FINE-GRAINED VERSUS COARSE-GRAINED WAVE RIPPLES GENERATED EXPERIMENTALLY UNDER LARGE-SCALE OSCILLATORY FLOW

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1 Journal of Sedimentary Research, 2009, v. 79, Research Article DOI: /jsr FINE-GRAINED VERSUS COARSE-GRAINED WAVE RIPPLES GENERATED EXPERIMENTALLY UNDER LARGE-SCALE OSCILLATORY FLOW DON I. CUMMINGS, 1 * SIMONE DUMAS, 1{ AND ROBERT W. DALRYMPLE 1 1 Department of Geological Sciences and Geological Engineering, Queen s University, Kingston, Ontario, K7L 3N6 Canada dcumming@nrcan.gc.ca ABSTRACT: Wave ripples were generated in a wave tunnel under large-scale oscillatory flow (orbital diameter m) using two different grain sizes, very fine sand and coarse sand. The geometry of bed configurations that were produced varied strongly as a function of grain size: small anorbital ripples (wavelengths, 10 cm, heights, 1 cm) formed exclusively in very fine sand at low oscillatory velocities, whereas large orbital ripples (wavelengths cm, heights 7 26 cm) formed in both very fine and coarse sand, but were subdued, sharp- to round-crested, and 2-D to 3-D in very fine sand, and steep, sharp-crested, and 2-D in coarse sand. The large ripples in fine sand, if aggraded, would deposit low-angle (5 15u) cross stratification resembling hummocky cross stratification, whereas the large ripples in coarse sand would deposit high-angle (15 25u) cross stratification that might be mistaken for the deposit of a dune because of its high dip angle and large set thickness (. 5 cm). These results support the hypothesis advanced by Leckie (1988) that large waves generate markedly different stratigraphic signatures in finegrained and coarse-grained sediment. INTRODUCTION The back-and-forth motion of water beneath waves commonly stirs sediment on the seafloor, shaping it into regularly spaced mounds termed wave ripples. Wave ripples are an invaluable tool to geologists: their presence indicates subaqueous deposition, and their shape, size, and stratification can be used to infer the size of the waves that deposited them, which, in turn, provides a general idea of the size of the water body in which they formed. This makes them useful for interpreting ancient depositional environments (e.g., open-ocean shoreface versus backbarrier lagoon) and for reconstructing the migration of ancient depositional environments with time, an important component of sequence stratigraphy. Unfortunately, compared to bedforms generated by unidirectional currents, for which robust phase diagrams have been developed (Southard and Boguchwal 1990), our understanding of the relationship between the form and generating conditions of wave ripples is still incomplete. This is particularly true for bed forms generated by large waves (e.g., storm waves in the ocean). The reasons for this are twofold. First, instruments deployed to monitor the seafloor during storms commonly fail or become damaged (e.g., Wright et al. 1994; cf. Traykovski et al. 1999; Hanes et al. 2001). Second, it is difficult to generate high (. 2m from peak to trough), long-period (. 6 s) waves like those present in the ocean in the laboratory because prohibitively long, deep flumes are required (e.g., Williams et al. 2004). A successful solution has been the use of wave tunnels enclosed ducts through which water is forced back and forth to study wave ripples generated by large-scale oscillatory flow. In * Present address: Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada { Present address: Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada previous experiments (e.g., Southard et al. 1990; Arnott and Southard 1990; Dumas et al. 2005; O Donoghue et al. 2006), a particular bed configuration consisting of low-relief, meter-wavelength mounds ( hummocks ) was reported in each case. Aggradation of these mounds produces low-angle (, 15u), gently undulating cross stratification that closely resembles hummocky cross stratification ( HCS ; Harms et al. 1982; Southard et al. 1990; Dumas and Arnott 2006). This observation supports the hypothesis, advanced by field geologists in the 1960s and 1970s (e.g., Cambpell 1966; Harms et al. 1975), that HCS is diagnostic of deposition under the influence of large waves in large water bodies. Despite the promise of wave tunnels for understanding bed forms generated by large waves, few have been built because they are big and expensive. Consequently, few experiments have been conducted, the result being that the variables that control the size, shape, and stratification of bed forms generated by large waves namely sediment grain size (D 50 ), oscillatory velocity (U o ), and orbital diameter (d 0 ) (Fig. 1) have not been explored comprehensively. In particular, the effects of grain size are poorly understood, because most large-scale wave-tunnel experiments have used very fine or fine sand, the grain size in which HCS is most commonly reported (Harms et al. 1982). What if coarser sand is used? Does HCS still form? Based on an analysis of modern and ancient waveripple data, Leckie (1988) argued that large straight-crested ripples form in place of hummocks in coarse sediment, an idea that appears to be supported by recent field and experimental data (Traykovski et al. 1999; O Donoghue and Club 2001; Hanes et al. 2001; O Donoghue et al. 2006). To test this hypothesis explicitly in a controlled laboratory setting, and to further our understanding of wave ripples in general, this study documents equilibrium bed forms generated under large-scale oscillatory motion in a wave tunnel at Queen s University, Canada, using first a bed of very fine sand, and then, under the same hydraulic conditions, a bed of coarse sand. Copyright E 2009, SEPM (Society for Sedimentary Geology) /09/ /$03.00

2 84 D.I. CUMMINGS ET AL. JSR FIG. 2. Ripple properties measured in this study. Ripple steepness (height/ wavelength), ripple symmetry (left-flank length/right-flank length), and maximum flank slope angles were measured from the side-wall viewing area (see Fig. 5 for traced photos). LWRs are defined as having wavelengths. 30 cm, whereas SWRs have wavelengths, 30 cm. FIG. 1. The three key controls on wave-ripple shape and size orbital diameter (d 0 ), maximum oscillatory velocity (U o ) and sediment grain size (D 50 ). (Many authors (e.g., Clifton 1976; Southard et al. 1990) substitute wave period (T) for d 0 since T 5 pd 0 /U o.) Other variables may be important under certain circumstances (Yalin and Russell 1962; Southard et al. 1990), especially in the nearshore zone (water depths, 10 m; see Clifton et al. 1971; Clifton 1976; Swift and Niedoroda 1985), where strong wave-orbital asymmetry and/or unidirectional currents may generate asymmetric ripples (e.g., the lunate megaripples of Clifton (1976) and Hay and Mudge (2005)). However, on vast expanses of the continental shelf where near-bed oscillatory motion is nearly symmetric and unidirectional currents negligible (i.e., less than 5 10 cm/s; see Clifton 1976; Arnott and Southard 1990; Dumas et al. 2005), d 0, U o, and D 50 are the first-order controls on wave-ripple shape and size. TERMINOLOGY, MATERIALS, AND METHODS Unlike current-generated bed forms (Ashley 1990), descriptive terms for wave ripples (e.g., steep, large) are not standardized. To facilitate description, therefore, an informal terminology is adopted (in part from Southard 1991). A bed form is defined as a distinct topographic element on the bed, and a bed configuration is the assemblage of all topographic elements on the bed at one time. The term wave ripple is applied to all wave-generated bed forms, irrespective of shape and size (cf. Carstens et al. 1969). Following Hanes et al. (2001), small wave ripples (SWRs) are defined as having wavelengths of, 30 cm, and large wave ripples (LWRs) are defined as having wavelengths. 30 cm (Fig. 2). Steep and subdued ripples have maximum flank slopes greater and less than 15u, respectively. Two-dimensional (2-D) ripples have regular crest-to-crest spacings along the length of two adjacent bedforms and sinuous to straight, continuous crests, whereas three-dimensional (3-D) ripples have irregular crest-tocrest spacings and highly sinuous to discontinuous crests. The term quasi two-dimensional (2.5-D) is used to designate a bed configuration characterized by the presence of both 2-D and 3-D wave ripples. It should be noted that, given the limited width of the tunnel used (0.5 m), all wave ripples exceeding several meters in wavelength appeared to be 2- D even though their true shape may have been more irregular. The wave tunnel at Queen s University is a large, fully enclosed, racetrack shaped duct through which water is forced back and forth in a simple harmonic motion by a motor-driven piston (Fig. 3). Prior to each series of runs, the bed was planed flat. The orbital diameter was then set by fixing the throw of the piston, and the oscillatory velocity was varied in increments by adjusting the angular speed of the motor that drives the piston. In each case, the bed was allowed to reach equilibrium before proceeding to the next increment of oscillatory velocity. An equilibrium bed was judged to have been reached when the average size and shape of the wave ripples stopped changing. In general, equilibrium was reached rapidly, usually within several tens of minutes to one hour. However, in one coarse-sand run just above the threshold of sediment motion, the bed required almost twelve hours to reach equilibrium. Runs were terminated relatively rapidly (, 3 seconds), which prevented any spin down modification of the equilibrium bed configuration. Once the run was stopped, the bed was photographed and the height (h), wavelength (l), flank length, and flank slope of any wave ripples that had formed were measured, to a precision of cm. These values are reported as averages for each run (Table 1). When the highest possible oscillatory velocity was achieved for a given orbital diameter, the orbital diameter was reset to a new value and another series of runs was performed. After the full range of orbital diameters and oscillatory velocities was explored, the limits of which were dictated by the mechanical limits of the wave tunnel apparatus, the sediment was changed from very fine to coarse sand, and a similar set of experiments was conducted. The range of hydraulic conditions explored in the experiments was 95 cm, d 0, 450 cm, 20 cm/s, U o, 170 cm/s, and 4.5 s, T, 14 s (Table 1). Such conditions are common during storms on open continental shelves, where waves may exhibit a wide range of periods (, 1 to 25 s) and heights (, 1 to 15 m) but are rarely encountered in lakes, rivers, and estuaries, where waves tend to be limited to periods of less than 4 seconds and maximum heights of 1 to 2 meters because of the small fetch (Komar 1974; Young 1999; US Army Corps of Engineers 2002; Goda 2003). The two sediment types used in the experiments had median grain sizes (D 50 ) of 0.8 mm (coarse sand) and 0.12 mm (very fine sand) (Fig. 4). In a very general way, they can be considered representative of the coarser sediment that is a common component of relict deposits (transgressive lags) on the continental shelf and finer sand commonly found in highstand deltas and highstand and transgressive shorefaces on the inner continental shelf (Wright and Nittrouer 1995; Geyer et al. 2005). The coarse-sand runs may also be relevant to carbonate shelves where coarse bioclastic material is abundant. In most cases, the oscillatory flow generated in the wave tunnel was very slightly asymmetric in magnitude (the maximum speed of the forward stroke was slightly more than that of the backward stroke) and time (the forward stroke took less time to complete than the backward stroke). (In all figures traced from the side-wall viewing area (e.g., Fig. 5), the shorter, faster stroke is to the right.) The asymmetry increased with increasing oscillatory velocity (because of increasing stress on the wavetunnel drive mechanism), but the difference between forward and backward strokes rarely exceeded several percent. As a consequence, bed forms were almost invariably symmetric (see ripple symmetry values in Table 1). An exception to this occurred near the transition to plane bed

3 JSR WAVE RIPPLES AND GRAIN SIZE 85 FIG. 3. Race-track style wave tunnel at the Coastal Hydraulics Laboratory, Queen s University, Canada. The side-wall viewing area is relatively large (12 m long, 1 m high, and 0.5 m wide) and has windows on both sides. See Brebner and Riedel (1973) for details. in the very fine sand runs, where substantial stress on the drive mechanism caused flow asymmetry to reach 10 15%. (The wave ripples generated in these runs were consequently slightly asymmetric; see Table 1.) Although flow asymmetry in general had no obvious effect on bed-form symmetry, it did cause the otherwise-symmetric wave ripples to migrate in the direction of the shorter, faster stroke, especially in runs with higher oscillatory velocities, where migration rates reached several centimeters per minute. Sand was therefore gradually mobilized out of the viewing area, and had to be shovelled back several times over the course of the experiments. TABLE 1. Experimental conditions and results. Grain Orbital Oscillatory Wave Bed Ripple Ripple Ripple Flank Slope of Slope of Crest Planform Wavelength Size diameter Velocity period configuration* wavelength 1 height 1 symme- Ripple shape in left right flank 1 shape in morpho- / orbital di- Run (mm) (cm) (cm/s) (s) (cm) (cm) try 1 steepness 1 profile* 1 flank 1 (u) (u) profile* logy ameter (%) S1R Plane bed S1R Plane bed S1R LWR CV 5 6 R 2-D** 77 S1R LWR R 2-D** 46 S2R Plane bed S2R LWR CV 7 8 R 2.5-D 62 S2R LWR and CV, CC R,S 3-D 36 SWRt S2R LWR and CC, CV 10 9 R,S 3-D 59 SWR D 3 S3R LWR R,S 2.5-D 59 S3R LWR and CV, CC 9 12 S,R 2.5-D 60 SWRt S3R LWR and D 76 SWR D 6 S3R SWR CV, CC D 5 S4R LWR CV, CC S 2.5-D 66 S4R LWR and F, CC S 2.5-D 66 SWRt S4R S5R1* S5R LWR CC 19u 19 S 2-D 45 S5R LWR CC 19u 20 S 2-D 49 S5R LWR F, CC 17u 18 S, one R 2-D 46 S5R5* S6R LWR F 18u 17 S, one R 2-D 40 S6R LWR F, CV 15u 17 S 2-D 40 S6R LWR F, CV 15u 18 R 2-D 40 S7R LWR CV, CC 16u 17 S,R 2-D 36 S7R Motor broke S8R NM S8R NM S8R LWR CC 19u 21 S 2-D 37 S8R LWR CC 17u 18 S 2-D 35 * Key to acronyms: LWR, large wave ripples (wavelength. 30 cm); SWR, small wave ripples (wavelength, 15 cm); SWRt, small, superimposed wave ripples in troughs of LWRs only; NM, no movement; CV, convex-up flanks; CC, concave-up flanks; F, flat flanks; R, rounded crests; S, sharp crests. Ripple symmetry was measured by dividing the length of the left flank by that of the right flank, as measured from the sidewall viewing area. The shorter, faster stroke of the oscillation was invariably towards the right. 1 Average value for the given run. ** Given the size of these ripples relative to the width of the wave tunnel, they appeared 2-D, but could have been 3-D.

4 86 D.I. CUMMINGS ET AL. JSR very fine sand, the run with the lowest possible oscillatory velocity (37 cm/s) generated SWRs from an initially flat bed. The threshold of sediment movement for very fine sand therefore lies somewhere below 37 cm/s. Small Wave Ripples (SWRs) Small symmetric wave ripples (4 cm, l ave, 26 cm; 0.2 cm, h ave, 3 cm) formed in very fine sand at low to moderate oscillatory velocities (37 cm/s, U o, 75 cm/s) over a wide range of orbital diameters (95 cm, d 0, 250 cm) (Fig. 5). SWRs formed at the start of some of the coarse-sand runs at low oscillatory velocities but invariably became unstable with time and were replaced by LWRs (see also Lofquist 1978), a transformation that may have been instigated by the progressive winnowing of a several-millimeter thick fine-grained suspension-fallout layer that inevitably accumulated between runs. SWRs did not form in runs with an orbital diameter of 450 cm, but this may not reflect an upper d 0 limit to the occurrence of SWRs because oscillatory velocity may have been too high during these runs (U o. 115 cm/s). Flow separated across SWRs, and flow reattachment in the lee of ripple crests was relatively gentle, with no significant upward injection of fluid and/or sediment into the core of the flow. Changes in orbital diameter did not obviously affect the size of SWRs. Rather, SWRs maintained average wavelengths of 10 cm (Fig. 8A) and average heights of 0.75 cm (Fig. 8B) irrespective of orbital diameter. However, SWRs decreased in height with increasing oscillatory velocity (Fig. 9). SWRs appeared 2-D to 2.5-D, but their crestal continuity was not measured systematically. FIG. 4. Grain-size distributions for the very fine sand (D mm) and coarse sand (D mm) used in the study. The sands had the density of standard siliciclastic sand (2.65 g/cm 3 ). They were obtained from aggregate pits just outside of Kingston that mine esker-associated subaqueous-outwash fans. Vf, f, m, c, and vc refer to very fine, fine, medium, coarse, and very coarse sand grain sizes. L and U refer to lower and upper divisions of each of these subclasses, respectively. For each run, the oscillatory velocity was measured using an acoustic Doppler velocimeter (ADV) positioned, 15 cm above the bed, and the orbital diameter was estimated visually by tracking neutrally buoyant particles in the viewing area. Measurements of oscillatory velocity are therefore believed to be accurate within several cm/s, whereas orbitaldiameter estimates are less accurate, with an estimated error of up to 6 0.1d 0. BED CONFIGURATIONS Depending on the hydraulic conditions and grain size, one of five equilibrium bed configurations was observed: (1) no movement, (2) small wave ripples (SWRs), (3) small wave ripples superimposed on large wave ripples (SWRs/LWRs), (4) large wave ripples (LWRs), or (5) a flat, horizontal bed (referred to here as oscillatory plane bed) (Figs. 5, 6; Table 1). The effects of U o, d 0, and D 50 on SWRs and LWRs are summarized in Figure 7. No Movement For coarse sand, and starting from an initially flat bed, the threshold of sediment movement was breached somewhere between an oscillatory velocity of 24 cm/s (no movement) and 37 cm/s (2-D LWRs) (Fig. 5). In SWRs Superimposed on LWRs SWRs coexisted with LWRs in very fine sand at moderate oscillatory velocities (57 cm/s, U o, 75 cm/s) (Fig. 5). SWRs disappeared first from LWR crests as oscillatory velocity was increased (U o. 64 cm/s), then from LWR troughs when U o. 80 cm/s. Large Wave Ripples (LWRs) Large symmetric wave ripples (55 cm, l ave, 270 cm; 6 cm, h ave, 26 cm) formed at moderate to high oscillatory velocities (37 cm/ s, U o, 122 cm/s), in both very fine sand and coarse sand, at all orbital diameters investigated (95 cm, d 0, 450 cm). When orbital diameter was increased, LWRs increased in size (Fig. 8) but did not obviously change in shape (Fig. 10). As a general rule, their wavelengths scaled to roughly 50% of the orbital diameter, with slightly lower proportionalities for coarse sand (l ave, 0.4d 0 ) and slightly higher proportionalities for very fine sand (l ave, 0.6d 0 ) (Fig. 8A). LWR heights (h) scaled to roughly 5% of the orbital diameter, with lower proportionalities for very fine sand (h ave d 0 ) and slightly higher proportionalities for coarse sand (h ave d 0 ) (Fig. 8B). In coarse sand, LWRs were typically sharp-crested, concave-up to straight-flanked, and steep (run-averaged flank slopes of 15 21u) and were invariably 2-D at the scale of the tunnel. Under similar hydraulic conditions, LWRs in very fine sand had similar wavelengths, but were much more subdued (run-averaged flank slopes of 5 15u), were either 2-D or 3-D, and were either sharp-crested or round-crested, commonly with convex-up flanks (Figs. 11, 12, 13). The effects of oscillatory velocity on LWR shape and size were not as obvious as the effects of grain size and orbital diameter. However, LWR height did decrease with increasing oscillatory velocity (Figs. 14, 15), especially for very fine sand, with the most subdued LWRs occurring just below the transition to oscillatory plane bed. It is unclear whether equivalent subdued LWRs are produced near the transition to plane bed

5 JSR WAVE RIPPLES AND GRAIN SIZE 87 FIG. 5. Phase diagrams for wave ripples generated in A) very fine sand and B) coarse sand under large orbital diameters ( m) and moderately low to high oscillatory velocities ( cm/s). The phase boundaries (horizontal lines) are solid where reasonably well constrained, dashed where extrapolated. The bed profiles shown are in general 12 m long and have no vertical exaggeration. They were traced from the viewing area.

6 88 D.I. CUMMINGS ET AL. JSR Oscillatory plane bed was not generated in runs that used coarse sand because sufficiently high oscillatory velocities could not be achieved. DISCUSSION FIG. 6. Histograms showing frequency distributions of A) crest-to-crest wavelength and B) crest-to-trough height of ripples observed in the experiments. Note that two wave-ripple populations can be distinguished, small wave ripples (SWRs) with wavelengths, 30 cm and heights typically, 1 cm (present in very fine sand only), and large wave ripples (LWRs) with wavelengths between 0.35 and 2.7 m and heights between 5 and 26 cm (present in both grain sizes; all observations combined). in coarse sand, because plane-bed conditions were not achieved in this grain size. Flow separated over all LWRs. Over steep LWRs (i.e., LWRs in coarse sand and some LWRs in very fine sand at low U o ), flow-separation bubbles were well developed, sediment-laden fluid was ejected violently up into the core of the flow, and flow reattachment was vigorous. By contrast, over subdued LWRs (LWRs in fine sand at high U o ), flow-separation bubbles were flattened and flow reattachment was relatively gentle. Oscillatory Plane Bed Oscillatory plane bed was produced in very fine sand at high oscillatory velocities (U o. 130 cm/s) (Fig. 5). The run with the next-lowest oscillatory velocity (U o cm/s) produced very subdued 3-D LWRs. The shape of LWRs generated in the experiments was strongly modulated by grain size (Fig. 16). Fine-grained LWRs were commonly hummocky (i.e., round crested, subdued and 3-D), whereas coarsegrained LWRs were invariably 2-D and almost always sharp crested. This appears to confirm Leckie s hypothesis (1988) that fine-grained and coarse-grained LWRs generated under large-scale oscillatory flow are morphologically different. Whether fine-grained and coarse-grained LWRs are genetically different is debatable, however, inasmuch as all LWRs generated in the experiments scaled strongly to orbital diameter. This suggests that they formed by a common mechanism related to the near-bed fluid flow and sediment transport set up by the oscillatory motion itself, an observation that has led previous authors to term similar bed forms oscillatory-current ripples (Southard et al. 1990) or orbital ripples (Clifton 1976). In addition to exerting an influence on the shape of LWRs, grain size also appeared to modulate LWR size. The wavelengths of fine-grained LWRs scaled on average to 0.6d 0, which is close to 0.65d 0, the most commonly reported scaling ratio for orbital ripples (Miller and Komar 1980; Southard et al. 1990; Wiberg and Harris 1994). Coarse-grained LWRs, by contrast, scaled on average to, 0.4d 0. Similar scaling ratios have been reported from recent wave-tank experiments (T s; d m), where ripples in fine sand (D mm) scaled to, 0.65d 0 and ripples in medium sand (D mm) scaled to 0.45d 0 (Williams et al. 2004). If representative, these observations suggest that coarser-grained orbital ripples are on average more closely spaced than finer-grained orbital ripples generated under the same hydraulic conditions. It is unclear why this occurs, although it may be related to the greater fall velocity of coarser sediment (reduced amount of time needed for grains to settle, increased amount of time needed for grains to be suspended), or the different style of fluid-flow and sediment-transport set up by the steeper (coarser) versus more subdued (finer) wave ripples. The SWRs generated in the experiments reported here are believed to be genetically distinct from the LWRs. Unlike LWRs, SWRs were stable only in very fine sand and at relatively low oscillatory velocities. In addition, SWRs were always associated with flow separation but not with violent flow reattachment or strong upward ejection of fluid into the core FIG. 7. Average response of large wave ripples (LWRs) and small wave ripples (SWRs) to increases in orbital diameter, oscillatory velocity, and grain size. A large arrow indicates a pronounced response, whereas a small arrow indicates a more subtle response. The responses apply only to the grain sizes and range of hydraulic conditions investigated (D or 0.8 mm; 0.95 m, d 0, 4.5 m; 0.2 m/s, U o, 1.7 m/s).

7 JSR WAVE RIPPLES AND GRAIN SIZE 89 FIG. 9. Plot showing decrease in SWR steepness (i.e., height/wavelength) with increasing oscillatory velocity. All of the runs plotted in this figure were conducted with an orbital diameter of 1.6 meters and used very fine sand. SWR wavelengths remained nearly constant on average (, 9 cm) in this series of runs, but their heights decreased on average from 0.9 cm (S3R1) to 0.4 cm (S3R3) as oscillatory velocity was increased. Ripple-profile triangles accurately represent ripple steepness but not ripple shape. FIG. 8. Plots of orbital diameter versus A) ripple wavelength and B) height. Note that the wavelength of large wave ripples (LWRs) scale to, 50% of the orbital diameter, whereas the small wave ripples (SWRs) retained the same wavelength irrespective of orbital diameter. of the flow, and they lacked obvious scaling relationships with the largerscale structure of the flow (i.e., the oscillations themselves). As such, it is tempting to think of them as two-sided current ripples they appeared to owe their existence to the current that reversed, not to the oscillations themselves. (However, SWRs decreased in height with increasing shear stress (oscillatory velocity), whereas current ripples may exhibit the opposite trend (Baas 1999).) Previous authors have referred to similar ripples as reversing-current ripples (Southard et al. 1990) or anorbital ripples (Clifton 1976). All LWRs in the experiments were orbital ripples, whereas all SWRs were anorbital ripples. Indeed, anorbital ripples in natural settings do appear to be invariably small and fine-grained (e.g., Hanes et al. 2001). However, the size range of orbital ripples in natural settings overlaps that of anorbital ripples, as orbital ripples range in wavelength from several centimeters (Evans 1941) to several meters (Cacchionne et al. 1984; Forbes and Boyd 1987; Traykovski et al. 1999; Yang et al. 2006) depending on the orbital diameter (and, as argued above, grain size) under which they form. As such, in fine-grained successions where both FIG. 10. Change in LWR profile with increasing orbital diameter at constant grain size and nearly constant oscillatory velocity. A) LWRs generated in coarse sand. Note increase of LWR size with orbital diameter. LWRs in all three runs are 2-D. These three runs were chosen because they had the closest oscillatory velocities to one another (oscillatory velocity was 70 cm/s in S8R4, 75 cm/s in S5R3, and 80 cm/s in S6R1). B) LWRs generated in very fine sand. Again, note that an increase in orbital diameter causes LWR size to increase. The asymmetry of ripples in S1R3 is due to asymmetry in the oscillatory flow that developed inherently in the wave tunnel at higher oscillatory velocities (although the symmetry of ripples in other high-velocity runs seems to have been less affected). As in Part A, the three runs in Part B were chosen because they had the nearest possible oscillatory velocities (oscillatory velocity was 80 cm/s in S3R4, 110 cm/s in S2R4, and 115 cm/s in S1R3). See Table 1 for details.

8 90 D.I. CUMMINGS ET AL. JSR FIG. 11. Change in LWR profile with increasing grain size at nearly constant orbital diameter and nearly constant oscillatory velocity (S5R2: d m, U o 5 65 cm/s; S2R1: d m, U o 5 60 cm/s). Compared to the LWRs generated in very fine sand, LWRs generated in the coarse sand were steeper, sharper-crested, lacked superimposed SWRs, were 2-D at the scale of the tunnel, and had a shorter wavelength. See Figure 13 for photos of wave ripples from these two runs. FIG. 14. Change in LWR profile with increasing oscillatory velocity in A) very fine sand and B) coarse sand. Orbital diameter is held constant in both examples (it is 2.5 m in S2R2 and S2R4 and 4.1 m in S6R1 and S6R2). Note that the effects of an increase in oscillatory velocity on LWR size and shape are more subtle than those caused by a change in orbital diameter (Fig. 6) and grain size (Fig. 8); however, the data suggest that LWRs do decrease slightly in steepness with increasing oscillatory velocity (see Fig. 15). FIG. 12. Average slope of the flanks of the large wave ripples (LWRs) for each run plotted as a function of grain size. Note that the coarse-grained LWRs had steeper flanks (. 15u) than the fine-grained LRWs (, 15u). (It is important to note that the LWR flank-slope values in this diagram are the averaged values for each run. If the slope of each individual ripple generated in the experiments was plotted, this plot would show greater variability and more overlap.) Ripple-profile triangles accurately represent ripple steepness but not ripple shape. types of ripples may be present, only LWRs should be used to infer paleoenvironmental information such as wave size and basin size, in that SWRs in fine sand may be generated by both large waves (as anorbital ripples) or by small waves (as orbital ripples). By contrast, all coarsegrained wave ripples appear to scale to orbital diameter, and therefore can be used to reconstruct wave size. Several detailed methods of extracting paleohydraulic information from wave ripples have been outlined by previous authors (Komar 1974; Clifton 1976; Clifton and Dingler 1984). As a simple rule of thumb, if symmetric ripples (fine- or coarse-grained) with wavelengths in excess of, 75 cm are identified in a FIG. 13. Representative photos of the LWRs generated in A) very fine sand and B) coarse sand. (Note that SWRs are superimposed on LWR in Part A only.) The LWRs in Parts A and B were generated under similar hydraulic conditions they are from the two runs depicted in Figure 11 (S2R1: D mm, d m, U o 5 60 cm/s, T 5 14 s; S5R2: D mm, d m, U o 5 65 cm/s, T 5 12 s). Note that the very-fine-sand LWR in Part A is round-crested, subdued, and three dimensional, and has superimposed SWRs, whereas the coarse-sand LWR in Part B is sharp-crested, steep and two-dimensional, and lacks superimposed SWRs.

9 JSR WAVE RIPPLES AND GRAIN SIZE 91 FIG. 17. Stratification traced from side-wall photos of fine-grained and coarse-grained LWRs. Note that cross-strata produced by the ripple in very fine sand are low-angle (, 5u) and convex-up, whereas those produced by the ripple in coarse sand are high-angle (, 25u) and concave-up. Unidirectional dip of crossstrata is due to slight flow asymmetry (less than a few percent), which in general had no obvious affect on bed-form symmetry but caused them to migrate slowly in one direction (to the right) in most runs. S1R1 and S5R2 refer to run numbers (see Table 1 for details). In nature, a similar translation of the bed form could be produced by the weak unidirectional currents that commonly accompany storms (Dumas et al. 2005) and might be termed anisotropic HCS (cf. Nottvedt and Kreisa 1987). FIG. 15. Plots showing decrease in LWR steepness (i.e., height/wavelength) with increase of oscillatory velocity (and orbital diameter) for runs conducted in A) very fine sand and B) coarse sand. Although data are limited, the trend appears to be slightly more pronounced in fine sand than in coarse sand. Ripple-profile triangles accurately represent ripple steepness but not ripple shape. stratigraphic unit, one can conclude with relative confidence that they formed in a fetch-unrestricted water body, such as an open-ocean continental shelf, because the requisite large, symmetric near-bed oscillations tend not to occur beneath smaller waves inherent to fetchlimited settings such as estuaries, rivers and most lakes (Komar 1974; Clifton 1976; US Army Corps of Engineers 2002; Goda 2003). Where preserved, cross stratification generated in the experiments by fine-grained and coarse-grained LWRs differed substantially (Fig. 17). As mentioned previously, a slight flow asymmetry (less than several percent) caused LWRs to migrate slowly (several millimeters to several centimeters per minute) in one direction in most runs; similar behaviour is likely common for wave ripples beneath shoaling waves (e.g., Clifton 1976; Traykovski et al. 1999) and ripples generated by purely oscillatory wave motion with a very weak superimposed unidirectional current (Arnott and Southard 1990; Dumas et al. 2005). As a result, the stratification within the LWRs in very fine sand resembled anisotropic, low-angle (5 15u) hummocky cross stratification (e.g., Nottvedt and Kreisa 1987; Dumas and Arnott 2006), whereas stratification within the LWRs in coarse sand resembled dune cross stratification because the set thickness potentially exceeded 5 cm and the dip angle was high (15 21u). These results suggest that the deposits formed by coarse-grained LWRs could be misinterpreted as having been formed by dunes in the absence of information on the external shape of the bed forms. In such cases, care must be taken when interpreting the shallow-marine stratigraphic record: vertical trends in the type of cross stratification in a given succession may simply reflect grain-size change, and not necessarily a change in the prevailing hydraulic conditions (e.g., Leckie 1988; Cheel and Leckie 1992; Yoshida et al. 2007). FIG. 16. Simplified cartoon showing succession of equilibrium bed configurations generated in very fine sand and coarse sand under increasing oscillatory velocity, for moderate to large-sized waves (i.e., orbital diameter. 1 m). Note that the transition between small anorbital ripples and large orbital ripples in very fine sand is in fact gradual (not shown), with small anorbital ripples occurring first on large orbital ripples before eventually disappearing. The transition from large orbital ripples to plane bed, as plotted, is completely conceptual, because plane-bed conditions were not achieved in the coarse-sand runs. Further experiments are required to determine the nature of this and other transitions between equilibrium bed configurations (cf. fig in Harms et al. 1982).

10 92 D.I. CUMMINGS ET AL. JSR CONCLUSION The results of our wave-tunnel experiments suggest that grain size exerts a first-order control on the geometry of ripples generated by large waves. Two main differences were observed in the equilibrium bed configurations generated in very fine and coarse sand. First, small anorbital ripples (wavelengths, 10 cm, heights 1 2 cm) formed and remained stable only in very fine sand, just above the threshold of sediment movement. Second, large orbital ripples (wavelengths cm, heights cm) formed and remained stable in both very fine and coarse sand and exhibited similar wavelengths under similar hydraulic conditions. However, their shapes differed substantially as a function of grain size: large ripples in coarse sand were steep, 2-D, and sharp-crested, whereas large ripples in very fine sand were subdued, 2-D or 3-D, and round- or sharp-crested. These results appear to confirm the hypothesis advanced by previous authors that coarse- and fine-grained large wave ripples have distinctly different shapes and, by extension, produce distinctly different styles of cross stratification (Leckie 1988; Cheel and Leckie 1992), at least over the range of variables investigated here. One possibility that the experiments did not rule out is that large hummocky ripples may form in coarse sediment at very high oscillatory velocities. This is because a significant amount of phase space exists between 125 cm/s, the highest oscillatory velocity tested in the coarsesand runs, and, 200 cm/s, the oscillatory velocity at which ripples on a coarse-sand bed become planed off (Clifton 1976). Further experiments are required to resolve this question. Nevertheless, given the results of the study, and taking into account data from modern and ancient environments (e.g., Cacchionne et al. 1984; Forbes and Boyd 1987; Leckie 1988; Traykovski et al. 1999; Cummings and Arnott 2005; Yang et al. 2006), it seems likely that the stratigraphic signature of large waves is typically different in fine-grained and coarse-grained sediment. ACKNOWLEDGMENTS The research was carried out when the first two authors were postdoctoral fellows at Queen s University. The wave tunnel was graciously made available to us by Kevin Hall and William Kamphuis of the Department of Civil Engineering, Queen s University. Stuart Seabrook, the Coastal Lab manager, facilitated access to the premises and guided us through the use of the wave tunnel. Paul Thrasher coordinated several major repairs to the motor that allowed us to complete our experiments. Constructive scientific reviews by Patricia Wiberg, Dale Leckie, and Paul Myrow and a thorough copy edit by John Southard helped improve the final manuscript. RWD acknowledges the continued support of the Natural Sciences and Engineering Research Council of Canada (NSERC) for his research. 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