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Analysis of titanomagnetite within weathered middle Pleistocene KMT tephra and its application for fluvial terrace chronology, Kanto Plain, central Japan
ARTICLE IN PRESS
Quaternary International 178 (2008) 119–127
Analysis of titanomagnetite within weathered middle Pleistocene KMT tephra and its application for fluvial terrace chronology, Kanto Plain, central Japan Takehiko Suzuki Department of Geography, Tokyo Metropolitan University, Minamiosawa 1-1, Hachioji, Tokyo 192-0397, Japan Available online 12 December 2006
Abstract The widespread Kaisho-Kamitakara tephra (KMT), derived from the source vent located in the Hida Mountains, is a favorable marker tephra for the chronological study of the middle Pleistocene in the Kanto Plain, central Japan. As the KMT in aeolian sediments is strongly weathered, the chemical composition of titanomagnetite, resistant to weathering, was determined for its identification. The age of the KMT was estimated at 0.62 Ma (MIS 16.0) by Nakazato [2006. Horizon and age of Kaisho-Kamitakara tephra from the Inubo Group in the northeastern part of Chiba Prefecture, central Japan. Programme and Abstracts, Japan Association for Quaternary Research 36, 106–107 (in Japanese)], revising a previous estimate of 0.58–0.69 Ma by Suzuki [2000. Kaisho-Kamitakara tephra erupted from the Hida Mountains in early half of middle Pleistocene and its significance for geomorphic chronology of central Japan. Geographical Review of Japan 73, 1–25 (in Japanese)] The stratigraphic position of the KMT in the Azuyama and Sayama terrace surfaces in southwest Kanto and the Kitsuregawa Upper terrace surface in north Kanto indicates that these terraces are correlative. It is assumed that the dissected fluvial terrace surfaces were formed a few tens of thousands of years prior to the deposition of the KMT. Alluvial fan deposits constituting these terrace surfaces were formed during the period from MIS 17.3 to 16.2, during a transition from interglacial to glacial. Geomorphic conditions during this period of the middle Pleistocene were stable, allowing the formation of broad fans. r 2006 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Tephra layers resulting from volcanic eruptions provide stratigraphic markers that assist in classifying terrace surfaces and sediments. Many studies have discussed the formation of fluvial terrace surfaces in response to either climate or sea level control using tephrochronology (e.g. Litchfield and Berryman, 2005). An advantage to studying fluvial terrace surfaces in the Japanese Islands, a volcanic island arc, is the ability to use tephrochronology. The Kanto Plain (Fig. 1), the widest in the Japanese Islands, is regarded as one of the standard areas for Japanese Quaternary tephra study (Yoshikawa et al., 1981). Intensive studies of the tephra and the marine and fluvial terrace surfaces in this area have revealed the detailed geomorphic development during the late PleistoTel.: +81 426 77 2590; fax: +81 426 77 2589
E-mail address: [email protected].
cene (Machida, 1975; Kaizuka et al., 1977). However, until the 1990s the chronological framework of the middle Pleistocene fluvial terrace surfaces had not been established in detail, despite considerable tephrochronological studies on the middle Pleistocene marine sequence in the Oiso hills and Boso Peninsula. The existing chronological studies had not confirmed the relationship of the fluvial terrace surfaces to oxygen marine isotope stages, in contrast to the established late Pleistocene chronostratigraphy. Tephrochronological studies focusing on the middle Pleistocene widespread tephra layers advanced in the 1990s (Machida, 1999a), providing several precise datum planes for this period covering the Kanto Plain. The widespread middle Pleistocene Kaisho-Kamitakara tephra (KMT), derived from the source vents located in the Hida Mountains (Fig. 1), central Japan, was recognized in marine strata on the Boso Peninsula. This played a significant role in the reconstruction of the geomorphic development in coastal to inland areas of the Kanto Plain
1040-6182/$ - see front matter r 2006 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2006.10.039
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T. Suzuki / Quaternary International 178 (2008) 119–127
Fig. 1. Satellite image of the Kanto Plain and the adjacent area showing the distribution and localities of Kaisho-Kamitakara tephra (KMT). Closed circles and numerals indicate localities where KMT was identified and locality numbers, respectively. Open circle indicates locality where Nakazato (2006) determined the age of KMT.
(Suzuki, 2000). However, in many cases, the KMT in aeolian sediments including tephric soil deposits (tephric loess) had been strongly weathered. Chemical weathering affected by the mid-latitude oceanic climate causes difficulties in identifying the KMT. Where volcanic glass shards are intensely altered or non-existent, glass geochemistry is not available for identification. In this study, the chemical composition of titanomagnetite was used for the identification of the KMT. Titanomagnetite is moderately stable under weathering, and is abundant in weathered KMT in tephric soil deposits. Additionally, although the measurement of the refractive indices of orthopyroxene and hornblende has been a conventional method for tephra characterization in Japan, this method is not applicable to the identification of the KMT. Plagioclase, biotite, and titanomagnetite are the dominant minerals, although orthopyroxene and hornblende are rarely present. Many studies of the chemical composition of titanomagnetite have been carried out since the 1960s. Using ilmenite and titanomagnetite, a combination of temperature and oxygen fugacity was estimated from their chemistry (Andersen and Lindsley, 1988; Andersen et al., 1991). Shane (2000) pointed out that these compositions are useful for the discrimination of tephras in New Zealand.
This paper demonstrates the identification of KMT mainly through chemical analysis of titanomagnetite, as well as by other methods. A tephrochronologically reconstructed geomorphic history of the middle Pleistocene in the Kanto Plain is presented, with special reference to sea level and climate changes. 2. Kaisho-Kamitakara tephra The KMT, erupted from the Kaisho source vent in the southwest part of the Hida Mountains in central Japan, is composed of the Kamitakara Pyroclastic Flow Deposit (440 km3) and a fallout tephra (440 km3) distributed from the Chubu to the south Tohoku areas, 300 km from the source (Harayama, 1990; Suzuki, 2000) (Fig. 1). The volcanic explosivity index (VEI; Simkin and Siebert, 1994) of the eruption forming the KMT is 6, indicating a paroxysmal eruption. 2.1. Characteristics of the proximal and distal deposits of KMT Analysis of individual glass shards by electron probe microanalysis (EPMA) is effective for the identification of tephra. Glass geochemistry and other characteristics indicate that the Kamitakara pyroclastic flow deposit of
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Locality 1 in the proximal area and distal deposits of Locality 6 in the Boso Peninsula are correlative. At Locality 1, the basal part comprises more than 3.5 m of non-welded pyroclastic flow deposit, which underlies strongly welded pyroclastic flow deposit up to 40 m thick. At Locality 6, the Kasamori 22 tephra (Ks 22) (Tokuhashi and Endo, 1984) in the Kasamori formation of the Plio-Pleistocene Kazusa Group forearc basin-fill deposits crops out as a gray glassy ash layer of 9 cm in thickness. They are composed of colorless glass shards; abundant biotite, titanomagnetite, plagioclase, and quartz; and small amounts of zircon, orthopyroxene, and hornblende. The major element compositions of glass shards in samples collected from the matrix of the basal part of the Kamitakara pyroclastic flow deposit at Locality 1 in the proximal area and from the well-preserved distal ash-fall deposit in the marine sediments of Locality 6, were determined by EPMA (Suzuki, 2000) (Table 1). The mean SiO2 content of 76.9–77.1 wt% and mean K2O content of 5.2–5.5 wt%, recalculated on a volatile-free basis, indicate a high-silica rhyolite with high alkali content. This major element probe data presented for two beds from separate locations (Localities 1 and 6) demonstrates their correlation to each other. Moreover, refractive indices of glass shards ranging within 1.497–1.500 (Suzuki, 2000) and the presence of zircon grains with high uranium content, 450 ppm (Locality 1) (Suzuki et al., 1998a) and 490 ppm (Locality 6) (Watanabe and Danhara, 1996) support their correlation. On the other hand, glass shards collected from the distal ash-fall deposits (Locality 6) are mostly pumice-type shards, in contrast to the abundance of bubble-walled shards in the matrix of the pyroclastic flow deposit (Locality 1) in the proximal area. This suggests that the emplacement of distal ash-fall deposits of KMT were associated with a Plinian eruption, and they are not coignimbrite ash-fall deposits (Cas and Wright, 1988). The mode of eruption and the recognition of KMT in the Boso Peninsula, 250 km southeast from the source, suggest the widespread occurrence of KMT in the whole area of the Kanto Plain.
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2.2. Correlation of the weathered KMT in tephric soil deposits Titanomagnetite chemistry analysis was applied to seven extremely weathered, clay-rich distal ash-fall deposits and to the pyroclastic flow deposit of Locality 1. These weathered tephras have no remaining volcanic glass shards, although they do contain some sand-sized titanomagnetite grains and abundant biotite. The abundant visible biotite grains are lithologically unusual. Previous studies named this tephra the Bio 1 (Locality 4; Shimizu and Horiguchi, 1994), the Biotite Zone (Locality 5; Minagawa and Machida, 1971) and the Hoshitoge Biotite tephra (HtB) (Localities 7–9; Koike et al., 1985). The stratigraphic positions of these tephra layers are shown in Fig. 2. The Biotite Zone is located above the Imokubo Gravel Bed forming the Sayama terrace surface in the Sayama hills. The Bio 1, correlated to the Biotite Zone by Shimizu and Horiguchi (1994), is located above the Upper Tooyoka Gravel Bed forming the Azuyama terrace surface in the Azuyama hills. The Hoshitoge Biotite tephra is located above the Sakaibayashi Gravel Bed forming the Kitsuregawa Upper terrace surface in the Kitsuregawa hills. The major element compositions of the titanomagnetite in samples were determined by EPMA (Table 2). To examine analysis and correlation of KMT, two points were checked. Firstly, the content of titanomagnetite grains with total weight percentage of 98–102%, recalculated under the condition that the grains lie on the ulvospinel-magnetite line, was considered. Secondly, the content of the grains with typical KMT chemical composition as KMT was considered. Despite pyroclastic flow deposit, 22 grains in 24 grains in the sample of collected from the basal part of the Kamitakara pyroclastic flow deposit at Locality 1 have total weight percentage of 98–102% on the ulvospinelmagnetite line. This indicates that the basal part was cooled immediately after the deposition, showing non-oxidation accompanying slow cooling. Consequently, the compositions of the titanomagnetite in samples at Locality 1 are available for identification. All collected samples except sample of Locality 5 had grains that were chemically very similar to those in the KMT. All grains from Locality 2 are
Table 1 Chemical composition of glass shards in KMT Tephra (Locality) Ks 22: upper part (Loc. 6: Senda, Chonan, Chiba) Ks 22: middle part (Loc. 6: Senda, Chonan, Chiba) Kamitakara pfl.: lower (Loc. 1: Sugo, Takayama, Gifu)
mean st. dev. mean st. dev. mean st. dev.
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
K2O
Na2O
Total
Analytical total
n
77.1 0.1 77.1 0.2 76.9 0.3
0.1 0.0 0.1 0.0 0.1 0.0
12.6 0.1 12.6 0.1 12.6 0.1
0.9 0.1 0.9 0.1 0.8 0.2
0.1 0.1 0.1 0.1 0.1 0.1
0.2 0.0 0.3 0.0 0.3 0.0
0.5 0.0 0.5 0.0 0.6 0.1
5.3 0.1 5.2 0.1 5.5 0.1
3.3 0.1 3.3 0.1 3.1 0.1
100.0
98.5 1.1 96.5 4.5 93.6 1.1
12
100.0 100.0
13 15
Reference: Suzuki (2000) FeO: total irons as FeO. Analyses recalculated to 100% on a volatile-free basis and presented as a mean and standard deviation of n shard analyses. Determined by a JEOL JSM-5200 (scanning electron microscope) and a JEOL JED-2001 energy dispersive X-ray spectrometry using a 0.3 nA current at 15 kV and a 10 mm beam diameter. Method and standards as in Suzuki (1996).
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8
1
0 0
Scale for Loc. 8 Kamitakara pyroclastic flow deposit
3
11 5m
5 10m
10
2
12
4 6
9 KSF KMT
NAG
Misawa, Takayama
Nashinoki, Shiojiri
Pumice lapilli Scoria lapilli Volcanic ash Pyroclastic flow deposit (welded) Pyroclastic flow deposit (non-welded)
IM Senda, UTG Chonan Kawabe, Kanekosaka, Tokorozawa Iruma
Brown tephric soil deposits Cracked zone
SAG
Adachi, Ittochi
Kamako, Higashi Yoshii, Ogawa
SAG
Yanoto, Nihonmatsu
Kabata, Otawara
Silt Gravel Basement rock
Fig. 2. Stratigraphic position of KMT. Locality is shown in Fig. 1. Columnar section of Locality 4 is after Shimizu and Horiguchi (1994). NAG: Nashinoki Gravel Bed; UTG: Upper Toyooka Gravel Bed; IMG: Imokubo Gravel Bed; KSF: Kasamori Formation; SAG: Sakaibayashi Gravel Bed.
of the KMT type. However, from Localities 8, 10–12, only 21–47% of grains were of the KMT type. In these cases, the thickness of KMT is less than 10 cm and blocks of ash are dispersed in tephric soil deposits. The small KMT content is explained by contamination of grains from the upper and lower horizons. A further fingerprinting method used the content of uranium in zircon presented in previous studies. Despite the low primary zircon content, it was well preserved due to its stability under weathering. A significant characteristic of the KMT is the presence of zircon grains with high uranium content: 450 ppm (Locality 1), 410 ppm (Locality 2), 480 ppm (Locality 3) (Suzuki et al., 1998a), 440 ppm (Locality 8) (Suzuki et al., 1998b), and 490 ppm (Locality 6) (Watanabe and Danhara, 1996). These values range between 410 and 490 ppm, confirming their correlation. Additionally, a dispersed and weathered biotite-rich tephra named the Biotite Zone, exposed as a 10 cm thick tephric soil deposits bearing abundant visible biotite grains (Locality 5), contains zircon grains with high uranium
content of 470 ppm (Suzuki et al., 1998a). Although KMT type titanomagnetite grains were not detected in this tephra, the content of uranium and presence of abundant biotite grains show that this tephra is correlative to KMT. The absence of KMT type titanomagnetite grains is most likely caused by strong contamination. 2.3. Age of the KMT Suzuki (2000) determined the age of the KMT to be 0.58–0.69 Ma using many radiometric ages from previous studies (Table 3) and the stratigraphic position in the marine sequence of Boso Peninsula. The broad range of this estimated age was caused by large errors in radiometric dating and uncertainty of the age determination on the marine sediments. Recent dating by Watanabe et al. (1999) has revised the estimated age of Suzuki (2000). Watanabe et al. (1999) determined K-Ar ages of 0.6470.03 and 0.6570.03 Ma for the distal ash-fall deposits in Boso Peninsula and 0.6270.06 and 0.6370.06 Ma for the
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Table 2 Chemical composition of titanomagnetite in KMT Locality
Usp. (1) / (2) SiO2 TiO2 Al2O3 V2O3 FeO MnO MgO CaO ZnO Analytical Ulvospinel basis FeO Total Mol. % (3) total Fe2O3
Loc. 1//Sugo, Takayama, Gifu pyroclastic flow deposit Loc. 2//Nashinoki, Shiojiri, Nagano ash-fall deposit Loc. 3//Misawa, Takayama, Gunma ash-fall deposit Loc. 5//Kawabe, Tokorozawa, Tokyo ash-fall deposit Loc. 8//Yoshii, Ogawa, Tochigi ash-fall deposit Loc. 10//Kamako, Higashi, Fukushima ash-fall deposit Loc. 11//Yanoto, Nihonmatsu, Fukushima ash-fall deposit Loc. 12//Ittochi, Adachi, Fukushima ash-fall deposit
20 24 20 20 14 30 0 14 6 30 8 30 6 30 8 22
/ 22 0.5 0.1 / 20 0.5 0.1 / 22 0.5 0.1 /8 — — / 28 0.6 0.0 / 17 0.5 0.0 / 17 0.6 0.1 / 19 0.5 0.0
6.0 0.2 6.3 0.5 6.1 0.2 — — 6.1 0.1 6.1 0.2 6.1 0.2 6.3 0.3
1.4 0.1 1.5 0.1 1.5 0.1 — — 1.4 0.1 1.4 0.1 1.6 0.5 1.4 0.1
0.7 0.1 0.7 0.1 0.7 0.1 — — 0.7 0.1 0.7 0.1 0.7 0.2 0.7 0.1
83.1 0.6 82.9 0.7 83.1 0.4 — — 83.4 0.5 82.8 0.3 82.3 0.8 83.2 0.6
1.6 0.1 1.9 0.1 1.6 0.2 — — 1.7 0.1 1.6 0.1 1.4 0.2 2.3 0.1
0.3 0.1 0.3 0.1 0.3 0.1 — — 0.2 0.1 0.2 0.1 0.5 0.5 0.3 0.1
0.0 0.0 0.0 0.0 0.0 0.0 — — 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 — — 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
93.6 0.6 94.2 0.6 93.8 0.3 — — 94.0 0.4 93.3 0.4 93.2 0.5 94.8 0.7
53.2 0.5 52.8 1.1 53.2 0.4 — — 53.3 0.4 52.9 0.2 52.5 0.4 53.4 0.5
35.2 0.4 35.3 0.5 35.3 0.3 — — 35.5 0.2 35.2 0.3 35.0 0.5 35.2 0.5
98.9 0.6 99.5 0.6 99.2 0.3 — — 99.4 0.5 98.6 0.4 98.5 0.5 100.1 0.7
19.4 0.6 20.1 1.5 19.3 0.6 — — 19.4 0.4 19.6 0.4 19.7 0.6 20.0 0.7
(1) Number of titanomagnetite grains with typical chemical composition as KMT; (2) Number of titanomagnetite grains with the total weight percentage of 98-102 % recalculated under the condition that the grains lie on the ulvospinel-magnetite join; (3) Total number of analyses. Chemical compositions were averaged of titanomagnetite for (1). Standard deviations are shown below average value. The apparatus for measurement is shown in Table 1.
Table 3 Radiometric ages of KMT determined by previous studies Locality
Material
Method
Age (Ma)
Reference
Loc. 1 Loc. 1 — Loc. 1 Loc. 1 Loc. 1 Kaisho source vent Kaisho source vent Loc. 5 1.6 km NE of Loc. 6 1.6 km NE of Loc. 6 500 m SE of Loc. 6 — — Loc. 7 Loc. 7 Loc. 8 Loc. 2 Loc. 3
PFD PFD PFD PFD PFD PFD WT WT AFA AFA AFA AFA AFA AFA AFA AFA AFA AFA AFA
FT FT (zeta) FT (zeta) K-Ar K-Ar K-Ar FT FT (zeta) FT (zeta) FT FT FT (zeta) K-Ar K-Ar FT FT (zeta) FT (zeta) FT (zeta) FT (zeta)
0.9270.11 0.5570.07 0.6970.07 0.6370.25 0.6270.06* 0.6370.06* 1.0570.1 0.5370.06 0.6470.06 0.4970.07 0.5370.04 0.6070.04 0.6470.03* 0.6570.03* 0.8370.07 0.5170.05 0.6670.04 0.7470.09 0.6670.09
Yamada et al. (1985) Suzuki et al. (1998a) Harayama et al. (1997) Shibata and Yamada (1977) Watanabe et al. (1999) Watanabe et al. (1999) Harayama (1990) Danhara and Iwano (1998) Suzuki et al. (1998a) Tokuhashi et al. (1983a) Tokuhashi et al. (1983b) Watanabe and Danhara (1996) Watanabe et al. (1999) Watanabe et al. (1999) Koike et al. (1985) Danhara and Iwano (1998) Suzuki et al. (1998b) Suzuki et al. (1998a) Suzuki et al. (1998a)
*Ages were not used for the estimation of KMT age in Suzuki (2000); —: not described in references; PFD: pyroclastic flow deposit; WT: welded tuff; AFA: ash-fall deposit; FT: fission track dating method; zeta: FT age with zeta calibration; K-Ar: Potassium-Argon dating using biotite phenocrysts.
pyroclastic flow deposit. They derived weight-mean ages of 0.6570.02 Ma (distal ash-fall deposits) and 0.6370.04 Ma (pyroclastic flow deposit) equivalent to 6–3% of the age, smaller errors than most of the radiometric ages referred to in Suzuki (2000). Moreover, Nakazato (2006) determined a more secure age for the KMT using stratigraphic position of KMT, a high-resolution oxygen-isotope stratigraphy, and calcareous nannofossil biostratigraphy in the Inubo Group, the most east part of the Kanto Plain (Fig. 1). This recent age
estimation shows that the age of KMT is positioned around 0.62 Ma, where marine isotope stage (MIS) 16.0 presents (Fig. 3), concordant with the weight-mean K-Ar age of 0.6370.04 Ma (pyroclastic flow deposit) by Watanabe et al. (1999). Although this age is a little younger than the weight-mean K-Ar age of 0.6570.02 Ma (distal ash-fall deposits) by Watanabe et al. (1999) and an estimated age of 0.64 Ma by Harayama (2005), Nakazato (2006)’s age of around 0.62 Ma is adopted in the discussion of this study.
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Paleomagnetic polarity delta18O
0
(‰) 2 1 0- 1- 2- 3
Tama River
Kinu River Naka River Younger Fan Main Fan
Tc1-3 M3 M2
0.1
5.5
6.2
M1 S
7.1
0.2 8.2
Ma
10.2
9.3
0.4
Hoshakuji
Brunhes
0.3
Kobiki
11.3 12.2
0.5
13.3 Kitsuregawa Lower
15.1
0.6
KMT 16.2
Azuyama, Sayama
Kitsuregawa Upper
17.3 18.2 19.3
0.8
Matuyama
0.7
Terrace surface Marker tephra
Fig. 3. Chronology of terrace surfaces in and around the Kanto Plain during the late Quaternary. Compiled by Minagawa and Machida (1971), Akagi and Koike (1978), and Kaizuka et al. (2000). The d18O curve is after Bassinot et al. (1994).
3. Discussion 3.1. Correlation of fluvial terrace surfaces The stratigraphic positions of the KMT in the terrace sequence indicate that the upper dissected fluvial terrace surfaces Azuyama, Sayama, and Kitsuregawa Upper are correlative. They were formed a few tens of thousands of years prior to the deposition of the KMT (Fig. 3). This is concordant with a paleomagnetic study conducted by Ueki (2006) reporting Brunhes Chron paleomagnetic polarity of tephric soil deposits between KMT and the Imokubo Gravel Bed (approximately 1 m in thickness) on the Sayama terrace surface. The distributions of the Azuyama and Sayama terrace surfaces, originally continuous, suggest that a broad alluvial fan of the Tama River, the origin of these surfaces, had developed in the area at least from the Azuyama hills to the Sayama hills (Figs. 4 and 5). Both the width and angle of this fan were larger than those of several late Pleistocene fans (Kaizuka et al., 2000) formed during MIS 6–5.5 (S terrace surface); MIS 5.1–4 (M2, M3 terrace surfaces); and MIS 3–2 (Tc1 3 terrace surface group). The reconstructed middle Pleistocene fan morphology is comparable to the M1 terrace surface (MIS 5.3) the broadest late Pleistocene fan surface.
In the north part of the Kanto Plain, a prominent fan surface of the Kitsuregawa Upper terrace surface along the Naka and Kinu Rivers had emerged a few tens of thousands of years preceding the deposition of the KMT. Although the depositional surface of the Kitsuregawa Upper terrace surface was fairly dissected, it is well preserved as a broad hill. The area of this terrace surface is larger than other younger alluvial fan surfaces in this area. The presence of these well-developed alluvial fans seems to indicate that there was a long and stable duration of favorable conditions for the formation of broad fans. 3.2. Condition and timing of the formation of broad fan surfaces Generally, in the southwest part of the Kanto Plain, where flights of emerged fan surfaces of different altitudes and ages are developed in the non-glaciated catchments, the present river beds in deeply excavated narrow valleys show deep incision (Fig. 4), as do the river beds of the last interglacial period (MIS 5.5) (Kaizuka et al., 2000). In contrast to this, several alluvial fans formed in the transitional stage from MIS 5.5 to MIS 2, the culmination of the last glacial stage, generally developed with longitudinal profiles characterized by steeper gradient and curvature. Typical terrace surfaces along the Tama River
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Fig. 4. Oblique aerial view of the Sayama and Azuyama hills originated from a continuous broad alluvial fan, which emerged between MIS 17.3 and 16.2, and the Tama River in excavated narrow valley. Taken from the south.
are M1 (MIS 5.3), M2 (MIS 5.1), M3 (MIS 4), and the Tc1 3 terrace surface group (MIS 3-2) (Kaizuka et al., 1977, 2000). These alluvial fan deposits originated from the sediments transported by the extended paleo-Tama River, flowing on an emerged delta and coastal plain associated by elevated sea level during MIS 5.5. The estimated age of KMT (ca. 0.62 Ma) and the stratigraphic relation of this tephra with the Azuyama, Sayama and Kitsuregawa Upper terrace surfaces suggests that these terrace surfaces emerged between MIS 17.3 and 16.2 (0.688–0.628 Ma). The period of MIS 17.3 to 16.2 is a remarkable transitional period from intense interglacial to intense glacial, similar to the transitions from MIS 5.5 to 2, 9.3 to 8.2, and 11.3 to 10.2. Thus, the alluvial fan of Azuyama and Sayama terrace surfaces most likely formed under similar geomorphic conditions as did the M1 to M3 terrace surfaces and Tc1 3 surface group. This is concordant with the similar characteristics of alluvial fan deposits. Thicknesses of the alluvial fan deposits forming the Azuyama and Sayama terrace surfaces are 10–12 m (Machida, 1973) and 9 m (Hatori and Juen, 1958), respectively. These thicknesses are similar to those of M1 to M3 terrace surfaces ranging 3–10 m, and are thicker than those of Tc1 3 surface group ranging 1.5–4 m (Machida, 1973, 1999b). Therefore, it is inferred that the original alluvial fan of the Azuyama and Sayama terrace surface was formed in a regressive stage after the peak of MIS 17.3. On the other hand, along the middle reach of the Naka River, prominent fluvial terrace surfaces originated from alluvial fans formed during MIS 3 to 2 (Main Fan surface) and MIS 2 (Younger Fan surface) (Fig. 3) (Akagi and Koike, 1978). These are well developed, and the present
river bed shows deep incision in the excavated narrow valley cutting these terraces. This landform development, similar to that of the southwest part of the Kanto Plain, indicates that the formation of broad alluvial fans represents a transitional stage from interglacial to glacial. Therefore, the alluvial fan deposits constituting the Kitsuregawa Upper terrace surface formed under the similar geomorphic conditions during the period from MIS 17.3 to 16.2. Suzuki (2000) concluded that the Kitsuregawa Upper terrace surfaces terraced during the termination from MIS16.2 to MIS15.1. This conclusion was based on the broad estimated age of the KMT to be 0.58–0.69 Ma. However, the adopted age in this study (ca. 0.62 Ma) indicates that the Kitsuregawa Upper terrace surfaces had formed and terraced during the termination from MIS 17.3 to MIS16.2. Despite the age of the Sayama terrace surface and its correlative, they are well preserved. This is most likely caused by the development of the original wide fan surface, a more acceptable explanation than suggesting that preservation from lateral erosion was by chance. This interpretation suggests that there was a long duration with stable conditions favorable for graded river development, associated with strong lateral erosion and minor changes of the river bed in altitude. A problem left in the study of geomorphic development in the middle Pleistocene is whether the transitional stage of MIS 17.3 to 16.2 was a specific event in the formation of middle Pleistocene landforms. Further research is needed to determine whether formation of the prominent alluvial fans of this age is general in the area affected by mid-latitude oceanic climates, such as the Japanese Islands.
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Mountains
Kasamori formation
Kitsuregawa hills
Quaternary volcano
Naka River
Kinu Rive r
Kanto Mountains Azuyama hills
Sayama hills Tokyo Tokyo Bay Tama River
1 2 3
Boso Peninsula
Sagami Bay
Hayama-Mineoka Island
4 5 6
0
25 km
50
Fig. 5. Paleogeography in and around the Kanto Plain before and after the deposition of KMT. Modified after Suzuki (2000). 1: paleo-shoreline; 2: present shoreline and boundary of lowland, terraces and hills; 3: Kazusa Trough; 4: hills formed preceding marine isotope stage 17; 5: Sayama terrace surface and its correlative terrace surfaces; 6: estimation of original distribution of Imokubo Gravel Bed and its correlative gravel bed.
4. Conclusions For the identification of strongly weathered KMT in aeolian sediments, the chemical composition of titanomagnetite was determined by electron probe microanalysis. Although volcanic glass shards are intensely altered or non-existent, the chemical composition of titanomagnetite, combined with uranium content measurements in zircon grains, are useful for the identification of strongly weathered KMT. The chemical composition of titanomagnetite is effective for the discrimination of weathered middle Pleistocene tephra in the Japanese Islands affected by mid-latitude oceanic climate. Evaluating all published age data for the KMT, the age of KMT is around 0.62 Ma (MIS 16.0). The geomorphic history of the middle Pleistocene in the Kanto Plain was tephrochronologically reconstructed.
Correlation of dissected fluvial terrace surfaces originating from broad alluvial fans, the Azuyama and Sayama terrace surfaces in southwest Kanto and the Kitsuregawa Upper terrace surface in north Kanto, suggests the presence of stable conditions for the formation of broad fans during the transition from interglacial (MIS 17.3) to glacial (MIS 16.2). Acknowledgments I wish to dedicate this paper to Professor John Westgate in commemoration of his retirement from the University of Toronto. Part of this study was financially supported by Grants-in-Aid for Science Research from the Ministry of Education, Science, Sports, and Culture (No. 09780137: Chemical analysis of major elements on titanomagnetite by EPMA and its applicative study for identification of tephra).
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References Akagi, T., Koike, K., 1978. Reconsideration of geomorphic development of fans in Nasunogahara. Abstract, Association of Japanese Geographers 14, 256–257 (in Japanese). Andersen, D.J., Bishop, F.C., Lindsley, D.H., 1991. Internally consistent solution models for the Fe–Mg–Mn–Ti oxides: Fe–Mg–Ti oxides and olivine. American Mineralogist 76, 427–444. Andersen, D.J., Lindsley, D.H., 1988. Internally consistent solution models for Fe–Mg–Mn–Ti oxides. American Mineralogist 73, 714–726. Bassinot, F.C., Labeyrie, L.D., Vincent, E., Quidelleur, X., Shackleton, N.J., Lancelot, Y., 1994. The astronomical theory of climate and the age of the Brunhes—Matuyama magnetic reversal. Earth and Planetary Science Reviews 126, 91–108. Cas, R.A.F., Wright, J.V., 1988. Volcanic Successions. Modern and Ancient. Chapman & Hall, London. Danhara, T., Iwano, H., 1998. Zircon fission-track ages of the Omachi Tephras in central Japan: a re-investigation (part 2). Abstracts of the 105th Annual Meeting of the Geological Society of Japan, p. 318 (in Japanese). Harayama, S., 1990. Geological Map of Kamikochi District and Explanatory Text. Geological Survey of Japan, Tsukuba (in Japanese). Harayama, S., 2005. Kamitakara-Kaisho volcano (Kamitakara pyroclastic flow deposits). In: Editorial Committee of Enlarged Edition of Regional Geology of Japan (Ed.), Enlarged Edition of Regional Geology of Japan. Kyoritsu Shuppan, Tokyo, pp. 194–195 (in Japanese). Harayama, S., Takahashi, M., Hoshi, H., 1997. Paleomagnetism of the Pleistocene Kamitakara Pyroclastic Flow Deposit in the Takayama area central Japan. Abstracts 1997 Japan Earth and Planetary Science Joint Meeting, p. 767 (in Japanese). Hatori, K., Juen, S., 1958. The Quaternary history of the western margin of Kanto Basin (II): the geology and topography of the Sayama, Kazumi hilly-lands. Journal of the Geological Society of Japan 64, 232–249 (in Japanese). Kaizuka, S., Koike, K., Endo, K., Yamazaki, H., Suzuki, T., 2000. Regional geomorphology of the Japanese Islands. Geomorphology of Kanto and Izu-Ogasawara. vol. 4. University of Tokyo Press, Tokyo (in Japanese). Kaizuka, S., Naruse, Y., Matsuda, I., 1977. Recent formation and their basal topography in and around Tokyo Bay, central Japan. Quaternary Research 8, 32–50. Koike, K., Iwasaki, T., Danhara, T., Momose, M., 1985. Fission-track ages and their geological meaning of Quaternary volcanic ashes (Shimotsuke-loam) covering the Kitsuregawa hills, Tochigi Prefecture, central Japan. Komazawa Geography 21, 39–67 (in Japanese). Litchfield, N.J., Berryman, K.R., 2005. Correlation of fluvial terraces within the Hikurangi Margin, New Zealand: implications for climate and baselevel controls. Geomorphology 68, 291–313. Machida, M., 1973. Tephrochronological study of the northern part of the Musashino Plateau and its adjacent areas. Journal of the Geological Society of Japan 79, 167–180 (in Japanese). Machida, H., 1975. Pleistocene sea-level of south Kanto, Japan, analysed by tephrochronology. In: Suggate, R.P., Cresswell, M.M. (Eds.), Quaternary Studies. The Royal Society of New Zealand, Christchurch, pp. 215–222. Machida, H., 1999a. The stratigraphy, chronology and distribution of distal marker tephras in and around Japan. Global and Planetary Change 21, 71–94.
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Machida, M., 1999b. Quaternary geology in the central to western part of the Kanto tectonic basin. Saito-Ho-on Kai Museum Research Bulletin 67, 1–93. Minagawa, K., Machida, M., 1971. Stratigraphy of Tama Loam in south Kanto. Earth Science 25, 164–176 (in Japanese). Nakazato, H., 2006. Horizon and age of Kaisho-Kamitakara tephra from the Inubo Group in the northeastern part of Chiba Prefecture, central Japan. Programme and Abstracts, Japan Association for Quaternary Research 36, 106–107 (in Japanese). Shane, P., 2000. Tephrochronology: a New Zealand case study. EarthScience Reviews 49, 223–259. Shibata, K., Yamada, N., 1977. K-Ar ages of the Takara volcanic rocks and the Ueno basalt from the eastern part of Gifu Prefecture, central Japan. Earth Science (Chikyu Kagaku) 31, 15–18 (in Japanese). Shimizu, Y., Horiguchi, M., 1994. Tama Loam located in the western part of Kanto Plain. The Journal of College of Liberal Arts, Saitama University 29, 107–119 (in Japanese). Simkin, T., Siebert, L., 1994. Volcanoes of the World, second ed. Smithsonian Institution, Washington, DC. Suzuki, T., 1996. Chemical analysis of volcanic glass by energy dispersive X-ray spectrometry with JEOL JED-2001 and JSM-5200: analytical procedures and application. Geographical Reports of Tokyo Metropolitan University 31, 27–36. Suzuki, T., 2000. Kaisho-Kamitakara tephra erupted from the Hida Mountains in early half of middle Pleistocene and its significance for geomorphic chronology of central Japan. Geographical Review of Japan 73, 1–25 (in Japanese). Suzuki, T., Fujiwara, O., Danhara, T., 1998a. Fission track of several Quaternary tephras in Kanto and Chubu regions, central Japan. Journal of Geography 107, 348–367 (in Japanese). Suzuki, T., Fujiwara, O., Danhara, T., 1998b. Fission track of ages eleven Quaternary tephras in north Kanto and south Tohoku regions, central Japan. The Quaternary Research 37, 95–106 (in Japanese). Tokuhashi, S., Danhara, T., Endo, H., Isoda, K., Nishimura, S., 1983a. Some experiments and problems on fission-track dating of geologically younger-age samples. Bulletin of the Geological Survey of Japan 34, 241–269 (in Japanese). Tokuhashi, S., Danhara, T., Endo, H., Isoda, K., Nishimura, S., 1983b. Fission-track dating of Ks22 in the Kasamori formation of Shimosa Group. Abstracts of the 90th Annual Meeting of the Geological Society of Japan, p. 105 (in Japanese). Tokuhashi S., Endo, H., 1984. Geological Map of Anesaki District and Explanatory Text. Geological Survey of Japan, Tsukuba (in Japanese). Ueki, T., 2006. Paleomagnetic age-estimate of the Kamikayama and Sayama surfaces along the western side of Kanto Plain, central Japan. The Quaternary Research 45, 81–97 (in Japanese). Watanabe, M., Danhara, T., 1996. Fission track ages of volcanic ash layers of the Kazusa Group in the Boso Peninsula, central Japan. Journal of the Geological Society of Japan 102, 545–556 (in Japanese). Watanabe, M., Takahashi, M., Harayama, S., Hoshi, H., Itoh, Y., 1999. K-Ar ages and paleomagnetism of a Pleistocene ash fall deposit (Ks22: Boso) and its source ignimbrite (Kamitakara: Takayama). Abstracts of the 106th Annual Meeting of the Geological Society of Japan, p. 196. Yamada, N., Adachi, M., Kajita, S., Harayama, S., Yamazaki, H., Bunno, M., 1985. Geological Map of Takayama District and Explanatory Text. Geological Survey of Japan, Tsukuba (in Japanese). Yoshikawa, T., Kaizuka, S., Ota, Y., 1981. The Landforms of Japan. University of Tokyo Press, Tokyo.
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Analysis of titanomagnetite within weathered middle Pleistocene KMT tephra and its application for fluvial terrace chronology, Kanto Plain, central Japan Takehiko Suzuki Department of Geography, Tokyo Metropolitan University, Minamiosawa 1-1, Hachioji, Tokyo 192-0397, Japan Available online 12 December 2006
Abstract The widespread Kaisho-Kamitakara tephra (KMT), derived from the source vent located in the Hida Mountains, is a favorable marker tephra for the chronological study of the middle Pleistocene in the Kanto Plain, central Japan. As the KMT in aeolian sediments is strongly weathered, the chemical composition of titanomagnetite, resistant to weathering, was determined for its identification. The age of the KMT was estimated at 0.62 Ma (MIS 16.0) by Nakazato [2006. Horizon and age of Kaisho-Kamitakara tephra from the Inubo Group in the northeastern part of Chiba Prefecture, central Japan. Programme and Abstracts, Japan Association for Quaternary Research 36, 106–107 (in Japanese)], revising a previous estimate of 0.58–0.69 Ma by Suzuki [2000. Kaisho-Kamitakara tephra erupted from the Hida Mountains in early half of middle Pleistocene and its significance for geomorphic chronology of central Japan. Geographical Review of Japan 73, 1–25 (in Japanese)] The stratigraphic position of the KMT in the Azuyama and Sayama terrace surfaces in southwest Kanto and the Kitsuregawa Upper terrace surface in north Kanto indicates that these terraces are correlative. It is assumed that the dissected fluvial terrace surfaces were formed a few tens of thousands of years prior to the deposition of the KMT. Alluvial fan deposits constituting these terrace surfaces were formed during the period from MIS 17.3 to 16.2, during a transition from interglacial to glacial. Geomorphic conditions during this period of the middle Pleistocene were stable, allowing the formation of broad fans. r 2006 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Tephra layers resulting from volcanic eruptions provide stratigraphic markers that assist in classifying terrace surfaces and sediments. Many studies have discussed the formation of fluvial terrace surfaces in response to either climate or sea level control using tephrochronology (e.g. Litchfield and Berryman, 2005). An advantage to studying fluvial terrace surfaces in the Japanese Islands, a volcanic island arc, is the ability to use tephrochronology. The Kanto Plain (Fig. 1), the widest in the Japanese Islands, is regarded as one of the standard areas for Japanese Quaternary tephra study (Yoshikawa et al., 1981). Intensive studies of the tephra and the marine and fluvial terrace surfaces in this area have revealed the detailed geomorphic development during the late PleistoTel.: +81 426 77 2590; fax: +81 426 77 2589
E-mail address: [email protected].
cene (Machida, 1975; Kaizuka et al., 1977). However, until the 1990s the chronological framework of the middle Pleistocene fluvial terrace surfaces had not been established in detail, despite considerable tephrochronological studies on the middle Pleistocene marine sequence in the Oiso hills and Boso Peninsula. The existing chronological studies had not confirmed the relationship of the fluvial terrace surfaces to oxygen marine isotope stages, in contrast to the established late Pleistocene chronostratigraphy. Tephrochronological studies focusing on the middle Pleistocene widespread tephra layers advanced in the 1990s (Machida, 1999a), providing several precise datum planes for this period covering the Kanto Plain. The widespread middle Pleistocene Kaisho-Kamitakara tephra (KMT), derived from the source vents located in the Hida Mountains (Fig. 1), central Japan, was recognized in marine strata on the Boso Peninsula. This played a significant role in the reconstruction of the geomorphic development in coastal to inland areas of the Kanto Plain
1040-6182/$ - see front matter r 2006 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2006.10.039
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T. Suzuki / Quaternary International 178 (2008) 119–127
Fig. 1. Satellite image of the Kanto Plain and the adjacent area showing the distribution and localities of Kaisho-Kamitakara tephra (KMT). Closed circles and numerals indicate localities where KMT was identified and locality numbers, respectively. Open circle indicates locality where Nakazato (2006) determined the age of KMT.
(Suzuki, 2000). However, in many cases, the KMT in aeolian sediments including tephric soil deposits (tephric loess) had been strongly weathered. Chemical weathering affected by the mid-latitude oceanic climate causes difficulties in identifying the KMT. Where volcanic glass shards are intensely altered or non-existent, glass geochemistry is not available for identification. In this study, the chemical composition of titanomagnetite was used for the identification of the KMT. Titanomagnetite is moderately stable under weathering, and is abundant in weathered KMT in tephric soil deposits. Additionally, although the measurement of the refractive indices of orthopyroxene and hornblende has been a conventional method for tephra characterization in Japan, this method is not applicable to the identification of the KMT. Plagioclase, biotite, and titanomagnetite are the dominant minerals, although orthopyroxene and hornblende are rarely present. Many studies of the chemical composition of titanomagnetite have been carried out since the 1960s. Using ilmenite and titanomagnetite, a combination of temperature and oxygen fugacity was estimated from their chemistry (Andersen and Lindsley, 1988; Andersen et al., 1991). Shane (2000) pointed out that these compositions are useful for the discrimination of tephras in New Zealand.
This paper demonstrates the identification of KMT mainly through chemical analysis of titanomagnetite, as well as by other methods. A tephrochronologically reconstructed geomorphic history of the middle Pleistocene in the Kanto Plain is presented, with special reference to sea level and climate changes. 2. Kaisho-Kamitakara tephra The KMT, erupted from the Kaisho source vent in the southwest part of the Hida Mountains in central Japan, is composed of the Kamitakara Pyroclastic Flow Deposit (440 km3) and a fallout tephra (440 km3) distributed from the Chubu to the south Tohoku areas, 300 km from the source (Harayama, 1990; Suzuki, 2000) (Fig. 1). The volcanic explosivity index (VEI; Simkin and Siebert, 1994) of the eruption forming the KMT is 6, indicating a paroxysmal eruption. 2.1. Characteristics of the proximal and distal deposits of KMT Analysis of individual glass shards by electron probe microanalysis (EPMA) is effective for the identification of tephra. Glass geochemistry and other characteristics indicate that the Kamitakara pyroclastic flow deposit of
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Locality 1 in the proximal area and distal deposits of Locality 6 in the Boso Peninsula are correlative. At Locality 1, the basal part comprises more than 3.5 m of non-welded pyroclastic flow deposit, which underlies strongly welded pyroclastic flow deposit up to 40 m thick. At Locality 6, the Kasamori 22 tephra (Ks 22) (Tokuhashi and Endo, 1984) in the Kasamori formation of the Plio-Pleistocene Kazusa Group forearc basin-fill deposits crops out as a gray glassy ash layer of 9 cm in thickness. They are composed of colorless glass shards; abundant biotite, titanomagnetite, plagioclase, and quartz; and small amounts of zircon, orthopyroxene, and hornblende. The major element compositions of glass shards in samples collected from the matrix of the basal part of the Kamitakara pyroclastic flow deposit at Locality 1 in the proximal area and from the well-preserved distal ash-fall deposit in the marine sediments of Locality 6, were determined by EPMA (Suzuki, 2000) (Table 1). The mean SiO2 content of 76.9–77.1 wt% and mean K2O content of 5.2–5.5 wt%, recalculated on a volatile-free basis, indicate a high-silica rhyolite with high alkali content. This major element probe data presented for two beds from separate locations (Localities 1 and 6) demonstrates their correlation to each other. Moreover, refractive indices of glass shards ranging within 1.497–1.500 (Suzuki, 2000) and the presence of zircon grains with high uranium content, 450 ppm (Locality 1) (Suzuki et al., 1998a) and 490 ppm (Locality 6) (Watanabe and Danhara, 1996) support their correlation. On the other hand, glass shards collected from the distal ash-fall deposits (Locality 6) are mostly pumice-type shards, in contrast to the abundance of bubble-walled shards in the matrix of the pyroclastic flow deposit (Locality 1) in the proximal area. This suggests that the emplacement of distal ash-fall deposits of KMT were associated with a Plinian eruption, and they are not coignimbrite ash-fall deposits (Cas and Wright, 1988). The mode of eruption and the recognition of KMT in the Boso Peninsula, 250 km southeast from the source, suggest the widespread occurrence of KMT in the whole area of the Kanto Plain.
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2.2. Correlation of the weathered KMT in tephric soil deposits Titanomagnetite chemistry analysis was applied to seven extremely weathered, clay-rich distal ash-fall deposits and to the pyroclastic flow deposit of Locality 1. These weathered tephras have no remaining volcanic glass shards, although they do contain some sand-sized titanomagnetite grains and abundant biotite. The abundant visible biotite grains are lithologically unusual. Previous studies named this tephra the Bio 1 (Locality 4; Shimizu and Horiguchi, 1994), the Biotite Zone (Locality 5; Minagawa and Machida, 1971) and the Hoshitoge Biotite tephra (HtB) (Localities 7–9; Koike et al., 1985). The stratigraphic positions of these tephra layers are shown in Fig. 2. The Biotite Zone is located above the Imokubo Gravel Bed forming the Sayama terrace surface in the Sayama hills. The Bio 1, correlated to the Biotite Zone by Shimizu and Horiguchi (1994), is located above the Upper Tooyoka Gravel Bed forming the Azuyama terrace surface in the Azuyama hills. The Hoshitoge Biotite tephra is located above the Sakaibayashi Gravel Bed forming the Kitsuregawa Upper terrace surface in the Kitsuregawa hills. The major element compositions of the titanomagnetite in samples were determined by EPMA (Table 2). To examine analysis and correlation of KMT, two points were checked. Firstly, the content of titanomagnetite grains with total weight percentage of 98–102%, recalculated under the condition that the grains lie on the ulvospinel-magnetite line, was considered. Secondly, the content of the grains with typical KMT chemical composition as KMT was considered. Despite pyroclastic flow deposit, 22 grains in 24 grains in the sample of collected from the basal part of the Kamitakara pyroclastic flow deposit at Locality 1 have total weight percentage of 98–102% on the ulvospinelmagnetite line. This indicates that the basal part was cooled immediately after the deposition, showing non-oxidation accompanying slow cooling. Consequently, the compositions of the titanomagnetite in samples at Locality 1 are available for identification. All collected samples except sample of Locality 5 had grains that were chemically very similar to those in the KMT. All grains from Locality 2 are
Table 1 Chemical composition of glass shards in KMT Tephra (Locality) Ks 22: upper part (Loc. 6: Senda, Chonan, Chiba) Ks 22: middle part (Loc. 6: Senda, Chonan, Chiba) Kamitakara pfl.: lower (Loc. 1: Sugo, Takayama, Gifu)
mean st. dev. mean st. dev. mean st. dev.
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
K2O
Na2O
Total
Analytical total
n
77.1 0.1 77.1 0.2 76.9 0.3
0.1 0.0 0.1 0.0 0.1 0.0
12.6 0.1 12.6 0.1 12.6 0.1
0.9 0.1 0.9 0.1 0.8 0.2
0.1 0.1 0.1 0.1 0.1 0.1
0.2 0.0 0.3 0.0 0.3 0.0
0.5 0.0 0.5 0.0 0.6 0.1
5.3 0.1 5.2 0.1 5.5 0.1
3.3 0.1 3.3 0.1 3.1 0.1
100.0
98.5 1.1 96.5 4.5 93.6 1.1
12
100.0 100.0
13 15
Reference: Suzuki (2000) FeO: total irons as FeO. Analyses recalculated to 100% on a volatile-free basis and presented as a mean and standard deviation of n shard analyses. Determined by a JEOL JSM-5200 (scanning electron microscope) and a JEOL JED-2001 energy dispersive X-ray spectrometry using a 0.3 nA current at 15 kV and a 10 mm beam diameter. Method and standards as in Suzuki (1996).
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8
1
0 0
Scale for Loc. 8 Kamitakara pyroclastic flow deposit
3
11 5m
5 10m
10
2
12
4 6
9 KSF KMT
NAG
Misawa, Takayama
Nashinoki, Shiojiri
Pumice lapilli Scoria lapilli Volcanic ash Pyroclastic flow deposit (welded) Pyroclastic flow deposit (non-welded)
IM Senda, UTG Chonan Kawabe, Kanekosaka, Tokorozawa Iruma
Brown tephric soil deposits Cracked zone
SAG
Adachi, Ittochi
Kamako, Higashi Yoshii, Ogawa
SAG
Yanoto, Nihonmatsu
Kabata, Otawara
Silt Gravel Basement rock
Fig. 2. Stratigraphic position of KMT. Locality is shown in Fig. 1. Columnar section of Locality 4 is after Shimizu and Horiguchi (1994). NAG: Nashinoki Gravel Bed; UTG: Upper Toyooka Gravel Bed; IMG: Imokubo Gravel Bed; KSF: Kasamori Formation; SAG: Sakaibayashi Gravel Bed.
of the KMT type. However, from Localities 8, 10–12, only 21–47% of grains were of the KMT type. In these cases, the thickness of KMT is less than 10 cm and blocks of ash are dispersed in tephric soil deposits. The small KMT content is explained by contamination of grains from the upper and lower horizons. A further fingerprinting method used the content of uranium in zircon presented in previous studies. Despite the low primary zircon content, it was well preserved due to its stability under weathering. A significant characteristic of the KMT is the presence of zircon grains with high uranium content: 450 ppm (Locality 1), 410 ppm (Locality 2), 480 ppm (Locality 3) (Suzuki et al., 1998a), 440 ppm (Locality 8) (Suzuki et al., 1998b), and 490 ppm (Locality 6) (Watanabe and Danhara, 1996). These values range between 410 and 490 ppm, confirming their correlation. Additionally, a dispersed and weathered biotite-rich tephra named the Biotite Zone, exposed as a 10 cm thick tephric soil deposits bearing abundant visible biotite grains (Locality 5), contains zircon grains with high uranium
content of 470 ppm (Suzuki et al., 1998a). Although KMT type titanomagnetite grains were not detected in this tephra, the content of uranium and presence of abundant biotite grains show that this tephra is correlative to KMT. The absence of KMT type titanomagnetite grains is most likely caused by strong contamination. 2.3. Age of the KMT Suzuki (2000) determined the age of the KMT to be 0.58–0.69 Ma using many radiometric ages from previous studies (Table 3) and the stratigraphic position in the marine sequence of Boso Peninsula. The broad range of this estimated age was caused by large errors in radiometric dating and uncertainty of the age determination on the marine sediments. Recent dating by Watanabe et al. (1999) has revised the estimated age of Suzuki (2000). Watanabe et al. (1999) determined K-Ar ages of 0.6470.03 and 0.6570.03 Ma for the distal ash-fall deposits in Boso Peninsula and 0.6270.06 and 0.6370.06 Ma for the
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Table 2 Chemical composition of titanomagnetite in KMT Locality
Usp. (1) / (2) SiO2 TiO2 Al2O3 V2O3 FeO MnO MgO CaO ZnO Analytical Ulvospinel basis FeO Total Mol. % (3) total Fe2O3
Loc. 1//Sugo, Takayama, Gifu pyroclastic flow deposit Loc. 2//Nashinoki, Shiojiri, Nagano ash-fall deposit Loc. 3//Misawa, Takayama, Gunma ash-fall deposit Loc. 5//Kawabe, Tokorozawa, Tokyo ash-fall deposit Loc. 8//Yoshii, Ogawa, Tochigi ash-fall deposit Loc. 10//Kamako, Higashi, Fukushima ash-fall deposit Loc. 11//Yanoto, Nihonmatsu, Fukushima ash-fall deposit Loc. 12//Ittochi, Adachi, Fukushima ash-fall deposit
20 24 20 20 14 30 0 14 6 30 8 30 6 30 8 22
/ 22 0.5 0.1 / 20 0.5 0.1 / 22 0.5 0.1 /8 — — / 28 0.6 0.0 / 17 0.5 0.0 / 17 0.6 0.1 / 19 0.5 0.0
6.0 0.2 6.3 0.5 6.1 0.2 — — 6.1 0.1 6.1 0.2 6.1 0.2 6.3 0.3
1.4 0.1 1.5 0.1 1.5 0.1 — — 1.4 0.1 1.4 0.1 1.6 0.5 1.4 0.1
0.7 0.1 0.7 0.1 0.7 0.1 — — 0.7 0.1 0.7 0.1 0.7 0.2 0.7 0.1
83.1 0.6 82.9 0.7 83.1 0.4 — — 83.4 0.5 82.8 0.3 82.3 0.8 83.2 0.6
1.6 0.1 1.9 0.1 1.6 0.2 — — 1.7 0.1 1.6 0.1 1.4 0.2 2.3 0.1
0.3 0.1 0.3 0.1 0.3 0.1 — — 0.2 0.1 0.2 0.1 0.5 0.5 0.3 0.1
0.0 0.0 0.0 0.0 0.0 0.0 — — 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 — — 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
93.6 0.6 94.2 0.6 93.8 0.3 — — 94.0 0.4 93.3 0.4 93.2 0.5 94.8 0.7
53.2 0.5 52.8 1.1 53.2 0.4 — — 53.3 0.4 52.9 0.2 52.5 0.4 53.4 0.5
35.2 0.4 35.3 0.5 35.3 0.3 — — 35.5 0.2 35.2 0.3 35.0 0.5 35.2 0.5
98.9 0.6 99.5 0.6 99.2 0.3 — — 99.4 0.5 98.6 0.4 98.5 0.5 100.1 0.7
19.4 0.6 20.1 1.5 19.3 0.6 — — 19.4 0.4 19.6 0.4 19.7 0.6 20.0 0.7
(1) Number of titanomagnetite grains with typical chemical composition as KMT; (2) Number of titanomagnetite grains with the total weight percentage of 98-102 % recalculated under the condition that the grains lie on the ulvospinel-magnetite join; (3) Total number of analyses. Chemical compositions were averaged of titanomagnetite for (1). Standard deviations are shown below average value. The apparatus for measurement is shown in Table 1.
Table 3 Radiometric ages of KMT determined by previous studies Locality
Material
Method
Age (Ma)
Reference
Loc. 1 Loc. 1 — Loc. 1 Loc. 1 Loc. 1 Kaisho source vent Kaisho source vent Loc. 5 1.6 km NE of Loc. 6 1.6 km NE of Loc. 6 500 m SE of Loc. 6 — — Loc. 7 Loc. 7 Loc. 8 Loc. 2 Loc. 3
PFD PFD PFD PFD PFD PFD WT WT AFA AFA AFA AFA AFA AFA AFA AFA AFA AFA AFA
FT FT (zeta) FT (zeta) K-Ar K-Ar K-Ar FT FT (zeta) FT (zeta) FT FT FT (zeta) K-Ar K-Ar FT FT (zeta) FT (zeta) FT (zeta) FT (zeta)
0.9270.11 0.5570.07 0.6970.07 0.6370.25 0.6270.06* 0.6370.06* 1.0570.1 0.5370.06 0.6470.06 0.4970.07 0.5370.04 0.6070.04 0.6470.03* 0.6570.03* 0.8370.07 0.5170.05 0.6670.04 0.7470.09 0.6670.09
Yamada et al. (1985) Suzuki et al. (1998a) Harayama et al. (1997) Shibata and Yamada (1977) Watanabe et al. (1999) Watanabe et al. (1999) Harayama (1990) Danhara and Iwano (1998) Suzuki et al. (1998a) Tokuhashi et al. (1983a) Tokuhashi et al. (1983b) Watanabe and Danhara (1996) Watanabe et al. (1999) Watanabe et al. (1999) Koike et al. (1985) Danhara and Iwano (1998) Suzuki et al. (1998b) Suzuki et al. (1998a) Suzuki et al. (1998a)
*Ages were not used for the estimation of KMT age in Suzuki (2000); —: not described in references; PFD: pyroclastic flow deposit; WT: welded tuff; AFA: ash-fall deposit; FT: fission track dating method; zeta: FT age with zeta calibration; K-Ar: Potassium-Argon dating using biotite phenocrysts.
pyroclastic flow deposit. They derived weight-mean ages of 0.6570.02 Ma (distal ash-fall deposits) and 0.6370.04 Ma (pyroclastic flow deposit) equivalent to 6–3% of the age, smaller errors than most of the radiometric ages referred to in Suzuki (2000). Moreover, Nakazato (2006) determined a more secure age for the KMT using stratigraphic position of KMT, a high-resolution oxygen-isotope stratigraphy, and calcareous nannofossil biostratigraphy in the Inubo Group, the most east part of the Kanto Plain (Fig. 1). This recent age
estimation shows that the age of KMT is positioned around 0.62 Ma, where marine isotope stage (MIS) 16.0 presents (Fig. 3), concordant with the weight-mean K-Ar age of 0.6370.04 Ma (pyroclastic flow deposit) by Watanabe et al. (1999). Although this age is a little younger than the weight-mean K-Ar age of 0.6570.02 Ma (distal ash-fall deposits) by Watanabe et al. (1999) and an estimated age of 0.64 Ma by Harayama (2005), Nakazato (2006)’s age of around 0.62 Ma is adopted in the discussion of this study.
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Paleomagnetic polarity delta18O
0
(‰) 2 1 0- 1- 2- 3
Tama River
Kinu River Naka River Younger Fan Main Fan
Tc1-3 M3 M2
0.1
5.5
6.2
M1 S
7.1
0.2 8.2
Ma
10.2
9.3
0.4
Hoshakuji
Brunhes
0.3
Kobiki
11.3 12.2
0.5
13.3 Kitsuregawa Lower
15.1
0.6
KMT 16.2
Azuyama, Sayama
Kitsuregawa Upper
17.3 18.2 19.3
0.8
Matuyama
0.7
Terrace surface Marker tephra
Fig. 3. Chronology of terrace surfaces in and around the Kanto Plain during the late Quaternary. Compiled by Minagawa and Machida (1971), Akagi and Koike (1978), and Kaizuka et al. (2000). The d18O curve is after Bassinot et al. (1994).
3. Discussion 3.1. Correlation of fluvial terrace surfaces The stratigraphic positions of the KMT in the terrace sequence indicate that the upper dissected fluvial terrace surfaces Azuyama, Sayama, and Kitsuregawa Upper are correlative. They were formed a few tens of thousands of years prior to the deposition of the KMT (Fig. 3). This is concordant with a paleomagnetic study conducted by Ueki (2006) reporting Brunhes Chron paleomagnetic polarity of tephric soil deposits between KMT and the Imokubo Gravel Bed (approximately 1 m in thickness) on the Sayama terrace surface. The distributions of the Azuyama and Sayama terrace surfaces, originally continuous, suggest that a broad alluvial fan of the Tama River, the origin of these surfaces, had developed in the area at least from the Azuyama hills to the Sayama hills (Figs. 4 and 5). Both the width and angle of this fan were larger than those of several late Pleistocene fans (Kaizuka et al., 2000) formed during MIS 6–5.5 (S terrace surface); MIS 5.1–4 (M2, M3 terrace surfaces); and MIS 3–2 (Tc1 3 terrace surface group). The reconstructed middle Pleistocene fan morphology is comparable to the M1 terrace surface (MIS 5.3) the broadest late Pleistocene fan surface.
In the north part of the Kanto Plain, a prominent fan surface of the Kitsuregawa Upper terrace surface along the Naka and Kinu Rivers had emerged a few tens of thousands of years preceding the deposition of the KMT. Although the depositional surface of the Kitsuregawa Upper terrace surface was fairly dissected, it is well preserved as a broad hill. The area of this terrace surface is larger than other younger alluvial fan surfaces in this area. The presence of these well-developed alluvial fans seems to indicate that there was a long and stable duration of favorable conditions for the formation of broad fans. 3.2. Condition and timing of the formation of broad fan surfaces Generally, in the southwest part of the Kanto Plain, where flights of emerged fan surfaces of different altitudes and ages are developed in the non-glaciated catchments, the present river beds in deeply excavated narrow valleys show deep incision (Fig. 4), as do the river beds of the last interglacial period (MIS 5.5) (Kaizuka et al., 2000). In contrast to this, several alluvial fans formed in the transitional stage from MIS 5.5 to MIS 2, the culmination of the last glacial stage, generally developed with longitudinal profiles characterized by steeper gradient and curvature. Typical terrace surfaces along the Tama River
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Fig. 4. Oblique aerial view of the Sayama and Azuyama hills originated from a continuous broad alluvial fan, which emerged between MIS 17.3 and 16.2, and the Tama River in excavated narrow valley. Taken from the south.
are M1 (MIS 5.3), M2 (MIS 5.1), M3 (MIS 4), and the Tc1 3 terrace surface group (MIS 3-2) (Kaizuka et al., 1977, 2000). These alluvial fan deposits originated from the sediments transported by the extended paleo-Tama River, flowing on an emerged delta and coastal plain associated by elevated sea level during MIS 5.5. The estimated age of KMT (ca. 0.62 Ma) and the stratigraphic relation of this tephra with the Azuyama, Sayama and Kitsuregawa Upper terrace surfaces suggests that these terrace surfaces emerged between MIS 17.3 and 16.2 (0.688–0.628 Ma). The period of MIS 17.3 to 16.2 is a remarkable transitional period from intense interglacial to intense glacial, similar to the transitions from MIS 5.5 to 2, 9.3 to 8.2, and 11.3 to 10.2. Thus, the alluvial fan of Azuyama and Sayama terrace surfaces most likely formed under similar geomorphic conditions as did the M1 to M3 terrace surfaces and Tc1 3 surface group. This is concordant with the similar characteristics of alluvial fan deposits. Thicknesses of the alluvial fan deposits forming the Azuyama and Sayama terrace surfaces are 10–12 m (Machida, 1973) and 9 m (Hatori and Juen, 1958), respectively. These thicknesses are similar to those of M1 to M3 terrace surfaces ranging 3–10 m, and are thicker than those of Tc1 3 surface group ranging 1.5–4 m (Machida, 1973, 1999b). Therefore, it is inferred that the original alluvial fan of the Azuyama and Sayama terrace surface was formed in a regressive stage after the peak of MIS 17.3. On the other hand, along the middle reach of the Naka River, prominent fluvial terrace surfaces originated from alluvial fans formed during MIS 3 to 2 (Main Fan surface) and MIS 2 (Younger Fan surface) (Fig. 3) (Akagi and Koike, 1978). These are well developed, and the present
river bed shows deep incision in the excavated narrow valley cutting these terraces. This landform development, similar to that of the southwest part of the Kanto Plain, indicates that the formation of broad alluvial fans represents a transitional stage from interglacial to glacial. Therefore, the alluvial fan deposits constituting the Kitsuregawa Upper terrace surface formed under the similar geomorphic conditions during the period from MIS 17.3 to 16.2. Suzuki (2000) concluded that the Kitsuregawa Upper terrace surfaces terraced during the termination from MIS16.2 to MIS15.1. This conclusion was based on the broad estimated age of the KMT to be 0.58–0.69 Ma. However, the adopted age in this study (ca. 0.62 Ma) indicates that the Kitsuregawa Upper terrace surfaces had formed and terraced during the termination from MIS 17.3 to MIS16.2. Despite the age of the Sayama terrace surface and its correlative, they are well preserved. This is most likely caused by the development of the original wide fan surface, a more acceptable explanation than suggesting that preservation from lateral erosion was by chance. This interpretation suggests that there was a long duration with stable conditions favorable for graded river development, associated with strong lateral erosion and minor changes of the river bed in altitude. A problem left in the study of geomorphic development in the middle Pleistocene is whether the transitional stage of MIS 17.3 to 16.2 was a specific event in the formation of middle Pleistocene landforms. Further research is needed to determine whether formation of the prominent alluvial fans of this age is general in the area affected by mid-latitude oceanic climates, such as the Japanese Islands.
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Mountains
Kasamori formation
Kitsuregawa hills
Quaternary volcano
Naka River
Kinu Rive r
Kanto Mountains Azuyama hills
Sayama hills Tokyo Tokyo Bay Tama River
1 2 3
Boso Peninsula
Sagami Bay
Hayama-Mineoka Island
4 5 6
0
25 km
50
Fig. 5. Paleogeography in and around the Kanto Plain before and after the deposition of KMT. Modified after Suzuki (2000). 1: paleo-shoreline; 2: present shoreline and boundary of lowland, terraces and hills; 3: Kazusa Trough; 4: hills formed preceding marine isotope stage 17; 5: Sayama terrace surface and its correlative terrace surfaces; 6: estimation of original distribution of Imokubo Gravel Bed and its correlative gravel bed.
4. Conclusions For the identification of strongly weathered KMT in aeolian sediments, the chemical composition of titanomagnetite was determined by electron probe microanalysis. Although volcanic glass shards are intensely altered or non-existent, the chemical composition of titanomagnetite, combined with uranium content measurements in zircon grains, are useful for the identification of strongly weathered KMT. The chemical composition of titanomagnetite is effective for the discrimination of weathered middle Pleistocene tephra in the Japanese Islands affected by mid-latitude oceanic climate. Evaluating all published age data for the KMT, the age of KMT is around 0.62 Ma (MIS 16.0). The geomorphic history of the middle Pleistocene in the Kanto Plain was tephrochronologically reconstructed.
Correlation of dissected fluvial terrace surfaces originating from broad alluvial fans, the Azuyama and Sayama terrace surfaces in southwest Kanto and the Kitsuregawa Upper terrace surface in north Kanto, suggests the presence of stable conditions for the formation of broad fans during the transition from interglacial (MIS 17.3) to glacial (MIS 16.2). Acknowledgments I wish to dedicate this paper to Professor John Westgate in commemoration of his retirement from the University of Toronto. Part of this study was financially supported by Grants-in-Aid for Science Research from the Ministry of Education, Science, Sports, and Culture (No. 09780137: Chemical analysis of major elements on titanomagnetite by EPMA and its applicative study for identification of tephra).
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