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Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan
Sedimentary Geology 135 (2000) 219–230 www.elsevier.nl/locate/sedgeo
Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan O. Fujiwara a,*, F. Masuda b, T. Sakai b, T. Irizuki c, K. Fuse d b
a Tono Geoscience Center, Japan Nuclear Cycle Development Institute, Toki 509-5102, Japan Department of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan c Department of Earth Sciences, Aichi University of Education, Kariya 448-8542, Japan d Daiwa Geological Laboratory Co. Ltd., Sendai 980-0824, Japan
Received 12 December 1998; accepted 29 July 1999
Abstract Tsunami have probably deposited sand sheets that are intercalated in Holocene bay-floor mud exposed on the Boso and Miura Peninsulas, facing the convergent boundary of the Pacific, Philippine Sea, and Eurasian Plates. We have identified seven sand sheets at four drowned valleys, and correlated these by 137 radiocarbon dates of shells and wood. The sheets consist of poorly sorted muddy sand and well-sorted sand layers in fining upward sequences, containing abundant transported shells, rip-up clasts and wood fragments. The sheets erosionally overlie bioturbated bay-floor mud that contains molluscan shells in life position. Most of the sheets are less than 20 cm thick and rarely more than 50 cm thick. Some molluscan shells are older in these layers than in underlying mud. Both landward and seaward paleocurrents are shown in a few cases by imbrication of shells and by low-angle wedge shaped lamination. At least five of the sand sheets contain molluscan fossils derived from rocky coasts or shore platforms, although they intercalated in mud deposited within bays, at depths of 10–15 m. Two other sand sheets are dominated by open seashore ostracode assemblages, although they were deposited in the brackish inner bay and muddy central bay. Five of the layers may correlate with emergences recorded by nearby Holocene marine terraces. These correlations suggest that great earthquakes triggered the inferred tsunami. The tsunami occurred at intervals of 300–2000 years beginning about 10,000 years ago. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: bay mud; Holocene; Japan; sedimentology; Tsunami deposit
1. Introduction The Pacific coast of central Japan faces the convergent boundary of the Pacific, Philippine Sea and Eurasian Plates. This boundary is marked by troughs and trenches including the Sagami Trough and the Japan Trench (Fig. 1). Great earthquakes (M ⱌ 8) in 1703 * Corresponding author. Fax: ⫹81-572-55-0180. E-mail address: [email protected] (O. Fujiwara).
and 1923 AD centered around the Sagami Trough uplifted the coastal area of the southern Kanto region (Matsuda et al., 1978), and generated destructive tsunami (Usami, 1996; Watanabe, 1998). Holocene marine terraces along the Boso and Miura Peninsulas of the southern Kanto region have been ascribed to uplift during prehistoric great earthquakes at intervals in the hundreds to thousand of years (Shimazaki and Nakata, 1980; Kumaki, 1985). Tsunami deposits attributed to these prehistoric
0037-0738/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0037-073 8(00)00073-7
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Fig. 1. Tectonic setting of the Japanese Islands (after Nakamura, 1984) and study areas. (A) Boso and Miura Peninsulas. (B) Distribution of the Holocene marine terraces (Numa I–IV) and outcrops (numerals) and drilling sites (TB3–TB6) in the Boso Peninsula. Classification of marine terraces was quoted from Nakata et al. (1980). (C) Distribution of the Holocene marine terraces (Nobi I–III) and drilling sites (MB1–MB3) in the Miura Peninsula. Classification of marine terraces was quoted from Geographical Survey Institute (1982) and Kumaki (1985).
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earthquakes were recently found beneath alluvial plains beside the marine terraces (Fujiwara et al., 1997, 1999). These sheets were deposited on the muddy bay floors at depths of about 10 m. Here we discuss the detailed sedimentology and depositional process of these tsunami deposits using their sedimentary facies, fossils, and spatial and temporal distribution. We show that submarine tsunami deposits can be used to infer paleo-seismic events which are otherwise known mainly from coastal marshes (e.g. Atwater, 1987; Long et al., 1989; Atwater and Moore, 1992; Clague and Bobrowsky, 1994) and lakes (e.g. Minoura et al., 1994). This finding expands the usefulness of tsunami deposits.
2. Geological setting The southern Kanto region (Fig. 1) is one of the most seismically active areas in Japan. Marine terraces which have been emerged by coseismic uplifts since the maximum stage of postglacial marine transgression, ca. 7200 cal. yr BP, are distributed in four and three levels of the Boso and Miura Peninsulas, respectively (Fig. 1B and C). The highest terrace reaches a maximum height of 30 m above sea level (Kayane and Yoshikawa, 1986). They reflect active seismicity around the Sagami Trough through the Holocene (Shimazaki and Nakata, 1980). Many valleys were drowned in the southern Kanto region by the postglacial marine transgression. These Holocene bays, now alluvial plains, were filled up by muddy sediments that contain marine fossils (e.g. Matsushima, 1974, 1976, 1984). Paleodepths of the bay centers, estimated from ostracode and molluscan assemblages, reached 10–20 m at the maximum sealevel rise (Frydl, 1982; Matsushima, 1984; Fujiwara et al., 1997, 1999; Irizuki et al., 1998). Coral and oyster beds were formed on the wave-cut benches and rocky coasts of some bays at this stage (Matsushima and Yoshimura, 1979; Matsushima, 1984). Subsequent emergence has exposed these bay-floor deposits in the banks of several streams. We studied four alluvial plains, herein called Tateyama, Tomoe, Miyata, and Zushi Bays (Fig. 1B and C). We chose them because of thick marine sediments and good exposure.
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Tateyama Bay was located on the west coast of southern Boso Peninsula (Fig. 1B). Outcrops, a few meters high, are continuous for about 1.5 km along the Heguri River, which flows through central part of the plain. We studied these outcrops and drilled three boreholes (TB3, TB4, TB6) along a line perpendicular to the bay coast. The Holocene bay-floor mud reaches about 9 m in thickness. Tomoe Bay was located along the Tomoe river valley on the west coast of southernmost portion of Boso Peninsula (Fig. 1B). Outcrops of bay-floor mud, height of a few meters, are scattered about 1.3 km along the Tomoe River. The exposed thickness of Holocene marine deposits is about 5 m. Miyata Bay, north of Miura City, was located on the west coast of the southern portion of Miura Peninsula (Fig. 1C). The thickness of Holocene marine deposits reaches about 13 m at inner bay (MB1 drilling site) and 20 m at the bay mouth (MB2 drilling site). At Zushi Bay, on the west coast of northern Miura peninsula (Fig. 1C), the Holocene bay-floor mud is about 28 m thick at the bay mouth (MB3 drilling site).
3. Methods We have examined the texture, structure, color, fossils in outcrops and drilling cores, diameters 80 and 95 mm, and took samples for 14C dating, micro and molluscan fossil studies and grain size analysis. One hundred and thirty-seven radiocarbon ages of marine shells, including in situ bivalves and wood, were obtained by proportional gas counting and AMS methods. The radiocarbon ages were converted to calendric ages (cal. yr BP) using the CALIB rev 3.0.3 (Stuiver and Braziunas, 1993). Numerical analyses of ostracode fauna in the 40 samples from three cores (MB1, MB2 and MB3) reveal seven assemblages which indicate following environments (Irizuki et al., 1998): Spinileberis quadriaculeata–Cytheromorpha acupunctata (SC: muddy inner brackish bay, about 2–5 m in depth), Loxoconcha uranouchiensis–Parakrithella pseudadonta (LP: sandy bay coast), Bicornucythere bisanensis– Pistocythereis bradyi (BP: muddy middle bay, 5– 10 m in depth), Loxoconcha viva–Nipponocythere bicarinata (LN: muddy middle bay, 10–15 m in
222 O. Fujiwara et al. / Sedimentary Geology 135 (2000) 219–230 Fig. 2. Sequences and calibrated radiocarbon ages of the Holocene marine sediments in the Boso and Miura Peninsulas. (A) TB3–TB6 drilling cores and outcrops along the Heguri River (Loc. 12-22, 25-32) and the Tomoe River (Loc. 34). Locations are shown in Fig. 1B. Elevation axis for the drilling cores and Loc. 12-22, 25-32. (B) MB1–MB3 drilling cores. Drilling sites are shown in Fig. 1C. T1–T7 are isochronal layers traced through the study areas. Radiocarbon ages were calibrated using CALIB rev 3.0.3 (Stuiver and Braziunas, 1993) and an error multiplier of 1. The horizontal lines represent the 1s age range. Rectangles show depositional ages of T1–T7 inferred from their stratigraphic positions along age–depth lines. Three candidate sheets for the layer T4 are present at Loc. 12-22.
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Fig. 3. Fossil compositions of the event deposits and the bay-floor mud. (A) Diagrams showing percentages of molluscan species belonging to each assemblage in the event deposits (I, IV) and Bay floor mud (II, III). I and II: Tateyama Bay. III and IV: Tomoe Bay. Sampling points are shown in Fig. 2A. Remarkable increases in number of species and mixing of assemblages are shown in the event deposits. (B) Diagrams showing percentages of the ostracode species belonging to each assemblage in the event deposits and bay-floor muds of MB3 core. Sampling points are shown in Fig. 2B. I: Sandy clay beneath and over layer T1. Brackish inner bay deposits. II: T1 layer. Spike of CP (e.g. Pontocythere subjaponica) and AX (e.g. Aurilaa munechikai) assemblages indicates the invasion of open marine water into the brackish inner bay.
depth), Trachyleberis–Pistocythereis bradyformis (TP: sandy mud bottom of outer bay), Callistocythere–Pontocythere subjaponica (CP: sandy foreshore-shoreface) and Aurila–Xestoleberis (AX: open rocky shore with calcareous algae and/or Zostera bed) assemblages. Qualitative analyses of molluscan fauna in the 85 samples from outcrops and six cores (MB1–MB3 and TB3–TB6) reveal four assemblages which indicate following environments (Fujiwara et al., 1997, 1999): TF (sandy or muddy bay-floor, intertidal
zone), SM (sandy mud bay-floor, shallower than 10 m in depth), CM (central muddy bay-floor, shallower than 20 m in depth) and RB (rocky bottom and/or wave-cut bench) assemblages.
4. Event deposits in bay-floor mud Many sand and gravel sheets are intercalated in the Holocene bay-floor mud of each paleo-bay (Fig. 2A and B). These sedimentary sequences show repeated
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Fig. 4. Type A event deposit in the MB2 core. (A) Layer T2 consists of poor-sorted gravely sand with RB assemblages (e.g. Chama sp., Spondylys sp.) and erosionally covers the bay mouth mud deposited around the depth of 15 m. (B) Layer T3 consists of poor-sorted gravely sand with RB assemblage and rip-up clasts, and erosionally covers the bay mouth mud deposited under the influence of open sea water, around the depth of 10–15 m. M: rip-up clast; S: shell fragment.
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colored poor-sorted massive sandy clay dominated by TF (e.g. Cyclina sinensis, Batillaria zonalis) and SC assemblages. This part formed in a brackish inner bay, probably on a tidal flat. The middle part resembles the bay-floor mud of Tateyama and Tomoe Bays (Fig. 4). CM, BP and LN assemblages are dominant. Depositional environment was middle bay to bay mouth, 10–15 m deep. The upper part mainly consists of massive muddy sand yielding SM, CP, TP and AX assemblages. This part was estimated to be deposited at the bay mouth under the influence of open sea water, around a depth of 10 m. 4.2. Event deposits Event deposits intercalated in the bay-floor mud can be classified into three types based on their sedimentary facies and fossils. Type A has muddy matrix, type B consists of cross laminated and sorted sand, and type C is a biofragment layer. Fig. 5. Grain size analysis for the sediments of Tateyama and Tomoe Bays. Event deposits have wide range both in grain size distribution and sorting grade covering that of fluvial to inner bay deposits.
burial events of muddy bay-floor by coarse sediments. These sheets are called “event deposits” in this report. 4.1. Bay-floor mud The deposits of Tateyama and Tomoe Bays consist of bioturbated massive muddy sand or sandy silt with occasional thin lenses of sand and clay. CM (e.g. Dosinella penicillata, Fulvia mutica) and SM (e.g. Macoma Praetexta, Nitidotellina nitidula) molluscan assemblages are dominant in the mud (Fig. 3). Some bivalves (e.g. Dosinella penicillata) are in life position. Depositional environments were bay center to bay mouth, around 10–20 m deep (Fujiwara et al., 1997). The deposits of Miyata and Zushi Bays can be divided into three superposed parts based on the sedimentary facies and fossil assemblages (Irizuki et al., 1998; Fujiwara et al., 1999). The lower part, ranging from several (MB1 and MB2 cores) to seven (MB3 core) meters in thickness, consists of olive-green
4.2.1. Type A Some of the event deposits of Tateyama, Miyata and Zushi Bays are composed of single or several units in fining upward sequences (rarely in coarsening upward). Each unit consist of poorly sorted muddy sand and/or sandy gravel with muddy sand matrix. Transported shells and rip-up clasts are common (Fig. 4). The sand has a wide range in both grain size and sorting, comparable to the ranges of fluvial and inner bay deposits (Fig. 5). Contacts between lower beds are sharp and commonly erosional. Type A beds grade upward into bay-floor mud in many cases, and are occasionally covered by white clay up to 10 cm thick. Wood fragments are included in the clay layer and upper part of sand sheet. Type A beds range up to 1 m in thickness, but vary markedly in thickness and taper into a thin layer of sand and/or shell fragments. Thick sand sheets are exposed for a distance of 100–300 m along the Heguri River. Convolute bedding is observed in a sand sheet exposed along the Heguri River (Fig. 2A). Paleocurrents are obtained from the sedimentary structures in some outcrop beds, although most beds show strong bioturbation (Fig. 2A). Both landward and seaward directions are shown by
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Fig. 6. Type B event deposit exposed along the Tomoe River. A sheet consists of well-sorted sand showing a fining-upward sequence and low angle wedge-shape lamination with many pebbles and cobbles (G) and shell fragments (S), erosionally covering bioturbated bay-floor mud yielding in situ Dosinella penicillata, deposited at a depth of 10–20 m. Plant fragment layers (black band) are visible in the upper part of the sand sheet (P). Wood fragments (W) are scattered in the mud just above the sand sheet. Basal part of the sand sheet is concealed under the river water. Scales in 10 cm. Loc. 34 of Figs. 1C and 2A.
imbrication of shells and dip of low-angle wedge shaped lamination. Type A beds contain shell beds and lenses. Most of shells are fractured. The number of individuals and species is much greater than in bounding mud beds (Fig. 3A). RB (e.g. Spondylus sp., Barnea manilensis, Petricola divergens (boring shell)) and TF (e.g. Batillaria zonalis) assemblages are mixed with dominant CM (e.g. Dosinella penicillata, Fulvia mutica) assemblages. Many Dosinella penicillata penetrate into the upper part of Type A beds in life position.
Layer T1 of the MB3 core (Fig. 2B) contains assemblages of fully marine ostracodes typical of sandy coasts (CP) and rocky coast (AX). In contrast, the bounding deposits consist of sandy clay of a brackish inner bay (Fig. 3B). The event deposit contains much more ostracode species than does the sandy clay, and many of the valves are poorly preserved and altered. Such an increase in species diversity and a spike of fully marine ostracodes (e.g. Neonesidea oligodentata, Aurila cymba) is also recognized in layer T2 intercalated in the middle bay mud of MB1
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Fig. 7. Type C event deposit exposed along the Heguri River. A bio-fragment layer erosionally overlies the oyster and coral bed. Upper branches of coral were broken by the layer. The event occurred around 7300 cal. BP and correlated to the layer T3 based on the radiocarbon age. Scale in 10 cm. Loc. 23 of Fig. 1B.
core. The marine ostracodes were probably derived from an open sandy coast and rocky shore, and were re-deposited in an inner bay. 4.2.2. Type B Event deposits of Tomoe Bay are composed of laminated moderately or well-sorted medium to coarse grained sand in fining upward sequences. Maximum thickness is about 70 cm (Fig. 6). The grain size distribution is similar to that of fluvial sand (Fig. 5). The sheets erosionally cover lower mud. Abundant shell fragments, pebbles and cobbles
ranging up to 15 cm in diameter, and rip-up clasts are included in the basal part. Wood fragment layers are intercalated in the upper part of the event deposits, and are also intercalated in the mud immediately overlying the event deposits (Fig. 6). Paleocurrents, in both landward and seaward directions, are shown by the dip of low-angle wedge shaped lamination in some beds. Hummocky cross stratification (HCS: Harms et al., 1975; Dott and Bourgeois, 1982; Cheel and Leckie, 1993) can be observed in the beds. Central muddy bay-floor assemblage, such as
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Fig. 8. Inferred correlations among the event deposits and the Holocene marine terraces. Emergent ages of the marine terraces were converted from Nakata et al. (1980) and Kumaki (1985) to calendric ages using the CALIB rev 3.0.3 (Stuiver and Braziunas, 1993).
Dosinella penicillata and Fulvia mutica, common in the lower bay-floor mud, and Rocky Bottom and/or wave-cut bench assemblage (e.g. Chlamys spp., Ostrea denselamelosa, Barnea manilensis (boring shell)) are mixed in the event deposits (Fig. 3A). The number of molluscan species exceeds that in the adjacent bay mud. Most of shells are fractured and flat-lying. 4.2.3. Type C This type bed consists of shell and coral fragments and averages 30 cm in thickness (Fig. 7). It erosionally overlies the oyster and coral bed built on the wave-cut bench at Loc. 23 of Tateyama Bay (Fig. 1B).
Miyata and Zhushi Bays. Layer T2, deposited about 8200 cal. BP, was observed at all four bays. Layer T3, formed about 7400–7200 cal. BP, was recognized at all but Zushi Bay. The type C bed of Tateyama Bay formed about this time. Layer T4, recognized at Tateyama and Miyata Bays, formed about 5600– 5300 cal. BP as did three event deposits at Tateyama Bay (Fig. 2A). Layer T5 recognized at Tateyama and Miyata Bays was formed about 5000–4700 cal. BP. Layers T6 and T7, recognized at all bays except Tomoe Bay, were deposited around 4000–3700 and 3100–2800 cal. BP, respectively.
6. Origin and depositional process of event deposits 5. Chronology and correlations Age–depth curves connecting the 14C ages were drawn for each locality, excluding the values incompatible with overall trends (Fig. 2A and B). The slopes of the curves show average accumulation rates at each locality. Depositional ages of event deposits were read from the age–depth curves. The ages imply that hundreds of years elapsed between the events in most cases. Seven isochronal layers, denoted T1–T7 in ascending order, can be traced among the bays (Fig. 8). Layer T1, formed around 9500 cal. BP, was found at
Most of the event deposits erosionally overlie mud that was deposited in low energy environments within a bay. They recorded erosion of submarine and coastal sediments and their transportation into bays by strong currents. Reversal of 14C ages (Fig. 2A and B) shows that the event deposits contain material eroded from older deposits. Reductions of accumulation rates around the event deposits also indicate submarine erosion. The materials in the event deposits were also derived from within and around the bays, as shown by abundant wood fragments. The muddy matrix and muddy-bay molluscs typical of type A beds indicate sediment sources within
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muddy bays. Material was also derived from wave-cut benches, as shown by their molluscan assemblage. Invasions of marine water into the inner bay are recorded by ostracode assemblages in the layer T1 of Zushi Bay and layer T2 of Miyata Bay. Tsunami is the one possible origin of the layers T1 and T2 which can explain the landward transportation and wide range distribution in the coastal area facing the Sagami Trough. Long-period oscillatory water currents probably account for hummocky cross-stratification seen in some type B beds. One possible source of the beds consistent with grain size data is a deltaic sand body. Type C bed was derived from an oyster bed attacked by strong currents. 7. Correlation with marine terraces Flights of the Holocene marine terraces on the coast of the southern Boso Peninsula and the Miura Peninsula have been classified into four (Numa I–IV terraces) and three (Nobi I–III terraces) levels, respectively, separated by scarps about 5 m high (Nakata et al., 1980; Kumaki, 1985). Their rapid, perhaps coseismic emergence (Fig. 8) occurred in prehistoric time around 7200 cal. BP (Numa I and Nobi I terraces), 5300 cal. BP (Nobi II terrace), 4900 cal. BP (Numa II terrace), 3500 cal. BP (Nobi III terrace), 2900 cal. BP (Numa III terrace). Coseismic uplift is the known cause of emergence for the Numa IV terrace which was raised by the AD 1703 earthquake. Layers T3–T7 have similar ages to the emergence ages of these terraces (Fig. 8). Layer T3 corresponds with the Numa I and Nobi I terraces. Layers T4–T7 show similar ages to the Nobi II, Numa II, Nobi III and Numa III terraces, respectively. The age agreements strongly suggest that these layers are earthquake-induced tsunami deposits. Kayane and Yoshikawa (1986) reported that each of Numa I–IV terraces on the rocky coast of the southeastern Boso Peninsula can be subdivided into several levels by low scarps, about one meter high. It is inferred that small scale coseismic uplift of the AD 1923 type has occurred several times in each interval between the large scale AD 1703 type coseismic events in past 7200 years (Kayane and Yoshikawa, 1986; Kumaki, 1988). One possible origin of the event deposits intercalated in the sections among
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layers T3–T7 is tsunami generated by the AD 1923 type earthquakes with smaller displacements of the coasts, although we cannot definitely correlate such events with marine terraces and rule out other possible depositional mechanisms, such as floods and storms. 8. Conclusions Tsunami well explains seven of sand and gravel sheets intercalated in the Holocene bay-floor mud of southern Kanto region facing the Sagami Trough. A tsunami origin is inferred from the sedimentary structures and fossil compositions of the sheets indicating erosive strong water currents in relatively low-energy settings, their presence over large areas, and their permissive correlation with rapid emergence of marine terraces. These tsunami deposits consist of muddy sand and/ or sand sheets in fining upward sequences, with abundant transported shells, rip-up clasts, pebbles, cobbles and wood fragments, erosionally covering the bayfloor mud deposited around 10 m in depth. Acknowledgements Prof. Makoto Okamura, Dept. Sci., Kochi Univ., has contributed to the coring using the vibration core sampler. The authors would like to thank Dr. Brian Atwater, USGS, for a critical reading of the manuscript. This study was partly supported by the grantin-aid for scientific research from the Japanese Ministry of Education, Science and Culture (No. 10740246). References Atwater, B.F., 1987. Evidence for great Holocene earthquakes along the outer coast of Washington State. Science 236, 942– 944. Atwater, B.F., Moore, A.L., 1992. A tsunami about 1000 years ago in Puget sound, Washington. Science 258, 1614–1616. Cheel, R.J., Leckie, D.A., 1993. Hummocky cross-stratification. Sediment. Rev. 1, 103–122. Clague, J.J., Bobrowsky, P.T., 1994. Tsunami deposits beneath tidal marshes on Vancouver island, British Colombia. Geol. Soc. Am. Bull. 106, 1293–1303. Dott Jr., R.H., Bourgeois, J., 1982. Hummocky stratification: significance of its variable bedding sequence. Geol. Soc. Am. Bull. 93, 663–680.
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Frydl, P., 1982. Holocene ostracods in the southern Boso Peninsula. Studies on Japanese Ostracoda, Hanai, T. (Ed.). Univ. Mus. Tokyo Bull. 20, 61–140. Fujiwara, O., Masuda, F., Sakai, T., Fuse, K., Saito, A., 1997. Tsunami deposits in Holocene bay-floor mud and the uplift history of the Boso and Miura Peninsulas. Quat. Res. 36, 73– 86 (in Japanese with English abstract). Fujiwara, O., Masuda, F., Sakai, T., Irizuki, T., Fuse, K., 1999. Holocene tsunami deposits detected by drilling in drowned valleys of the Boso and Miura Peninsulas. Quat. Res. 38, 41– 58 (in Japanese with English abstract). Geographical Survey Institute, 1982. The distribution of Holocene terraces and radiocarbon dates in the coastal area of southern Kanto. 76pp. Harms, J.C., Southard, J.B., Spearing, D.R., Walker, R.G., 1975. Depositional environments as interpreted from primary sedimentary structures and stratification sequences, SEPM Short Course, 2, 161. Irizuki, T., Fujiwara, O., Fuse, K., Masuda, F., 1998. Paleoenvironmental changes during the last post glacial period on the western coast of the Miura Peninsula, Kanagawa Prefecture, Central Japan: Fossil ostracode fauna and event deposits in bore hole cores. Fossils 64, 1–22 (in Japanese with English abstract). Kayane, S., Yoshikawa, T., 1986. Comparative study between present and emergent erosional landforms on the southeast coast of Boso Peninsula, Central Japan. Geogr. Rev. Japan 59A, 18–36 (in Japanese with English abstract). Kumaki, Y., 1985. The deformations of Holocene marine terraces in southern Kanto, Central Japan. Geogr. Rev. Japan 58B, 49–60. Kumaki, Y., 1988. Mean recurrence time of the 1923 type Kanto earthquake as deduced from the Holocene shoreline data in the Boso peninsula, central Japan. J. Geogr. 97, 20–31 (in Japanese with English abstract). Long, D., Smith, D.E., Dawson, A.G., 1989. A Holocene tsunami deposit in eastern Scotland. J. Quat. Sci. 4, 61–66. Matsuda, T., Ota, Y., Ando, M., Yonekura, N., 1978. Fault mechanism and recurrence time of major earthquakes in southern Kanto
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Tsunami deposits in Holocene bay mud in southern Kanto region, Pacific coast of central Japan O. Fujiwara a,*, F. Masuda b, T. Sakai b, T. Irizuki c, K. Fuse d b
a Tono Geoscience Center, Japan Nuclear Cycle Development Institute, Toki 509-5102, Japan Department of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan c Department of Earth Sciences, Aichi University of Education, Kariya 448-8542, Japan d Daiwa Geological Laboratory Co. Ltd., Sendai 980-0824, Japan
Received 12 December 1998; accepted 29 July 1999
Abstract Tsunami have probably deposited sand sheets that are intercalated in Holocene bay-floor mud exposed on the Boso and Miura Peninsulas, facing the convergent boundary of the Pacific, Philippine Sea, and Eurasian Plates. We have identified seven sand sheets at four drowned valleys, and correlated these by 137 radiocarbon dates of shells and wood. The sheets consist of poorly sorted muddy sand and well-sorted sand layers in fining upward sequences, containing abundant transported shells, rip-up clasts and wood fragments. The sheets erosionally overlie bioturbated bay-floor mud that contains molluscan shells in life position. Most of the sheets are less than 20 cm thick and rarely more than 50 cm thick. Some molluscan shells are older in these layers than in underlying mud. Both landward and seaward paleocurrents are shown in a few cases by imbrication of shells and by low-angle wedge shaped lamination. At least five of the sand sheets contain molluscan fossils derived from rocky coasts or shore platforms, although they intercalated in mud deposited within bays, at depths of 10–15 m. Two other sand sheets are dominated by open seashore ostracode assemblages, although they were deposited in the brackish inner bay and muddy central bay. Five of the layers may correlate with emergences recorded by nearby Holocene marine terraces. These correlations suggest that great earthquakes triggered the inferred tsunami. The tsunami occurred at intervals of 300–2000 years beginning about 10,000 years ago. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: bay mud; Holocene; Japan; sedimentology; Tsunami deposit
1. Introduction The Pacific coast of central Japan faces the convergent boundary of the Pacific, Philippine Sea and Eurasian Plates. This boundary is marked by troughs and trenches including the Sagami Trough and the Japan Trench (Fig. 1). Great earthquakes (M ⱌ 8) in 1703 * Corresponding author. Fax: ⫹81-572-55-0180. E-mail address: [email protected] (O. Fujiwara).
and 1923 AD centered around the Sagami Trough uplifted the coastal area of the southern Kanto region (Matsuda et al., 1978), and generated destructive tsunami (Usami, 1996; Watanabe, 1998). Holocene marine terraces along the Boso and Miura Peninsulas of the southern Kanto region have been ascribed to uplift during prehistoric great earthquakes at intervals in the hundreds to thousand of years (Shimazaki and Nakata, 1980; Kumaki, 1985). Tsunami deposits attributed to these prehistoric
0037-0738/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0037-073 8(00)00073-7
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Fig. 1. Tectonic setting of the Japanese Islands (after Nakamura, 1984) and study areas. (A) Boso and Miura Peninsulas. (B) Distribution of the Holocene marine terraces (Numa I–IV) and outcrops (numerals) and drilling sites (TB3–TB6) in the Boso Peninsula. Classification of marine terraces was quoted from Nakata et al. (1980). (C) Distribution of the Holocene marine terraces (Nobi I–III) and drilling sites (MB1–MB3) in the Miura Peninsula. Classification of marine terraces was quoted from Geographical Survey Institute (1982) and Kumaki (1985).
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earthquakes were recently found beneath alluvial plains beside the marine terraces (Fujiwara et al., 1997, 1999). These sheets were deposited on the muddy bay floors at depths of about 10 m. Here we discuss the detailed sedimentology and depositional process of these tsunami deposits using their sedimentary facies, fossils, and spatial and temporal distribution. We show that submarine tsunami deposits can be used to infer paleo-seismic events which are otherwise known mainly from coastal marshes (e.g. Atwater, 1987; Long et al., 1989; Atwater and Moore, 1992; Clague and Bobrowsky, 1994) and lakes (e.g. Minoura et al., 1994). This finding expands the usefulness of tsunami deposits.
2. Geological setting The southern Kanto region (Fig. 1) is one of the most seismically active areas in Japan. Marine terraces which have been emerged by coseismic uplifts since the maximum stage of postglacial marine transgression, ca. 7200 cal. yr BP, are distributed in four and three levels of the Boso and Miura Peninsulas, respectively (Fig. 1B and C). The highest terrace reaches a maximum height of 30 m above sea level (Kayane and Yoshikawa, 1986). They reflect active seismicity around the Sagami Trough through the Holocene (Shimazaki and Nakata, 1980). Many valleys were drowned in the southern Kanto region by the postglacial marine transgression. These Holocene bays, now alluvial plains, were filled up by muddy sediments that contain marine fossils (e.g. Matsushima, 1974, 1976, 1984). Paleodepths of the bay centers, estimated from ostracode and molluscan assemblages, reached 10–20 m at the maximum sealevel rise (Frydl, 1982; Matsushima, 1984; Fujiwara et al., 1997, 1999; Irizuki et al., 1998). Coral and oyster beds were formed on the wave-cut benches and rocky coasts of some bays at this stage (Matsushima and Yoshimura, 1979; Matsushima, 1984). Subsequent emergence has exposed these bay-floor deposits in the banks of several streams. We studied four alluvial plains, herein called Tateyama, Tomoe, Miyata, and Zushi Bays (Fig. 1B and C). We chose them because of thick marine sediments and good exposure.
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Tateyama Bay was located on the west coast of southern Boso Peninsula (Fig. 1B). Outcrops, a few meters high, are continuous for about 1.5 km along the Heguri River, which flows through central part of the plain. We studied these outcrops and drilled three boreholes (TB3, TB4, TB6) along a line perpendicular to the bay coast. The Holocene bay-floor mud reaches about 9 m in thickness. Tomoe Bay was located along the Tomoe river valley on the west coast of southernmost portion of Boso Peninsula (Fig. 1B). Outcrops of bay-floor mud, height of a few meters, are scattered about 1.3 km along the Tomoe River. The exposed thickness of Holocene marine deposits is about 5 m. Miyata Bay, north of Miura City, was located on the west coast of the southern portion of Miura Peninsula (Fig. 1C). The thickness of Holocene marine deposits reaches about 13 m at inner bay (MB1 drilling site) and 20 m at the bay mouth (MB2 drilling site). At Zushi Bay, on the west coast of northern Miura peninsula (Fig. 1C), the Holocene bay-floor mud is about 28 m thick at the bay mouth (MB3 drilling site).
3. Methods We have examined the texture, structure, color, fossils in outcrops and drilling cores, diameters 80 and 95 mm, and took samples for 14C dating, micro and molluscan fossil studies and grain size analysis. One hundred and thirty-seven radiocarbon ages of marine shells, including in situ bivalves and wood, were obtained by proportional gas counting and AMS methods. The radiocarbon ages were converted to calendric ages (cal. yr BP) using the CALIB rev 3.0.3 (Stuiver and Braziunas, 1993). Numerical analyses of ostracode fauna in the 40 samples from three cores (MB1, MB2 and MB3) reveal seven assemblages which indicate following environments (Irizuki et al., 1998): Spinileberis quadriaculeata–Cytheromorpha acupunctata (SC: muddy inner brackish bay, about 2–5 m in depth), Loxoconcha uranouchiensis–Parakrithella pseudadonta (LP: sandy bay coast), Bicornucythere bisanensis– Pistocythereis bradyi (BP: muddy middle bay, 5– 10 m in depth), Loxoconcha viva–Nipponocythere bicarinata (LN: muddy middle bay, 10–15 m in
222 O. Fujiwara et al. / Sedimentary Geology 135 (2000) 219–230 Fig. 2. Sequences and calibrated radiocarbon ages of the Holocene marine sediments in the Boso and Miura Peninsulas. (A) TB3–TB6 drilling cores and outcrops along the Heguri River (Loc. 12-22, 25-32) and the Tomoe River (Loc. 34). Locations are shown in Fig. 1B. Elevation axis for the drilling cores and Loc. 12-22, 25-32. (B) MB1–MB3 drilling cores. Drilling sites are shown in Fig. 1C. T1–T7 are isochronal layers traced through the study areas. Radiocarbon ages were calibrated using CALIB rev 3.0.3 (Stuiver and Braziunas, 1993) and an error multiplier of 1. The horizontal lines represent the 1s age range. Rectangles show depositional ages of T1–T7 inferred from their stratigraphic positions along age–depth lines. Three candidate sheets for the layer T4 are present at Loc. 12-22.
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Fig. 3. Fossil compositions of the event deposits and the bay-floor mud. (A) Diagrams showing percentages of molluscan species belonging to each assemblage in the event deposits (I, IV) and Bay floor mud (II, III). I and II: Tateyama Bay. III and IV: Tomoe Bay. Sampling points are shown in Fig. 2A. Remarkable increases in number of species and mixing of assemblages are shown in the event deposits. (B) Diagrams showing percentages of the ostracode species belonging to each assemblage in the event deposits and bay-floor muds of MB3 core. Sampling points are shown in Fig. 2B. I: Sandy clay beneath and over layer T1. Brackish inner bay deposits. II: T1 layer. Spike of CP (e.g. Pontocythere subjaponica) and AX (e.g. Aurilaa munechikai) assemblages indicates the invasion of open marine water into the brackish inner bay.
depth), Trachyleberis–Pistocythereis bradyformis (TP: sandy mud bottom of outer bay), Callistocythere–Pontocythere subjaponica (CP: sandy foreshore-shoreface) and Aurila–Xestoleberis (AX: open rocky shore with calcareous algae and/or Zostera bed) assemblages. Qualitative analyses of molluscan fauna in the 85 samples from outcrops and six cores (MB1–MB3 and TB3–TB6) reveal four assemblages which indicate following environments (Fujiwara et al., 1997, 1999): TF (sandy or muddy bay-floor, intertidal
zone), SM (sandy mud bay-floor, shallower than 10 m in depth), CM (central muddy bay-floor, shallower than 20 m in depth) and RB (rocky bottom and/or wave-cut bench) assemblages.
4. Event deposits in bay-floor mud Many sand and gravel sheets are intercalated in the Holocene bay-floor mud of each paleo-bay (Fig. 2A and B). These sedimentary sequences show repeated
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Fig. 4. Type A event deposit in the MB2 core. (A) Layer T2 consists of poor-sorted gravely sand with RB assemblages (e.g. Chama sp., Spondylys sp.) and erosionally covers the bay mouth mud deposited around the depth of 15 m. (B) Layer T3 consists of poor-sorted gravely sand with RB assemblage and rip-up clasts, and erosionally covers the bay mouth mud deposited under the influence of open sea water, around the depth of 10–15 m. M: rip-up clast; S: shell fragment.
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colored poor-sorted massive sandy clay dominated by TF (e.g. Cyclina sinensis, Batillaria zonalis) and SC assemblages. This part formed in a brackish inner bay, probably on a tidal flat. The middle part resembles the bay-floor mud of Tateyama and Tomoe Bays (Fig. 4). CM, BP and LN assemblages are dominant. Depositional environment was middle bay to bay mouth, 10–15 m deep. The upper part mainly consists of massive muddy sand yielding SM, CP, TP and AX assemblages. This part was estimated to be deposited at the bay mouth under the influence of open sea water, around a depth of 10 m. 4.2. Event deposits Event deposits intercalated in the bay-floor mud can be classified into three types based on their sedimentary facies and fossils. Type A has muddy matrix, type B consists of cross laminated and sorted sand, and type C is a biofragment layer. Fig. 5. Grain size analysis for the sediments of Tateyama and Tomoe Bays. Event deposits have wide range both in grain size distribution and sorting grade covering that of fluvial to inner bay deposits.
burial events of muddy bay-floor by coarse sediments. These sheets are called “event deposits” in this report. 4.1. Bay-floor mud The deposits of Tateyama and Tomoe Bays consist of bioturbated massive muddy sand or sandy silt with occasional thin lenses of sand and clay. CM (e.g. Dosinella penicillata, Fulvia mutica) and SM (e.g. Macoma Praetexta, Nitidotellina nitidula) molluscan assemblages are dominant in the mud (Fig. 3). Some bivalves (e.g. Dosinella penicillata) are in life position. Depositional environments were bay center to bay mouth, around 10–20 m deep (Fujiwara et al., 1997). The deposits of Miyata and Zushi Bays can be divided into three superposed parts based on the sedimentary facies and fossil assemblages (Irizuki et al., 1998; Fujiwara et al., 1999). The lower part, ranging from several (MB1 and MB2 cores) to seven (MB3 core) meters in thickness, consists of olive-green
4.2.1. Type A Some of the event deposits of Tateyama, Miyata and Zushi Bays are composed of single or several units in fining upward sequences (rarely in coarsening upward). Each unit consist of poorly sorted muddy sand and/or sandy gravel with muddy sand matrix. Transported shells and rip-up clasts are common (Fig. 4). The sand has a wide range in both grain size and sorting, comparable to the ranges of fluvial and inner bay deposits (Fig. 5). Contacts between lower beds are sharp and commonly erosional. Type A beds grade upward into bay-floor mud in many cases, and are occasionally covered by white clay up to 10 cm thick. Wood fragments are included in the clay layer and upper part of sand sheet. Type A beds range up to 1 m in thickness, but vary markedly in thickness and taper into a thin layer of sand and/or shell fragments. Thick sand sheets are exposed for a distance of 100–300 m along the Heguri River. Convolute bedding is observed in a sand sheet exposed along the Heguri River (Fig. 2A). Paleocurrents are obtained from the sedimentary structures in some outcrop beds, although most beds show strong bioturbation (Fig. 2A). Both landward and seaward directions are shown by
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Fig. 6. Type B event deposit exposed along the Tomoe River. A sheet consists of well-sorted sand showing a fining-upward sequence and low angle wedge-shape lamination with many pebbles and cobbles (G) and shell fragments (S), erosionally covering bioturbated bay-floor mud yielding in situ Dosinella penicillata, deposited at a depth of 10–20 m. Plant fragment layers (black band) are visible in the upper part of the sand sheet (P). Wood fragments (W) are scattered in the mud just above the sand sheet. Basal part of the sand sheet is concealed under the river water. Scales in 10 cm. Loc. 34 of Figs. 1C and 2A.
imbrication of shells and dip of low-angle wedge shaped lamination. Type A beds contain shell beds and lenses. Most of shells are fractured. The number of individuals and species is much greater than in bounding mud beds (Fig. 3A). RB (e.g. Spondylus sp., Barnea manilensis, Petricola divergens (boring shell)) and TF (e.g. Batillaria zonalis) assemblages are mixed with dominant CM (e.g. Dosinella penicillata, Fulvia mutica) assemblages. Many Dosinella penicillata penetrate into the upper part of Type A beds in life position.
Layer T1 of the MB3 core (Fig. 2B) contains assemblages of fully marine ostracodes typical of sandy coasts (CP) and rocky coast (AX). In contrast, the bounding deposits consist of sandy clay of a brackish inner bay (Fig. 3B). The event deposit contains much more ostracode species than does the sandy clay, and many of the valves are poorly preserved and altered. Such an increase in species diversity and a spike of fully marine ostracodes (e.g. Neonesidea oligodentata, Aurila cymba) is also recognized in layer T2 intercalated in the middle bay mud of MB1
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Fig. 7. Type C event deposit exposed along the Heguri River. A bio-fragment layer erosionally overlies the oyster and coral bed. Upper branches of coral were broken by the layer. The event occurred around 7300 cal. BP and correlated to the layer T3 based on the radiocarbon age. Scale in 10 cm. Loc. 23 of Fig. 1B.
core. The marine ostracodes were probably derived from an open sandy coast and rocky shore, and were re-deposited in an inner bay. 4.2.2. Type B Event deposits of Tomoe Bay are composed of laminated moderately or well-sorted medium to coarse grained sand in fining upward sequences. Maximum thickness is about 70 cm (Fig. 6). The grain size distribution is similar to that of fluvial sand (Fig. 5). The sheets erosionally cover lower mud. Abundant shell fragments, pebbles and cobbles
ranging up to 15 cm in diameter, and rip-up clasts are included in the basal part. Wood fragment layers are intercalated in the upper part of the event deposits, and are also intercalated in the mud immediately overlying the event deposits (Fig. 6). Paleocurrents, in both landward and seaward directions, are shown by the dip of low-angle wedge shaped lamination in some beds. Hummocky cross stratification (HCS: Harms et al., 1975; Dott and Bourgeois, 1982; Cheel and Leckie, 1993) can be observed in the beds. Central muddy bay-floor assemblage, such as
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Fig. 8. Inferred correlations among the event deposits and the Holocene marine terraces. Emergent ages of the marine terraces were converted from Nakata et al. (1980) and Kumaki (1985) to calendric ages using the CALIB rev 3.0.3 (Stuiver and Braziunas, 1993).
Dosinella penicillata and Fulvia mutica, common in the lower bay-floor mud, and Rocky Bottom and/or wave-cut bench assemblage (e.g. Chlamys spp., Ostrea denselamelosa, Barnea manilensis (boring shell)) are mixed in the event deposits (Fig. 3A). The number of molluscan species exceeds that in the adjacent bay mud. Most of shells are fractured and flat-lying. 4.2.3. Type C This type bed consists of shell and coral fragments and averages 30 cm in thickness (Fig. 7). It erosionally overlies the oyster and coral bed built on the wave-cut bench at Loc. 23 of Tateyama Bay (Fig. 1B).
Miyata and Zhushi Bays. Layer T2, deposited about 8200 cal. BP, was observed at all four bays. Layer T3, formed about 7400–7200 cal. BP, was recognized at all but Zushi Bay. The type C bed of Tateyama Bay formed about this time. Layer T4, recognized at Tateyama and Miyata Bays, formed about 5600– 5300 cal. BP as did three event deposits at Tateyama Bay (Fig. 2A). Layer T5 recognized at Tateyama and Miyata Bays was formed about 5000–4700 cal. BP. Layers T6 and T7, recognized at all bays except Tomoe Bay, were deposited around 4000–3700 and 3100–2800 cal. BP, respectively.
6. Origin and depositional process of event deposits 5. Chronology and correlations Age–depth curves connecting the 14C ages were drawn for each locality, excluding the values incompatible with overall trends (Fig. 2A and B). The slopes of the curves show average accumulation rates at each locality. Depositional ages of event deposits were read from the age–depth curves. The ages imply that hundreds of years elapsed between the events in most cases. Seven isochronal layers, denoted T1–T7 in ascending order, can be traced among the bays (Fig. 8). Layer T1, formed around 9500 cal. BP, was found at
Most of the event deposits erosionally overlie mud that was deposited in low energy environments within a bay. They recorded erosion of submarine and coastal sediments and their transportation into bays by strong currents. Reversal of 14C ages (Fig. 2A and B) shows that the event deposits contain material eroded from older deposits. Reductions of accumulation rates around the event deposits also indicate submarine erosion. The materials in the event deposits were also derived from within and around the bays, as shown by abundant wood fragments. The muddy matrix and muddy-bay molluscs typical of type A beds indicate sediment sources within
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muddy bays. Material was also derived from wave-cut benches, as shown by their molluscan assemblage. Invasions of marine water into the inner bay are recorded by ostracode assemblages in the layer T1 of Zushi Bay and layer T2 of Miyata Bay. Tsunami is the one possible origin of the layers T1 and T2 which can explain the landward transportation and wide range distribution in the coastal area facing the Sagami Trough. Long-period oscillatory water currents probably account for hummocky cross-stratification seen in some type B beds. One possible source of the beds consistent with grain size data is a deltaic sand body. Type C bed was derived from an oyster bed attacked by strong currents. 7. Correlation with marine terraces Flights of the Holocene marine terraces on the coast of the southern Boso Peninsula and the Miura Peninsula have been classified into four (Numa I–IV terraces) and three (Nobi I–III terraces) levels, respectively, separated by scarps about 5 m high (Nakata et al., 1980; Kumaki, 1985). Their rapid, perhaps coseismic emergence (Fig. 8) occurred in prehistoric time around 7200 cal. BP (Numa I and Nobi I terraces), 5300 cal. BP (Nobi II terrace), 4900 cal. BP (Numa II terrace), 3500 cal. BP (Nobi III terrace), 2900 cal. BP (Numa III terrace). Coseismic uplift is the known cause of emergence for the Numa IV terrace which was raised by the AD 1703 earthquake. Layers T3–T7 have similar ages to the emergence ages of these terraces (Fig. 8). Layer T3 corresponds with the Numa I and Nobi I terraces. Layers T4–T7 show similar ages to the Nobi II, Numa II, Nobi III and Numa III terraces, respectively. The age agreements strongly suggest that these layers are earthquake-induced tsunami deposits. Kayane and Yoshikawa (1986) reported that each of Numa I–IV terraces on the rocky coast of the southeastern Boso Peninsula can be subdivided into several levels by low scarps, about one meter high. It is inferred that small scale coseismic uplift of the AD 1923 type has occurred several times in each interval between the large scale AD 1703 type coseismic events in past 7200 years (Kayane and Yoshikawa, 1986; Kumaki, 1988). One possible origin of the event deposits intercalated in the sections among
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layers T3–T7 is tsunami generated by the AD 1923 type earthquakes with smaller displacements of the coasts, although we cannot definitely correlate such events with marine terraces and rule out other possible depositional mechanisms, such as floods and storms. 8. Conclusions Tsunami well explains seven of sand and gravel sheets intercalated in the Holocene bay-floor mud of southern Kanto region facing the Sagami Trough. A tsunami origin is inferred from the sedimentary structures and fossil compositions of the sheets indicating erosive strong water currents in relatively low-energy settings, their presence over large areas, and their permissive correlation with rapid emergence of marine terraces. These tsunami deposits consist of muddy sand and/ or sand sheets in fining upward sequences, with abundant transported shells, rip-up clasts, pebbles, cobbles and wood fragments, erosionally covering the bayfloor mud deposited around 10 m in depth. Acknowledgements Prof. Makoto Okamura, Dept. Sci., Kochi Univ., has contributed to the coring using the vibration core sampler. The authors would like to thank Dr. Brian Atwater, USGS, for a critical reading of the manuscript. This study was partly supported by the grantin-aid for scientific research from the Japanese Ministry of Education, Science and Culture (No. 10740246). References Atwater, B.F., 1987. Evidence for great Holocene earthquakes along the outer coast of Washington State. Science 236, 942– 944. Atwater, B.F., Moore, A.L., 1992. A tsunami about 1000 years ago in Puget sound, Washington. Science 258, 1614–1616. Cheel, R.J., Leckie, D.A., 1993. Hummocky cross-stratification. Sediment. Rev. 1, 103–122. Clague, J.J., Bobrowsky, P.T., 1994. Tsunami deposits beneath tidal marshes on Vancouver island, British Colombia. Geol. Soc. Am. Bull. 106, 1293–1303. Dott Jr., R.H., Bourgeois, J., 1982. Hummocky stratification: significance of its variable bedding sequence. Geol. Soc. Am. Bull. 93, 663–680.
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