4.1 Geological cross sections
This section contains a description of the lithologies and N values of the Kazusa and Shimosa groups and the post-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 incised-valley fills on the basis of the sedimentary facies and radiocarbon dates obtained from the stratotype cores and previous studies (Fig. S1). Based on these data, the depth 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 and mud beds with N values > 50. The post-LGM incised-valley fill contains the BG (N value = 50), the LC (N value = 5–20), the MS (N value = 5–30), the UC (N value < 5), the US (N value = 5–20), and the UA (N value < 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 upward-finning (deepening) and -coarsening (shallowing) sand and 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 post-LGM incised-valley fills is regarded as a sequence boundary (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 is regarded as part of the transgressive meandering river sediments (Fig. 5; Tanabe et al. 2010), the LC in the Tama River Lowland consists of mud with rootlets and burrows that yield a mixture of marine, brackish, and freshwater diatoms (Fig. S1; Tanabe 2020). Therefore, the LC in the Tama River Lowland is interpreted as salt marsh sediments (Tanabe and Nakashima 2016). These salt marsh sediments are marine sediments, and form part of the estuary sediments.
A radiocarbon date of 35.7 kyr BP shows that the base of the GS-KKW-1 core contains pre-LGM deposits (Figs. S1, 6; Tanabe and Nakashima 2016). Radiocarbon dates from sediments overlying the Kazusa and Shimosa groups 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 Tama River Valley (Fig. 6). These dates demonstrate that the estuary 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 and Nakashima 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 the west. Undulating topography can be identified at the base of the LGM incised valley (Fig. 6).
4.1.2 Section BB’
Section BB’ is a transverse cross section of the Tama River 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 the valley axis at a depth of − 50 m TP, the Tachikawa buried terrace at − 20 m TP, and the Shimosa group at − 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 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 constitute delta 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 sea-level of the LGM (20.5 ka; Yokoyama et al. 2018), and may be correlated to MIS 3. The platform at − 5 m TP is regarded 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 Tama River Valley in the northwest of the Tama River Lowland (Fig. 2). In this section, the post-LGM incised-valley 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 are dated 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 that consists 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 (N value > 50). The post-LGM incised-valley fill consists of the UC (N value < 5), the US (N value = 5–20), and the UA (N value < 5), in ascending order. The lower portion of the UC is correlated to the estuary sediments, and the upper portion of the UC, the US, and the UA are correlated to the delta sediments (Fig. 5). To the west, the sand bed of the US changes laterally into a sandy 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 a flood-tidal delta to the bay head portion of the Paleo-Tsurumi Bay during the middle Holocene highstand (Matsushima 1987). The unconformity between the Kazusa group and the post-LGM incised-valley fills is regarded 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 the base (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 (N values > 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 and the 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 age of the MxFS corresponds with the middle Holocene sea-level highstand of 7 ka in an embayment with low sediment discharge in the Tokyo Lowland. Sugimoto and Umehara (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) lithological succession associated with upward increase in reworked shell fragments (Tanabe et al. 2015).
The buried terrace and BG are absent from this cross section. The bases of the LGM incised valleys are above − 45 m TP (Fig. 6).
4.1.6 Section FF’
In section FF’ across the Tokyo Lowland (Fig. 2), the post-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 sand bed (N value > 50); the upper half contains a sand bed (N value = 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 consists of 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 (N value < 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 to the 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 the estuary 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 Paleo-Tokyo and Kanda river valleys (Fig. 6). This platform is regarded 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 is dated to 24.3 ka, and the Hk-TP tephra (MIS 4; Machida and Arai 2003) has been obtained from the Kanto Loam beneath 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 incised valleys and the buried terraces in the Tama, Tsurumi, 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 River valleys 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 not differ 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 used in 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 the north 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 Research Group, 2000; Bureau of Port and Harbour, Tokyo Metropolitan Government 2001). The locations at which the Kanto Loam is identified in borehole logs are marked with white dots in Fig. 7. The Kanto Loam is common in the northern part of the T1 between the Paleo-Tokyo and Kanda river valleys, which supports the correlation of the platform with the MIS 5a Musashino Terrace by the Port of Tokyo Geological Research Group (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 of the 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 the outcrop 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 or 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 or in 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 are widely present in the Tama River Lowland, and locally in the Tsurumi, Owoka, and Katabira river lowlands and the Tokyo Lowland. Gravel beds are widespread in the Tama 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, the gravel 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 sand and mud of the Kazusa and Shimosa groups, and there is no source of coarse-grained materials originating from hard rock (Fig. 1c). In the Tokyo Lowland, the gravel bed is absent on the T1, but is locally present on the 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 are outcrops 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 the gravels 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 bases of 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 basis of the depth of the modern Tama River channel (ca. 5 m; Fig. 4); 2 m and 10 m are approximately half and double 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. This difference 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 the Tama 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 indicates that the gravel clasts in these BGs are mudstone from the Kazusa Group. The gravel beds on the slope and at the base of the Kanda River Valley are 5–10 m thick, similar to those of the Shimosa Group (Fig. 6, section FF’).