Characteristics of sediments and their dispersal systems along theshelf and slope of an active forearcmargin, eastern Hokkaido,

northern Japan�

Atsushi Noda a,∗ Taqumi TuZino aaGeological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Higashi 1-1-1, Ibaraki 305-8567,

Japan

Abstract

This study revealed sediment distributions and their dispersal systems in a shelf–slope setting along an active forearc marginoff eastern Hokkaido, northern Japan, where the Pacific Plate is being subducted beneath the Okhotsk (North American) Plate atthe 7 km deep Kuril Trench. The findings were based on analyses of the textures and structures of surface sediments, componentsof the sampled sand fractions, high-resolution acoustic reflection records, and sediment budgets. The studied margin has a20–30 km wide shelf and steep (5–10◦) slope marked by faults and folds associated with transpressional tectonics. The shelfsediments are classified into gravelly coarse to fine sand from the shelf edge to the uppermost slope (110–1,000 m water depth),gravel and coarse sand on the shelf off Hanasaki, fine sand on the middle–outer shelf off Kiritappu, and very fine sand on theshelf off Akkeshi. The coarse sediments on the eastern shelf off Hanasaki and the shelf margin are interpreted to represent lagdeposits from coastal erosion associated with a post-glacial transgression across topographic highs. The two depocenters of shelfsediments subsequent to the Last Glacial Maximum are situated in the middle–outer shelf off Kiritappu and in the inner–middleshelf off Akkeshi; these are interpreted as transgressive–highstand sequences whose locations were controlled by ridges on theshelf margin and topographic highs on the shelf. The shelf sediment thickness reaches ∼20 m, and the total volume is estimatedat ca. 6 km3. The slope sediments are grouped into medium–fine sand on the uppermost and upper slopes (150–2,000 m waterdepth) and mud-dominated sediments on the upper to middle slope (1,000–3,000 m water depth). The sandy sediments on theslope at water depths greater than 1,000 m are regarded as being derived from the shelf break by gravitational redistribution.The occurrence of diamictite-like sediments around gullies that incise into the slope indicates that mass movement and sedimentgravity flows play an important role in the redistribution of sediments upon the slope. On the upper–middle slope, hemipelagicfallout and advection of turbid water from the shelf edge provide a supply of suspended material. The main contributor ofsediment supply to the area is coastal erosion (1.81 Mt y−1), along with a lesser contribution from fluvial input (0.10 Mt y−1).During the latest Quaternary, the rates of sediment accumulation upon the shelf are estimated to have been ∼0.47 Mt y−1. Theremainder of the material transported into the sea is probably redistributed to deeper parts of the slope via gravitational sedimentflows or transported outside of the study area by alongshore and tidal currents. The studied shelf that kept less than a quarterof the mass from land is comparable with other active margin shelves of New Zealand and California, although they are fed byhigh sediment-laden rivers.

Key words: sediments, shelf, slope, grain size, sand, composition, budget, Japan

� NOTICE: this is the author’s version of a work that was acceptedfor publication in Sedimentary Geology. Changes resulting from peerreview are reflected, but editing, formatting, and pagination from thepublishing processes are not included in this document. A definitive

version will be published in DOI: 10.1016/j.sedgeo.2007.07.002.∗ Corresponding author. Fax: +81 29 861 3653.Email address: a.noda@aist.go.jp (Atsushi Noda).

Preprint submitted to Sedimentary Geology 17 July 2007

1. Introduction

Shelves and slopes at forearc margins are some ofthe most active regions in the world in terms of the ero-sion, transport, and deposition of sediment (e.g. Milli-man and Syvitski, 1992). Sediment dynamics in suchsettings interplay with the characteristic occurrence offaulting and folding associated with regional tecton-ics and the creation of accommodation space related tothese structures, in addition to hydrographic conditions,sea-level change, and climate. Several studies have in-vestigated sediment dispersal systems on tectonicallyactive marginal shelves and slopes such as the Cali-fornia margin, USA (Nittrouer, 1999; Edwards, 2002;Spinelli and Field, 2003; Sommerfield and Lee, 2004),Papua New Guinea (Kineke et al., 2000; Walsh andNittrouer, 2003; Keen et al., 2006), and the northernHikurangi margin of New Zealand (Lewis, 1973; Fos-ter and Carter, 1997; Orpin, 2004; Orpin et al., 2006).Such studies have emphasized the importance of struc-tural controls on the distribution of shelf–slope sedi-ments (Spinelli and Field, 2003; Orpin et al., 2006):thick sediment packages accumulate in structural lows,whereas little or no sediment is deposited upon struc-tural highs. In other words, the preservation of lowstandand transgressive deposits is controlled by structures orincised valley systems (e.g. Zaitlin et al., 1994; Ercillaand Alonso, 1996). Although comprehensive and quan-titative studies are necessary in order to gain a betterunderstanding of shelf–slope sediment dynamics alongactive forearc margins, only limited areas have beenwell studied.The area of eastern Hokkaido, northern Japan, is an

active forearc margin where the Pacific Plate is beingsubducted along the Kuril Trench (Fig. 1). The purposeof this study is to reveal sediment characteristics anddispersal systems upon the shelf and slope quantita-tively. This paper presents new information on the tex-ture, volume, and accumulation rate of sediments uponthe shelf and slope of the eastern Hokkaido forearc mar-gin, based on granulometric and mineralogic analyses ofsurface sediments and high-resolution seismic records.The sediment budget and factors controlling the ob-served sediment-dispersal systems are then discussed.The observations reported in this study will serve as auseful tool in understanding the interactions among thesupply, redistribution, and accumulation of sedimentsand sedimentary rocks upon shelves and slopes alongtectonically active forearc margins.

2. Physical setting

2.1. Geology

The study area is situated on the eastern Hokkaidomargin, which descends to the 7,000 m deep KurilTrench (Fig. 1). In this region, the Pacific Plate is beingsubducted beneath the Okhotsk (North American) Plateat about 8.0 cm y−1 toward ca. N62◦W (DeMets et al.,1990, 1994; Seno et al., 1996), with eastern Hokkaidomoving to the WSW at ∼3 cm y−1 (Ito et al., 2000)under the present tectonic regime. Oblique subductionof the Pacific Plate has caused dextral strike-slip move-ment within the Kuril forearc sliver (Fitch, 1972).Although geodetic data reveal steady subsidence at

average rates of 5–10 mm y−1 over the past 100 years(Shimazaki, 1974; Kasahara, 1975; Kasahara and Kato,1981), during the late Quaternary the area of easternHokkaido has risen slightly more than it has fallen. Theuplift rate and accrued elevation over the past 125,000years (Interglacial Stage 5e) are estimated to be 0.16–0.24 mm y−1 and 20–30 m, respectively, based on theelevations of coastal terraces (Okumura, 1996). The rateof tectonic uplift continued to equal or exceed tectonicsubsidence during the Holocene (Maeda et al., 1992;Sawai, 2001; Atwater et al., 2004). The uplift rate inthis area is one or two orders of magnitude less thanthe general rate of increase in sea level over the periodfrom the Last Glacial Maximum (LGM: ca. 18 ka) tothe sea level maximum of about 6 ka (ca. 120 m/12 ky= 10 mm y−1).The coastal land area shown in Fig. 2 is underlain

by the Nemuro Group, which comprises a sequenceof Cretaceous–Eocene marine clastic and hyaloclasticrocks that total approximately 3,000 m in thickness.The marine clastic sequence is composed mainly ofhemipelagic mudstone, turbidites, and submarine slumpdeposits (Kiminami, 1983; Naruse, 2003). The hyalo-clastic deposits in the lower part of the Nemuro Groupare overwhelmingly intermediate–mafic in composition,and the sedimentary rocks contain a large amount of de-tritus derived from these igneous rocks. Sandstones ofthe lower Nemuro Group are therefore rich in feldspar(35–80%), especially plagioclase, and rock fragments,while being quartz-poor (< 10%) (Kumon and Kimi-nami, 1994). In contrast, sandstones of the upper Ne-muro Group are relatively rich in quartz (30–60%) andK-feldspar (< 10%), and are characterized by addi-tional sources of granitic rocks (Okada, 1974; Kimi-nami, 1979). These sequences form homoclinal struc-tures that strike parallel to the coastline (east–west) and

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Fig. 1. (A) Index map of the survey area and location in Japan. (B) Bathymetry of the eastern Hokkaido forearc along the Kuril Trench. KSC,Kushiro submarine canyon.

dip gently to the south.The Nemuro Group is overlain by the Kushiro Group

and Kutcharo pyroclastic flow deposits. The former iscomposed of Pleistocene conglomerate and sandstone,while the latter consists of Pleistocene pumiceous rhy-olitic pyroclastic deposits (Okumura, 1991). ThesePleistocene strata are partly covered by HoloceneMashu volcanic ashes (Katsui, 1962).

2.2. Geography

Pleistocene marine terraces are common along thePacific coast in the present study area, forming cliffs ashigh as 80 m above the current sea level. The terrace

heights indicate that the elevation of the paleo-shorelinegradually decreases eastward from Akkeshi to Hanasaki(Okumura, 1996) (Fig. 2). No rivers enter the shelf be-tween Kiritappu and Hanasaki, except for very shortephemeral streams; however, short rivers (10–20 km inlength) and estuarine mouths are found on the coast be-tween Akkeshi and Kiritappu (Fig. 2).The shelf has a steep and rocky coastline with a com-

plex outline due to the presence of several capes, suchas Cape Ochiishi, and numerous islets. The bathymetryis characterized by a moderate (20–30 km wide) shelfand steep (up to 10◦) slope (Figs. 2 and 3A). The shelfbreak off Hanasaki occurs in 130–150 m of water depthand deepens westward, reaching 170–180 m depth offAkkeshi (Fig. 2). The shelf off Akkeshi is relatively

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Fig. 2. Sampling localities of seafloor surface sediments and survey lines of the seismic and Parasound sub-bottom profiling records. Solidcircles and open squares represent sampling points visited during Cruises GH03 (May–June, 2003) and GH04 (July–August, 2004), respectively.Solid and dashed lines represent acoustic survey lines surveyed during Cruises GH03 and GH04, respectively. The transects labeled x–x’ andy–y’ show the locations of topographic cross-sections oriented parallel to the coast, while the transects a–a’ and b–b’ indicate cross-sectionsoriented perpendicular to the coast (Fig. 3). Bold solid lines represent the Parasound sub-bottom and seismic profiles shown in Figs. 11 and 12.

gentle and has low relief, whereas the inner shelf offHanasaki and Cape Ochiishi is more irregular due tothe presence of topographic highs (line a–a’ in Fig. 3Band C).The slope is subdivided into three parts in this study:

the uppermost (shelf break to 1,000 m water depth), up-per (1,000–2,000 m water depth), and middle (2,000–3,000 m depth) parts. The uppermost slope has the high-est dip (average 5–6◦), reaching 10◦ in places (Fig. 3A),whereas the middle slope is less steeply dipping (1–3◦).No deep canyons cut into the outer shelf and slope;

instead, a series of gullies runs down the uppermost andupper slopes, oriented perpendicular to the main trendof the shelf. The gullies are concentrated in the area offCape Ochiishi, originating in the uppermost part (200–500 m) of the slope (Figs. 2 and 3D).

2.3. Climate and oceanography

The oceanography of the northwestern Pacific is dom-inated by the Oyashio Current, which is part of thewestern boundary current of the subarctic gyre (Ohtani,1991; Yasuda et al., 1996). The present-day axis of thecurrent lies within 42◦–42◦15’N, outside of the studyarea. The Oyashio Current moves to the southwest atca. 20–40 cm s−1 (e.g. Isoguchi et al., 1997; Ohshimaet al., 2005).The Off-Tokachi nearshore current (Sugiura, 1956)

also flows along the coastline toward the southwest orwest-southwest (Fig. 4). The average and maximumvelocities for this current are ca. 25–50 cm s−1 andca. 100 cm s−1, respectively (Maritime Safety Agency,1987, 1998, 1999). The northeast–southwest compo-nents in Fig. 4 represent local tidal currents.

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Fig. 3. Bathymetric profiles across the study area. (A) Profiles acrossthe shelf–slope along lines a–a’ and b–b’. The dip of the uppermostslope exceeds 10◦ in parts, and continues to a depth of more than2,000 m. (B) Details of across-shelf profiles along lines a–a’ andb–b’. Topographic highs occur off Cape Ochiishi (line a–a’). (C)Along-shelf profile along line x–x’. The seafloor off Cape Ochiishiis the shallowest area, and deepens both eastward and westward.(D) Along-slope profile along line y–y’. Sets of gullies cut the slopefrom off Cape Ochiishi to off Akkeshi. The locations of all thecross-sections are shown in Fig. 2.

3. Methods

In May and June 2003 (Cruise GH03) and July andAugust 2004 (Cruise GH04), 82 stations were visited onthe Pacific side of eastern Hokkaido as part of a cruiseundertaken by R/V Hakurei-maru No. 2 (Fig. 2 andTable 1). The targeted area was 42◦35’N–43◦20’N and144◦45’E–146◦15’E, with positions being determinedusing differential GPS.The cruise involved sampling surface sediment using

a grab sampler, taking photographs of the sea floor, col-lecting sediment cores using gravity and piston corers,and measuring oceanographic data (temperature, con-ductivity, pH, ORP, DO, and turbidity). Oceanographicdata for the bottom water were measured 2 m above theseafloor, and the turbidity of the water column was mea-sured at every sampling locality (69 points). The spec-ifications of the equipment precluded measurements ofturbidity at localities with water depth in excess of

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2,000 m.Seismic reflection profiles were collected during

Cruises GH03 and GH04 using a GI gun (250 in3

generator and 105 in3 injector airgun) with a 6 chstreamer cable. The survey speed was generally 8 knots(14.8 km h−1) and the shooting interval was 6 sec(ca. 25 m). The grid spacing was 2 miles (3.7 km)E–W and 4.5 miles (8.3 km) N–S (Fig. 2). Parasoundsub-bottom profiles (Atras, USA) were obtained atthe same time as the seismic profiles; these provedto be useful in analyses of sediment distribution (e.g.Damuth, 1980; Kuhn and Weber, 1993).The grab sampler used in this study typically recov-

ered sediments from the upper 10–15 cm of sand or 20–30 cm of mud, and in most cases preserved the stratig-raphy to a satisfactory degree. The uppermost 3–5 cmof the sediments was removed for analyses of grain-size and sand composition. Grain-size analyses of ma-terials coarser and finer than 4φ (62.5 mm) were madeusing dry sieve and hydrometer methods, with intervalsof 0.25 and about 0.5φ, respectively. These grain-sizedata were combined to produce complete grain-size dis-tributions. Textural parameters such as mean grain size,sorting coefficient, and skewness were calculated basedon Folk and Ward (1957).To study the structures within the sediment, soft X-

radiographs were taken from slab subsamples of thegrab samples using a Sofron Type STA-1005 operatedat a voltage of 45 kV, current of 3 mA, and irradiationtime of 30–90 s.The compositions of medium-sand (1.75–2.00φ) frac-

5

Table 1Sampling localities, grain sizes, and sand compositions of surface sediments surface sediments.

St. Longitude Latitude Mn Srt (ø) G VCS CS MS FS VFS Slt Cly Lm Hm Lf Vol Ben Plank Glauc Plant

1 145.817 43.300 36 -1.16 3.39 0.20 50.2 15.9 11.0 7.9 1.0 1.0 6.7 6.4 24.1 23.7 46.4 1.3 4.5 0.0 0.0 0.02 145.850 43.233 63 2.25 0.73 -0.41 0.0 0.7 7.4 19.3 62.1 7.0 3.5 0.0 33.0 33.9 21.5 10.3 1.3 0.0 0.0 0.03 145.766 43.200 62 2.54 0.78 0.11 0.0 0.5 2.7 12.3 66.6 11.7 3.2 3.1 45.1 16.0 23.5 13.1 2.3 0.0 0.0 0.04 145.800 43.134 93 1.81 0.86 -0.15 0.1 2.6 15.4 35.3 39.4 4.0 3.2 0.0 36.1 14.9 38.9 9.6 0.5 0.0 0.0 0.05 145.833 43.067 1276 145.867 43.000 889 2.36 1.67 -0.09 4.7 2.8 6.2 20.0 37.9 18.3 6.8 3.3 27.4 8.1 30.9 32.7 0.9 0.0 0.0 0.07 145.917 42.900 1893 4.63 2.77 0.68 0.0 0.4 1.0 4.0 27.8 22.7 29.3 14.9 10.9 3.2 15.0 65.9 3.6 1.4 0.0 0.08 145.967 42.800 2337 6.59 2.92 0.15 0.0 0.0 0.0 3.0 4.5 11.3 54.9 26.4 32.2 9.4 22.8 32.7 0.5 2.5 0.0 0.09 146.017 42.700 2679 7.28 3.20 0.04 0.0 0.2 0.6 0.9 6.0 9.0 46.2 37.3 15.4 2.4 8.2 66.3 4.8 2.4 0.0 0.5

10 146.066 42.600 2809 7.28 3.08 0.04 0.0 0.0 0.0 1.0 6.4 6.8 49.5 36.4 4.6 1.5 4.2 81.2 3.4 5.0 0.0 0.011 145.667 43.200 4512 145.700 43.133 71 2.36 1.10 -0.11 0.5 1.0 7.6 21.7 46.5 14.7 5.9 2.2 48.5 25.8 15.5 9.0 1.3 0.0 0.0 0.013 145.733 43.067 105 1.99 1.09 0.18 0.3 0.8 8.1 40.0 40.7 3.6 3.0 3.6 35.3 10.2 43.3 10.7 0.5 0.0 0.0 0.014 145.767 43.000 344 2.28 1.55 0.03 2.9 1.8 5.9 22.7 49.7 8.7 4.2 4.3 24.2 2.9 43.5 29.5 0.0 0.0 0.0 0.015 145.800 42.934 1034 4.61 2.89 0.70 0.0 0.0 0.0 6.2 28.9 25.0 24.2 15.7 32.3 12.9 21.2 32.3 1.4 0.0 0.0 0.016 145.850 42.833 2086 6.03 2.97 0.22 0.1 0.2 0.3 1.2 10.7 18.8 45.5 23.4 6.9 1.8 6.0 71.0 0.9 12.9 0.0 0.517 145.899 42.735 2449 6.28 3.20 0.05 0.0 0.8 2.0 4.4 7.2 9.1 45.0 31.6 47.4 9.5 29.4 10.0 0.9 2.8 0.0 0.018 145.950 42.633 2686 6.60 3.46 -0.06 0.0 0.9 1.4 5.3 10.2 8.4 41.2 32.6 5.3 0.9 8.3 83.3 0.9 1.3 0.0 0.019 145.600 43.134 4820 145.634 43.067 82 1.91 1.07 0.18 0.4 1.6 16.7 36.4 29.8 9.7 3.2 2.2 54.9 12.7 24.6 3.7 4.1 0.0 0.0 0.021 145.667 43.000 11222 145.700 42.933 596 2.17 1.88 -0.18 6.4 3.7 7.9 19.8 37.6 16.0 4.6 4.0 31.8 17.6 37.3 12.9 0.4 0.0 0.0 0.023 145.733 42.867 1677 5.02 2.89 0.58 0.3 0.6 1.3 3.0 18.7 26.3 31.9 17.9 26.8 2.3 1.8 52.3 13.6 2.7 0.0 0.524 145.783 42.767 2209 6.33 2.93 0.19 0.0 0.0 0.0 0.5 6.7 15.9 52.4 24.5 4.8 0.0 5.7 84.3 1.3 3.9 0.0 0.025 145.833 42.667 2587 6.26 3.11 0.08 0.0 0.0 0.0 3.0 12.2 11.6 45.2 28.1 31.8 13.9 26.5 26.0 0.9 0.9 0.0 0.026 145.533 43.067 52 0.81 2.22 -0.29 18.4 7.2 13.8 34.4 16.2 3.9 3.8 2.4 30.0 9.2 31.3 1.8 27.7 0.0 0.0 0.027 145.567 43.001 95 2.02 0.90 0.02 0.7 2.1 7.4 36.5 43.2 5.1 2.0 3.2 30.0 7.7 55.9 5.9 0.5 0.0 0.0 0.028 145.599 42.933 277 2.47 1.20 0.31 0.0 0.2 2.3 17.0 62.3 9.6 4.3 4.4 14.9 2.7 12.2 69.7 0.5 0.0 0.0 0.029 145.639 42.866 1174 3.30 4.16 0.05 15.3 4.3 5.3 6.3 10.2 18.5 28.0 12.1 40.5 16.4 19.8 16.8 5.6 0.9 0.0 0.030 145.668 42.803 1811 5.10 2.76 0.52 0.0 0.0 0.0 3.0 17.2 23.5 39.6 16.7 12.8 1.8 14.6 61.1 8.0 1.8 0.0 0.031 145.717 42.700 2183 6.01 2.90 0.21 0.0 0.0 0.0 0.5 10.6 18.6 48.5 21.8 2.5 0.4 2.1 73.3 3.3 18.3 0.0 0.032 145.766 42.600 2703 6.24 2.85 0.18 0.0 0.0 0.0 1.7 5.6 14.8 53.5 24.4 31.5 7.5 12.2 40.4 6.6 1.9 0.0 0.033 145.434 43.067 41 -0.58 2.46 -0.25 33.7 14.1 14.4 25.6 6.4 1.4 4.5 0.0 39.0 2.7 31.8 0.4 26.0 0.0 0.0 0.034 145.467 43.000 80 1.02 2.07 -0.15 14.5 8.2 19.3 36.4 12.1 4.2 1.9 3.4 56.8 7.7 33.8 1.3 0.4 0.0 0.0 0.035 145.501 42.934 130 2.25 0.58 -0.09 0.0 0.0 3.1 23.6 64.5 4.6 4.3 0.0 23.2 4.5 40.9 30.9 0.5 0.0 0.0 0.036 145.550 42.833 1258 4.28 2.46 0.69 0.1 0.2 0.7 4.3 26.5 32.0 24.9 11.3 9.5 1.4 12.2 65.6 8.6 1.8 0.5 0.537 145.600 42.734 1911 5.27 2.78 0.43 0.0 0.0 0.0 4.0 11.8 22.8 44.0 17.4 16.2 5.1 8.3 66.7 3.7 0.0 0.0 0.038 145.650 42.633 2270 6.20 2.95 0.15 0.0 0.0 0.5 1.4 8.1 16.8 50.0 23.3 16.9 1.8 11.6 56.0 0.9 12.9 0.0 0.039 145.333 43.066 38 2.87 1.25 0.37 0.0 0.2 0.3 1.6 62.8 23.7 5.4 6.1 14.3 2.8 31.3 12.9 38.7 0.0 0.0 0.040 145.366 43.000 64 2.87 0.85 0.24 0.0 0.2 0.7 2.0 59.4 29.1 4.5 4.2 12.6 0.0 24.4 52.6 8.1 1.5 0.0 0.741 145.399 42.934 111 2.41 0.87 0.05 0.8 0.5 3.4 16.3 62.5 10.4 2.5 3.5 33.5 15.0 38.5 12.0 1.0 0.0 0.0 0.042 145.433 42.867 336 2.60 1.42 -0.13 2.3 1.7 4.8 13.1 41.8 28.0 3.8 4.6 11.1 1.4 14.0 73.4 0.0 0.0 0.0 0.043 145.483 42.767 1566 6.14 3.07 0.24 0.0 0.0 0.0 2.1 10.0 18.6 45.4 23.9 22.4 8.6 35.7 30.5 1.4 1.4 0.0 0.044 145.533 42.667 2042 6.87 2.68 0.21 0.0 0.0 0.0 0.2 1.2 8.8 62.6 27.1 35.8 8.7 17.9 16.8 0.6 19.7 0.0 0.645 145.266 43.000 54 2.98 0.90 0.37 0.0 0.1 0.3 1.4 51.0 36.6 6.0 4.5 15.9 4.2 21.5 57.0 0.9 0.0 0.0 0.546 145.299 42.933 92 2.77 0.41 0.01 0.1 0.2 1.1 2.8 67.9 23.8 4.2 0.0 2.7 1.8 4.9 88.4 2.2 0.0 0.0 0.047 145.332 42.867 154 1.54 1.84 -0.54 14.8 2.3 5.9 21.7 42.1 8.9 4.4 0.0 42.6 11.8 28.9 16.7 0.0 0.0 0.0 0.048 145.367 42.800 749 2.86 1.43 0.04 0.5 1.6 3.9 10.9 32.7 38.6 7.3 4.6 32.6 5.1 20.0 41.4 0.9 0.0 0.0 0.049 145.417 42.700 1759 5.73 2.88 0.25 0.1 0.1 0.4 1.3 12.8 19.7 45.2 20.5 26.4 2.4 28.8 36.1 4.8 1.4 0.0 0.050 145.465 42.599 2197 6.90 3.04 0.14 0.0 0.0 0.1 0.4 5.2 11.8 53.4 29.2 1.8 0.0 5.0 81.3 2.7 9.1 0.0 0.051 145.167 43.000 46 2.97 0.96 0.31 0.0 0.3 0.6 3.3 49.4 35.3 7.0 4.0 22.4 2.2 38.1 13.9 22.9 0.4 0.0 0.052 145.200 42.934 83 3.08 0.98 0.52 0.0 0.1 0.2 0.9 47.6 39.2 7.6 4.5 1.3 0.4 1.3 95.2 1.8 0.0 0.0 0.053 145.232 42.867 123 2.77 1.14 0.29 0.4 1.0 2.6 7.4 58.5 20.4 5.8 4.0 7.7 1.0 9.1 81.3 1.0 0.0 0.0 0.054 145.267 42.800 459 2.37 1.88 -0.33 6.9 4.8 5.7 10.3 32.8 29.6 6.2 3.6 39.8 17.0 25.7 17.4 0.0 0.0 0.0 0.055 145.301 42.733 1312 5.22 2.98 0.49 0.1 0.2 1.3 5.4 14.6 22.2 38.6 17.6 30.0 8.3 22.1 33.6 2.3 3.7 0.0 0.056 145.349 42.634 1976 6.10 2.90 0.14 0.0 0.2 0.2 0.8 8.6 21.2 49.6 19.5 16.7 0.0 12.7 63.3 1.1 6.2 0.0 0.057 145.098 42.933 63 2.67 2.44 -0.01 5.7 4.3 4.5 8.1 33.6 29.1 7.9 6.8 19.2 3.9 28.4 23.1 25.3 0.0 0.0 0.058 145.132 42.866 110 3.14 1.24 0.53 0.1 0.3 0.8 2.2 46.1 36.2 9.0 5.2 11.9 1.0 15.7 70.5 0.5 0.5 0.0 0.059 145.167 42.801 148 0.46 2.43 0.06 32.1 12.3 9.7 14.8 19.4 5.3 2.9 3.6 30.8 11.8 34.8 22.6 0.0 0.0 0.0 0.060 145.201 42.733 806 2.93 2.22 0.29 0.7 1.8 5.8 21.9 28.1 22.5 11.8 7.3 55.8 10.7 22.8 9.8 0.0 0.0 0.9 0.061 145.234 42.667 1488 4.90 3.01 0.36 0.5 0.9 3.3 5.7 13.1 20.9 40.2 15.5 34.0 3.1 34.0 25.9 2.3 0.8 0.0 0.062 145.000 42.933 56 3.13 0.92 0.55 0.0 0.1 0.1 1.1 44.4 41.5 9.0 3.8 1.5 0.8 12.8 62.4 19.5 1.5 0.0 1.563 145.032 42.867 93 3.26 1.13 0.63 0.1 0.0 0.3 1.4 43.0 38.7 11.9 4.6 21.6 0.6 13.2 59.3 1.8 1.8 0.0 1.864 145.067 42.800 130 2.14 1.65 -0.16 5.3 5.0 6.4 18.5 43.8 13.6 3.8 3.6 42.6 16.7 34.3 5.1 0.0 0.0 1.4 0.065 145.100 42.734 370 1.94 1.16 -0.11 1.3 5.0 14.5 28.6 33.7 12.4 4.6 0.0 30.1 7.4 29.6 30.6 1.4 0.9 0.0 0.066 145.133 42.667 1207 5.33 3.15 0.26 0.3 0.8 1.8 6.8 13.2 15.2 41.2 20.7 34.1 0.4 33.2 31.4 0.4 0.4 0.0 0.067 145.167 42.600 1614 6.15 2.93 0.07 0.3 1.0 1.4 3.3 4.8 12.7 53.3 23.2 40.2 10.2 40.6 7.1 1.6 0.4 0.0 0.068 144.932 42.867 87 3.18 0.87 0.60 0.0 0.1 0.2 1.0 42.0 45.2 7.3 4.2 6.1 0.0 5.7 87.4 0.9 0.0 0.0 0.069 144.968 42.800 115 2.85 1.31 0.24 0.6 0.5 1.3 7.3 53.5 26.7 4.7 5.5 31.9 11.0 33.3 22.9 0.5 0.5 0.0 0.070 145.000 42.733 174 1.85 0.98 -0.17 2.7 3.8 10.2 36.1 37.1 6.4 3.7 0.0 56.2 14.6 24.8 4.4 0.0 0.0 0.0 0.071 145.033 42.667 849 2.94 1.08 0.22 0.1 0.5 1.0 9.2 43.7 34.3 7.3 3.9 36.3 4.9 43.5 14.4 0.9 0.0 0.0 0.072 145.067 42.600 1356 4.71 2.82 0.45 0.0 0.2 0.6 6.2 22.1 19.8 36.9 14.1 0.4 0.0 0.0 94.8 0.9 3.9 0.0 0.073 144.837 42.867 82 3.36 1.33 0.72 0.0 0.2 0.2 1.1 34.8 46.4 11.5 5.8 1.7 0.4 0.9 83.2 6.0 2.2 0.0 5.674 144.867 42.799 119 3.09 1.19 0.47 0.0 0.1 0.4 2.8 44.9 39.5 7.1 5.2 3.9 0.4 5.7 86.5 2.2 0.4 0.0 0.975 144.900 42.734 167 2.36 1.29 0.31 0.0 0.4 3.2 27.7 49.3 11.6 3.2 4.6 49.5 9.5 30.0 11.0 0.0 0.0 0.0 0.076 144.934 42.667 582 2.61 1.52 -0.08 1.4 3.3 5.4 13.5 38.0 29.2 5.0 4.3 28.2 4.8 47.1 18.1 0.0 1.3 0.0 0.477 144.967 42.600 1154 5.07 2.87 0.34 0.0 0.2 0.5 5.3 17.5 18.5 41.9 16.0 1.6 0.0 2.0 93.3 0.4 2.8 0.0 0.079 144.767 42.800 109 3.64 1.66 0.81 0.0 0.1 0.2 1.5 33.0 43.4 13.4 8.4 27.6 1.4 16.8 51.9 0.9 0.5 0.0 0.980 144.799 42.733 163 2.59 0.75 0.20 0.0 0.1 1.4 13.3 57.6 20.6 3.8 3.2 45.2 3.3 30.0 20.7 0.7 0.0 0.0 0.081 144.833 42.666 432 2.02 1.39 -0.43 6.9 5.8 6.0 17.8 42.0 16.9 4.6 0.0 32.1 8.1 21.1 38.3 0.5 0.0 0.0 0.082 144.867 42.600 970 4.38 2.57 0.45 0.0 0.5 1.3 6.6 20.9 26.2 32.9 11.6 13.0 2.5 9.0 73.0 1.0 1.5 0.0 0.087 144.767 42.600 884 4.15 2.31 0.46 0.2 0.4 1.1 5.9 20.7 31.4 32.3 8.0 32.6 4.7 15.8 45.6 0.0 1.4 0.0 0.0

Waterdepth

Weight percent of each size fraction (wt%) Sand composition of fine sand fraction (%)Sk

Abbreviations, Mn: mean grain size, Srt: sorting, Sk: skewness, G: gravel, VCS: very coarse sand, C: coarse sand, MS: medium sand, FS: fine sand, VFS: very fine sand, Slt: silt, Cly: clay, Lm: light minerals, Hm: heavy minerals, Lf: lithic fragments, Vol: volcaniclastics, Ben: benthis remains, Plank: planktonic remains, Glauc: glauconite, Plant: marine plant fragments.

6

tions were determined from 200–400 points countedunder a stereomicroscope. Sand grains were classifiedinto eight categories: light minerals, heavy minerals,lithic fragments, volcaniclastics (pumice and volcanicglass), benthic remains, planktonic remains, glauconite,and marine plant fragments.The sediment budget was calculated to gain a semi-

quantitative understanding of the active sediment-dispersal systems. Coastal erosion was estimated onthe basis of coastline retreat measured via a compari-son of dated (published in 1922) and recent (publishedin 1996) topographic maps. To estimate the mass oferoded material, we assumed that the elevation anddry bulk density of rocks eroded from sea cliffs were20 m and 2.6 g cm−3 (Suda et al., 1991), respectively.Values of height and density adopted for beaches were3 m and 1.4 g cm−3 (using an average porosity forsandy sediments of 49%, as proposed by Pryor 1973),respectively.The volumes of shelf sediments were calculated

based on measurements of sediment thickness and dis-tribution in sub-bottom profiling records and geologicalmaps previously reported by Maritime Safety Agency(1987, 1998, 1999). To estimate the mass, a dry bulkdensity of 1.4 g cm−3 was assumed for the post-glacialshelf sediments. Sedimentation rates on the slope wereestimated with reference to historical volcanic ash lay-ers (dating from the 17th century) that are intercalatedwith the sediments. The value of dry bulk density was0.40 g cm−3, as calculated from the water content(200% wet weight) of muddy surface sediments in coresamples obtained close to the study area (Noda et al.,2005; Kuroyanagi et al., 2006).

4. Results

4.1. Textures

Surface sediments were not recovered at station num-ber 5, 11, 19, and 21 because the seafloor at these siteswas rocky (Table 1); the nature of the seafloor was con-firmed from photographs. The mean grain sizes of theinner shelf from off Hanasaki to off Cape Ochiishi andthe shelf edge from off Akkeshi to off Kiritappu are allcoarser than coarse sand (0–1φ) (Fig. 5A). Medium sand(1–2φ) was recorded on the outer shelf off Hanasaki,while fine (2–3φ) to very fine sand (3–4φ) occurs on theshelf off Kiritappu and Akkeshi. Fine sand also coversmuch of the uppermost slope (< 1,000 m), with veryfine sand to medium silt (5–6φ) upon the upper slope(1,000–2,000 m). Fine silt (6–7φ) and very fine silt (7–

8φ) are distributed upon the middle slope (> 2,000 m).The degree of sorting within the study area ranges

from well sorted (0.4) to extremely poorly sorted (4.2)(Fig. 5B). Almost all of the shelf sediments are classi-fied as moderately (0.71–1.0) or poorly sorted (1.0–2.0),with the exception being gravelly sediments on the in-ner shelf from off Hanasaki to off Cape Ochiishi. Wellsorted (0.35–0.5) to moderately well sorted (0.5–0.71)sediments occur on the outer shelf off Kiritappu; siltysediments on the slope are generally very poorly sorted(> 2.0).Skewness values for shelf sediments off Hanasaki

are strongly negative (< −0.3) to negative (−0.3–−0.1)(Fig. 5C), as are those for shelf-edge sediments offKiritappu–Akkeshi. In contrast, sediments on the shelfoff Akkeshi show strongly positive skewness (> 0.3).The obtained values reveal along-shelf variations, witha trend of gradually increasing skewness to the west(Fig. 5C). The upper slope sediments (1,000–2,000 m)are also strongly positively skewed, while those on themiddle slope (deeper than 1,500–2,000 m) are positivelyskewed (0.1–0.3).High gravel contents (> 10 wt%) are recorded for sed-

iments on the inner shelf from off Hanasaki to off CapeOchiishi and on the shelf edge off Kiritappu (Fig. 6).The sediments from the shelf edge to the uppermostslope contain in excess of 1 wt% gravel. Mud contentis generally low (< 10 wt%) on the shelf off Hanasakiand the shelf edge, whereas the mud contents of shelfsediments off Kiritappu to Akkeshi and part of the areaoff Hanasaki (St. 1) exceed 10 wt%; on the middle shelfoff Akkeshi, mud contents exceed 15 wt% (Fig. 6).On the slope, mud contents rapidly increase from theuppermost to upper slope. Sediments on the slope offHanasaki in water depths greater than 1,800–2,000 mcontain more than 60 wt% mud, as do sediments offAkkeshi at depths of 1,200–1,500 m .

−200

−200

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−100

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−1000

145˚00' 145˚20' 145˚40' 146˚00'

42˚40'

43˚00'

43˚20'

0 10 20

km

Gravel (> 10 wt%)

Rocky bottom

Gravel (1–10 wt%)Mud content

10%

10%

10%30%

10%

30%

60%

10%

15%

Cape Ochiishi

Hanasaki

AkkeshiBay

Hokkaido

KiritappuAkkeshi

Fig. 6. Gravel and mud contents (wt%).

7

−0.4

0.4

42˚40'N

42˚50'N

43˚00'N

43˚10'N 0 10 20

km

AkkeshiKiritappu

Hanasaki

−0.6

−0.4

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0.0

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0.4

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0.8Skewness

1

1

2

42˚40'N

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43˚00'N

43˚10'N 0 10 20

km

AkkeshiKiritappu

Hanasaki

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Sorting1

2

6

145˚00 E 145˚20 E 14˚540 E 146˚00 E

42˚40'N

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km

AkkeshiKiritappu

Hanasaki

−1

0

1

2

3

4

5

6

7

Mean (ø)

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77

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11

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3

33

3

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0

–0.4–0.4

0

0

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0

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0.4

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0.4

0.40.4

0

0

0

0

0

–0.4

–0.4–0.4–0.4

0

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1

11

1

1

1

1

2

2

2

2

22

3

3

33

3

02

2

2

2

3

1

3

3

4

4

56

7

OchiishiOchiishiOchiishi

OchiishiOchiishiOchiishi

OchiishiOchiishiOchiishi

Fig. 5. Spatial distribution of (A) mean grain size, (B) degree of sorting, and (C) skewness of the surface sediments.

8

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145˚00' 145˚20' 145˚40' 146˚00'

42˚40'

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43˚20'

0 10 20

km

Benthic remains (> 20%)

Pumice & volcanic glass(> 50%)

Heavy minerals(> 10%)

Planktonic remains(> 5%)

Marine plant fragments(> 1%)

Cape Ochiishi

Hanasaki

KiritappuAkkeshi

Hokkaido

Fig. 7. Areas of concentrations of selected components within themedium-sand fraction (1.75–2.0φ) of the sampled sediments.

4.2. Sand composition

The main components of the sand-sized fraction arelight and heavy minerals, lithic fragments, and volcani-clastic grains (Table 1); in combination, these compo-nents make up more than 80% of the total grains in themajority of the analyzed samples. Quartz and feldsparare both classified as light minerals, as their similarshapes and densities mean that they exhibit similar be-haviors under sea water. The concentrations of lightminerals are relatively high (> 30%) in shelf sedimentsfrom offHanasaki to off Cape Ochiishi, at the shelf edgeoff Akkeshi, and on the upper slope.The heavy minerals observed within the analyzed

samples are dominantly orthopyroxene and clinopy-roxene, with lesser amounts of amphibole, mica, andopaque minerals such as magnetite. High concentra-tions of heavy minerals (> 10%) are recorded from theshelf to the uppermost slope off Hanasaki, and from theshelf edge to the uppermost slope off the region fromKiritappu to Akkeshi (Fig. 7). Concentrations of heavyminerals in excess of 15% are recorded on the shelf offHanasaki (St. 1, 2, 3, and 12), the shelf edge (St. 64),and the uppermost slope (St. 22, 29, and 54).Lithic fragments are also one of the main components

within the sand fraction. Fragments of intermediate–mafic igneous rocks and sedimentary rocks (chert, mud-stone, and sandstone) are dominant in this category. Thesediments on the shelf off Hanasaki and the shelf edgerecord the highest percentages of lithic fragments (morethan 30%).Many sediment samples obtained from the shelf and

slope contain high concentrations of pumice and vol-

canic glass. Pumice grains have a very low density, andare sometimes transported by suspension on the sea sur-face. The shelf sediments off Akkeshi and those on theupper and middle parts of the slope have high pumicecontents (> 50%) (Fig. 7).Benthic remains observed within the analyzed sam-

ples include shells, foraminifera, sponges, bryozoa, os-tracoda, and echinoid spines. Shell fragments are abun-dant in the shelf sediments, while foraminifera are dom-inant in the slope sediments. The inner shelf sedimentsfrom off Cape Ochiishi to off Akkeshi have high con-tents (more than 20%) of benthic bioclasts (Fig. 7).Planktonic remains include radiolarians, diatoms, and

foraminifera. Diatoms and radiolarians are dominantin the study area, whereas planktonic foraminifera arerare. Areas of high concentrations of planktonic remains(> 5%) are zonally distributed along the middle slope(Fig. 7).Marine plant fragments consist mainly of algae and

sea grass fragments. Sediments on the inner shelf offAkkeshi contain more than 1% plant fragments (Fig. 7).

4.3. Turbidity

Turbidity generally decreases with increasing waterdepth. Relatively high (> 1.0 ppm) bottom-water turbid-ity was observed on the inner shelf from off Kiritapputo off Akkeshi (St. 51, 62, 73) and on the shelf edgeoff Kiritappu (St. 41 and 47) (Fig. 8A). The shelf fromoff Hanasaki to off Akkeshi and shelf edge off Kiri-tappu recordedmoderate turbidity values (0.5–1.0 ppm),whereas very low values were recorded for the bottomwater on the upper and middle slope. Exceptionally highturbidity was recorded at a single station on the upperslope (St. 7). Intermediate and bottom nepheloid layersfrom the shelf edge to areas off the shelf were identifiedin the vertical turbidity profiles (Fig. 8B).

4.4. Sediment structure

Sediment structures within the surface sedimentswere examined using soft X-radiographs. Fine sandsupon the middle–outer shelf off Hanasaki (St. 2 and 3)lack gravel and contain weakly developed parallel lam-inae and burrows (Fig. 9). The medium to coarse sandsand gravels from off Hanasaki to off Cape Ochiishi(Figs. 5 and 6) are devoid of cross and ripple lami-nations. Muddy sands on the shelf from off Kiritapputo off Akkeshi contain numerous burrows near thesediment–water interface (0–10 cm below the seafloor)(Fig. 9). The lower sediments at St. 73 contain lami-

9

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145˚00' 145˚20' 145˚40' 146˚00'

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0 10 20

km

Akkeshi Cape Ochiishi

Hanasaki

Kiritappu

Turbidity (ppm)

> 2.0

1.0–2.0

0.5–1.0

0.2–0.5

< 0.2

45 46 47 48 49

600

400

200

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800

Wat

er d

epth

(m)

Station number

1.0–2.0 ppm

0.5–1.0 ppm

> 2.0 ppm

0.2–0.5 ppm

< 0.2 ppm

X Y

X

Y

(A)

(B)

Fig. 8. (A) Spatial distribution of turbidity of the bottom water (2 mabove the seafloor). (B) Vertical profiles of turbidity along the lineX–Y.

nated sands and shell fragments (Fig. 9). The gradualupward reduction in the brightness of the radiographsindicates an upward reduction in grain size with ongo-ing accumulation.Surface sediments on the upper slope contain uncon-

formities at 3–10 cm below the seafloor (Fig. 9). Theseboundaries are easily identified in the radiographs aslayers of sharp contrast. The unconformities generallyconsist of poorly sorted gravelly and muddy (diamictite-like) sediments that overlie semi-consolidated muddydeposits.

4.5. Sub-bottom profiles

The classification scheme adopted for the Parasoundsub-bottom profiles (SBPs) obtained in the present studywas based on acoustic classes related to the form andsignature of the echo from the seafloor. Two broad fa-cies were identified based on the intensity of the surfacereflection: distinct (Facies I) and indistinct (Facies II).Facies I showed strong reflection with no penetrationor internal reflection, while Facies II exhibited a cer-tain width of reflection with internal reflectors or sub-bottom reflection beneath the surface reflection. Facies

I was further subdivided into two sub-facies based onthe form of the surface reflection: irregular (Facies IA)and smooth (Facies IB) (Fig. 10). Facies II was furthersubdivided into the following four types as Facies I:continuous (Facies IIA), discontinuous (Facies IIB), hy-perbolic (Facies IIC), and weak (Facies IID), with eachof these being subdivided in turn into the following fiveclasses based on the observed internal reflection pattern:no internal or sub-bottom reflectors (a), sub-bottom butnot internal reflectors (b), several continuous stratifiedreflectors (c), several discontinuous reflectors (d), andseveral weak reflectors (e) (Fig. 10). The facies mapswere then compiled based on data obtained from theN–S survey lines.Facies IA occurs in parts of the inner shelf from off

Hanasaki to off Akkeshi, along the shelf margin fromoff Hanasaki to off Kiritappu Ochiishi, and on the outershelf off Akkeshi (Fig. 10). This facies corresponds tooutcrop or gravelly sediment (Fig. 11). Facies IB, whichrepresents fine to coarse sand, is largely distributedon the shelf off Cape Ochiishi and the outer shelf offAkkeshi.Facies IIAa, IIAb, and IIAc are distributed on the

shelf off Hanasaki, Kiritappu, and Akkeshi, as well asareas upon the middle slope (Fig. 10). These facies aredeposited on the shelf upon the erosional surface of theLast Glacial Maximum (Fig. 11). The facies offAkkeshiBay change to the west from IIAb to IIAa and IIAc(Fig. 11). Facies IIAa and IIAb represent fine to mediumsand, while Facies IIAc is silt to very fine sand.Facies IIDa and IIDe are characteristic of the upper-

most (average gradient in excess of 4–6◦) and upper(2–4◦) slope, respectively. The lack of penetration ofacoustic waves reflects the steepness of the slope. FaciesIIAd and IIBd are recognized around gullies developedupon the middle slope.

4.6. Seismic profiles

A seismic profile across the shelf to the slope revealsa clear two-part division of the strata based on an onlapunconformity (Fig. 12). The lower unit (U) dips steeplyseaward and is characterized by continuous but weakreflectors; accordingly, it cannot be traced to deeperlevels. Unit U is evident in the SBP records around theshelf edge (Fig. 11A), which represents the Paleogenefrom the shelf to the upper slope (TuZino et al., 2004,2005). The unit was uplifted and tilted subsequent tothe late Paleogene, as the uppermost part of the NemuroGroup extends to the Eocene where exposed onland,and Unit L onlapped the underlying Unit U (TuZino

10

0 5

cm

St. 67(1614 m)

St. 29(1174 m)

Laminae

Burrows?

(62 m)St. 3

Weak laminae

Burrows

(63 m)St. 2

(109 m)St. 79

Burrows

(82 m)St. 73

Burrows

Shell

Hanasaki Akkeshi Slope

Fig. 9. Soft X-radiographs of the shelf sediments off Hanasaki (St. 2 and St. 3), off Akkeshi (St. 73 and St. 79), and slope sediments (St. 29and St. 67). Fine sands on the outer shelf off Hanasaki contain weakly developed laminae and burrows. The muddy sands on the shelf offAkkeshi contain numerous bioturbation traces. Unconformities are recognized 4–6 cm below the surface of the slope sediments, where poorlysorted gravelly and muddy (diamictite-like) sediments are deposited upon semi-consolidated muddy sediments.

et al., 2004, 2005).The upper unit can be subdivided into subunits Qa,

Qb, P, M, and L, which are characterized by strong sub-horizontal reflectors. Based on a comparison with theacoustic units, more than 4,000 m of drill core obtainedsouthwest of Kushiro (Sasaki et al., 1985), and onlandexposures, the units Qa, Qb, P, M, and L are assignedto the Quaternary, the Pliocene, the upper Miocene, themiddle Miocene, and the lower Miocene, respectively(TuZino et al., 2004, 2005).Anticlines trending ENE–WSW are observed in the

middle slope at water depths of 2,000–2,500m (Fig. 12).Although the length of each fold axis is 15–20 km, sev-eral anticlines occur in the middle slope (Fig. 10). Theseismic record shown in Fig. 12 reveals that a greaterthickness of sediments was deposited on the landwardside of the anticline. The thickness of sediment alongthe anticline axis is similar to that in the limbs for unitsM to P; in contrast, a greater thickness is recorded alongthe limbs for unit Qb and Qa. This indicates that the an-ticline developed mainly during deposition of unit Qb

(Pliocene).

4.7. Sediment budget

Comparisons of dated and recent topographic mapsrevealed that the combined erosion of sea cliffs(2.414 km2) and beaches (2.814 km2) over the past75 years considerably exceeds the amount of coastalaccretion over that time (0.228 km2) (Fig. 13). The netrate of coastline erosion was calculated to be 0.066 km2

y−1 (4.94 km2/75 years), while the total erosion rate inthe study area was estimated to be 1.67 Mt y−1 for seacliffs and 0.14 Mt y−1 for beaches, representing a totalof 1.81 Mt y−1 (Fig. 13). The volumes and rates ofcoastal erosion determined for three sections (Hanasaki,Kiritappu, and Akkeshi) were 0.0136 km2 y−1 and0.46 Mt y−1, 0.0467 km2 y−1 and 1.08 Mt y−1, and0.0103 km2 y−1 and 0.27 Mt y−1, respectively.Although there is no data for the sediment supply of

small streams entering the shelf, the transported vol-umes are known for the Kushiro River and its tribu-

11

Nemuro

Nemuro Peninsula

Akkeshi −200

−200

−100−100−100

−100

−200

0

−2000

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−1000

−1000

'

42˚40'N

42˚50'N

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0 10 20

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Cape Ochiishi

Hanasaki

AkkeshiBay

HokkaidoHokkaidoHokkaido

Kiritappu

AkkeshiAkkeshiAkkeshi

Distinct, irregular, and no internal reflectors. Rough topography on the shelf and shelf edge.Distinct, smooth, and prolonged bottom reflectors. No internal reflectors. Inner–outer shelf.

Indistinct, smooth, prolonged bottom reflector. No subbottom and internal reflectors. Inner–outer shelf.

Indistinct, smooth, prolonged bottom reflector with subbottom. No internal reflectors. Inner to middle shelf.

Indistinct, smooth, prolonged bottom reflector. Continuous and parallel internal reflectors. Inner and middle shelf, and middle slope.

Indistinct, smooth, and prolonged bottom reflector. Discontinuous internal reflectors. Occurs in vicinity of facies IIAc.

Indistinct and discontinuous bottom and internal reflectors, associated with rough topography, especially channelized slope.

Hyperbolae bottom reflector. No internal and subbottom reflectors. Rough topography on slope.

Weak bottom reflector without internal reflector. Very steep slope.

Weak bottom reflector with low penetration. Stratified or discontinuous internal reflectors. Steep slope.

Anticline

Gully

IA

IB

IIAa

IIAb

IIBd

IICa

IIDa

IIDe

IIAc

IIAd

Fig. 10. Spatial distribution of the acoustic facies determined from the Parasound sub-bottom profiler records.

12

IAIIAa IIAbIIAc

WSW ENE(B) Line 1002

IAIIAa IIAbIIAbIAIBa

SSW NNE(A) Line 23

50 m

5 km 0

Fig. 11. Parasound sub-bottom profiling records for (A) a transect across the shelf off Kiritappu and (B) a transect along the shelf off Akkeshi.The maximum penetration depth is about 15 m. The location of the survey line is shown in Fig. 2. Facies labels are shown in Fig. 10.

taries (2,510 km2), flowing in the west of the studyarea (Fig. 1). The annual rate of the transported ma-terials is about 14,930 m3 y−1 (ca. 0.21 Mt y−1 if thedensity is 1.4 g cm−3) (Ohtsuka, 2003). We applied thevalue of the Kushiro River for estimation of the inputvolumes and rates in the drainage areas of each sec-tions (41 km2 for Hanasaki, 202 km2 for Kiritappu, and942 km2 for Akkeshi). The approximation resulted in0.004, 0.017, and 0.079 Mt y−1; the first two were twoorder of magnitude less than the values of coastal ero-sion. The last value in Akkeshi area was about one forthof the coastal erosion, and the total sediment input wastherefore 0.35 Mt y−1 (Fig. Fig. 13).If possible, volumes of sediment deposited on

the 17th-century tephras or the transgressive surfaceshould be used for estimation of accumulation rateson the shelf, because the sediment supply related to

the sea level might be stable during the Holocene inthis area (Ohta et al., 1990). Nevertheless, we hereused the volumes since the Last Glacial Maximum(ca. 18,000 years), as the tephra layers or the transgres-sion surface were hardly identified in the shelf sedi-ments or the seismic data, like other margins aroundJapan (cf. Saito, 1994). The volumes of sediment ac-cumulated on the shelf since the LGM were calculatedto be 0.37 km3 off Hanasaki, 2.19 km3 off Kiritappu,and 3.47 km3 off Akkeshi (Fig. 13). The average massaccumulations for each area were therefore 0.03, 0.17,and 0.27 Mt y−1 (using an average dry bulk densityof 1.4 g cm−3), respectively. The sum of these values(0.47 Mt y−1) represents approximately 25% of thesediment supply from land.The sedimentation rate on the upper–middle slope

over the past 340 years was 0.032–0.088 cm y−1, as con-

13

0

1.0

2.0

3.0

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500 1000 1500 2000

10 km

VE: 8.8

NNW SSE

TW

T (

s) AnticlineAnticline

LL

PP

MM

QaQaQaQbQbQb

UU

Anticline

CDP

Fig. 12. Representative seismic profile for the area off Cape Ochiishi. Horizontal axis, CDP, is ca. 25 m; Vertical axis TWT is ca. 750 m.Labels for seismic units are explained in the text.

0.088 cm y0.088 cm y-1-1

(0.035 g cm y(0.035 g cm y-1-1)

0.032 cm y0.032 cm y-1-1

(0.012 g cm y(0.012 g cm y-1-1)

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(0.013 g cm y(0.013 g cm y-1-1)

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144˚50'E 145˚00'E 145˚10'E 145˚20'E 145˚30'E 145˚40'E 145˚50'E 146˚00'E 146˚10'E

42˚40'N

42˚50'N

43˚00'N

43˚10'N

43˚20'N

Akkeshi

Sediment thickness

2.5 m

10 m

5 m

15 m

Sedimentationrate

Sediment gravityflow deposits

SBP facies IA

Kiritappu

3.47 km3.47 km3 (0.27

(0.27 Mt Mt y-1-1 )

2.19 km2.19 km3

2.19 km2.19 km3

(0.0.17 (0.0.17 Mt Mt y

-1-1 )

0.37 km

0.37 km3

(0.0(0.03 3 Mt Mt y

-1-1 )

0.047 cm y0.047 cm y-1-1

(0.018 g cm y(0.018 g cm y-1-1)0.032 cm y0.032 cm y-1-1

(0.012 g cm y(0.012 g cm y-1-1)3.47 km3 (0.27 Mt y-1 )

2.19 km3

(0.17 Mt y-1 )

0.37 km3

(0.03 Mt y

-1 )

Coastal erosion0.27 Mt y-1

Fluvial input0.079 Mt y -1

Coastal erosion1.08 Mt y-1

Fluvial input0.017 Mt y -1

Coastal erosionCoastal erosion0.46 Mt y0.46 Mt y-1-1

Fluvial inputFluvial input0.004 Mt y0.004 Mt y -1 -1

Coastal erosion0.46 Mt y-1

Fluvial input0.004 Mt y -1

0.088 cm y-1

(0.035 g cm y-1)

0.047 cm y-1

(0.019 g cm y-1)0.032 cm y-1

(0.013 g cm y-1)

0.032 cm y-1

(0.013 g cm y-1)

0.034 cm y-1

(0.014 g cm y-1)

Fig. 13. Estimated rates of eroded mass along the coast and fluvial input (Mt y −1). Also shown are sediment thicknesses (m), volumes (km3),and mass accumulation rates (Mt y−1) for the modern shelf sediments and sedimentation rates (cm y−1) for the slope sediments. Data onsediment thickness upon the shelf were derived from Maritime Safety Agency (1987, 1998, 1999).

14

strained from the 17th-century tephras. Assuming a drybulk density of 0.4 g cm−3 for muddy sediments on theseafloor, the rate of sediment accumulation is estimatedto be ca. 0.024 g cm−2 y−1 (0.013–0.035 g cm−2 y−1).

5. Discussion

5.1. Sediment characteristics and dispersal systems

5.1.1. Shelf area off HanasakiFrom off Hanasaki to Cape Ochiishi, high concen-

trations of heavy minerals in the sand fraction reflectthe fact that the sediments are coarse-grained (Fig. 7).The occurrence of abundant intermediate–mafic vol-canic and sedimentary lithic fragments and pyroxenessuggests that the sands were originally derived from theNemuro Group, as exposed upon the Nemuro Penin-sula. Transgressive erosion of the cliffs along the coastis likely to be a major contributor to the gravelly andsandy detritus accumulating on the inner shelf, mainlyat depths of less than 20 m (Figs. 13 and 14B).Under the present conditions, the small stream inputs

result in low mud contents in sediments of the middle–outer shelf (Fig. 6), so that relict sediments remain as athin (∼2.5 m) cover over the middle–outer shelf. A smallamount of mud, partly found in the gravelly bottom onthe inner shelf (Fig. 6), is considered to be derived fromsuspended muddy particles produced mainly by coastalerosion (Fig. 13).

5.1.2. Shelf area off KiritappuThe sediments are characterized by high percent-

ages of benthic remains in the inner-shelf sedimentsand pumice grains in the outer shelf. The volcani-clastic grains are possibly derived from the Kutcharoand Mashu pyroclastic flow deposits (Katsui, 1962)or the other Holocene airfall tephras that originatedfrom western Hokkaido (Furukawa and Nanayama,2006). The Holocene tephras are widespread off easternHokkaido and are intercalated as 1–5 cm thick layerswithin surface marine sediments between Hiroo andNemuro (Noda et al., 2003, 2004). Because of their lowdensity, pumice grains generally behave in the samemanner as finer grains.Sediments in excess of 15 m thickness are deposited

in the middle–outer shelf (Fig. 13). The location of thedepocenter suggests that the distribution of postglacialshelf deposits was controlled by ridges on the shelfmargin (SBP Facies IA). This depositional area mightbe generated by north-trending normal faults associatedwith strike-slip (transtensional) tectonics from oblique

subduction of the Pacific Plate (Fitch, 1972; Kimura andTamaki, 1986), as observed in the shelf along the east-ern Nankai subduction zone (Yamaji et al., 2003), inthe submarine Aleutian forearc (Geist et al., 1988), andin the Tibet Plateau (McCaffrey and Nabelek, 1998).Several buried channels might have acted as active con-duits to the outer shelf and beyond during lowstand pe-riods (Fig. 14A), which are buried by transgressive orhighstand sediments and connected to the depocenters(Fig. 13). These sediments are interpreted to represent adiscrete sand sheet within a transgressive systems tract.The nearshore area is covered by ∼5 m of sediments

with minimal input from streams, has probably beendirectly derived from coastal erosion. Strong winter-time storm waves attack the cliffs and remove beachsand to offshore areas. Landslides that develop withinthe coastal cliffs, especially earthquake-induced slopefailures, also frequently deliver sediment into the sea(Tajika et al., 1994a,b; Amemiya and Tajika, 1999).The middle–outer shelf sediments have relatively low

mud contents (< 10% in Fig. 6), and some (St. 34 and46) are moderately well sorted to well sorted (Fig. 5).These features indicate that little mud is deposited in thearea deeper than the storm wave base (ca. 40 m in Suna-mura, 1987), although a zone of high turbidity (St. 41and 47) extends from the nearshore area to the outershelf off Kiritappu–Akkeshi (Fig. 8). The lack of mudcan be explained by the fact that the suspended parti-cles within the bottom water (Fig. 8B) are transportedto deeper waters as both bottom and intermediate neph-eloid layers, as observed in other slope settings (Mc-Cave, 1972; Pak and Zaneveld, 1977; Pak et al., 1980;McGrail and Carnes, 1983; Cacchione and Drake, 1986;Durrieu de Madron et al., 1990; Walsh and Nittrouer,1999, 2003).In addition, the resuspension of fine particles by phys-

ical processes and subsequent offshore transport fromthe slope may leave behind only sandy sediment. TheOff-Tokachi nearshore current flows westward at speedsof up to 100 cm s−1, making it capable of transport-ing suspended material and redistributing previously de-posited sediments (Fig. 4). The simple calculation pro-posed byMiller et al. (1977), in which u100 = 122.6D0.29

for D ≤ 0.2 cm, where u100 is the average current ve-locity 100 cm above the seafloor and D is grain diam-eter, suggests that the threshold flow velocities for 3φand 4φ grains are 34.4 cm s−1 and 14.4 cm s−1, respec-tively. Although the velocity just above the seafloor isunknown, the Off-Tokachi nearshore current is almostcertainly able to transport fine fractions from within theshelf sediments (Fig. 14B).

15

010

20km

010

20km

A

B

Sediments derived from coastal erosion

Along shelf transport by oceanic current

Suspended transport pathway

Gravitational transport pathway

Fig. 14. (A) A schematic diagram of paleoenvironment upon the shelf during the early transgression stage. (B) Present-day distribution ofsediments and their dispersal systems.

5.1.3. Shelf area off AkkeshiThe depocenter in this area is located along the coast

in water depths of between 40 and 80 m, extending tothe west (Figs. 10 and 13). This area is also surroundedby topographic highs similar to those off Kiritappu. Thepresence of a high-turbidity area (Fig. 8) suggests thatsuspended particles are being supplied to the area, prob-ably from coastal erosion, river discharge, and the es-tuarine mouths of Lake Akkeshi and Akkeshi Bay viawave or tidal action (cf. Hubbard et al., 1979; Dalrym-ple et al., 1992). The material supplied to the ocean isprobably deposited on the seafloor at depths below thestorm wave base (ca. 40 m), following transportation byalongshore and tidal currents. Accordingly, these sedi-ments are interpreted to represent modern sediments ofpresent highstand systems tracts.

5.1.4. Shelf margin–uppermost slopePoorly sorted medium to gravelly sands are dis-

tributed in water depths of between 110 and 1,000 m(Fig. 5), and contain gravel contents of more than 1wt% (Fig. 6); in addition, outcrops were recognizedat a number of localities off Hanasaki (St. 5 and 21).The geometry in the area around the shelf break is

irregular, bearing features such as ridges (Fig. 11) andslump scours (Fig. 2). The water depths at the shelfedge (130–180 m) are too deep to be affected by thetypes of modern storm waves that typically producelag deposits.The coarse sediments around the shelf edge are re-

garded as relicts of the period from the last glacialage to the early transgression stage (Fig. 14A). At thistime, river mouths might have connected to gullies thatincised into the slope during lowstand periods, andthereby might directly transport terrigenous detritus tothe deep sea (Fig. 14A). The gullies developed furtherwith repeated submarine slope failures that occurred inresponse to the large amount of material produced dur-ing the following transgression with failure triggeredby earthquakes related to the subduction of the PacificPlate.The coarse sandy sediments extend to the uppermost

slope (1,000 m water depth) where hemipelagic muddysediments are generally deposited (e.g. Doyle et al.,1979; Stanley et al., 1983). The slope is characterizedby its steepness and the presence of incised gullies thatrepresent repeated downcutting by gravity flows (e.g.Field et al., 1999). The large amounts of gravel and

16

heavy minerals, in combination with small amounts ofplanktonic remains, suggest that the uppermost slopesediments were supplied from the shelf edge under theinfluence of gravity mainly during the Last Glacial Max-imum. The steepness (5–10◦) of the slope might haveprevented hemipelagic suspended particles from settlingto the sediment surface.

5.1.5. Upper slopePoorly sorted and positively skewed sandy sediments

are deposited on the upper slope (Fig. 5). The texturalcharacteristics of these sediments can be explained bythe mixing of coarse deposits and fine hemipelagic par-ticles. The coarse fractions may have been gravitation-ally derived from the shelf edge or the uppermost slope.The advection of turbid water (Fig. 8) implies inter-mediate and bottom nepheloid-layer transport from theshelf edge. Nepheloid-layer transport involves the sea-ward movement of shelf-generated turbid water alongisopycnal surfaces (Fig. 8). The turbid water that led tothe development of intermediate and bottom nepheloidlayers may originally have been produced mainly bycoastal erosion, resuspension by storm waves and cur-rents, and perhaps infrequently by river discharge, andestuarine plumes (Fig. 14B). The particles within theturbid water and hemipelagic fallout could have con-tributed to the slope sediment, producing poorly sortedand positively skewed fine-grained deposits. The loca-tion of the highest accumulation rate within the slope,situated at the extension of the high-turbidity area offKiritappu, indicates that nepheloid-layer transport is animportant component of the off-shelf sediment disper-sal system.

5.1.6. Middle slopeMuddy sediments are distributed on the upper to mid-

dle slope at water depths in excess of 1,000 m (Fig. 5).High proportions of volcaniclastic detritus and plank-tonic remains are interpreted to reflect primary depo-sition by the precipitation of surface-plume fallout ornepheloid transport (Fig. 7). Sediments on the steepslope and in areas around gullies occasionally containunconformities, with the deposits immediately abovethe unconformities being very poorly sorted muddy sed-iments (Figs. 9 and 13). These observations suggest thatmass movements or sediment gravity flows might erodeand redeposit the surface sediments upon the slope.Rapid subduction of the Pacific Plate beneath Hokkaidoacts to compress the forearc margin (Tada and Kimura,1987), which may in turn act to steepen and deformthe slope (Fig. 12). In addition, large earthquakes oc-

cur frequently along the subduction zone (Kanamori,1970; Shimazaki, 1974; Kikuchi and Fukao, 1987; Hi-rata et al., 2003; Yamanaka and Kikuchi, 2003), andthese events may reduce the strength of the slope sedi-ments (e.g. Lee and Edward, 1986; Normark and Piper,1991; Hampton et al., 1996). Such seismicity could po-tentially trigger mass movements or sediment gravityflows. In the eastern Hokkaido forearc (offKushiro), therecurrence interval for sediment gravity flows over thepast 340 years is estimated to be ca. 60–70 years (Nodaet al., 2004); accordingly, sediment gravity flows are in-terpreted to be an important component of the sedimentdispersal system upon the steep part of the slope.

5.2. Sediment budget

5.2.1. Shelf areaThemass accumulation rates offHanasaki (0.03Mt y−1)

and Kiritappu (0.17 Mt y−1) represent approximately6% and 16% of the sediment supply around Hanasaki(0.46 Mt y−1) and Kiritappu (1.08 Mt y−1), respectively.Coastal erosion can therefore explain the entire post-glaciation sediment mass on the shelf. The remainderof the material derived from coastal erosion must havebeen transported to deeper areas as a nepheloid layeror moved out of the study area by nearshore and tidalcurrents. This implies that the majority of sediment in-put is dispersed seaward to the slope, as reported fromother active margins (cf. Sommerfield and Nittrouer,1999; Orpin et al., 2006). Small inputs of terrigenousmaterial could have not covered much of the outershelf, particularly in the east.The accumulation rate off Akkeshi (0.27 Mt y−1) is

nearly balanced by the sediment supply (0.35 Mt y−1)in the Akkeshi area. The higher ratio of accumulation toinput rates than other areas could be attributed to trans-portation by the Off-Tokachi nearshore current from theeast. Sedimentation with along-shelf transport would bemore important, as the shelf widened with sea-level rise.

5.2.2. SlopeAn accumulation rate of about 0.032–0.088 cm y−1

(mass accumulation rate of 0.013–0.035 g cm−2 y−1)was estimated for the middle slope (Fig. 13). The lowrate of mass accumulation reflects the low dry bulk den-sity (0.4 g cm−3) of the surface sediments, which mayin turn reflect high levels of diatom productivity withinthe Oyashio Current during the Holocene (Shiomotoet al., 1994; Narita et al., 2002; Ikehara et al., 2006).Assuming that the sedimentation area on the slope is1,000–2,000 km2, the sedimentation rate is calculated

17

to be 0.24–0.48 Mt y−1. The total accumulation rateon the shelf–slope (0.71–0.95 Mt y−1) is insufficient toexplain the sediment budget from land (1.91 Mt y−1),even if productivity in the ocean is not considered. It istherefore interpreted that most of the sediments on theupper–middle slope were transported to the deeper partof the slope or the trench by sediment gravity flows; al-ternatively, the westerly Off-Tokachi nearshore currenton the shelf may have transported suspended particlesout of the study area. The transported fines might havebeen concentrated in submarine canyons by currents ortides. Such an interpretation is supported by the veryhigh sedimentation rates (40 cm y−1, 16 g cm−2 y−1from Noda et al., 2004) recorded in the bottom of theKushiro submarine canyon off Kushiro (Fig. 1).

5.3. Comparison of the eastern Hokkaido forearc shelfwith other active margin shelves

We compared the eastern Hokkaido margin with theintensively studied shelves–slopes at Poverty Bay, alongthe northern Hikurangi margin, New Zealand, and theEel shelf along the northern California margin, USA(Fig. 15). Both of these margins are located in tectoni-cally active regions and are similar in scale to the easternHokkaido shelf; however, both receive high terrigenoussediment fluxes from large rivers: the Waipaoa River(delivering 15 Mt y−1 of mud) in the case of PovertyBay (Hicks et al., 2000) and the Eel River (15.3 Mt y−1)in the case of the northern California margin (Sommer-field and Nittrouer, 1999). The deposits on both shelvesare distributed in the middle–outer shelves off the rivermouths; the thickness are controlled by anticlines and/orsynclines (Foster and Carter, 1997; Spinelli and Field,2003). The shelf sediments in eastern Hokkaido havebeen similarly deposited in topographic lows on themiddle–outer shelf, and barriered by topographic highs.These geomorphic/tectonic controls of sediment distri-bution may be common in active margin settings.The sediment loads of these rivers are significantly

larger than that of coastal erosion along the easternHokkaido margin (1.81 Mt y−1). The sediment thick-nesses and mass accumulation rates on the Poverty Bayshelf are 45 m and 0.9–1.7 Mt y−1, respectively (Fosterand Carter, 1997; Orpin et al., 2006), while the equiv-alent values for the Eel shelf are more than 60 m and3 Mt y−1 (Sommerfield and Nittrouer, 1999; Spinelliand Field, 2003); these values are much larger thanthose recorded in the present study area (20 m and0.47 Mt y−1). Despite the differences, the ratio of thetrapped load in the shelf to the total input load in the

0 10 20

km

178˚00'E 178˚20'E

39˚00'S

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–100

–200

Fig. 15. Comparison of sediment distributions and thicknesses deter-mined in the study area with those obtained for other active marginshelves. (A) Data for the present study area. (B) Data for PovertyBay, northern Hikurangi margin, New Zealand (modified from Orpinet al., 2006). (C) Data for the Eel River margin, offshore northernCalifornia, USA (modified from Spinelli and Field, 2003).

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present study (25% in total; 6–16% off Hanasaki toKiritappu) is comparable with the values reported forPoverty Bay shelf (7–11%) and the Eel shelf (20%).This suggests that active margins without large riverscould store less than a quarter of derived materials fromland during the transgressive and highstand sea level, aswell as active margins with high sediment-laden rivers.

6. Conclusions

The surface sediments on the shelf and slope of east-ern Hokkaido, northern Japan, were grouped into graveland coarse sand on the inner shelf off Hanasaki, finesand on the middle–outer shelf off Kiritappu, and veryfine sand on the middle–outer shelf off Akkeshi. Thearea from the shelf margin to the uppermost slope (<500 m water depth) was covered by fine to coarse grav-elly sand, although some areas were devoid of sedi-ment. Medium–fine sand on the upper slope was de-posited even though the water depth exceeded 500 m.The upper–middle slope (< 1,000 m water depth) wasdraped by silt–clay.The composition of sand within the sediments reflects

hydraulic conditions and sediment sources. Pumicegrains and plant fragments were concentrated in areasof modern sedimentation with high mud content, whileplanktonic remains were concentrated in the middle-slope sediments. In contrast, high concentrations ofheavy minerals were observed around the shelf marginand topographic highs off Hanasaki, where coarse togravelly sand was distributed.Rapid transgression at the beginning of the deglacia-

tion might have produced coarse gravelly depositsaround the shelf edge. The subsequent sea-level rise de-posited sandy sediments as transgressive and highstanddeposits within topographic lows and incised valleysoff Kiritappu and Akkeshi, whereas topographic highsoff Hanasaki and Cape Ochiishi limited transgressivecover. The distribution of sediments is influenced byseafloor topography that might originally have formedunder the control of regional transpressional tectonics.Sediments upon the slope were derived from

hemipelagic fallout, nepheloid-layer transport, andmass movement or sediment gravity flows. The tectoniccompression generated the steep dip of the slope (5–10◦), thus enabling the redistribution of medium–finesands to water depths of 1,000 m or greater. These grav-itational events were possibly associated with frequentsubduction-related earthquakes that occur in the region.The sediment supply to the shelf has been domi-

nated by coastal erosion (1.81 Mt y−1) than fluvial dis-

charge (0.10 Mt y−1). The total mass derived from landis not balanced by the mass accumulation rates on theshelf (0.47 Mt y−1, ∼25% of the total) and slope (0.24–0.48 Mt y−1, 13–27% of the total). The remainder ofthe inputs may be transported to deeper parts of theslope–trench by gravitational flows, or removed fromthe study area by alongshore and tidal currents. Thestudied shelf that kept less than 25% of the mass fromland is comparable with other active margins of NewZealand and California, although they are fed by highsediment-laden rivers.

Acknowledgments

We are indebted to the officers, crew, and researchstaff of the R/V Hakurei-maru No. 2 for their kind sup-port during the survey. The sea-beam data used for un-dersea topography were obtained by the Hydrographicand Oceanographic Department of the Japan CoastalGuard. This manuscript has benefited from constructivereviews by K. B. Lewis and B. P. Roser. This study waspart of the “Marine Geological Mapping Project of theContinental Shelves Around Japan” program supportedby the Geological Survey of Japan/AIST.

References

Amemiya, K., Tajika, J., 1999. Slope movements caused by the threelarge earthquakes of Hokkaido, 1993–1994. Journal of JapanLandslide Society 35 (4), 26–33, (in Japanese with English ab-stract).

Atwater, B. F., Furukawa, R., H., E., Ikeda, Y., Kashima, K.,Kawase, K., Kelsey, H. M., Moore, A. L., Nanayama, F.,Nishimura, Y., Odagiri, S., Ota, Y., Park, S.-C., Satake, K., Sawai,Y., Shimokawa, K., 2004. Seventeenth-century uplift in easternHokkaido, Japan. The Holocene 14, 487–501.

Cacchione, D. A., Drake, D. E., 1986. Nepheloid layers and internalwaves over continental shelves and slopes. Geo-Marine Letters6, 147–152.

Dalrymple, R. W., Zaitlin, B. A., Boyd, R., 1992. Estuarine faciesmodels; conceptual basis and stratigraphic implications. Journalof Sedimentary Petrology 62, 1130–1146.

Damuth, J. E., 1980. Use of high-frequency (3.5–12 kHz) echogramsin the study of near-bottom sedimentation processes in the deep-sea: a review. Marine Geology 38, 51–75.

DeMets, C., Gordon, R. G., Argus, D. F., Stein, S., 1990. Currentplate motions. Geophysical Journal International 101, 425–478.

DeMets, C., Gordon, R. G., Argus, D. F., Stein, S., 1994. Effectof recent revisions to the geomagnetic reversal time scale onestimates of current plate motions. Geophysical Research Letters21, 2191–2194.

Doyle, L. J., Pilkey, O. H., Woo, C. C., 1979. Sedimentation onthe eastern United States continental slope. In: Doyle, L. J.,Pilkey, O. H. (Eds.), Geology of continental slopes. Vol. 27 ofSpecial Publication - Society of Economic Paleontologists and

19

Mineralogists. SEPM (Society for Sedimentary Geology), Tulsa,OK, United States (USA), pp. 119–129.

Durrieu de Madron, X., Nyffeler, F., Godet, C. H., 1990. Hydro-graphic structure and nepheloid spatial distribution in the Gulf ofLions continental margin. Continental Shelf Research 10, 915–929.

Edwards, B. D., 2002. Variations in sediment texture on the north-ern Monterey Bay National Marine Sanctuary continental shelf.Marine Geology 181, 83–100.

Ercilla, G., Alonso, B., 1996. Quaternary siliciclastic sequencestratigraphy of western Mediterranean passive and tectonicallyactive margins; the role of global versus local controlling factors.In: de Batist, M., Jacobs, P. (Eds.), Geology of siliciclastic shelfseas. Geological Society Special Publications 117. GeologicalSociety, London, pp. 125–137.

Field, M. E., Gardner, J. V., Prior, D. B., 1999. Geometry andsignificance of stacked gullies on the northern California slope.Marine Geology 154, 271–286.

Fitch, T. J., 1972. Plate convergence, transcurrent faults, and internaldeformation adjacent to southeast Asia and the western Pacific.Journal of Geophysical Research 77, 4432–4460.

Folk, R. L., Ward, W. C., 1957. Brazons River bar: a study inthe significance of grain size parameters. Journal of SedimentaryPetrology 27, 3–26.

Foster, G., Carter, L., 1997. Mud sedimentation on the continentalshelf at an accretionary margin; Poverty Bay, New Zealand. NewZealand Journal of Geology and Geophysics 40 (2), 157–173.

Furukawa, R., Nanayama, F., 2006. Holocene pyroclastic fall de-posits along the Pacific coastal region of eastern Hokkaido. Bul-letin of Volcanological Society of Japan 51 (6), 351–371, (inJapanese with English abstract).

Geist, E. L., Childs, J. R., Scholl, D. W., 1988. The origin of summitbasins of the Aleutian Ridge; implications for block rotation ofan arc massif. Tectonics 7 (2), 327–341.

Hampton, M. A., Lee, H. J., Locat, J., 1996. Submarine landslides.Reviews of Geophysics 34, 33–59.

Hicks, D. M., Gomez, B., Trustrum, N. A., 2000. Erosion thresh-olds and suspended sediment yields, Waipaoa River Basin, NewZealand. Water Resources Research 36, 1129–1143.

Hirata, K., Geist, E. L., Satake, K., Tanioka, Y., Yamaki, S.,2003. Slip distribution of the 1952 Tokachi-Oki earthquake(M 8.1) along the Kuril trench deduced from tsunami wave-form inversion. Journal of Geophysical Research 108 (B4), doi:101029/2002JB001976.

Hubbard, D. K., Oertel, G., Nummedal, D., 1979. The role of wavesand tidal currents in the development of tidal-inlet sedimen-tary structures and sand body geometry; examples from NorthCarolina, South Carolina, and Georgia. Journal of SedimentaryPetrology 49, 1073–1091.

Ikehara, K., Ohkushi, K., Shibahara, A., Hoshiba, M., 2006. Changeof bottom water conditions at intermediate depths of the Oyashioregion, NW Pacific over the past 20,000 yrs. Global and PlanetaryChange 53 (1–2), 78–91.

Isoguchi, O., Kawamura, H., Kono, T., 1997. A study on wind-driven circulation in the subarctic North Pacific using theTOPEX/Poseidon altimeter data. Journal of Geophysical Research102, 12457–12468.

Ito, T., Yoshioka, S., Miyazaki, S., 2000. Interplate coupling innortheast Japan deduced from inversion analysis of GPS data.Earth and Planetary Science Letters 176, 117–130.

Kanamori, H., 1970. Synthesis of long-period surface waves and itsapplication to earthquake source studies-Kurile Islands earthquake

of October 13, 1963. Journal of Geophysical Research 75, 5011–5027.

Kasahara, K., 1975. Aseismic faulting following the 1973 Nemuro-Oki earthquake, Hokkaido, Japan (a possibility). Pure and AppliedGeophysics 113 (1), 127–139.

Kasahara, K., Kato, T., 1981. Aseismic faulting following the 1973Nemuro-oki earthquake, Hokkaido, Japan (a retrospective study).Pure and Applied Geophysics 119, 392–403.

Katsui, Y., 1962. Explanatory text of the geological map of Japan,Kutcharo (scale 1:50000). Hokkaido Development Agency, 42pp.,(in Japanese with English abstract).

Keen, T. R., Ko, D. S., Slingerland, R. L., Riedlinger, S., Flynn, P.,2006. Potential transport pathways of terrigenous material in theGulf of Papua. Geophysical Research Letters 33 (L04608), doi:10.1029/2005GL025416.

Kikuchi, M., Fukao, Y., 1987. Inversion of long-period P-wavesfrom great earthquakes along subduction zones. Tectonophysics144 (1-3), 231–247.

Kiminami, K., 1979. Sedimentary petrography of the Nemuro Group.Chikyu Kagaku (Earth Science) 33 (3), 152–162, (in Japanesewith English abstract).

Kiminami, K., 1983. Sedimentary history of the late Cretaceous–Paleocene Nemuro Group, Hokkaido, Japan: a forearc basin ofthe Paleo-Kuril arc-trench system. The Journal of the GeologicalSociety of Japan 89, 607–624.

Kimura, G., Tamaki, K., 1986. Collision, rotation and back-arcspreading in the region of the okhotsk and japan seas. Tectonics5, 389–401.

Kineke, G. C., Woolfe, K. J., Kuehl, S. A., Milliman, J. D., Del-lapenna, T. M., Purdon, R. G., 2000. Sediment export from theSepik River, Papua New Guinea; evidence for a divergent sedi-ment plume. Continental Shelf Research 20, 2239–2266.

Kuhn, G., Weber, M. E., 1993. Acoustic characterization of sedi-ments by parasound and 3.5 kHz systems: Related sedimentaryprocesses on the southeastern Weddell Sea continental slope,Antarctica. Marine Geology 113, 201–217.

Kumon, F., Kiminami, K., 1994. Model and chemical compositionsof the representative sandstones from the Japanese Islands andtheir tectonic implications. In: Kumon, F., Yu, K. M. (Eds.),Proceedings of the 29th International Geological Congress, PartA. VSP BV, Utrecht, The Netherlands, pp. 135–151.

Kuroyanagi, A., Kawahata, H., Narita, H., Ohkushi, K., Aramaki,T., 2006. Reconstruction of paleoenvironmental changes based onthe planktonic foraminiferal assemblages off Shimokita (Japan)in the northwestern North Pacific. Global and Planetary Change53 (1–2), 92–107.

Lee, H. J., Edward, B. D., 1986. Redional method to assess offshoreslope stability. Journal of Geotechnical Engineering 112, 489–509.

Lewis, K. B., 1973. Sediments on the continental shelf and slopebetween Napier and Castlepoint, New Zealand. New ZealandJournal of Marine and Freshwater Research 7 (3), 183–208.

Maeda, Y., Nakada, M., Matsumoto, E., Matsuda, I., 1992. Crustaltilting derived from Holocene sea-level observations along theeast coast of Hokkaido in Japan and upper mantle rheology.Geophysical Research Letters 19, 857–860.

Maritime Safety Agency, 1987. Basic map of the sea in coastalwaters (1:50,000), Hanasaki. Tokyo, Japan.

Maritime Safety Agency, 1998. Basic map of the sea in coastalwaters (1:50,000), Akkeshi. Tokyo, Japan.

Maritime Safety Agency, 1999. Basic map of the sea in coastalwaters (1:50,000), Kiritappu. Tokyo, Japan.

20

McCaffrey, R., Nabelek, J., 1998. Role of oblique convergence in theactive deformation of the Himalayas and southern Tibet Plateau.Geology 26 (8), 691–694.

McCave, I. N., 1972. Transport and escape of fine-grained sedimentfrom shelf areas. In: Swift, D. J. P., Duane, D. B., Pilkey, O. H.(Eds.), Shelf sediment transport: process and pattern. Dowden,Hutchinson and Ross, Stroudsburg, PA, pp. 225–248.

McGrail, D. W., Carnes, M., 1983. Shelfedge dynamics and thenepheloid layer in the northwestern Gulf of Mexico. In: Stan-ley, D. J., Moore, G. T. (Eds.), The shelfbreak; critical interfaceon continental margins. Vol. 33 of Special Publication - Soci-ety of Economic Paleontologists and Mineralogists. Society ofEconomic Paleontologists and Mineralogists, pp. 251–264.

Miller, M. C., McCave, I. N., Komar, P. D., 1977. Threshold ofsediment motion under unidirectional currents. Sedimentology24, 507–527.

Milliman, J. D., Syvitski, J. P. M., 1992. Geomorphic/tectonic controlof sediment discharge to the ocean; the importance of smallmountainous rivers. Journal of Geology 100, 525–544.

Narita, H., Sato, M., Tsunogai, S., Murayama, M., 2002. Biogenicopal indicating less productive northwestern North Pacific duringthe glacial ages. Geophysical Research Letters 29 (22), doi:10.1029/2001GL014320.

Naruse, H., 2003. Cretaceous to Paleocene depositional history ofNorth-Pacific subduction zone; reconstruction from the NemuroGroup, eastern Hokkaido, northern Japan. Cretaceous Research24, 55–71.

Nittrouer, C. A. (Ed.), 1999. The formation of continental-marginstrata. Vol. 154 of Marine Geology. Elsevier, Amsterdam, Nether-lands (NLD), 1–420pp.

Noda, A., Katayama, H., Ikehara, K., 2003. Surface sediments offTokachi. In: Okamura, Y. (Ed.), Marine Geological and Geophys-ical Studies on the Collision Zone of Kurile and Northeast JapanArcs—Off Tokachi Area—. Preliminary reports on researches inthe 2002 fiscal year, GSJ Interim Report, no. 26. GeologicalSurvey of Japan, AIST, pp. 54–67, (in Japanese).

Noda, A., Katayama, H., Ikehara, K., 2005. Paleoseismicity de-duced from sediment cores in the submarine fan off Nemuro. In:Katayama, H. (Ed.), Marine Geological and Geophysical Studieson the Collision Zone of Kurile and Northeast Japan Arcs—OffNemuro and Hidaka Area—. Preliminary reports on researchesin the 2004 fiscal year, GSJ Interim Report, no. 33. GeologicalSurvey of Japan, AIST, pp. 84–97, (in Japanese).

Noda, A., Tsujino, T., Furukawa, R., Yoshimoto, N., 2004. Character,provenance, and recurrence intervals of Holocene turbidites inthe Kushiro Submarine Canyon, eastern Hokkaido forearc, Japan.The Memoirs of the Geological Society of Japan 58, 123–135.

Normark, W. R., Piper, D. J. W., 1991. Initiation processes and flowevolution of turbidity currents; implications for the depositionalrecord. In: Osborne, R. H. (Ed.), From shoreline to abyss; con-tributions in marine geology in honor of Francis Parker Shep-ard. Vol. 46 of Society of Economic Paleontologists and Min-eralogists Special Publication. Society for Sedimentary Geology,Tulsa, OK, pp. 207–230.

Ohshima, K. I., Wakatsuchi, M., Saitoh, S., 2005. Velocity field ofthe Oyashio region observed with satellite-tracked surface driftersduring 1999-2000. Journal of Oceanography 61, 845–855.

Ohta, Y., Umitsu, M., Matsushima, Y., 1990. Recent Japanese re-search on relatice sea level changes in the Holocene and relatedproblems – review of studies between 1980 and 1988 –. Daiyonki-Kenkyu (The Quaternary Research) 29, 31–48, (in Japanese withEnglish abstract).

Ohtani, K., 1991. To confirm again the characteristics of Oyashio.Bulletin of the Hokkaido National Fisheries Research Institute55, 1–24, (in Japanese with English abstract).

Ohtsuka, J., 2003. Constraint of sediment inputs into thekushiro mire. In: Research conference of national land,infrastructure, and transport in the 2003 fiscal year. Min-istry of National Land, Infractructure, and Transport, p. 8,http://www.mlit.go.jp/chosahokoku/h17giken/program/kadai/pdf/ippan/kan1-04.pdf, (in Japanese).

Okada, H., 1974. Migration of ancient arc–trench systems. In: Dott,R. J., Shaver, R. H. (Eds.), Modern and Ancient GeosynclinalSedimentation; Problems of palinspastic restoration. Vol. 19 ofSociety of Economic Paleontologists and Mineralogists SpecialPublication. Society of Economic Paleontologists and Mineralo-gists, SEPM (Society for Sedimentary Geology), Tulsa, UnitedStates, pp. 311–320.

Okumura, K., 1991. Quaternary tephra studies in the Hokkaidodistrict, northern Japan. Daiyonki-Kenkyu (The Quaternary Re-search) 30, 379–390, (in Japanese with English abstract).

Okumura, K., 1996. Tephrochronology, correlation, and deformationof marine terraces in eastern Hokkaido, Japan. GeographicalReports of Tokyo Metropolitan University 31, 19–26.

Orpin, A. R., 2004. Holocene sediment deposition on the Poverty-slope margin by the muddy Waipaoa river, East Coast NewZealand. Marine Geology 209, 69–90.

Orpin, A. R., Alexander, C., Carter, L., Kuehl, S., Walsh, J. P., 2006.Temporal and spatial complexity in post-glacial sedimentation onthe tectonically active, Poverty Bay continental margin of NewZealand. Continental Shelf Research 26, 2205–2224.

Pak, H., Zaneveld, J. R. V., 1977. Bottom nepheloid layers andbottom mixed layers observed on the continental shelf off Oregon.Journal of Geophysical Research 82, 3921–3931.

Pak, H., Zaneveld, J. R. V., Kitchen, J., 1980. Intermediate neph-eloid layers observed off Oregon and Washington. Journal ofGeophysical Research 85, 6697–6708.

Pryor, W. A., 1973. Permeability–porosity patterns and variationsin some Holocene sand bodies. The American Association ofPetroleum Geologists Bulletin 57, 162–189.

Saito, Y., 1994. Shelf sequence and characteristic bounding surfacesin a wave-dominated setting; latest Pleistocene–Holocene exam-ples from Northeast Japan. Marine Geology 120, 105–127.

Sasaki, A., Kachi, T., Sasaoka, T., Iguchi, T., 1985. Stratigraphyof the Kisoshisui Tokachi-oki well—on the study of Mioceneturbidite facies in eastern Hokkaido—. Journal of the JapaneseAssociation for Petroleum Technology 50, 53–63, (in Japanesewith English abstract).

Sawai, Y., 2001. Episodic emergence in the past 3000 years at theAkkeshi Estuary Hokkaido, northern Japan. Quaternary Research56, 231–241.

Seno, T., Sakurai, T., Stein, S., 1996. Can the Okhotsk plate bediscriminated from the North American plate? Journal of Geo-physical Research 101, 11305–11315.

Shimazaki, K., 1974. Nemuro-oki earthquake of June 17, 1973: alithospheric rebound at the upper half of the interface. Physicsof the Earth and Planetary Interiors 9, 314–327.

Shiomoto, A., Sasaki, K., Shimoda, T., Matsumura, S., 1994. Primaryproductivity in the offshore Oyashio in the spring and summer1990. Journal of Oceanography 50, 209–222.

Sommerfield, C. K., Lee, H. J., 2004. Across-shelf sediment trans-port since the last glacial maximum, Southern California margin.Geology 32 (4), 345–348.

Sommerfield, C. K., Nittrouer, C. A., 1999. Modern accumulation

21

rates and a sediment budget for the Eel River shelf, USA; aflood-dominated depositional environment. Marine Geology 154,227–241.

Spinelli, G. A., Field, M. E., 2003. Controls of tectonics and sed-iment source locations on along-strike variations in transgres-sive deposits on the northern California margin. Marine Geology197 (1-4), 35–47.

Stanley, D. J., Addy, S. K., Behrens, E. W., 1983. The mudline;variability of its position relative to shelfbreak. In: Stanley, D. J.,Moore, G. T. (Eds.), The shelfbreak; critical interface on continen-tal margins. Vol. 33 of Special Publication - Society of EconomicPaleontologists and Mineralogists. SEPM (Society for Sedimen-tary Geology), Tulsa, OK, United States (USA), pp. 279–298.

Suda, Y., Murata, Y., Kikuchi, T., Hanaoka, N., 1991.Physical Properties Data-Base System of Rocks (PROCK).Vol. 155 of Open File Report of Geological Surveyof Japan. Geological Survey of Japan, Tsukuba, 231pp.,(http://www.aist.go.jp/RIODB/prock/welcomee.html).

Sugiura, J., 1956. A note on current branches in the Oyashio area.Journal of Oceanography 12 (4), 117–119, (in Japanese withEnglish abstract).

Sunamura, T., 1987. Coastal cliff erosion in Nii-Jima Island, Japan.In: Gardinar, V. (Ed.), International Geomorphology 1986, Part2. Wiley, New York, pp. 1192–1212.

Tada, T., Kimura, G., 1987. Collision tectonic and crustal defor-mation at the southwestern margin of the Kuril Arc. Zisin 40,197–204, (in Japanese with English abstract).

Tajika, J., Fukami, H., Okazaki, N., Ozawa, S., Endo, Y., Kuro-sawa, K., Ohtsu, S., Ogino, T., Ishimaru, S., Akita, F., 1994a.Ground failures and disasters caused by the 1993 Kushiro-Okiearthquake. Vol. 23 of Geological Survey of Hokkaido SpecialReport. Geological Survey of Hokkaido, Sapporo, Japan, 105pp.,(in Japanese with English abstract).

Tajika, J., Okamura, T., Tsuboyama, A., Yamagishi, H., 1994b. Ge-ologic control for deep-seated landslides along coastal slopes andtheir morphologic features; landslides in Kushiro–Akkeshi area,Hokkaido. Vol. 22 of Geological Survey of Hokkaido SpecialReport. Geological Survey of Hokkaido, Sapporo, Japan, 45pp.,(in Japanese with English abstract).

TuZino, T., Arai, K., Okamura, Y., 2004. Seismic survey off Kushiroand Hidaka. In: Katayama, H. (Ed.), Marine Geological andGeophysical Studies on the Collision Zone of Kuril and NortheastJapan Arc—Off Kushiro and Hidaka Area—. Preliminary Reportson Researches in the 2004 Fiscal Year, GSJ Interim Report, no. 30.Geological Survey of Japan, AIST, pp. 20–36, (in Japanese).

TuZino, T., Arai, K., Okamura, Y., 2005. Seismic survey off CapeOchiishi and Cape Erimo. In: Katayama, H. (Ed.), Marine Geo-logical and Geophysical Studies on the Collision Zone of Kuriland Northeast Japan Arc—Off Nemuro and Hidaka Area—. Pre-liminary Reports on Researches in the 2004 Fiscal Year, GSJInterim Report, no. 33. Geological Survey of Japan, AIST, pp.23–31, (in Japanese).

Walsh, J. P., Nittrouer, C. A., 1999. Observations of sediment flux tothe Eel continental slope, Northern California. Marine Geology154, 55–68.

Walsh, J. P., Nittrouer, C. A., 2003. Contrasting styles of off-shelfsediment accumulation in New Guinea. Marine Geology 196,105–125.

Yamaji, A., Sakai, T., Arai, K., Okamura, Y., 2003. Unstable forearcstress in the eastern Nankai subduction zone for the last 2 millionyears. Tectonophysics 369 (1-2), 103–120.

Yamanaka, Y., Kikuchi, M., 2003. Source processes of the recurrent

Tokachi-oki earthquake on September 26, 2003, inferred fromteleseismic body waves. Earth, Planets and Space 55, e21–e24.

Yasuda, I., Okuda, K., Shimizu, Y., 1996. Distribution and modifica-tion of North Pacific Intermediate Water in the Kuroshio–Oyashiointerfrontal zone. Journal of Physical Oceanography 26, 448–465.

Zaitlin, B., Dalrymple, R., Boyd, R., 1994. The stratigraphic organi-zation of incised-valley systems associated with relative sea-levelchange. In: Dalrymple, B., Boyd, R., Zaitlin, B. (Eds.), IncisedValley Systems, Origin and Sedimentary Sequences. Vol. 51 ofSEPM Special Publication. SEPM (Society for Sedimentary Ge-ology), Tulsa, OK, United States (USA), pp. 45–60.

22