Sedimentary Geology 226 (2010) 1–8

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Sedimentary Geology

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Are coarse-grained sediment waves formed as downstream-migrating antidunes?Insight from an early Pleistocene submarine canyon on the Boso Peninsula, Japan

Makoto ItoDepartment of Earth Sciences, Chiba University, Chiba 263-8522, Japan

E-mail address: mito@faculty.chiba-u.jp.

0037-0738/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2010.02.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 June 2009Received in revised form 6 January 2010Accepted 18 February 2010Available online 1 March 2010

Communicated by G.J. Weltje

Keywords:Coarse-grained sediment waveDownstream-migrating antiduneSubmarine canyonTurbidity currentsPleistocene

Gently undulating waveforms were identified in conglomerates and pebbly sandstones of the lowerPleistocene Higashihigasa Formation, which represents an infill of a submarine canyon on the BosoPeninsula, Japan. On the basis of dimension, geometry, and texture, these waveforms are interpreted asbedforms that are analogous to coarse-grained sediment waves in modern deep-water environments. Thestudied waveforms exhibit symmetrical or dune-like asymmetrical forms, in association with minorantidune-like asymmetrical forms with the lee sides being longer than the stoss sides. The waveforms arealso characterized internally by bedding gently inclined in the downstream direction to the northeast.Laterally, washout-dune and humpback-dune deposits are locally developed in association with planarbedding. The coarse-grained sediment-wave deposits are interpreted to have developed as downstream-migrating antidunes in the upper-flow-regime condition of turbidity currents. On the basis of the empiricalrelationship between the wavelengths and mean flow depths of downstream-migrating gravel antidunes,the waveforms are interpreted to have been formed by waves at the interface between the denser bottom-flow and the less-dense upper-flow within a turbidity current. It is possible that some of the coarse-grainedsediment waves in modern deep-water environments may have been formed as downstream-migratingantidunes, regardless of their plan-view geometry.

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1. Introduction

Various types of scours and bedforms have been documentedfrommodern deep-water environments mainly on the basis of marinegeophysical techniques (e.g., Normark et al., 1979; Piper andNormark, 2001), and have been used for the reconstruction of thedynamic condition of turbidity currents and related sediment-gravityflows (e.g., Normark et al., 1980; Morris et al., 1998). These scours andbedforms commonly exceed outcrop scales and are therefore difficultto recognize in stratigraphic successions (e.g., Normark et al., 1979,2002; Wynn et al., 2002a). Gently undulating wave-like bedforms,which appear to consist mainly of sands and gravels, have been foundin channels, canyons, and channel-to-lobe transitional zones, andare called coarse-grained sediment waves or gravel waves (e.g.,Malinverno et al., 1988; Hughes Clarke et al., 1990; Normark andPiper, 1991; Wynn et al., 2002a). Because of their coarse-grained tex-ture, it is difficult to obtain core samples from coarse-grained sedi-ment waves, and their seismic resolution is usually poor comparedwith that of fine-grained sediment waves (Wynn et al., 2002a). Thus,side-scan-sonar data and multibeam-bathymetry data have mainlybeen used for the estimation of their dimension, composition, andmigration directions (e.g., Malinverno et al., 1988; Savoye et al., 1993;

Wynn et al., 2002a; Smith et al., 2007), as well as some data obtainedby submersible dives (Shor et al., 1990). Coarse-grained sedimentwaves have dimensions similar to those of some large subaqueousdunes, and are smaller than those of fine-grained sediment waves,which are commonly documented in levees of submarine-fan chan-nels and have been interpreted as a bedform formed under antiduneor cyclic-step flow conditions in fine-grained turbidity currents(Normark et al., 1980, 2002; Wynn and Stow, 2002; Fildani et al.,2006). Thus, coarse-grained sediment waves have the potential tobe examined for their geometry and internal organization using analo-gous outcrop features for a better understanding of their genesis(Piper and Kontopoulos, 1994; Vicente Bravo and Robles, 1995; Itoand Saito, 2006), which still remains controversial compared withthe genesis of fine-grained sediment waves (Wynn et al., 2002a).The surface morphology of modern coarse-grained sediment wavesis interpreted to show both upstream-migration similar to that ofantidunes and downstream-migration resembling dunes (e.g., Wynnet al., 2002a; Smith et al., 2007).

Ito andSaito (2006)describedwaveforms in a coarse-grained infill ofthe early Pleistocene submarine canyon on the Boso Peninsula of Japan,and suggested that the waveforms are analogous to coarse-grainedsediment waves in modern deep-water environments. This paperfocuses on the description of the internal structures of the studiedcoarse-grained sediment-wave deposits with the aim to gain insight inhow these waveforms are formed in a deep-water environment.

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2. Study area

Ancient analogues of coarse-grained sediment waves were found inthe lower Pleistocene infill of a submarine canyon exposed on the BosoPeninsula of Japan (Ito and Saito, 2006). The infill is named theHigashihigasa Formation (Sato and Koike, 1957; Yamauchi et al., 1990;Nakajima andWatanabe, 2005), and represents a unit in the lower partof the Kazusa Group,which developed in the Kazusa forearc basin in theperiod between 2.5 Ma and 0.45 Ma as a response to the subduction ofthe Pacific andPhilippine Seaplates beneath theEurasia plate at the Izu–Bonin Trench (Katsura, 1984; Ito and Masuda, 1988) (Fig. 1A). The baseof the formation locally steeply incises into slope and shelf-margindeposits, and clastic sediments are interpreted to have been funneledthrough this submarine canyon and deposited in submarine-fansystems. These submarine-fan systems are represented by the OtadaiandUmegase Formations in the farther northeasternoffshorearea in theperiod between 1.1 Ma and 0.9 Ma (Hirayama and Nakajima, 1977)(Fig. 1B). The northeastward-directed paleocurrents were dominantduring the deposition of these formations and are also observed in theHigashihigasa Formation submarine-canyon system (Ito and Saito,2006) (Fig. 1C). The estimated dimension of the ancient submarinecanyon is about 8 km in length, about 1 km in maximum width, upto 150 m in depth, and more than 3–4° in slope, on the basis of thedistribution pattern and thickness of the Higashihigasa Formation andthe paleowater depth of the adjacent slope and outer-shelf deposits (Itoand Saito, 2006).

The Higashihigasa Formation consists mainly of coarse-grainedsiliciclastic sediments, such as pebble-sized conglomerates, pebblysandstones, and very coarse- to medium-grained sandstones (Fig. 1C).Conglomerates are commonly clast-supported with a matrix of verycoarse sands and granules, and locally contain many sandy siltstoneclasts of up to 200 cm in maximum diameter. The formation alsolocally contains slumped deposits of up to 20 m thick, and exhibits anoverall fining-upward pattern over a thickness of 150 m (Ito and Saito,2006) (Fig. 1C). Coarse-grained deposits of the formation wereinterpreted to have been formed from high-density turbidity currents(sensu Lowe, 1982) or concentrated density flows (sensu Mulder and

Fig. 1. (A) Plate-tectonic framework of the Kazusa forearc basin, Japan. (B) Geologic sketch mcomposite section of the Higashihigasa Formation and a rose diagram of paleocurrents. N =

Alexander, 2001), which flowed in the northeastern downstreamdirection (Katsura, 1984; Yamauchi et al., 1990; Ito and Saito, 2006)(Fig. 1C). The studied outcrops are located at 35°13′07″ N–35°13′55″N in latitude and 139°56′56″ E–139°58′21″ E in longitude (Fig. 1B).

3. Description of waveforms in outcrops

Gently undulating waveforms are observed in conglomerates andoverlying pebbly sandstones of the Higashihigasa Formation parallelto the general paleocurrent directions in the northeastern downslopearea (Figs. 2, 3, 4, and 5). The dimensions of the waveforms arevariable, and thewavelengths are from 7 to 63 mwithwave heights of0.4 to 2.2 m (Fig. 6). Some of the outcrop examples show a dimensionsimilar to that of modern coarse-grained sediment waves, whereasthe other outcrop examples are smaller than the modern examples(Fig. 6). The symmetry index (SI) (Tanner, 1967) and wave steepnessare also variable among the 23 examples (Fig. 7). Nearly symmetrical(SI=0.6–1.4) waveforms are dominant (56%), in association withdune-like asymmetrical (SIN1.4) forms (35%), and minor antidune-like asymmetrical (i.e., lee sides are longer than stoss sides) (SIb0.6)forms (9%) (Fig. 7). The wave steepness is in the range of 0.03–0.08and does not show any apparent relationship with the symmetryindex (Fig. 7).

The bases of the conglomerates are marked by shallow trough-likeerosional surfaces (Fig. 2) in local associations with distinct erosionalstructures such as groove-like scours (ca. 5 m wide and 1.5 m deep)and flute-like scours (ca. up to 24 m long and 5 m deep) (Fig. 3).Everywhere, these erosional surfaces are not overlain by drapes offiner-grained sediments, and conglomerates rest directly on theerosional surfaces in local association with sandy siltstone clasts(Figs. 4 and 5A). Erosional surfaces, which face in the southwesternupstream direction, are locally overlain by conglomerates withconvex-up backset bedding (Figs. 2 and 3). These conglomeratesalso locally contain many sandy siltstone clasts, and the upper parts ofthe backset bedding are truncated by the erosional bases of overlyingwaveforms. Along laterally mappable erosional surfaces, several

ap of the central part of the Boso Peninsula. The rectangle indicates the study area. (C) Athe number of measurements. Modified from Ito (1998), and Ito and Saito (2006).

Fig. 2. (A) Outcrop features of coarse-grained sediments of the Higashihigasa Formation submarine-canyon-fill succession at Tagura, Futtsu City. (B) Interpretation ofmajor erosionalbases (1–5), waveforms, and internal bedding in the coarse-grained sediments of the Higashihigasa Formation submarine-canyon-fill succession shown in A. This outcrop is orientedlargely parallel to the paleocurrent directions (see Fig. 1C for paleocurrent data). The white arrow indicates the mean paleocurrent direction.

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waveforms with different dimensions and asymmetry are in line inthe downstream direction (Figs. 2 and 3).

Internally, the waveforms commonly contain bedding gentlyinclined in the downstream directions and some of the bedding showsfeatures similar to those of the foreset bedding in dune deposits inassociationwith climbing forms (Figs. 3B and 5A). Furthermore, someofthe foreset bedding has sigmoidal geometry and the brink points areoffset from themaximumelevation, showing features similar to those offoreset bedding in humpback-dune deposits (Fig. 5B). Dune-likeasymmetrical forms of conglomerates and pebbly sandstones are locallydeveloped as isolated bodies on erosional surfaces and are associatedlaterally with planar bedding in both the upstream and downstreamdirections (Figs. 4C and 5B). In a section oriented orthogonal to thepaleocurrent direction, the internal bedding of some waveformsexhibits a geometry similar to that of the trough cross-bedding found

Fig. 3. (A) Outcrop features of coarse-grained sediments of the Higashihigasa Formation subnortheastern (45°)-downstream area from the outcrop shown in Fig. 2. (B) Interpretation osediments of the Higashihigasa Formation submarine-canyon-fill succession shown in A. Tpaleocurrent data). The white arrow indicates the mean paleocurrent direction. Each of thFig. 3B.

in three-dimensional-dune deposits (Fig. 5C). The upper parts of thewaveforms are characterized by pebbly sandstones, which showsinusoidal bedding and pass gradationally upward into very coarse- tomedium-grained, sandstones (Figs. 4A and C and 5). These sandstonesshow better sorting than do the underlying pebbly sandstones andconglomerates, and are massive or weakly normally graded with dishstructures and convolute bedding (Ito and Saito, 2006). Couplets ofconglomerates and overlying pebbly sandstones and sandstones do notgenerally contain any distinct internal erosional surface, except for atthe bases of the conglomerates (Figs. 4A and C and 5).

4. Genesis of the waveforms

In terms of their dimension, geometry, and texture, the studiedwaveforms show similarity to coarse-grained sediment waves, which

marine-canyon-fill succession at Tagura, Futtsu City. This outcrop is located in a 120 m-f major erosional bases (1–5), waveforms, and internal bedding in the coarse-grainedhis outcrop is oriented largely parallel to the paleocurrent directions (see Fig. 1C fore major erosional surfaces (1–5) in Fig. 2B is correlated with the erosional surfaces in

Fig. 4. Outcrop features of waveforms and their constituent deposits. See Fig. 2B for legend. (A) Close-up view of the stoss side of a waveform, which consists of stratifiedconglomerates and overlying pebbly sandstones and very coarse- to medium-grained sandstones. A couplet of conglomerates and overlying pebbly sandstones and sandstones isdefined by erosional surfaces in the base and top. The black arrow indicates the paleocurrent direction. Note the occurrence of dewatering structures in the pebbly sandstones andsandstones. (B) Gently undulating waveforms and overlying pebbly sandstones and sandstones. The black arrow indicates the paleocurrent direction, and the black arrowheadpoints at a position similar to that in C. (C) Close-up view of the waveform in B. Note that wave-formed conglomerates change laterally into planar-bedded pebbly sandstones (PL),and are overlain by pebbly sandstones with sinusoidal bedding (SS), which fine upward gradationally into very coarse- to medium-grained sandstones. Locally, the conglomeratescontain many sandy siltstone clasts.

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have been reported from modern submarine canyons, submarinechannels, and channel-to-lobe transition zones (e.g., Normark andPiper, 1991; Wynn et al., 2002a) (Ito and Saito, 2006). However, someof the outcrop examples are smaller in dimension than moderncoarse-grained sediment waves that have been imaged by the marinegeophysical techniques (Fig. 6). Therefore, there still remains aresolution mismatch between outcrop studies and modern seafloorstudies. In terms of the spectrum of sediment-gravity flow processes,the present outcrop examples can fill the gap in dimension andformative processes between bedforms, which have been studied in

flumes and in turbidite successions, and large-scale coarse-grainedsediment waves in modern deep-water environments.

Because the present waveforms are overlain gradationally bypebbly sandstones and very coarse- to medium-grained sandstones(Figs. 4 and 5), the generation of waveforms is interpreted to havebeen unaffected by the reworking and/or reshaping of gravellydeposits by subsequent lower-concentration turbulent flows in asingle depositional event or by later events, in contrast to the findingsreported in previous studies (e.g., Winn and Dott, 1977; Piper andKontopoulos, 1994; Wynn et al., 2002a). The erosional bases of

Fig. 5. Lateral variations in the geometry and bedding styles of a coarse-grained sediment-wave deposit. The erosional base E1 in A corresponds to that in B and C. (A) A gentlyundulating waveform of conglomerates and pebbly sandstones with foreset bedding (FS) and sinusoidal bedding (SS). The white arrow indicates the paleocurrent direction. (B) Anisolated dune-like deposit with sigmoidal bedding (SG) in association with planar bedding (PL). The white arrow indicates the paleocurrent direction. (C) Trough-like cross-bedding(TX) in conglomerates shown in A. The face of this outcrop is oriented oblique to the paleocurrent direction. A and B are from Ito and Saito (2006).

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coarse-grained sediment-wave deposits, which are locally associatedwith groove-like and flute-like erosional structures (Fig. 3B), areinterpreted to indicate active scouring by turbulent flows. Large-scale

Fig. 6. Dimensions of modern and ancient coarse-grained sediment waves. The verticaland horizontal bars represent the ranges of data. Data for modern examples are fromMalinverno et al. (1988), Normark and Piper (1991), Wynn et al. (2002a,b), and Smithet al. (2007). Data for ancient examples are from (1) Piper and Kontopoulos (1994),(2) Vicente Bravo and Robles (1995), and (3) Wynn et al. (2002a).

erosional structures have also commonly been reported fromchannel-to-lobe transitional areas and canyon and channel floors inmodern deep-water environments (e.g., Normark et al., 1979; Morriset al., 1998; Wynn et al., 2002b). These erosional structures arealso locally associated with coarse-grained sediment waves and areinterpreted to have been formed by strong turbidity currents that

Fig. 7. Geometrical features of coarse-grained sediment-wavedeposits of theHigashihigasaFormation.

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carried coarse-grained sediments over long distances and thatresulted in the development of a variety of erosional structuresduring their passages (Morris et al., 1998;Wynn et al., 2002b). Becausethe present erosional surfaces are overlain directly by conglomeratesrather than by drapes of finer-grained sediments, the erosion andsubsequent deposition of coarse-grained sediments are interpreted tohave occurred by single events of depositional flows, although it ispossible that drapes were easily eroded away during the subsequentdeposition of coarse-grained sediments (e.g., Kane et al., 2009). Thesequence of erosion and deposition is interpreted to have developedthe waveforms in the Higashihigasa Formation submarine-canyonsystem and to have resulted from sediment-gravity flows, whichmay have been partly turbulent, such as high-density turbidity cur-rents (sensu Lowe, 1982) or concentrated flows (sensu Mulder andAlexander, 2001). In the spectrum of high-density sediment-gravitycurrents, the fluidal parts of each current are interpreted to have thepotential to develop significant basal erosion and tractional structures,as indicated by the findings of previous outcrop- and experiment-based studies (e.g., Postma and Roep, 1985; Postma et al., 1988; Muttiet al., 1999; LeClair and Arnott, 2005; Kane et al., 2009).

The dune-like, asymmetrical cross-sectional geometry of coarse-grained sediment-wave deposits, in association with foreset bedding,is interpreted to have been formed under tractional-flow conditions ofturbidity currents, similar to currents responsible for the formation ofsubaqueous dunes (e.g., Winn and Dott, 1977; Normark and Piper,1991; Wynn et al., 2002a; Ito and Saito, 2006). Tidal currents are analternative possible process for the development of waveforms in thesubmarine canyon. However, the speeds of tidal currents are notstrong enough to form coarse-grained dunes in modern canyons (e.g.,Smith et al., 2007). Thus, the waveforms are not considered to havebeen generated by tidal currents (Ito and Saito, 2006). Although theplan-view geometry of some modern coarse-grained sediment waveshas been interpreted to indicate upstream-migration of bedforms,similar to that of antidunes (e.g., Normark and Piper, 1991; Wynnet al., 2002a; Smith et al., 2007), most of the present examples arecharacterized by internal bedding inclined generally in the down-stream direction, regardless of their wave symmetry (Figs. 2, 3, and 4),and are therefore interpreted to have developed as downstream-migrating bedforms. Thus, the antidune-like plan-view geometry ofsome modern coarse-grained sediment waves can alternatively beinterpreted to represent the antidune-like asymmetrical geometry(i.e., lee sides are longer than stoss sides) of downstream-migratingbedforms, as recognized in some of the present examples (Fig. 7).

In terms of lateral variations in the dune-like waveforms, foresetbedding locally shows geometry typical in humpback-dune deposits,and also changes laterally in planar bedding (Figs. 4C and 5B).Furthermore, some of the present dune-like forms are isolated andhave a cross-sectional geometry similar to that of washout-dunedeposits (Fig. 5B). The spatial relationships between the variousbedding styles are interpreted to indicate that the dune-like wave-forms were formed under the upper-flow-regime condition, whichhas been interpreted to be the transition between dunes and upper-plane beds (Saunderson and Lockett, 1983; Roe, 1987; Sarkar et al.,1999; Fielding, 2006). Locally observed backset bedding on erosionalsurfaces is interpreted to have developed as a response to a hydraulicjump along a shoot-and-pool structure (e.g., Jopling and Richardson,1966; Postma and Roep, 1985; Nemec, 1990; Massari, 1996;Alexander et al., 2001). The backset bedding in the present examplesis characterized by convex-up, gently inclined geometry in a crosssection parallel to the paleocurrent directions (Figs. 2 and 3), andshows a bedding style similar to that of the upstream side of ahydraulic jump unit bar, as defined by Macdonald et al. (2009).

Although Postma et al. (2009) described some distinctivelithofacies features, which are interpreted to have been formed inresponse to an internal hydraulic jump, the present outcrop examplesdo not show these lithofacies features, except for the backset bedding.

Because hydraulic jumps are interpreted to have locally occurred inassociation with the formation of the waveforms, the downstream-migrating waveforms are interpreted to have been formed when thedensimetric Froude number is above unity, although Huang et al.(2009) recently claimed that the critical densimetric Froude numberis not a constant value (of approximate unity); instead, it varies withthe level of ambient-fluid entrainment and sediment exchange withthe channel bed. Furthermore, because turbidity currents in mostcanyons are interpreted to be supercritical (e.g., Komar, 1971; Garciaand Parker, 1989; Postma et al., 2009), the formation of thewaveforms in the Higashihigasa Formation submarine-canyon sys-tem, which has a slope exceeding 3–4°, is interpreted to have beencontrolled by Froude-supercritical currents. Alternatively, the nearlysymmetrical cross-sectional geometry and sinusoidal bedding ofconglomerates and pebbly sandstones suggest that the waveformswere also formed in non-breaking in-phase standing-wave conditions(Cheel, 1990; Vicente Bravo and Robles, 1995; Ito and Saito, 2006).Thus, the present waveforms are interpreted to have developed asdownstream-migrating antidunes in the upper-flow-regime condi-tions (e.g., Cheel, 1990). Large symmetrical dunes in the Fraser Riverin Canada have a geometry similar to that of humpback dunes, and arealso interpreted to have formed in the upper-flow-regime conditionfor the stability fields for upper-plane beds and/or for antidunes(Kostaschuk and Villard, 1996).

Downstream-migrating antidunes are interpreted to have a dune-like asymmetrical geometry with distinct foreset bedding as aresponse to the development of a separation zone in the lee side(e.g., Fukuoka et al., 1982; Cheel, 1990; Carling and Shvidchenko,2002). Downstream-migrating asymmetrical bedforms similar tocurrent ripples have also been formed by Froude-supercritical tur-bidity currents in flumes (e.g., Parker et al., 1987; Kostic et al., 2002;Spinewine et al., 2009). Furthermore, downstream-migrating anti-dunes are interpreted to develop when the Froude number is close tounity, and to be represented by the wave steepness (N0.035) largerthan that of upstream-migrating antidunes (Fukuoka et al., 1982;Cheel, 1990), although a flow condition characterized by higherFroude numbers is also interpreted to be responsible for thedevelopment of downstream-migrating symmetrical gravel antidunes(Carling and Shvidchenko, 2002). Because incipient antidunes in non-breaking standing waves are also symmetrical (Carling and Shvid-chenko, 2002), the symmetrical waveforms of the present examplesare alternatively interpreted to represent bedforms developed underFroude numbers lower than those for downstream-migrating sym-metrical gravel antidunes. Thus, the coarse-grained sediment wavesidentified in the Higashihigasa Formation are interpreted to havedeveloped as downstream-migrating antidunes in association withtransitional bedforms such as washout dunes and humpback dunes,which are interpreted to develop at the transition between dunes andupper-plane beds, and are also characterized by Froude numbers closeto or higher than unity (Saunderson and Lockett, 1983; Nnadi andWilson, 1995).

Although both supercritical and subcritical flow conditions havebeen inferred for the development of some modern coarse-grainedsediment waves, the estimated Froude numbers (≥0.88) indicate theupper-flow-regime conditions (Piper et al., 1988; Mulder et al., 1997).In the reconstruction of the flow conditions, the mean flow depth ofturbidity currents was estimated to exceed 20 m (up to 400 m) (Piperet al., 1988; Mulder et al., 1997). In the Higashihigasa Formationsubmarine-canyon system, the average flow depth of turbiditycurrents (or sediment-gravity flows in a broad sense) is unclear, butappears to have been at least more than 10 m, because some erosionalbases locally incise into underlying deposits by more than 10 m (Itoand Saito, 2006). The mean flow depth (h) responsible for thedevelopment of downstream-migrating gravel antidunes is inter-preted to follow the empirical relationship with the wavelength (L)(i.e., L=2πh), although the data obtained from asymmetrical

Fig. 8. Schematic illustration of the generation of coarse-grained sediment waves in anancient submarine canyon. First, coarse-grained sediments are interpreted to have beentransported into a canyon by turbidity currents, which locally developed distinct scours,such as large-scale groove-like and flute-like scours (A). Subsequently, coarse-grainedsediments, which had been transported as bed loads and suspended loads, developeddownstream-migrating waveforms as a response to the downslope movement ofdenser lower flow in association with waves in the interface between the denser lower-flow and the less-dense upper-flow within a turbidity current (B).

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downstream-migrating antidunes are plotted below the equation(Carling and Shvidchenko, 2002). Thus, the wavelengths measured inthe outcrops can be used to estimate paleoflow depth. In the presentexamples, the wavelengths vary from 7 to 63 m with an average of24 m (Fig. 6). The average wavelength infers the averagemean flow tobe a depth of 3.8 m. This average mean flow depth is shallower thanthe supposed mean flow depth of waveform-generating turbiditycurrents. One of the possible explanations for this shallow flow depthis that coarse-grained sediment wavesmost likely were not formed bywaves at the upper surface of turbidity currents but by those at theinterface between the denser bottom-flow and the less-dense upper-flow within a turbidity current (i.e., Kelvin Helmholtz waves) (cf.Postma et al., 1988; Mulder et al., 1997) (Fig. 8). Alternatively, forsome of the present examples, the successive deposition of coarse-grained sediments from sustained, thin turbidity currents may havedeveloped downstream-migrating antidunes as a response to wavemotion at the upper surface of turbidity currents.

5. Conclusions

Gently undulating waveforms were identified in conglomeratesand pebbly sandstones of the lower Pleistocene HigashihigasaFormation, which represents an infill of a submarine canyon on theBoso Peninsula, Japan. The waveforms are inferred to be analogous tocoarse-grained sediment waves in modern deep-water environments,because the outcrop examples have a dimension, geometry, and tex-ture similar to those of somemodern coarse-grained sediment waves.The present examples of coarse-grained sediment-wave deposits ex-hibit nearly symmetrical or dune-like asymmetrical geometry in crosssections, together with minor asymmetrical features, with the leesides being longer than the stoss sides. Internally, the present coarse-grained sediment-wave deposits are characterized by bedding gentlyinclined in the downstream direction. Locally, the waveforms changelaterally into humpback-dune and washout-dune deposits in associ-ation with planar bedding. Thus, the coarse-grained sediment-wavedeposits are interpreted to represent bedforms that developed asdownstream-migrating gravel antidunes in the upper-flow-regimecondition. On the basis of the empirical relationship between thewavelengths and mean flow depths for the development of down-stream-migrating gravel antidunes, the present waveforms appear to

have been formedbywaves at the interface between the denser lower-flow and the less-dense upper-flow within a turbidity current.

Acknowledgements

This research was supported in part by a Grant-in-Aid for ScientificResearch from the Japan Society for the Promotion of Science (nos.15340166, 18340154) and by a research grant from the TechnologyResearch Center for Japan Oil, Gas and Metals National Corporation(JOGMEC). I would like to acknowledge the quarry companies at thestudy area for their kind cooperation in allowing me continued accessto the outcrops. An early version of the manuscript received thebenefit of many constructive comments by G. Postma, R.B. Wynn, ananonymous reviewer, and the Editor (G.J. Weltje).

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