THE SIGNIFICANCE OF HUMMOCKY CROSS-STRATIFICATION (HCS) WAVELENGTHS: EVIDENCE FROM AN OPEN-COAST TIDAL FLAT, SOUTH KOREA

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1 Journal of Sedimentary Research, 2006, v. 76, 2 8 Current Ripples DOI: /jsr THE SIGNIFICANCE OF HUMMOCKY CROSS-STRATIFICATION (HCS) WAVELENGTHS: EVIDENCE FROM AN OPEN-COAST TIDAL FLAT, SOUTH KOREA BYONGCHEON YANG, 1 ROBERT W. DALRYMPLE, 1 AND SEUNGSOO CHUN 2 1 Department of Geological Sciences and Geological Engineering, Queen s University, Kingston, Ontario, K7L 3N6, Canada 2 Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Kwangju , Korea bcyang@geoladm.geol.queensu.ca ABSTRACT: Although hummocky cross-stratification (HCS) is one of the most common and widely recognized structures in ancient storm-dominated successions, the stratigraphic variability and environmental significance of HCS wavelength (l) are still not widely appreciated. New evidence from an open-coast intertidal flat where HCS might not have been expected to occur shows that the HCS becomes smaller in a landward direction because of a decrease of wave size. This confirms previous suggestions that the bedform responsible for HCS is a type of orbital ripple. A review of new and previously published data indicate that HCS wavelength is controlled by the bottom orbital diameter (d 0 ) according to the relationship l < 0.75 d 0. These observations imply that the maximum size of HCS should increase with decreasing water depth from the shelf to the surf zone (breaking point) but may then decrease landward of this point because wave size is depth limited. This suggests that it may be possible to use HCS size in paleo-environmental reconstructions to a greater degree than previously. INTRODUCTION Since Harms et al. (1975) first described and defined hummocky cross stratification (HCS), it has become one of the most widely recognized structures in modern and ancient, shallow-water, stormdominated environments. Much has been written about its morphology and origin (e.g., Dott and Bourgeois 1982; Brenchley 1985; Greenwood and Sherman 1986; Duke et al. 1991; Cheel and Leckie 1993; Li and Amos 1999). It is generally believed that HCS is generated by oscillatory motion under large waves during storms. The extent to which unidirectional currents are important in its formation has been the subject of considerable speculation (Southard et al. 1990; Duke et al. 1991; Cheel and Leckie 1993; Li and Amos 1999), but flume studies suggest that the formative conditions must be oscillation dominated (Arnott and Southard 1990; Dumas et al. 2005). HCS varies enormously in size, with wavelengths ranging from several meters to perhaps as small as cm (Harms et al. 1975; Dott and Bourgeois 1982; Brenchley 1985; Greenwood and Sherman 1986; Southard et al. 1990; Cheel and Leckie 1993; Banerjee 1996; Li and Amos 1999; Ito et al. 2001). Surprisingly, the environmental factors responsible for this variability have not been evaluated systematically, although it has been implied with varying degrees of confidence that HCS wavelength is related to wave orbital diameter, because the large, 3D ripples thought to form HCS increase in size as the wave orbital diameter increases (Harms et al. 1975; Arnott and Southard 1990; Southard et al. 1990; Wiberg and Harris 1994; Dumas et al. 2005). Perhaps as a result of the lack of appreciation of the controls on HCS wavelength, there have been very few attempts to use the characteristics of HCS to refine environmental interpretations. The most detailed studies to date are those by Banerjee (1996) and Ito et al. (2001), who have utilized the stratigraphic distribution of HCS size to make inferences about changing water depth and wave climate. However, these studies made no attempt to develop a general model for the spatial variability of HCS wavelength. Here, we report new observations on the spatial distribution of HCS wavelength (l) on a sandy, open-coast (unbarred) intertidal flat, west coast of Korea (Fig. 1A), that is subjected to intense wave action during the winter. Such a shallow-water environment, where HCS might not be expected to occur at all, provides a powerful test of previous ideas concerning the factors governing its size. Using in situ storm-wave data and theoretical considerations of wave action in very shallow water, we demonstrate that, as proposed by previous workers, HCS is formed by a type of orbital wave ripple. Then, using a synthesis of data on HCS wavelength derived from flumes and other modern environments, we develop a quantitative relationship between HCS wavelength and waveorbital diameter. From this, we propose a general model of the spatial and stratigraphic variation of HCS size in wave-dominated coastal environments. STUDY AREA The study area (the Baeksu tidal flat) is located on the southwestern coast of Korea and consists of an open-coast intertidal flat that is up to 5 km wide at low tide (Fig. 1B). It is characterized by a smooth, concaveup morphology that is featureless except for subtle undulations, cm high with a spacing of m, which are interpreted to be intertidal swash bars (Yang et al. 2005). The tidal-flat slope averages but increases landward from to The storm and wave climates are strongly seasonal, with generally calm conditions prevailing in the summer, whereas frequent, intense storms occur during the winter. Storms, which are defined here as times with wind speeds m/s, occur only 2 3 days/month in the summer but more than 8 days/month in the winter (Korea Meteorological Administration 1998). Consequently, the conditions in the intertidal zone Copyright E 2006, SEPM (Society for Sedimentary Geology) /06/ /$03.00

2 JSR RECONSIDERATION OF HCS WAVELENGTH AND ITS SEDIMENTOLOGICAL MEANING 3 FIG. 1. A) Location of study area (Baeksu tidal flat) on the southwestern coast of Korea. B) Map of the study area; extent of intertidal zone at spring low water shown in gray. C) Typical grain-size distributions for various locations across the tidal flat (northern survey line; Fig. 1B) in winter. Note that the modal grain size is uniform across the tidal flat but that the size distributions become more positively skewed in a seaward direction. alternate between muddy, tide-dominated deposition in the summer and sandy, wave-dominated sedimentation in the winter (Chun et al. 2000; Yang et al. 2005). During the winter, storm waves with a mean significant height of m approach the coast orthogonally. Because of the absence of barriers, these waves impinge directly on the tidal flat, where they dissipate their energy by breaking. Because of the low gradient, the surf zone can occupy the entire width of the intertidal zone at high tide during storms. Surface sediments during the winter are well-sorted very fine sand with a median grain size of 0.1 mm (Fig. 1C). The modal grain size is almost uniform across the intertidal flat, showing only a slight seaward-fining trend (Fig. 1C). The tides are semidiurnal with a spring tidal range of ca. 6.8 m. Tidalcurrent speeds vary from 20 to 40 cm/s, but wind action during storms can reinforce the current speed, creating peak speeds of 60 cm/s (Chun et al. 2000; Kim 2003). METHOD AND DATA COLLECTION Measurement of slope profiles was carried out using a Sokkia B2 1 theodolite along two permanent survey lines that extended from the shoreline to near the low-tide level. Surface-sediment samples and small cancores (30 cm deep 3 18 cm wide) (Fig. 2) were taken at 100 m and m intervals, respectively, along these lines (Yang et al. 2005). Most of the cancores were oriented parallel to the survey line, with a few oriented perpendicular to the line (i.e., parallel to the coast). In addition, a series of five large cancores (45 cm deep 3 40 cm wide) were taken side by side, at four locations, 1.7 km, 2.1 km, 2.7 km, and 3.0 km seaward of the mainland coast, in order to produce continuous sections up to 2 m long (Fig. 3). The grain size of the surface sediments was analyzed using sieves and a Sedigraph, after the organic matter and calcium carbonate were removed. The primary sedimentary structures in the cancores were studied using epoxy relief peels. The wavelength of HCS was estimated from the cancores only in cases where the geometry of the lamination indicated that more than half of the full wavelength was present, assuming that the structure was more or less symmetrical (Fig. 3). Although undulatory lamination with wavelengths less than 10 cm occurred in some instances (Fig. 3), we excluded wavelengths smaller than 30 cm from the data set because most bedforms of this size appeared to be combined-flow ripples, which are commonly characterized by an asymmetric rounded formset and sigmoidal foreset laminae (Figs. 2, 3) (e.g., Harms et al. 1975; Yokokawa et al. 1995; Meene et al. 1996). Even though many of the cancores were taken immediately after a storm, the surficial morphology of the hummocks was not preserved, so that the position of the cancore relative to the crest of the hummock was not known. For this reason, the measured wavelengths may deviate from the average wavelength by an unknown amount; however, the seriousness of the discrepancy is uncertain because we do not know the precise planform

3 4 B.C. YANG ET AL. JSR FIG. 2. Selected images of wave-generated structures from the Baeksu tidal flats. A) Hummocky cross-stratification passing upward, apparently gradationally, into wave-ripple cross lamination that shows chevron upbuilding and offshoot lamination. B) Combined-flow and climbing-ripple cross lamination. Note that combined-flow ripples are characterized by an asymmetric rounded formset (see well-preserved ripple formset below mud drape: arrows) and sigmoidal foreset laminae. Climbing ripples are probably produced by the successive solitary wave bores that cross the tidal flats (Yang et al. 2005). shape and spatial organization of the hummocks and swales. If the hummocks are assumed to be circular with the closest possible spacing (i.e., the planform equivalent of rhombohedral packing of spherical sand grains; Fig. 4A), then the average wavelength would be ca. 1.3 (where a value of 1.0 is the shortest possible spacing between crests), giving a possible deviation from the mean value of the order of 6 30%. The spatial equivalent of cubic packing would give a deviation of ca. 6 20% (Fig. 4B), whereas random spacing would yield higher values (Fig. 4C). Highly random spacing of the hummocks is not expected, however, given the reported geometry of bedding-plane exposures of HCS (e.g., Brenchley 1985; Cheel and Leckie 1993; Li and Amos 1999). Therefore, we suggest that our measurements of HCS wavelength are likely to deviate from the mean value by substantially less than 6 50%, which is insufficient to invalidate the spatial variability that we report below. HUMMOCKY CROSS-STRATIFICATION The thickness of the storm beds containing HCS ranges approximately from 10 to 30 cm and decreases in a landward direction across the tidal flat. Each one begins with an erosional surface, which is planar to gently undulatory (Figs. 2A, 3). Relief on such surfaces may reach up to 20 cm and tends to increase as the thickness of the bed increases in a seaward direction. Such surfaces are underlain by a thin mud layer where they are not erosional and are overlain commonly by mud pebbles and scattered shell fragments. These lag deposits are, in turn, overlain locally by planar lamination and then by undulatory lamination (Fig. 3) that we consider to represent HCS. The inclination of the undulatory lamination is nowhere more than 15u. Individual laminae, which range in thickness from 0.5 to 5 mm, thicken toward either the crest or the swale, but uniform draping over undulating surfaces is common. Low-angle onlap against erosion surfaces is also present. The angle of the inclined lamination gradually decreases upward through an individual bed, ending in planar lamination (Fig. 3). Storm beds are commonly capped by wave and/or combined-flow ripples, which typically show a landward direction of migration (Fig. 2B), followed by a thin mud layer. The three types of HCS shown by Cheel and Leckie (1993; scourand-drape, accretionary, and migrating), are all present in the Baeksu tidal flat, but the scour-and-drape type, which is characterized by quasiconformable mantling of undulatory erosion surfaces (Figs. 2A, 3), is the most abundant. This geometry is sometimes referred to as swaly crossstratification (SCS), which was originally described by Leckie and Walker (1982) as occurring in coarser sand and having more gentle dips than HCS. However, the fine grain size and relatively steep dips of some examples suggest that the use of the term HCS is more appropriate.

4 JSR RECONSIDERATION OF HCS WAVELENGTH AND ITS SEDIMENTOLOGICAL MEANING 5 FIG. 3. Series of five adjoining cancore peels taken 2.7 km from the mainland coast: the lower two peels join on the right end of the upper set of three peels. Most structures are concave-up and are interpreted as the swales between hummocks. HCS wavelength was measured from one crest to the adjacent crest. In cases where less than the complete wavelength was visible, the wavelength was calculated to be twice the distance between the crest and the bottom of the swale, on the assumption that the feature was symmetrical. Features in which less than half of the form was visible were not measured. The largest swale, which extends across nearly the entire width of the five panels and contains abundant mud pebbles in its base, has an estimated wavelength of slightly less than two meters (ca. 180 cm). Deformation at the sediment surface was caused during coring. Irregular markings on the surface of the peel are an artifact of the peel-making process. The HCS in the study area has wavelengths that range from 30 to 200 cm (cf. Fig. 3). Observations in our peels show that the range of HCS wavelengths decreases in a landward direction across the tidal flat (Fig. 5). This restriction in the range occurs because the largest wavelengths, which were measured in the long, composite peels (Fig. 3), decrease toward the land. The landward decrease in the number of data points is caused mainly by increased bioturbation, which obliterates sedimentary structures. The cancores indicate that the occurrence of HCS ends approximately 1.0 km from the shoreline (Fig. 5), on the basis of the arbitrary, lower wavelength limit of 30 cm. In more landward locations, FIG. 4. Schematic representation of the spatial distribution of hummocks. A) rhombohedral spacing, B) cubic spacing, and C) random spacing; cf. Allen 1985, p ) with estimates of the variability of wavelength (i.e., from one structural high to the next) measurements as obtained from random 2D sections: A, 6 30%; B, 6 20%; C, 6 75%.

5 6 B.C. YANG ET AL. JSR FIG. 5. Lateral variation of measured HCS wavelengths (l) across the tidal flat, as determined from large, composite peels. Maximum wavelength decreases landward because wave size decreases due to energy dissipation by breaking and bottom friction. Wave and combined-flow ripples are the only bedforms present in the inner 1 km. Dotted lines show calculated wave-orbital diameter (d 0 ) and 0.7 d 0, respectively, for incident waves 4 m high, as derived from the relevant curve in Figure 6 using Equation 4. wave and/or combined-flow ripple cross-lamination is abundant (see also Yang et al. 2005). WAVE-SIZE DISTRIBUTION ON THE TIDAL FLATS The landward decrease in storm-bed thickness and HCS wavelength (Fig. 5), which is accompanied by a landward increase in the abundance of ripples and the degree of bioturbation, strongly suggests that these trends are caused by a landward decrease in wave energy, which is, in turn, caused by wave-energy dissipation by bottom friction and breaking in shallow water (e.g., Thornton and Guza 1982; Le Hir et al. 2000). During storms, the waves break continuously across the shallowly submerged tidal flat; consequently, the size and energy of these breaking waves decrease progressively (e.g., Dally et al. 1985). Because the size of breaking waves is limited by water depth, the wave spectrum in the breaker zone is said to be saturated (Thornton and Guza 1982; Le Hir et al. 2000), and it is possible to predict the spatial distribution of wave size at any given time in a tidal cycle using wave theory, if the size of the waves arriving from the open ocean is known. The following analysis is performed only for high tide when water depth and wave size at any given location are greatest. According to Le Hir et al. (2000), the variation of wave height (H) with distance across a gently sloping, shallow-submerged tidal flat (i.e., water depth less than ca. 5 m) depends on the distance from shore (x), and the wave friction factor ( f w ), and can be determined from the relationship: d dx (H2 x 1=2 ) ~ 2 f w 3p b 2 H3 x {3=2 ð1þ where b is the tidal-flat gradient. This equation, which is based on energy conservation, can be conveniently converted into a dimensionless form using h 0, the water depth at the outer limit of wave breaking: d dx (H2 0 X 1=2 ) ~ 2 f w 3p b H3 0 X {3=2 ð2þ where H 0 5 H/h 0 and X 5 x b /h 0. Rearranging and solving for H 0 gives H 0 ~ 4 fw 15p b X {1 z H sea {1 { 4 fw {1 X 1=4 ð3þ 15p b where H sea is in the ratio of incident wave height (H) to water depth (h) at the outer breaking limit. To calculate the wave-height distribution from Equation 3, f w /b is set equal to 50, as suggested by Le Hir et al. (2000). Figure 6 shows that incoming waves experience a rapid, exponential decrease in height across the tidal flat in response to the decreasing water depth. FIG. 6. Predicted wave-height variation across the Baeksu tidal flat for different incident wave heights (1 m, 2 m, 3 m, and 4 m), as determined using Equations 2 and 3. MLWL is mean low water level. Note that the equations used to calculate wave height are not valid for deeper water (i.e., greater than approximately 5 m), so the subtidal extensions of the curves are inaccurate. All of the curves should flatten seaward and approach the appropriate value of the incident wave height.

6 JSR RECONSIDERATION OF HCS WAVELENGTH AND ITS SEDIMENTOLOGICAL MEANING 7 range of m (National Fisheries Research and Development Institute 1998; Kim 2003; Kim et al. 2003). Therefore, in order to approximate peak-storm conditions when waves are largest, we have assumed a typical incident-wave height of 4 m. Wave periods measured during storms vary over the tidal cycle because of changing water depth, but an average value of T 5 5 s is typical on the tidal flat at high tide (Kim 2003). The calculated distribution of d 0 values across the tidal flat is shown in Figure 5. From this, it is evident that the largest measured HCS wavelengths fall approximately along the line l d 0. Clearly, therefore, there is a direct correlation between the spatial distribution of the wavelength of the largest HCS and the peak size of the storm waves. The smaller HCS that co-occurs with the large HCS is believed to form during the falling and/or rising tide when water depths and wave heights are less than at high tide (Yang et al. 2004). This is supported by the fact that the largest HCS occurs at the bases of storm beds and is overlain by shorter-wavelength hummocks in the same storm bed (Fig. 3). DISCUSSION FIG. 7. Plot of published measurements of HCS wavelength, modified from Wiberg and Harris (1994). A) Relationship between l/d against d 0 /D, where D 5 mean grain size. The shaded areas show the locus of data points plotted by Clifton (1976) and Wiberg and Harris (1994). B) Relationship between l and d 0 for occurrences of HCS. l d 0 is the best-fit regression line; r 5 regression coefficient; N 5 number of data points. The bottom orbital diameter (d 0 )atanypointacrossthetidalflatcanbe determined from the predicted storm-wave heights (Figs. 5, 6) by the equation rffiffiffi d 0 ~ HT g = 2p ð4þ h where T is the wave period and g is the acceleration of gravity. Equation 4 indicates that bottom orbital diameter (d 0 ) is mainly a function of wave height and water depth if wave period (T) is assumed to be constant. During storms, significant wave heights in the offshore area are in the It is generally believed that HCS is formed by what many researchers term large ripples (Harms et al. 1975; Arnott and Southard 1990; Southard et al. 1990; Myrow and Southard 1991; Li and Amos 1999; Dumas et al. 2005). If we plot data on the wavelength and orbital diameter for reported occurrences of large ripples and/or HCS from flumes (Southard et al. 1990; Dumas et al. 2005) and modern environments (Sherman and Greenwood 1989; Li and Amos 1999; this study), we see that they fall along an extension of the trend defined by small orbital ripples (Fig. 7A). Given that HCS generally occurs in a narrow range of grain size (fine to very fine sand), these data can also be plotted in a dimensional plot of l versus d 0 (Fig. 7B). There is considerable scatter, with l 5 d 0 as the upper bound, but the best-fit linear regression yields the relationship l d 0, which has a slightly higher proportionality constant (0.75) than that suggested by our data alone (0.7; Fig. 5). This, in turn, supports the suggestions of previous workers that the bedform responsible for HCS is a type of orbital ripple (e.g., Harms et al. 1975; Southard et al. 1990). If this is the case, we should expect the wavelength of HCS to vary spatially in an offshore onshore transect (Fig. 8). As the seabed rises toward the shoreline, wave orbital diameter at the bed will increase in the predictable manner shown in textbooks, reaching a maximum value in shallow water, at the seaward edge of the breaking zone. Farther landward, wave size will decrease because of energy dissipation by breaking and bottom friction (cf. Fig. 6). According to the l d 0 relationship shown in Figure 7B, the wavelength of the HCS being formed at any given time should change in a similar manner. Temporal variability of wave size means that a range of HCS wavelengths will be formed at any location (cf. Fig. 5); however, the stratigraphic distribution of maximum size should show the trend illustrated in Figure 8. Therefore, HCS wavelength may be useful for paleo-environmental reconstructions in the manner pioneered by Banerjee (1996) and Ito et al. (2001). CONCLUSIONS Observations of HCS on the open-coast intertidal flat at Baeksu, Korea, combined with theoretical considerations of wave behavior in shallow water, indicate that the bedform responsible for HCS records the landward decrease in wave orbital diameter that occurs because of frictional dissipation. This confirms previous suggestions that HCS is formed by a type of orbital wave ripple. When our observations are combined with data from previous studies, we find that HCS wavelength and the wave orbital diameter are related by the equation l d 0. This finding suggests that the (maximum) wavelength of HCS varies in a predictable manner from the shelf to the shoreline: HCS wavelength will

7 8 B.C. YANG ET AL. JSR FIG. 8. Schematic diagram illustrating the probable variation of bottom orbital diameter and HCS wavelength from the shelf to the coast. The vertical distribution of HCS wavelength (right side) refers to the maximum values because short-wavelength HCS can form in any water depth. In very shallow water, the landward and upward decrease in both variables is caused by energy dissipation caused by wave breaking and bottom friction. This zone will be of limited lateral and vertical extent in normal beach environments, but will be more important in low-gradient, open-coast tidal flats such as those in the study area. increase as the seafloor shallows from the shelf to the outer edge of the breaker zone, but will decrease landward of this point, if there is a broad, shallow intertidal zone in which the wave spectrum becomes saturated (i.e., wave height is depth limited) as it does in the study area. This opens the possibility that the stratigraphic distribution of HCS can be used for paleo-environmental reconstruction more than it has until now. ACKNOWLEDGMENTS We thank Dr. HeeJun Lee (Korea Ocean Research and Development Institute), and Mike Johnson and Duncan Mackay (Queen s University) for their helpful comments. We also thank JongKwan Kim and KangSuk Jang (Chonnam National University) for help with field and laboratory work. This work was supported by grants from the Natural Science and Engineering Research Council of Canada (# ; RWD) and the Korea Research Foundation (KRF C00593; SSC). We appreciate the constructive comments by D. Dominic, P. Hill, C.P. North, and an anonymous reviewer. We also thank Dr. J.B. Southard and M. Lester for technical review and editorial assistance. REFERENCES ALLEN, J.R.L., 1985, Principles of Physical Sedimentology: London, Allen & Unwin, 272 p. 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