1 堆 積 学 研 究,53号,1-15,2001 Sed.Soc.Japan,No.53,1-15,2001J. Sedimentary structures of antidunes : An overview Tadashi Araya* and Fujio Masuda* There seem to be some misconceptions about the definition and the criteria of anti- dunes and their deposits. Antidune is one of the upper-flow-regime bedforms, whose definition was limited originally to upstream-migrating bedwaves but recently includes any bedwaves in-phase with surface gravity waves in the overlying flow. The term " antidune" represents a bedform, not a sedimentary structure or a deposit. Thin lenticular laminaset and HCS-like structure ("HCS mimics" ) may be formed as antidunes as well as upstream-dipping, relatively low-angle stratification (backsets). HCS-like structure may be formed as three-dimensional antidunes in upper-flow-regime conditions, particularly in abundant suspension fallout. However, the paleocurrent direction or regional facies assemblage, for example, will be needed in practice for the recognition of antidune deposits, besides descriptions of their internal sedimentary structures and grain fabric. Most studies were based on limited examples of diverse antidunes or only small portions of antidune deposits. The classification of antidune geometries is so insufficient that the processes, hydraulic conditions and mechanisms of the formation of sedimentary structures have been poorly understood. Detailed descriptions and analysis of the bedforms, resultant deposits and grain fabrics should be conducted based on a more sophisticated classification, from experimental or theoretical approaches. Key words : antidune, bedform, HCS mimics, sedimentary structure, upper flow regime have been many studies reporting sedimentary INTRODUCTION structures of antidune origin (or those inferred Antidune is one of the bedforms stable in up- so) in modern and ancient deposits in various per-flow-regime conditions, and is characterized environments. by upstream migration of the bedform and the Although there have been a few studies deal- resultant formation of backset cross stratific- ing with antidunes and their deposits in detail, ation. Antidunes are supposed to be rarely pre- the definition and the criteria of antidune depos- served in ancient deposits (cf. Allen, 1982 ; Collin- its seem to be confused. This study will review son and Thompson,1982), because their deposits the definitions of antidunes and the characteris- are destroyed by subsequent waning flows and tics of antidune deposits. Furthermore, some only reworked lower-flow-regime sedimentary examples of antidune-induced sedimentary structures are likely to remain. However, there structures in modern and ancient deposits will be reported. Received : March 19, 2001 Accepted : June 6, 2001 * Department of Geology and Mineralogy DEFINITION OF ANTIDUNE , Graduate School of Science, Kyoto University, Kyoto 606-8502, Antidune was originally defined by Gilbert Japan 2 Tadashi Araya and Fujio Masuda 2001 (1914), which was based on the observations of on the upstream side of the antidune ; and (2) bedforms in natural rivers and laboratory instability of the surface waves as they increase flumes. He defined antidune as a wavy, in-phase in height. bedform migrating upstream in uni-directional HYDRAULIC CONDITIONS flows. OF ANTIDUNE FORMATION On the other hand, the definitions by Langbein (1942), Simons and Richardson (1961, 1962), Ken- Laboratory experiments indicate that anti- nedy (1961, 1963), Middleton (1965) and Reineck dunes are stable in higher flow velocities (or and Singh (1973, 1980) attached greater impor- Froude numbers) than those of upper-flow- tance to in-phase nature of antidunes and in- regime plane bed conditions (e. g. Harms and cluded those migrating downstream or standing Fahnestock, 1965 ; Simons et al., 1965 ; Reineck (stationary). Such broader definition is now and Singh, 1973, 1980 ; Allen, 1982 ; Harms et al., more popular (e. g. Middleton, 1965 ; Allen, 1982 ; 1982; Southard and Boguchwal, 1990). Cheel Barwis and Hayes, 1985) without some excep- (1990) predicted based on flume experiments that tions (e. g. Cheel, 1990). That is, most of them with increasing flow strength the gradual se- have been recognized to be antidunes without quence of bed phases in uni-directional flows is : limitation to those migrating upstream as de- 1) plane bed, 2) low-relief, downstream-migrating fined by Gilbert (1914). in-phase waves, 3) stationary in-phase waves and 4) upstream-migrating antidunes (note that he MECHANISM OF UPSTREAM MIGRATION called only upstream-migrating bedforms "anti- Some hypotheses have been proposed about dunes" and what he called "in-phase waves" cor- the mechanisms of upstream migration of anti- responds to antidunes in a wide sense). dunes (e.g. Gilbert, 1914 ; Kennedy, 1961, 1963 ; Simons and Richardson (1960) suggested that Middleton, 1965 ; Simons et al., 1965 ; Allen, antidunes form at Froude numbers (F) greater 1982). Gilbert (1914), Kennedy (1961, 1963), Mid- than 1. Here F is given by : dleton (1965), Langford and Bracken (1987), Yagishita et al. (1988), Yagishita and Taira (1989) and Yagishita (1994) suggested that the up- stream migration is caused by erosion on the lee where U is the mean velocity, g is the gravity side (where the water is accelerating) and deposi- acceleration and d is the flow depth. Antidunes tion on the stoss side (where the water is deceler- can exist only for 0.844<F<1.77 (cf. Allen, 1982). ating) of antidunes, and Kennedy (1963, 1969) Kennedy (1963) reported that downstream- suggested that the greater angle between the migrating antidunes form under lower Froude slope and the center of gravity of the particles numbers than upstream-migrating ones. Barwis makes re-entrainment more difficult on the up- and Hayes (1985) suggested that standing anti- stream limb. Recently, Kubo and Yokokawa dunes occur at flow velocities intermediate be- (2001) explained upstream migration of anti- tween those for the two migrating forms. Some dunes with quasi-cyclic breaking of water sur- researchers (e, g. Shirasuna, 1972) suggest that face wave as a result of intermittent hydraulic antidunes are not formed in a closed (i, e., with- jumps moving upstream at the stoss side. out free water surface) experimental flume. Breaking of water surface wave and partial Antidune bedforms, i, e., the wavelength (L) destruction of the antidune may be due to two and the height (H) of antidunes reflect the hy- processes (Kennedy, 1963) : (1) creation of ad- draulic conditions. Kennedy (1963) showed ex- verse pressure gradients causing the onset of perimentally and theoretically that the mini- flow separation and subsequent flow instability mum wavelength of two-dimensional antidunes J.Sed.Soc.Japan,No.53Sedimentary structures of antidunes 3 formed in open channels may be given by : tary structures. Various sedimentary structures are supposed to be formed as antidunes (Fig. 1). "Upstream- dipping low-angle cross stratification" , "lenticu- whilst the maximum antidune steepness could lar or wavy stratification dipping both upstream be determined from and downstream" or "HCS-like structure" may be characteristic of antidune deposits (Table 1). For example, Alexander et al. (2001) closely ob- served bedforms and associated sedimentary The dominant wavelengths of antidunes may be structures formed under supercritical water given by (Kennedy, 1963) : flows over aggrading sand bed in a laboratory flume, and they described three-dimensional ge- ometry of the sedimentary structures of anti- dunes and chutes-and-pools (Fig. if). They re- where k is 2ƒÎ/L. For antidunes produced by ported that the structures associated with anti- density currents, they occur not at the free water dunes are represented by : (1) lenticular lami- surface, but at a density interface within the nasets with concave-upward erosional bases in fluid (Hand et al., 1972). Hand (1974) denoted which laminae generally dip upstream or fill the experimentally that antidunes, breaking anti- troughs symmetrically, which are associated dunes and chute-and-pools can be duplicated with growth and upstream migration of water- with density underflows (saline water flows). surface waves and antidunes, and with surface- Hand et al. (1972) presented a modified equation wave breaking and filling of antidune troughs, relating density current velocity and antidune respectively ; (2) sets of downstream-dipping wavelength as following : laminae produced by rapid migration of asym- metrical bedwaves ; (3) rare convex-upward laminae defining the shape of antidunes that developed under stationary water-surface where ƒÏ is the density of the current and •¢ƒÏ is waves. Most studies identify antidunes in an- the density difference between the density cur- cient deposits on the following grounds : 1) cross rent and the ambient fluid. Furthermore, assum- stratification dipping opposite (upstream) to the ing the resistance coefficient (f), bed slope (S) paleocurrent direction determined from lower- may be estimated using the Darcy-weisbach flow-regime sedimentary structures or regional equation : f acies assemblage ; and 2) parallel stratification adjacent laterally or vertically, suggesting depo- sition in upper-flow-regime conditions. Grain fabric may be also effective (see below). Antidune cross laminae are vague and rela- RECOGNITION OF ANTIDUNE DEPOSITS tively low-angle, and thus hard to recognize in The criteria of antidune deposits seem rather general (e. g. Middleton,1965 ; Barwis and Hayes, confused or ambiguous. The term "antidune" 1985).