Sedimentation on a wave-dominated, open-coast tidal flat, southwestern Korea: summer tidal flat –...

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Sedimentation on a wave-dominated, open-coast tidal flat, south-western Korea: summer tidal flat – winter shoreface

B. C. YANG*, R. W. DALRYMPLE* and S. S. CHUN

*Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, OntarioK7L 3N6, CanadaFaculty of Earth Systems and Environmental Sciences, Chonnam National University, Kwangju 500-757, Korea (E-mail: [email protected])

ABS TR ACT

Sedimentation on the open-coast tidal flats of south-western Korea is controlled by seasonal variation in the intensity of onshore-directed winds and waves. As aresult, an environmental oscillation takes place between tide-dominatedconditions in summer and wave-dominated conditions in winter. In summer,thick muddy deposits, including sporadic storm deposits, accumulate in

response to low wave energy, weak currents, and intense solar insolation thatpromotes consolidation of the mud at low tide. Bioturbation is minimal becauseof rapid sedimentation and soft substrate. During the autumn, the summer muddeposits experience erosion due to increasingly strong onshore winds andwaves, until only small mud patches and mud pebbles remain. Theconcentration of ebb runoff between the mud patches produces small,ephemeral tidal creeks. In winter, storm waves occur frequently (ca 10 daysa month) and dominate sedimentation in the intertidal zone, producingextensive wave-generated parallel lamination and short-wavelength (0Æ3–2 m)hummocky cross-stratification. The prevalence of strong onshore windsdecreases in spring, allowing longer and more frequent intervals of calmweather, during which time muddy sediments are deposited by tidal processes.

Over the long term, winter storm waves dominate sedimentation and thepreserved deposits consist of amalgamated storm beds that resemble thosegenerally associated with shorefaces. This raises the question of how manyancient ‘shorefaces’ are, in fact, open-coast tidal flats.

Keywords Hummocky cross-stratification, open-coast tidal flat, seasonalvariation, south-western Korea, storm deposits, tidal-flat sedimentation.

INTRODUCTION

Although the coastal classification schemes of Hayes (1979) and Boyd et al. (1992) indicate that

a gradation exists between wave- and tide-dom-inated coastal environments (Fig. 1), most re-search has focused on the two end-membersettings, namely sheltered tidal flats and exposed beaches and shorefaces. In contrast, non-barred,open-coast tidal flats have received little atten-tion, despite their widespread occurrence, espe-cially in South-east Asia (Reineck & Cheng, 1978;Mukherjee et al., 1987; Chun et al., 2000; Li et al.,2000; Yang & Chun, 2001). According to standarddefinitions, tidal flats are tide-dominated

environments with a gentle slope, negligible waveinfluence and well-developed tidal creeks (Klein,1985; Dalrymple, 1992), whereas beaches andshorefaces have relatively steep slopes and

experience strong wave action (Davis, 1985;Walker & Plint, 1992). As a result, these twoenvironments are considered to be distinct:muddy heterolithic facies, including flaser, wavy,and lenticular bedding, characterize tidal-flatdeposits, whereas wave and storm-generateddeposits, including hummocky cross-stratifica-tion (HCS), are typical of beach and shorefacedeposits. Indeed, these two environments arealmost invariably considered in separate chaptersin textbooks (e.g. Davis, 1985; Reading, 1996).

Sedimentology (2005) 52, 235–252 doi: 10.1111/j.1365-3091.2004.00692.x

2005 International Association of Sedimentologists 235


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Recent studies along the Chinese and SouthKorean coasts reveal that wave processes are animportant agent of sedimentation on open-coasttidal flats (Li et al., 2000; Yang & Chun, 2001; Fan& Li, 2002). The macrotidal flats at Baeksu (meantidal range 3Æ9 m; maximum range 6Æ8 m), south-western South Korea, are typical of large parts of the Korean west coast (Fig. 2). Like the Chinesetidal flats described by Li et al. (2000) and Fan &

Li (2002), they are non-barred. However, theydiffer from the Chinese examples by being muchmore sandy, and in being subjected to a moreintense monsoon wind regime because of theirexposed location. Because of this later factor,sedimentation on the Baeksu tidal flat alternates between tide-dominated in summer and wave-dominated in winter, and the resulting depositsmore closely resemble those of shorefaces thanthey do those of typical (i.e. sheltered) tidal flats.The purposes of this paper are to describe theseasonal pattern of sedimentation on the Baeksu

tidal flat, and to discuss those features that mayhelp to distinguish sandy, open-coast tidal flatsfrom shorefaces in the rock record.

MATERIALS AND METHODS

Two shore-normal sampling transects wereestablished across the Baeksu tidal flat (Fig. 2),along which metal stakes were installed at100 m intervals to permit consistent sampling

over the study period. At each station, sampleswere collected from the sediment surface (ca1 cm deep) using a spatula, and elevation datawere obtained using a Sokkia B21 level at 2–3 month intervals over a 2 year period, in orderto characterize the seasonal variation in tidal-flat characteristics. In addition, a Van Veen grabsampler was used to collect a more regional gridof samples from the intertidal and shallow

subtidal zones during the summer and winterseasons (Fig. 2). Grain-size analyses were car-ried out after organic matter and calciumcarbonate were removed. The dried sand frac-tion was analysed using a sieve interval of 0Æ5ø,while the mud fraction was analysed by thepipette method. Cancores (30 cm deep · 18 cmwide · 8 cm thick) were collected along bothsurvey lines (Fig. 2) on a seasonal basis, andepoxy relief peels were obtained from them inorder to document the sedimentary structures.Most peels are oriented in the shore-normal

direction, but some were collected with a shore-parallel orientation to determine the 3D geo-metry of the structures.

DESCRIPTION OF STUDY AREA

Geomorphology

The Baeksu tidal flat is 4–6 km wide and 8–10 kmlong (Figs 2 and 3). It faces north-west directly

Fig. 1. Classification of clasticcoastal environments, modifiedfrom Boyd et al. (1992). Open-coasttidal flats are subjected to the influ-ence of both waves and tides and,hence, fall in the transition betweenwave-dominated shorefaces andsheltered, tide-dominated tidal flats.Tidal beaches (Masselink & Short,1993), which are probably some-what more strongly wave-influencedthan open-coast tidal flats, are alsopart of the spectrum of transitionalenvironments.

236 B. C. Yang, R. W. Dalrymple and S. S. Chun

2005 International Association of Sedimentologists, Sedimentology , 52, 235–252


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onto the Yellow Sea without a barrier. Two coast-oblique sand ridges border the seaward margin of the tidal flat, near the low-water line. Beyondthis, the shallow inner shelf is broken by small bedrock islands. The tidal flat is bordered on itslandward side by rocky coastal cliffs in the north(Figs 2 and 3A) and by artificial dykes in thesouth (Fig. 3B) that have reclaimed a formerestuary.

Topographic surveys along the two survey lines

(Fig. 4) show that the tidal flat has a very gentleseaward slope, which averages 0Æ07 in the north(Line BA) and 0Æ06 in the south (Line YS). Suchgentle gradients are very similar to those of sheltered Korean tidal flats (Frey et al., 1989;Wells et al., 1990; Kim et al., 1999). There is,however, an overall concave-up morphology,with the slope decreasing seaward, from 0Æ1 inthe inner part to 0Æ05 on the outer part (Fig. 4).The landward end of Line BA shows a flatteningof the profile that is not evident on Line YS. This

area corresponds to the inner mud flat (IMF)(Fig. 2) that occupies the most sheltered part of the coast. The remainder of the tidal flat isdominantly sandy. On both lines, the tidal flatspass gradually into the subtidal zone without aprominent break in slope (Fig. 3C).

Both topographic profiles show gentle undula-tions with a wavelength of 100–300 m (Fig. 4 –see especially Line YS). They are typically about0Æ3 m high, but become larger in a landward

direction, reaching 0Æ5–1 m in height at the coast.Measurements over a 5 year period (1997–2001)show that these features, which we interpret as being wave-generated swash bars (King, 1972;McCave & Geiser, 1978), migrate landward at arate of ca 75 m year)1, with most of the movementoccurring during the winter. At the landward endof Line YS, the swash bars weld onto the coast toform a small, incipient strand plain that is ca150 m in length parallel to the coast, with a steepseaward slope of 0Æ5 (Fig. 4). In the area where

Fig. 2. Map of the Baeksu tidal flat and map of the Yellow Sea (inset) showing location of the study area. Note thatthe tidal flat faces directly onto the Yellow Sea without any protective barrier. Field surveys were carried out alongtwo, coast-perpendicular transect lines (BA and YS) where surface-sediment samples, sedimentation-rate measure-ments and cancores have been collected seasonally over a 2 year period (1997–1998). Dots indicate positions of regional grab samples. Ticks on survey lines give locations of surface-sediment samples (100 m spacing); numbersindicate distance from coast (·100 m). Bathymetric contours based on the Approaches to Anma Kundo and WidoNautical Chart No. 344 (National Oceanographic Research Institute, 1983); datum is mean high low-water level. SRM,

sedimentation rate measurement; IMF, inner mud flat.

Sedimentation on open-coast tidal flats 237



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the IMF is present, the swash bars stall on theouter part of the mud flat, forming isolated ridgestermed ‘cheniers’ (Lee et al., 1994; Figs 2 and 3B)that are 50–100 m long, 5–10 m wide and 0Æ5–1 mhigh, composed of fine to medium sand andshells.

Physical processes

Tides on the Baeksu tidal flat are semi-diurnal(diurnal inequality about 1 m) with a mean tidalrange of 3Æ9 m. Because of the limited cross-shoretidal prism, the maximum current velocity is onlyca 40 cm sec)1 during spring tide and ca25 cm sec)1 during neap tide (Kim, 2003). Thestrongest currents flow obliquely across the flatsin a NW–SE direction, but current speed anddirection are influenced strongly by winds (Chunet al., 2000).

The wind regime shows a pronounced season-ality associated with the Asian monsoon (Figs 5

and 6A). During the winter, winds blow mainlyonshore from the NW to NNE, with a mean speedof ca 10 m sec)1. In summer, by contrast, winds blow mainly from the south, in an obliquelyoffshore direction, with a mean speed of only 2–3 m sec)1 (Korea Meteorological Administration,1997, 1998). The frequency of storms, definedas times with wind speeds > 13Æ9 m sec)1, isalso strongly seasonal, ranging from less than2–3 days a month during summer to morethan 10 days a month during winter (Table 1;

Fig. 3. General morphological features of the Baeksutidal flat. (A) View landward on northern survey lineBA (Fig. 2), showing inner mud flat (to right) and Cre-taceous bedrock outcrops. (B) Small, isolated chenier(swash bar composed of sand and shells) on the inner

mud flat, near the inner end of survey line YS. (C)General view of the featureless outer sand flat in thefall, showing erosional remnant of summer mud layer.

Fig. 4. Topographic profiles and the thickness of muddeposits along the two survey lines in summer. Surfaceundulations marked by ‘SB’ represent intertidal swash

bars (Fig. 3B). Note that thickness of the summer mudgenerally decreases landward, and is thicker in thetroughs of the swash bars and thinner over their crests.A–F indicate positions of cancores shown in Figs 9 and11–13. MSL, mean sea-level.




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Fig. 6A). Typhoons occur mainly during thesummer, with one to three occurring each year

(Table 1).No systematic wave data exist for the Baeksu

area. However, the pronounced seasonal variationin wind speed (Fig. 6A) should produce a com-parable variation in wave size. Although ty-phoons can sometimes produce waves up to 5 mhigh, the summer season is generally character-ized by waves with significant heights of only0Æ5–1Æ0 m (Korea Meteorological Administration,1997, 1998; National Fisheries Research andDevelopment Institute, 1997, 1998). In winter, by contrast, the strong, onshore-directed winds

generate significant wave heights of 2–3 m (Kim,2003).

Satellite images indicate that the nearshorezone along the entire south-west coast of Koreais highly turbid, but no general suspended-sedi-ment concentration (SSC) data exist. Near-beddata collected over a single tidal cycle on theBaeksu tidal flat during winter show that SSCvalues range from 1 to 7 g l)1 as a function of thewind speed and direction (Chun et al., 2000). Thesuspended sediments are composed mostly of silt

grains (80–95%), with subordinate amounts of clay (5–15%) and sand (2–5%).

Sea water temperatures, measured at a waterdepth of 5 m near Kunsan, 60 km north of thestudy area, vary from ca 5 C in the winter to27 C in the summer (Fig. 6B; National FisheriesResearch and Development Institute, 1997, 1998).Sea water temperature never falls below thefreezing point and ice has never been observedon the tidal flats.

CHARACTERISTICS OF SURFACESEDIMENTS

General zonation

Most of the area does not show the standardfacies zonation expected of tidal flats (Klein,1985; Amos, 1995). Mudflats and salt marshes,which normally occur near the high-water level,are of limited extent, occurring only in the mostsheltered area in the central part of the studyarea (Fig. 2); the absence of these facies ismainly a result of extensive reclamation. Where

Fig. 5. Summary of wind data overa 5 year period (1994–1998), meas-ured 15 m above mean sea-level at astation located 15 km north of thestudy area (source: Korea Meteoro-

logical Administration). Note thatrose diagrams denote the directionfrom which the winds blew. Spring:March–May; Summer: June–August;Fall: September–November; Winter:December–February. A pronouncedseasonality is evident. Dashed andsolid arrows indicate the coastlineorientation and offshore directionrespectively.




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Table 1. Summary of storm frequency for the study area.

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Total per year

Occurrence of

typhoons (1951–1998)

0Æ5 0Æ2 0Æ5 0Æ7 0Æ9 1Æ8 4Æ0 5Æ7 5Æ0 4Æ1 2Æ6 1Æ3 27Æ3

Effective typhoons(1904–1999)

– – – – 0Æ01 0Æ18 0Æ88 1Æ2 0Æ78 0Æ08 – – 3Æ13

Occurrence of stormwind (1997–1998)

10 6 6 4 2 2 3 2 6 6 7 9 63

Storms and typhoons are defined as periods with wind speeds 13 Æ9–17 and > 17 m sec)1 respectively. The top rowindicates the average number of typhoons each month that approach the Korean peninsula. Effective typhoons arethose that strike the west coast of Korea. Although typhoons, which occur primarily in summer and early fall, havehigher wind speeds than winter storms, only 17% of typhoons directly impact the west coast of Korea (KoreaMeteorological Administration, 1997, 1998). As a result, the more frequent but less intense winter storms causehigher sustained wave energy.

Fig. 6. Seasonal variation of wind velocity (A; source: Korea Meteorological Administration), and water and airtemperatures obtained near Kunsan, 60 km north of the study area (B; National Fisheries Research and DevelopmentInstitute, 1998). All values are daily averages. The irregular dashed line in (A) is the running average, calculatedusing a 25 day interval. Values above the horizontal dashed line in (A) are classified as storms. The highest tem-peratures and lowest wind velocity occur during summer, resulting in deposition of muddy sediments.




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present, this IMF consists primarily of muddydeposits that are extensively bioturbated(Fig. 7). Small tidal channels extend seawardfrom points of land drainage, but die outrapidly in a seaward direction. The sand flatsare featureless, other than the subtle swash barsmentioned above. These flats display a pro-

nounced seasonality of sediment type: sandspredominate in winter, whereas muddy depositsare widely developed in summer. This seasonalpattern becomes less pronounced in a landwarddirection.

Seasonal sedimentation cycle

Summer In summer, the surficial deposits on the tidal flatsconsist chiefly of mud with minor amounts of sand (Fig. 8A). Sand occurs mainly in a coast-

parallel belt just seaward of the IMF, with onlylocalized patches of sand in more seaward areas.Cancore peels (Figs 7A and 9) show that thesurficial layer of soft mud rests either gradation-ally or abruptly on sand. Measurements show thatthis layer is thickest in the middle to outer part of the flats (Fig. 4A), where it locally exceeds 40 cmthick, and gradually decreases in thicknesstowards the landward margin of the sand flats. Itis thicker again on the IMF (Fig. 4A). Meansediment sizes are in the medium- and coarse-silt range (4–5Æ5ø), while the finest sediments

(finer than 5Æ5ø) occur in both the shallowsubtidal zone and the IMF. Thus, the meansediment size shows a seaward-fining trend, withthe exception of the IMF. The muddy depositsrange from moderately to poorly sorted (Fig. 8C),in part because of the inclusion of more than asingle lamina or thin bed in each sample. The

sands are moderately sorted.The cancore peels (Figs 7A and 9) show that the

mud layer consists of thinly interbedded tointerlaminated sand and mud. Overall, it typic-ally displays a pronounced, upward decrease insand content (Fig. 10), which results from acombined upward decrease in the thickness andnumber of sand layers and a correspondingincrease in mud-layer thickness. In many in-stances, the upward-fining trend is punctuated bytwo or three, sharp-based sandier intervals, eachof which is followed by a renewed upward

decrease in sand content over a thickness of 3–10 cm (Figs 9C and E, and 10).Sand-layer thickness ranges from 2 to 5 cm

near the base of the summer mud layer, or atthe base of the smaller-scale upward-finingsuccessions, to less than a millimetre at thetop of these successions. Mud layers are thick-est both near the base and at the top of thesesmall-scale successions. The thickest sands aregenerally laterally continuous and containgently undulatory lamination and ripple cross-lamination that gives them a pinch-and-swell

A B C D

Fig. 7. Cancore peels from the landward end of Line BA showing deposits of the inner mud flat. Cores collectedseasonally: (A) summer; (B) fall; (C) winter; and (D) spring. Thin units of ripple cross-laminated sand are formedduring winter and early spring (C and D) by storm waves. Biological activity during summer and fall (A and B)destroys the winter sand layer partially to completely. Inferred season of deposition of older deposits shown at rightof each peel: W, winter; Sr, spring; S, summer; and F, fall. Landward is to the left in all peels.




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geometry. Some of the thicker sands are broken by one to three erosional discordances thatimply amalgamation. Slightly thinner sand

layers higher in these successions are lenticular because of the presence of starved-ripple form-sets. Most of the cross-lamination dips towardsthe land, but bipolar dips do occur. Somelaminae drape continuously over the adjacentripple crest, in the ‘offshoot’ geometry described by De Raaf et al. (1977). Chevron-upbuilding of ripple crests is also present, but is not common.At the top of each succession, the sand laminaeare 0Æ5–1 mm thick and occur in flat lenses 5–10 cm long encased in mud (Figs 9C and 10).Bioturbation is present only near the top of

these successions, generally occurring as isola-ted burrows (e.g. Fig. 9B). Pervasive bioturba-tion occurs only in the IMF (Fig. 7).

Fall Field observations show that the summer mudlayer is gradually eroded during the fall, exposingthe underlying sand (Fig. 3C). Erosion starts atdiscrete points and slowly expands until onlyisolated patches of mud, a few metres in dia-meter, remain. Mud pebbles are abundant. They

begin as large, angular clasts, but become smallerand more rounded with time. These mud balls aretypically 1–5 cm long, but they may reach up to

15 cm in length.As might be expected, cancores collected during

the fall show significant spatial and temporalvariability. Cores through remnant mud patches(Fig. 11B–F) show features similar to those des-cribed above, although the structures are generallyless distinct because of a greater degree of bioturbation (Fig. 11E). In some cases, the upper part of the mud layer has been truncated leaving only the basal portion of the summer deposit (Fig. 11D).Cores from areas where the mud layer has beenremoved entirely consist exclusively of sand

(Fig. 11A). In the later stages of erosion, remnantmud patches are commonly buried by rippled sandderived from the surrounding areas (Fig. 3C).

Winter Except for the IMF, the tidal flats in winter arecharacterized by sandy sediment (Fig. 8B) thatgenerally becomes finer in a seaward direction.Sediment coarser than 3ø is present primarily atthe shoreline in the southern part of the studyarea, and in isolated patches associated with

Fig. 8. Spatial variation of surface-sediment texture during the summer of 1998 (A and C) and the winter of 1999(B and D). See Fig. 2 for sample locations.




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swash-bar crests. Sediment with a mean size of 3–3Æ5ø occurs in a coast-parallel band immediatelyseaward of the IMF, whereas the middle andouter flats consist of sediment with mean sizes of 3Æ5–4ø. Sediment sorting is better during winterthan in the summer (Fig. 8C and D). More than

70% of the area is moderately to well sorted,whereas poorly sorted sediment dominates in thesouthern part of the study area and in the IMF(Fig. 8D). The poorly sorted sediment in the southmay be caused by the local supply of mud by asmall stream that drains the reclaimed estuary.

Cancores show that, during winter, the IMF ismantled by a layer of sand, typically < 5 cm thick,which overlies the bioturbated muddy deposits of the summer and fall (Fig. 7C). Small mud ballsand shell fragments are locally present at the base

of this sand layer. The sand layer contains ripplecross-lamination and, rarely, gently undulatinglamination. Cross-lamination commonly shows both landward and seaward dips, but landward-directed dips predominate. Migratory chevronupbuilding (i.e. the crestal overlap point is

displaced laterally through time) is observedrarely. Offshoot lamination is also present. Dis-crete vertical burrows are present, but are not asabundant as in the summer.

The deposits of the inner part of the sand flatsare dominated by ripple cross-lamination(Fig. 12A) and gently inclined lamination(Fig. 12D), with lesser amounts of parallel lamin-ation. The inclined lamination shows both sea-ward and landward dips; the latter predominateon the landward (lee) side of swash bars and dip

Fig. 9. Cancore peels from the sand flats (see Fig. 4 for locations) collected in the summer. Note that a surficial mudlayer is widespread, overlying sand. Tidal bedding, heterolithic facies and structureless mud are the dominantstructures in this mud layer. Bioturbation is present, but rarely obliterates the bedding. The vertical burrow in B wasproduced by the brachiopod Lingula, which is common on the middle and upper tidal flats. The underlying sandsrepresent the deposits of previous winters and/or springs. Sd, storm deposit; Fd, fair-weather deposit. See text fordiscussion. Landward is to the left in all peels.




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more steeply (5–10) than the seaward-dippinglamination (2–5) that is most abundant on theseaward (stoss) side of the swash bars. Alternat-ing parallel lamination and ripple cross-lamin-ation are present beneath the crests of swash bars.The degree of bioturbation ranges from rare tomoderate.

In the middle and outer parts of the sand flat,the sedimentary structures are dominated by

parallel lamination and gently inclined to curvedlamination, with lesser amounts of ripple cross-lamination (Fig. 12B,C,E and F). The gentlyinclined to undulatory lamination is continuous(Fig. 12B–F), although more discontinuous lam-ination does occur in some cores (Fig. 12F, belowsecond dashed line). Dips rarely exceed 5.Curved, tangential downlap onto the set base ispresent (Fig. 12F), but the laminae commonlydrape over undulatory erosional surfaces(Fig. 12C). The internal laminae generally showsubtle lateral variations in thickness. Locally, the

thickest laminae occur on the convex-up crests, but more commonly the laminae are thickest inthe depressions so that the synoptic relief decrea-ses upward (Fig. 12C). Although it is difficult tomeasure the wavelength of the undulationsexactly because of the small width of the can-cores, it is estimated that wavelengths vary from30 to 200 cm in the outer part of the flat and from30 to 100 cm in the middle flat. Because similarfeatures are present in cancores oriented bothparallel and perpendicular to the coast, this

undulatory lamination is interpreted as HCS (Dott& Bourgeois, 1982; Harms et al., 1982).

The ripple cross-lamination typically occurs insets 1–3 cm thick, in units only one set thick insome cores, but more commonly in cosets of threeto five sets. Steeply climbing ripples are moder-ately common (Fig. 12E). The ripple migrationdirections are highly variable, but onshore migra-tion predominates. The presence of bidirectional

ripples in a single coset (Fig. 12B) might be takento suggest that these ripples were formed by tidalcurrents, but the presence of symmetrical crestsand offshoot lamination (De Raaf et al., 1977;Meene et al., 1996) indicate, instead, that theywere generated by waves and/or combined flow.Bioturbation is rarely present and consists only of vertical burrows.

Individual beds are defined by a regular suc-cession of structures. Each bed begins with aplanar to undulating erosion surface that islocally overlain by mud pebbles (Fig. 12B and

C) and/or shell fragments. The bulk of the bedconsists of one or more sets of HCS, the dip of which generally decreases upward. Ripple cross-lamination and/or a discontinuous, thin muddrape are present at the top of some beds. Bedthickness is most commonly 10–15 cm, but thepotential for amalgamation is high in cases whereseveral sets of HCS are stacked on each other(Fig. 12C). These beds are interpreted as repre-senting deposition during the waning stages of winter storms.

Fig. 10. Schematic representations of lamina successions seen in summer mud deposits: (A) and (B) two super-imposed, small-scale upward-fining successions, each 3–10 cm thick; (C) one thicker upward-fining succession.Ripple formsets produce lenticular bedding in (A). The smaller upward-fining successions are interpreted as beingformed during the waning stage of storms and during the immediate post-storm period, whereas the mud at the top of each succession was deposited during the intervening fair-weather period. Each of the sand laminae within the

successions represents an individual tide, while the thick mud layers near the base of each succession may have beendeposited from high-concentration suspensions produced by intense wave action (Kim, 2003). M, mud; S, sand.




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Spring The sedimentary structures observed in cancorestaken in the spring (Fig. 13) are similar to thoseseen in the winter, except for a greater abundanceof small-scale cross-lamination on the surface.The degree of bioturbation is somewhat higher

than in winter cores. Three types of small-scalecross-lamination are recognized: (1) trough cross-lamination, produced mainly by the migration of current ripples (Fig. 13C); (2) climbing-ripplecross-lamination (Fig. 13F), similar to that seenin the winter deposits; and (3) offshoot and gentlyundulating lamination that suggests deposition by wave action (Fig. 13C). The climbing-ripplecross-lamination shows both stoss-erosional andstoss-depositional varieties; the angle of climbtypically ranges from 10 to 15. Most ripple

cross-lamination shows landward-directed migra-tion. Later in spring, during the increasinglylengthy periods of minimal wave action, slack-water mud drapes begin to be preserved, initi-ating the transition to the summer mud layer.Accumulation of the mud typically begins in the

outer intertidal zone and gradually expands in alandward direction.

DEPOSITIONAL PROCESSES

Sedimentation during summer is characterized byweak winds with a prevalent offshore direction(Figs 5B and 6A); as a result, the waves reachingthe tidal flats are small, except during theinfrequent typhoons. The long intervals of

Fig. 11. Cancore peels from the sand flats (see Fig. 4 for locations) collected in the fall. Although the frequency andintensity of storms has increased, the summer mud deposit is still widespread. The thick (typically > 1 cm), homo-geneous but unburrowed mud layer at the surface in some cores (e.g. C and E) may be a fluid-mud deposit formedfollowing storms and/or typhoons that occurred in later summer or early fall. Landward is to the left in all peels.




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relative calm allow mud to be deposited. Theintense solar insolation and high temperaturesthat occur at this time (Fig. 6B) promote theconsolidation of the mud drapes during exposure(Anderson & Howell, 1984), enhancing their

preservation potential. The high water tempera-tures (ca 27 C; Fig. 6B) may also promote thesettling of the coarser silt-sized particles, becauseof the lowered viscosity (Krögel & Flemming,1998), although these low viscosities may inhibitthe deposition of the clay fraction becauseflocculation is reduced (Lau, 1990).

The upward decrease in sand content in the basal part of the summer mud layer (Figs 9 and 10)reflects the more or less gradual change of winddirection from winter to summer (Fig. 5), and the

accompanying decrease in wind strength (Fig. 6A)and wave energy. The upward decrease in sandcontent is also promoted by the landward expan-sion of the mud layer and the progressive reduc-tion in the source area for sand. As the soft mud

layer grows in thickness, it probably also tends toattenuate the wave energy (Wells, 1983), furtherdecreasing the ability to transport sand.

Sand occurs within the summer mud as dis-crete layers, which may in turn be grouped intosharp-based successions wherein the thickness of each individual sand layer decreases upward(Figs 9 and 10). The prevalence of ripples alsodecreases upward through each succession.These features suggest that the successions reflectdeposition during periods of waning energy.

Fig. 12. Cancore peels from the sand flats (see Fig. 4 for locations) collected in the winter. Hummocky cross-lamination and parallel lamination are the main structures. The vertical burrow in B was produced by a polychaeteworm that is common on the middle and outer tidal flats. In general, however, bioturbation is extremely rare. Sp,parallel-laminated sand; Scr, cross-laminated sand; HCS, hummocky cross-lamination. Small circles indicate mudpebbles. Landward is to the left in all peels.




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These successions superficially resemble tidal-rhythmite successions described by variousauthors (Nio & Yang, 1991; Lanier & Tessier,1998; Dalrymple et al., 2003). In such an inter-pretation, each sand–mud couple would repre-sent a single tidal cycle, while the largersuccession would represent the passage from

spring to neap tide. This explanation is notfavoured, however, for several reasons: there aregenerally only two to three such successions inthe summer mud layer, while there are six toeight neap-spring cycles during the 3–4 monthperiod in which the summer mud layer accumu-lates; upward thickening successions of sandlayers would be expected to form during thepassage from neap to spring tide, but they areabsent; and the relatively low speed of the tidalcurrents (almost never > 40 cm sec)1) makes it

unlikely that the sand comprising each layercould be transported hundreds of metres or morefrom an exposed sand source, across the mud-covered intertidal zone, during a single tide.Some of these objections are overcome if the sandlayers represent spring tides and the interveningmud layers are neap-tide deposits; however, the

regularity of the upward-fining trend seen in eachsuccession (Fig. 10) cannot be explained by thisprocess.

Instead, it is proposed here that the laminasuccessions represent deposition during and inthe immediate aftermath of typhoons and/orsummer storms (Figs 6A and 10). Most of theripples in the sand layers show gently undulatinglamination, chevron up-building, and a paucity of angle-of-repose lamination, attributes which indi-cate that they are of wave or combined-flow origin

Fig. 13. Cancore peels from the sand flats (see Fig. 4 for locations) collected in the spring. Landward-migratingclimbing ripples are particularly well developed at the top of core F. Landward is to the left in all peels.




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(De Raaf et al., 1977; Meene et al., 1996). Theabrupt start to the successions reflects non-depos-ition or erosion as the storm intensifies, whereasthe upward decrease in the prevalence of ripplesin each succession suggests that depositionoccurred rapidly as the storm waned. It issuspected here that each sand–mud couple repre-

sents a single tidal cycle, with the sand derivedfrom local areas where the mud layer was removedduring the storm. The thick, homogeneous mudlayers in the lower part of these successions(Fig. 10) may represent fluid-mud deposits(Dalrymple et al., 2003). The scarcity of bioturba-tion in these laminated successions is also con-sistent with rapid accumulation following storms.The only deposits that represent inter-storm peri-ods are the nearly homogenous mud at the top of each upward-fining succession, which are moder-ately to intensely bioturbated and contain isolated

silty to sandy laminae (Fig. 9C,E and F).The increasingly frequent occurrence of stormsand onshore winds through the fall and into thewinter (Figs 5 and 6A) leads to the erosion of thesummer mud layer. Erosion starts at local defects(e.g. burrow openings and stakes associated withfishing activities) and other weak areas (e.g.slightly lower, wetter places where desiccation

was less effective at consolidating the mud).Cooler water temperatures (Fig. 6B) and wind-and wave-induced turbulence hamper the settlingof mud. Whenever mud drapes do form, they arere-eroded almost immediately because desicca-tion-produced consolidation is negligible andwave action is nearly constant. As a result, mud

deposition is restricted to the inner part of theIMF where the influence of waves is negligible.The outer part of the IMF is frequently influenced by storm waves that produce wave and combined-flow ripples (Fig. 7A and D).

Unlike typical tidal flats that show a landward-fining trend, the sandy tidal flats coarsen land-ward, with the coarsest sand occurring justseaward of the IMF (Fig. 8B). In this, they aremore similar to shorefaces and beaches (Davis,1985). Indeed, the coarsest sand generally occurswhere the convex-up topographic profile is steep-

est near the high-tide elevation (Fig. 4): presum-ably wave energy is greatest at this location because of reduced attenuation by bottom frictionwhen water depths are greatest (Kim, 2003), and because water levels change slowly leading to theconcentration of wave energy. At any given time,however, wave attenuation causes wave energy todecrease landward (Fig. 14). As a result, the

Fig. 14. Schematic depositional model for a sandy, open-coast tidal flat like that at Baeksu. The frictional attenu-ation of storm waves as they propagate across the gently sloping tidal flat leads to a systematic change in the nature of the deposits: the wavelength of the HCS decreases landward because of the decrease in wave size; and the abundanceof wave ripples and bioturbation increase landward because of the decreasing wave energy. Summer mud depositsare only preserved in the innermost part of the tidal flat, landward of the place where swash bars weld to theshoreline, leading to the development of an incipient-strand plain/beach face. Further seaward, erosion of thesummer mud by winter storms generates mud-pebble lags at the base of storm beds.




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sedimentary structures produced during the win-ter change across the tidal flats, from ripple cross-lamination and gently inclined lamination on theinner sand flats (Fig. 12A and D), to parallellamination and HCS with minor ripples on theouter flats (Figs 12B,C,E and F, and 14).

The HCS present in the sand flats appears

qualitatively to decrease in wavelength towardsland, and even the longest wavelengths (ca 2 m)are shorter than the values commonly reportedfor shelf deposits (typically > 2 m; Dott &Bourgeois, 1982; Harms et al., 1982; Walker &Plint, 1992; Cheel & Leckie, 1993). This indicatesthat the wavelength of the HCS is correlated insome way with the wavelength of the waves thatproduced the structures. During storms, the wavespectrum on the tidal flats is likely to be ‘satur-ated’, meaning that the waves are continuously breaking and thus are at their depth-limited size

across the entire tidal flat (Wright & Short, 1984;Le Hir et al., 2000; Kim, 2003). Consequently, thewave height and spacing decrease landward as aresult of energy dissipation, causing the HCS to become smaller also. The presence of weak tidalcurrents throughout the period of submergenceduring storms (Kim, 2003) indicates that the HCSforms under combined-flow conditions.

The predominance of landward-directed ripplecross-lamination, much of it climbing at moder-ately steep angles (Figs 12E and 13F), cannot beattributed to tidal currents, because the flood andebb currents are nearly equal in speed and

duration (Kim, 2003). Instead, these structuresare attributed to wave action. As the wavesapproach the water’s edge on these gently slopingtidal flats, each one generates a solitary bore.Wave oscillatory motion in such circumstances ishighly asymmetric, with stronger landward-direc-ted currents (Clifton et al., 1971), especially dur-ing the flooding tide. Such motion, whencombined with the intense turbulence generated by the breaking waves, is believed to be respon-sible for the landward-directed climbing ripples.

DISCUSSION AND CONCLUSIONS

The Baeksu tidal flat is transitional betweensheltered (i.e. ‘true’) tidal flats and beaches/shorefaces (Fig. 1; Hayes, 1979; Short, 1991; Boydet al., 1992). In the summer, when wave action isminimal, it behaves much like a typical tidal flat:sedimentation is tide dominated and heterolithicstratification (Fig. 9), including deposits thatresemble tidal, wavy and lenticular bedding.

However, even during this period, storms playan important role in the development of rhyth-mite-like successions of interbedded sand andmud (Figs 9 and 10). During the winter, bycontrast, the tidal flats behave more like a shore-face. Sedimentation is dominated by breakingwaves that form graded storm beds and HCS

(Figs 12 and 13), deposits that strongly resemblethose considered diagnostic of shorefaces (Walker& Plint, 1992). The concave-upward topographicprofile (Fig. 4) and onshore increase in sandgrain size (Fig. 8B) are also similar to featuresseen in shorefaces, although the presence of theIMF in sheltered areas (Fig. 2) is more character-istic of tidal flats. The general scarcity of muddydeposits in the deeper parts of the cancores fromthe outer flats (Figs 9 and 11–13), regardless of the season, indicates that the summer mud israrely preserved and that, in the longer term, the

environment is wave dominated.Mixed-energy coastal environments that lie between the end members of sheltered tidal flatsand beaches/shorefaces are probably much morecommon than the available literature wouldsuggest. Sandy open-coast tidal flats such asdescribed here (see also Reineck & Cheng, 1978;Mukherjee et al., 1987) and ‘tidal beaches’(Short, 1991; Masselink & Short, 1993) that formin meso- to macrotidal settings where waveenergy is high represent variants within thistransition. All of these settings display a con-cave-up profile. The tidal beaches described by

Short (1991) and Masselink & Short (1993) havea relatively steep beach face with slopes thatrange between ca 0Æ5 and 3; the open-coasttidal flats of north-eastern India have a slope of ca 0Æ15 (Mukherjee et al., 1987); whereas theBaeksu tidal flat has an average slope of 0Æ06–0Æ07. In general, swash bars become moreprominent as the slope increases, whereas thepresence of an IMF is more common on low-gradient coasts. This spectrum of environmentspresumably reflects a gradient of wave energy,with the steeper slopes occurring where the

wave energy is higher (Wright & Short, 1984).However, sediment grain size also influences thecoastal gradient, coarser grain sizes promotingsteeper slopes than finer grain size (Davis, 1985;Short, 1991). Thus, open-coast tidal flats andtidal beaches may well form under broadly thesame wave-tide regime.

As already discussed, the preserved deposits of the Baeksu tidal flat show significant similaritieswith those typical of shorefaces. However, thereare a number of features that should permit the




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distinction between open-coast tidal flats andshorefaces in ancient deposits:

1. The muddy deposits that form during peri-ods of low wave energy may have relatively lowpreservation potential; nevertheless, they do oc-cur, either in the form of in situ remnants or, more

commonly, as mud pebbles concentrated at the bases of storm beds. The erosional remnants areparticularly important for making an environ-mental distinction, because they may contain ti-dal rhythmites, thereby attesting to the influenceof tidal processes.

2. Other indicators of tidal action, such as her-ringbone cross-lamination, are present in smallamounts in open-coast tidal flats.

3. Landward-directed climbing ripples aremoderately abundant in open-coast tidal flats.Their origin is ascribed to the formation of land-ward-propagating wave bores (Le Hir et al., 2000).

Such bores are of less importance on steeperslopes and should not be as prevalent in beach/shoreface settings (Komar, 1998). Indeed, land-ward-directed climbing ripples have not beenreported from more wave-dominated environ-ments (e.g. Clifton et al., 1971; Walker & Plint,1992). However, the prevalence of such climbingripples may be grain-size dependent, becomingmore abundant in finer sand that is more easilysuspended.

4. The HCS that occurs in open-coast tidalflats has shorter wavelengths than are con-

sidered typical of shoreface to shelf deposits:< 2 m on open-coast tidal flats (Figs 9 and 11–13; see also Mukherjee et al., 1987) versus> 2 m on marine shorefaces and shelves (e.g.Dott & Bourgeois, 1982; Cheel & Leckie, 1993;Amos et al., 1996). The smaller size is attrib-uted to the reduction in wave size that occursas they propagate into the intertidal zone (Clif-ton et al., 1971). Therefore, progradation of acoastline like that at Baeksu should produce asuccession with an upward decrease in the sizeof the HSC. The presence of small-wavelengthHCS by itself should not be taken as indicativeof an intertidal origin, because Allen (1981) andGreenwood & Sherman (1986) have reported theoccurrence of small-wavelength HCS from la-custrine shorefaces, presumably because theincoming waves are small as a result of thelimited fetch.

5. Gently inclined parallel lamination, such asoccurs on beaches, is not common in open-coasttidal flats. The steeper gradient of beaches andshorefaces leads to less frictional retardation of

the waves and promotes swash and backwashprocesses (Masselink & Short, 1993). As a result,parallel lamination is favoured instead of waveripples (Clifton et al., 1971).

6. Beaches and shorefaces should not pass up-ward gradationally into muddy tidal-flat deposits,whereas open-coast tidal flats may do so.

In summary, the sandy open-coast tidal flats atBaeksu, South Korea, experience a pronouncedseasonal variation in sedimentary conditions,oscillating between tidal-flat sedimentation insummer and shoreface-like conditions in winter, because of the monsoonal control on the wind andwave climate. In the longer term, wave-dominateddeposits predominate. These deposits are similarto, but subtly different from those of true shore-faces. Such an open-coast tidal-flat setting hasrarely been recognized in the rock record; havethey been misidentified as shorefaces?

ACKNOWLEDGEMENTS

This study was supported by grants from theKorea Science and Engineering Foundation (no.F01-2001-000-10010; SSC) and the Natural Sci-ence and Engineering Research Council of Can-ada (no. 7553-01; RWD). We thank C. D. Jung,C. S. Shon, Y. S. Baek, J. K. Kim and K. S. Jang(Chonnam National University) for their assist-ance in the field and laboratory. We appreciate

the constructive review comments by Paul Car-ling and Carl L. Amos.

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Manuscript received 2 October 2003;

revision accepted 2 October 2004



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