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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- and their deposits. Antidune is one of the upper-flow-regime , 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 , 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 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 and laboratory instability of the surface waves as they increase . 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 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 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

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). The origin of individual laminae is sup- means a bedform. Its definition contains the posed to be partly due to shear sorting (Barwis relation between the bedform and the formative and Hayes, 1985) and partly due to low-relief current (e, g. in-phase or out-of-phase). There- (millimeter-high) bedwaves (Alexander et al., fore, the definition of antidune described above 2001). Some researchers attributed their faint- is not applicable to ancient deposits or sedimen- ness to the absence of grain sorting that accom- 4 Tadashi Araya and Fujio Masuda 2001

panies avalanching (Leeder, 1982) or to the well- dominated shallow marine deposits. The name "HCS sorted nature of sands composing antidune de- mimics" (Rust and Gibling, 1990; Masuda posits (Leeder, 1982 ; Barwis and Hayes, 1985) et al., 1993) emphasizes that the sedimentary en- and regarded it as one of the reasons why anti- vironment or process of formation is quite diffe- dunes have been scarcely discovered in ancient rent from that of HCS. deposits. Rust and Gibling (1990) found a HCS-like three- "HCS MIMICS" AND ANTIDUNES dimensional sedimentary structure in the braid- ed-fluvial South Formation (Pennsylvanian), "HCS (h ummocky cross stratification) mimics" Canada. The unit is near the top of a 4.7-m-thick is one of the sedimentary structures formed by abandoned channel sandstone. The wavelengths antidunes. It is very similar in appearance to of the bedform were 50-110 cm, and the ampli- HCS (Harms et al., 1975) characteristic of storm- tudes ranged 5-10 cm. They called this structure J. Sed. Soc. Japan, No. 53 Sedimentary structures of antidunes 5

Fig. 1 Various sedimentary structures in antidune deposits. Data are shown in Table 1. (a) Fisher and Waters, 1970. Hammer for scale. (b) Prave and Duke, 1990. Height of the figure is about 30 cm. (c) Barwis and Hayes, 1985. (d) Langford and Bracken, 1987. (e) Cotter and Graham, 1991. Ruler is 15.2 cm long. (f) Massari, 1996. Hammer is 35 cm long. (g) Yokokawa et al., 1999. (h) Alexander et al., 2001. Flow-parallel vertical sections of antidune deposits produced under uni-directional supercritical flows in a flume experiment. Upstream dipping laminae (ud), down- stream dipping laminae (dd), laminae filling trough symmetrically (sf ), structureless part of trough fill (s), convex-upward laminae with trough- shaped base (cu) and small bedwaves on antidunes on bed surface (sbw) are identified.

"HCS mimics" because the original definition by (Fig. lb). This structure is characterized by Harms et al. (1975) restricted HCS to those wavy laminae continuous across crests and formed by storm waves in shallow marine envi- troughs with the wavelengths ranging between ronments. They interpreted this structure as a 20 and 70 cm (51 cm in average) and the ampli- trace of three-dimensional antidune bedforms tudes between 2 and 6 cm (3 cm in average). This under upper-flow-regime conditions, because occurs near the Bouma B-C boundary and grade they contained well-developed current lineations laterally and vertically into flat, planar lamina- parallel to the unidirectional paleoflow and ac- tions suggesting the in upper-flow- companied wavy parallel lamination. regime conditions. Thus, they interpreted this Prave and Duke (1990) reported a "small-scale structure as a form of antidune stratification hummocky cross-stratification" in Upper Creta- generated by standing waves along the interface ceous turbidites in the western Basque Pyrenees of a denser underflow and an overlying low- 6 Tadashi Araya and Fujio Masuda 2001

Table 1 Major previous studies on the sedimentary J. Sed. Soc. Japan, No. 53 Sedimentary structures of antidunes 7

structures or the bedform geometries of antidunes.

* later recognized as antidunes ** re-interpreted as reflected turbidites (Pickering and Hiscott,1985) 8 Tadashi Araya and Fujio Masuda 2001

density layer in a . In Japan, Masuda et al. (1993) reported "HCS mimics" in the tidal deposits of the Pleistocene Shimosa Group and in the flood deposits of the Pleistocene Osaka Group. The former occurs in a sandy tidal deposit in a tidal flat or tidal creek with the wavelength of 10-40 cm and the ampli- tude less than 5 cm. The latter is in a braided- fluvial flood deposit (2 m thick) composed of well- sorted fine sand, and the wavelengths are less than several 10 cm and the amplitudes less than a few centimeters. Both these "HCS mimics" accompany climbing ripple cross-lamination, suggesting rapid deposition in a dense flow with high suspended load. They inferred that these Fig. 2 (a) Bed profiles showing evolution of bed structures were formed as three-dimensional an- tidunes under upper-flow-regime conditions profile during run involving breaking antidunes and chutes-and-pools (after fig. 6A of Hand, based partly on the fact they laterally or verti- 1974). Current from right to left. Numbers cally grade into parallel lamination. indicate time sequence. (b) Resultant - "HCS mimics" indicate that HCS-like sedimen- ary structures of antidunes after removal of tary structures may be formed by some currents parts of profiles that were subsequently eroded other than storm-wave oscillatory or combined (modified after fig. 6B of Hand, 1974). (c), (d) Results of deposition at a constant flows in shallow marine environments, suggest- rate, supposing no modification of the bedforms. ing that HCS is not indicative of a particular Imaginary "HCS mimics" may develop at a high condition or depositional environment (Prave net aggradation rate. and Duke, 1990 ; Masuda et al., 1993 ; Masuda, 2000, 2001). In many cases, "HCS mimics" is inferred to be formed as three-dimensional anti- imaginarily under rapid net aggradational con- dunes in upper-flow-regime conditions, particu- ditions (see Figs. 2c and 2d), if the bedforms larly in abundant suspension fallout (Masuda et suffer no modification in spite of high suspend- al., 1993 ; Yokokawa, 1994 ; Yokokawa et al., ed-load fallout. 1999). This inference would be thought- GRAIN FABRIC OF ANTIDUNE DEPOSITS provoking for those investigating the hydraulic conditions or the processes of formation of HCS Grain fabric of antidune deposits has been or "HCS mimics". studied by Johansson (1976), Taira (1976, 1989), HCS-like structure or "HCS mimics" might be Yagishita et al. (1988), Yagishita and Taira (1989), explained by rapid aggradation of antidunes or Yokokawa and Masuda (1990), Yagishita (1994) chutes-and-pools. Fig. 2a is an observation of the and other researchers. Upstream-dipping imbri- bed profiles under a uni-directional density cur- cation is supposed to be dominant in antidune rent (saline water underflow) during a flume ex- deposits (e. g. Yagishita et al., 1988 ; Yagishita periment (Hand, 1974), representing the develop- and Taira, 1989) as in upper-plane-bed parallel ment of antidunes form an initially plane bed laminae, but probably the imbrication angle is (profile 1), their conversion to chutes-and-pools somewhat steeper in antidune deposits (Johans- and finally a return to simple antidunes migrat- son, 1976 ; Taira, 1976, 1989 ; Pickering and His- ing upstream. "HCS mimics" might be formed cott,1985). However, this would be the case only J. Sed. Soc. Japan, No. 53 Sedimentary structures of antidunes 9

in those formed by antidunes representing GRAIN SIZES AND SEDIMENTARY smooth aggradation and gradual, slow migration ENVIRONMENTS OF ANTIDUNE DEPOSITS without breaking of surface waves.

Grain fabric in HCS is supposed to be charac- Antidunes and antidune deposits have been

terized by quasi-cyclic vertical variations of im- reported in a wide range of grain sizes from brication angles reflecting the oscillatory compo- gravel to very fine sand or silt (Table 1). At least nent of the flow (e. g. Cheel,1991). Yagishita et al. in sand-sized , antidunes may develop

(1992) insisted that HCS and SCS formed by in any grain size of the bed surface. storm-generated combined flows are quite diffe- Antidunes can be formed in various sedimen-

rent from HCS-like antidune deposits in their tary environments from terrestrial to deep sea internal imbrication tendencies. They reported (Table 1). Antidunes are best known from that the grain fabric in HCS is characterized by beaches (e. g. Timmermans, 1935 ; Van Straaten,

the imbrication subparallel to the undulate lami- 1953 ; Reineck, 1963 ; Panin and Panin, 1967 ;

nation and in SCS by weak upcurrent imbrica- Wunderlich,1972 ; Hayes and Kana, 1976 ; Allen,

tion relative to the lamination, whereas that in 1982 ; McCubbin,1982) where they are created by antidune deposits is characterized by high-angle backwash in foreshore region and overwash in

upcurrent imbrication (up to 30•‹ from the hori- backshore (Reineck and Singh, 1973, 1980 ; Allen,

zontal plane) (Yagishita et al., 1988 ; Yagishita 1982 ; McCubbin, 1982). In barrier-island sys-

and Taira, 1989). On the other hand, Yokokawa tems, antidunes have been also reported on

(1994) pointed out the periodic reversals of imbri- washover fans (Barwis and Tankard, 1983 ; Bar- cation directions in "HCS mimics" formed in a wis and Hayes,1985). There are also many stud-

laboratory flume, just as reported by Cheel (1991) ies describing antidunes in fluvial, alluvial fan or

in Cretaceous HCS sandstone. Masuda (2000, fan delta environments (e. g. Power, 1961; Collin-

2001) also reported similar imbrication patterns son, 1966 ; Hand et al., 1969 ; Shaw and Keller-

of the "HCS mimics" in the turbidites of the hals, 1977 ; Langford and Bracken, 1987 ; Rust

Cretaceous Uwajima Group, Ehime Prefecture, and Gibling, 1990 ; Cotter and Graham, 1991;

Japan. He suggested that such characteristics Massari, 1996 ; Alexander and Fielding, 1997 ;

imply oscillatory currents generated by con- Blair, 2000 ; Suzuki, 2000). It may be partly be-

structive waves or hydraulic jumps during cause the current directions are estimated with

three-dimensional antidunes develop. ease and backset beddings are readily recogniz-

As a result, "HCS mimics" and HCS seem to be able. Some researchers reported antidune depos-

difficult to distinct only from their grain fabrics its in turbidites (e. g. Walker, 1967 ; Prave and

at present. Only limited examples of diverse Duke, 1990 ; Yagishita, 1994), although what

antidune deposits or only limited parts on the Skipper (1971) and Skipper and Bhattacharjee thin sections of antidune deposits have been in- (1978) inferred to be antidune deposits were later vestigated in grain fabric analysis. For example, re-interpreted as megaripple cross-stratification

antidune deposits with higher-angle backset formed under reflected turbidity currents (Pick-

bedding and those with HCS-like structure may ering and Hiscott, 1985).

represent different grain fabrics ; foresets and EXAMPLES OF ANTIDUNE DEPOSITS backsets may differ in grain imbrication angles.

The definition and classification of antidune bed- Case 1 : modern fluvial deposits This sedimen-

forms and their deposits should be established tary structure (Fig. 3) was observed in July 1998

and more examples are needed to be analyzed to at the Otaki (Otaki Village, Nagano Prefec-

clarify their sedimentation processes and hy- ture, Japan) and is interpreted to have been

draulic conditions. formed by antidunes, as previously reported by 10 Tadashi Araya and Fujio Masuda 2001

Fig. 3 Three-dimensional antidune deposit in modern fluvial deposits at the Otaki River, Nagano Prefecture. Paleocurrent direction from left to right on the photograph, judging from the flow direct- ion of the river.

Masuda (2000, 2001). The bedform geometry (Fig. ping both seaward and landward) low-angle 3a) is characterized by wavy mounds spaced cross lamination and thin lenses characteristic of evenly in the flow direction, whose wavelength antidune deposits were recognized on the verti- is 0.5-2 m and the amplitude is about 30 cm in cal section normal to the shoreline (Fig. 4b). maximum. The antidune deposit is mainly com- Such antidunes in beach environments were posed of ill-sorted medium-grained sand to gran- formerly called "regressive sand waves" (Bu- ule. In the vertical section parallel to the flow cher, 1919 ; Van Straaten, 1953 ; Panin and Panin, (Fig. 3), cross lamination dipping both upstream 1967), "terugloop-zandgolven" (Timmermans, and downstream was observed, which is charac- 1935), "Sandwellen" (Reineck, 1963) or "back- teristic of antidune deposits. Some of the lami- wash ripples" (Komar, 1976 ; Broome and Komar, nae were continuous from hummocks to swales 1979). Antidunes are readily formed by swash laterally, as in hummocky cross stratifications. and backwash in sandy foreshore, where limited The antidunes were partly modified into dunes water depths enlarge the Froude numbers. Ken- under the subsequent waning flow. nedy (1963) reported that foreshore antidunes are Case 2 : modern foreshore deposits Antidunes easily formed when the steepness of the waves is left on the beachface (Fig. 4a) were observed at 0.13-0.16. the Kujukuri coast, lioka Town, Chiba Prefec- Case 3 : ancient turbidites This structure (Fig. ture, Japan on May 29, 2000. This is a two- 5) occurs in the Upper Pliocene Katsuura Forma- dimensional, gently-inclined wavy bedform (the tion (Sawada, 1939 ; Tokuhashi, 1992), Kazusa wavelength was about 30 cm and the amplitude Group in the Katsuura City, Chiba Prefecture, was about 2 cm in maximum) with the crestlines Japan. The lower part of the sandstone bed was subparallel to the shoreline. The antidune de- "massive" or vaguely parallel -laminated (Bouma posit was composed mainly of heavy-mineral A-B divisions), which was overlain by the peb- rich, well-sorted fine sand. Bi-directional (dip- bly layer with low-angle cross lamination dip- J. Sed. Soc. Japan, No. 53 Sedimentary structures of antidunes 11

relatively low-angle and some laminae are con- tinuous from the hummocks to the adjacent swales in this example.

CONCLUDING REMARKS

The term "antidune" represents a bedform, not a sedimentary structure or a deposit. The crite- ria of antidune deposits are rather confused. An- tidunes are thought to form various sedimentary structures : thin lenticular structure and HCS- like structure ("HCS mimics") are formed as anti- dunes as well as backsets/foresets. However, paleocurrent directions or regional facies assem- blage, for example, will be needed for the recog- nition of antidune deposits as well as their sedi- mentary structures and grain fabrics. The classification of antidune geometries is so insufficient that the processes and mechanisms of the formation of sedimentary structures have been poorly understood. Detailed descriptions and analysis of the bedforms and the resultant deposits should be conducted based on a more Fig. 4 (a) Two-dimensional foreshore antidunes established classification from experimental or at the Kujukuri coast, lioka Town, Chiba Pre- hydraulic approaches. fecture. Crestlines are subparallel to the shore- line. Scale is about 30 cm long. (b) Vertical sect- ACKNOWLEDGEMENTS ion of the antidune deposit normal to the shore- We thank Dr. Miwa Yokokawa (Faculty of In- line. formation Science, Osaka Institute of Technol- ogy) and Dr. Yu'suke Kubo (Deep Sea Research ping both upstream and downstream, which Department, Japan Marine Science and Technol- graded upward into gently-undulated or subpar- ogy Center) for their valuable guidance about allel lamination. This Formation is interpreted antidunes. We extend our appreciation to Dr. as lower fan to basin plain deposits sensu Walker Shuichi Tokuhashi (Institute for Geo-Resources (1978) (Katsura, 1984), so it is not plausible that and Environment, National Institute of Ad- this structure was formed by storm-induced os- vanced Industrial Science and Technology) for cillatory or combined currents. introducing the outcrop of the Katsuura Forma- This structure may be formed as antidunes in tion, Kazusa Group. a turbidity current, by analogy with the exam- REFERENCES ples in Prave and Duke (1990) and Cotter and Graham (1991). This is different from those re- Alexander,J., Bridge, J.S., Cheel,R.J. and Leclair, S.F., 2001, garded by Skipper (1971) and Skipper and Bhat- Bedformsand associatedsedimentary structures formed tacharjee (1978) as antidune backsets in turbid- under supercritical water flows over aggrading sand beds. Sedimentology,48, 133-152. ites and later re-interpreted by Pickering and Alexander,J. and Fielding, C., 1997,Gravel antidunes in the Hiscott (1985) as megaripple cross-stratification tropical BurdekinRiver, Queensland,Australia. Sedimen- in reflected turbidites, since cross lamination is tology,44, 327-337. 12 Tadashi Araya and Fujio Masuda 2001

Fig. 5 Antidune deposits in a turbidite in the Upper Pliocene Katsuura Formation at Katsuura City, Chiba Prefecture. Paleocurrent direction from right to left on the photograph, as recognized based on the directions of the sole marks, imbrication dips and the foreset direction of the current ripple cross laminae (Loc. 7 of Tokuhashi, 1992).

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ア ン テ ィ デ ュ ー ン 堆 積 物 の 堆 積 構 造

荒 谷 忠 ・増 田富 士 雄,2001,堆 積 学 研究,No.53,1-15 Araya, T. and Masuda, F.,2001:Sedimentary structures of antidunes:An overview. Jour. Sed. Soc. Jopan, No.53,1-15

ア ン テ ィ デ ュ ー ン (反 砂 堆,antidune)や そ の 堆 積 物 の 定 義 や 認 定 基 準 に 関 し て は 一 般 に

誤 解 が 見 受 け ら れ る.そ も そ も,ア ン テ ィ デ ュ ー ン と い う用 語 は 流 れ の 高 領 域(upper flow regime)の ベ ッ ドフ ォ ー ム の 一 種 を 示 す も の で あ っ て,特 定 の 堆 積 構 造 あ る い は 堆 積 物 を 指

す わ け で は な い.上 流 側 に 比 較 的 低 角 で 傾 斜 した 斜 交 葉 理(層 理)の み な ら ず,薄 い レ ン ズ 状 の ラ ミナ セ ッ トや ハ ン モ ッ ク 状 斜 交 層 理 に 似 た 構 造("HCS mimics", HCSも ど き)も ア ン テ ィ デ ュ ー ン で 形 成 さ れ る.し か し,実 際 に ア ン テ ィ デ ュ ー ン堆 積 物 を 認 定 す る に は,そ の 堆

積 構 造 や 粒 子 配 列 だ け で は な く,古 流 向 や 堆 積 相 の 分 布 を 調 べ る 必 要 が あ る.