regarded as
roughly younger than 30 ka.
5.2 Interpretation of undulating topography and basal
gravel bed

The thalwegs of the Tama and Tsurumi river valleys calculated from the base of the post-LGM incised-valley
fills indicate that the base of the valleys undulate along
stream with a series of concave- and convex-upward
topography (Fig. 12). The concave-upward topography(scours) are spaced at 1–2-km intervals, and are 1–2 km
long, < 1 km wide, and 5–10 m deep. In the Tama andTsurumi river valleys, multiple borehole logs are available from both the bottoms and the margins of scours,suggesting that the undulating topography is not a result

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Page 17 of 21

Fig. 11 Sea-level change during the period of formation of the BG. Sea-level curves (a), (b), and (c) are after Lambeck et al. (2014), Siddall et al.
(2003), and Yokoyama et al. (2018), respectively. The gray shaded area marks the period of formation of the BG. RSL relative sea-levelof abnormal values calculated by mathematicalinterpolation (Fig. 9a, b, c). However, the undulating
topography in the Kanda River Valley, for example,
might result from abnormal values calculated by mathematical interpolation because the distribution of borehole logs there is sparse (Figs. 7, 9d).
The undulating topography in the Tama and Tsurumi
river valleys is comparable with pools and riffles ofbraided rivers, which are commonly observed in both
flume experiments and modern analogues (Shepherd
and Schumm 1974; Keller and Melhorn 1978; Thompson 2018), with pools located at bends and riffles locatedin straighter river tracts between bends. Pools are deep
with low velocities of current during baseflow, while riffles are shallow with high velocities during high flow
(Thompson 2018). However, the undulating topography
in the Tama and Tsurumi river valleys are large (valley
scale) compared to those of simple pools and riffles(channel scale). Furthermore, the gradients of the Tamaand Tsurumi river valleys are steeper (3/1000) thanthose of rivers forming pools and riffles (2/1000;
Thompson 2018). Additionally, undulating topography
formed by pools and riffles is visible on the tops ofgravel beds rather than at their bases (Thompson 2018).
Best and Ashworth (1997) reported scours, 2 km long,
400 m wide, and > 30 m deep in the channel confluencesof the Jamuna and Ganges rivers in Bangladesh. Theserivers have a combined annual mean discharge of 40,000
m3/s and a sediment transport rate of 38,000 kg/s per
year (Milliman and Farnsworth 2011). The scours areformed during monsoon flood period. Scours at channel
confluences have different magnitude due to the scale ofgravel bed braided rivers (Best 1988). In a braided river,abundant confluences are formed (e.g., Lane et al. 2003),
and scours may have been formed at these confluences.
There are a few studies demonstrating basal topographyof the gravel beds of the modern braided rivers. Therefore, future study is required; however, the undulating
topography in the Tama and Tsurumi river valleys canbe tentatively attributed to scours at channel confluencesin ancient times, rather than pools and riffles.
The undulating topography, which was formed during
the LGM, is more pronounced in the Tsurumi River
Valley than in the Tama River Valley (Fig. 12); furthermore, the river incision is deeper in the Tsurumi River
Valley than in the Tama River Valley in areas more than
20 km upstream of the Tama River mouth. River erosion, including incision, is controlled by, among otherfactors, bedrock geology and variations in water andsediment discharges (Whipple et al. 2013). In the Tamaand Tsurumi river valleys, the bedrock geology consistsof the consolidated Kazusa and Shimosa groups (N value> 50), but the distribution of the BG differs in the twovalleys. The high sediment discharge and the cover effectof gravel beds on the channel floor (the armoring) resulted in less-pronounced undulating topography andshallower river incision in the Tama River Valley. The
cover effect of coarse materials has been commonly detected in abrasion mill experiments (Sklar and Dietrich2001), modeling (Sklar and Dietrich 2004), flume experiments (Johnson and Whipple 2010), and field observations (Johnson et al. 2009). The cover effect of gravels
might also have influenced the sinuosity of the channel
forming the valley: the Tama River Valley is less sinuousTanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Page 18 of 21

Fig. 12 Thalwegs of the Tama (black line, XX’) and Tsurumi (red line YY’) river valleys

than the Tsurumi River Valley, likely because erosion in
the undercut slope in the Tama River Valley was small
due to the cover effect of the abundant gravels. The
applicability of the cover effect to both horizontal andvertical river erosion has been confirmed in flume experiments (Shepherd 1972) and theoretically (Turowski2018).

The gradient of the BG is 3/1000 (Fig. 12), which isthree to six times larger than that of the meanderingriver segment of the modern Tama River (Kadomura
1961); therefore, river bed erosion coeval with gravel deposition might be enhanced during the sea-level lowering in the LGM with consequent increasing topographic
gradient rather than the present day sea-level highstandTanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

(Suzuki 2012, 2013, 2015). The BG majorly consists ofgravels larger than 6 cm in diameter (Fig. S1; Tanabe
and Nakashima 2016), and the grain size is almost sameto that in the modern braided-river segment in theTama River (Kanto Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism
2017a). These gravels were transported and deposited by
bedload during floods (Suzuki 2012, 2013, 2015). The
grain size and river gradient of the LGM and present
braided rivers are almost same (Figs. 4 and 12). However, the LGM climate is considered to have been cooland dry, and the flood intensity and the sediment discharge by rivers have been regarded as low comparedwith those of today (Chabangborn et al. 2014); therefore,
the grain size of the gravels of the BG should be smaller
than that in the present braided-river segment. To explain the relatively large volume of the BG deposited
during the LGM (Fig. 10), recycle of gravels from thepost-MIS 5a fluvial terraces must be considered (Fig.
1c). Comparably large gravels of the BG instead of lessintensity of floods during the LGM can be also explainedby this recycle.

6 Conclusions
In this study, we revealed the detailed topography of the
LGM incised valley of the Tama River Lowland, central
Japan, by analyses of 4702 borehole logs. The spatial distribution of the incised valley was reconstructed bymathematically interpolating (Kriging) the depth of thebase of the incised valley identified in each borehole log.
From this analysis, undulating topography at the base of
the incised valley was confirmed. This undulating topography, which consists of a series of pits 1–2 km long, < 1
km wide, and 5–10 m deep that occur at 1–2-km intervals, can be identifiable as scouring at braided-river
channel confluences. At least one borehole log is available for each 187 m × 187 m grid cell and multiple borehole logs are available from both bottom and margin
portions of the scouring; therefore, this topography is
not a representation of abnormal values calculated by
Kriging.
In the Tama River Valley, the T2 at − 20 m TP, whichis overlain by the AT tephra, is dissected by the T3 at −40 m TP and the LGM incised valley at − 50 m TP, neither of which are overlain by the AT tephra. The TamaRiver Lowland has been tectonically stable since MIS 5e.
Therefore, these fluvial terraces might have formed as aresult of step-wise sea-level lowering before the lowest
sea-level during the LGM. The T2, the T3, and the
LGM incised valley might have formed during sea-level
plateaus at > 30.0, 30.0–21.5, and 21.5–20.5 ka, respectively. The last plateau corresponds to the lowest sealevel during the LGM. This is a first finding, which support the LGM sea-level plateau at 30.0–21.5 ka in Japan.

Page 19 of 21

The Tama River Valley (depth − 70 m TP) and the
Tsurumi River Valley (depth − 40 m TP) lie beneath theTama River Lowland. These valleys merge 10 km upstream of the modern Tama River mouth. The bedrockof both valleys is the consolidated Kazusa and Shimosagroups; however, the undulating topography is more
pronounced and the river incision depth and sinuosity
are greater in the Tsurumi River Valley than in theTama River Valley. The major differences between thetwo valleys arose from differences in sediment discharge
and the presence of the BG, which in turn led to a difference in the cover effect. The undulating topography is
pronounced and the river incision depth and sinuosity
are larger in the Tsurumi River Valley because it lacked
the cover effect of the BG. Clear evidence of the covereffect on the LGM incised valleys has been never demonstrated before.
In this study, we have confirmed that the basal age of
the BG is < 30 ka because the LGM incised valleys comprising the BG at the bases dissect the T2. In other riverine coastal plains, the LGM incised valleys and T3 might
be difficult to discriminate because the distribution of
the T3 is presumably local; therefore, the basal age of
the BG can be also regarded as roughly younger than 30
ka there. However, furthermore age data is required, andgeomorphological and depositional settings must be
considered for the adaptation of this age to the BGs inother riverine coastal plains.

7 Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40645-021-00411-0.
Additional file 1: Fig. S1. Sedimentary logs of five stratotype coresobtained by the Geological Survey of Japan (Tanabe and Nakashima,2016). Table S1. Sampling locations of stratotype cores used in thisstudy. Table S2. Radiocarbon dates from the stratotype cores. Measured
14
C ages of GaK were converted into conventional 14C age by assuming
delta 13C values of shells and peats as 0 and -27.5‰, respectively.

Acknowledgements
We thank the Civil Engineering Support and Training Center, Bureau ofConstruction, Tokyo Metropolitan Government; the Bureau of Port andHarbour, Tokyo Metropolitan Government; and the Bureau of Environment,
Kawasaki City, for providing the XML data of borehole logs. We thankvaluable comments from Yoshiki Saito, Marco Mancini, and two anonymous
reviewers, which greatly improved the manuscript.
Availability of data and material
Please contact author for data requests.
Authors’ contributions
ST designed and performed this study and wrote the manuscript. YI createdthe voxel model. Both authors read and approved the final manuscript.
Funding
This work was financially supported by the National Institute of Advanced
Industrial Science and Technology (AIST) research program “Geological andactive faults survey project in coastal areas of Japan”.

Tanabe and Ishihara Progress in Earth and Planetary Science(2021) 8:20

Competing interests
The authors declare that they have no competing interest.
Author details
1
Geological Survey of Japan, AIST, Central 7, Higashi 1-1-1, Tsukuba 305-8567,
Japan. 2Fukuoka University, Nanakuma 8-19-1, Jonan-ku, Fukuoka 814-0180,
Japan.
Received: 7 October 2020 Accepted: 27 January 2021

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