of large gravel dunes, Altai Mountains, Siberia. P. A. CARLING ... Internal structure indicates that flow over 2-D dunes was relatively uniform but over 3-D ...
Sedirnentology (1996) 43, 647-664
Morphology, sedimentology and palaeohydraulic significance of large gravel dunes, Altai Mountains, Siberia P. A. CARLING Hydrodynamics and Sedimentology Laboratory, Department of Geography, Lancaster University, Lancaster LA1 4YB, UK ABSTRACT
Coarse-gravel bedforms which resulted from Pleistocene glacial outburst floods are identified as subaqueous dunes. Comparison of the morphology of these ‘fossil’ structures with modern dunes shows that the form of two-dimensional (2-D) transverse dunes and 3-D cuspate and lunate dunes developed in coarse gravels is comparable with sand-dune morphology within lesser-scale geophysical flows. The similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform. Internal structure indicates that flow over 2-D dunes was relatively uniform but over 3-D bedforms flow was locally variable. Flow separation and complex streaming of flow occurred over the steepest 3-D dunes. Cross-beds are thin and few approach the angle of repose; consequently most dunes did not migrate primarily by avalanching but by stoss-entrained gravel transported over the crests rolling-down and depositing on the lee slopes. Lee-side sediments are often finer than the stoss-slope sediments, which indicates the lee formed when flood power was waning. Some dunes were sIightly planed-down during falling stage because lee-side cross-beds tend to be steeper than the angle of the preserved lee slope. However, silt-rich caps indicate that any height reduction was contemporary with the final deposition of foresets. Post-flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs.
INTRODUCTION
Understanding the development of bedforms in coarse clastic sediments is important for engineering purposes and for the interpretation of ancient sedimentary sequences. Within wide and shallow rivers, coarse gravels may frequently form lowamplitude quasiperiodic bedforms le.g. Allen, 1983; Dinehart, 1992b). However, the mobilization of coarse gravel to develop pronounced dune-topography requires high shear stresses and great depths to accommodate strongly twodimensional (2-D) bedforms. As such flows are rare there are few published studies of steep gravel dunes. Consequently this paper describes large-scale ‘fossil’ subaqueous gravel dunes within a context of the palaeoflow conditions.
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The typology of gravel bedforms and their relationship to sandy dunes is unclear but there probably is no limit to the grain size in which steep dunes can form given sufficient depths (e.g. Simons & Sentiirk, 1977; Middleton & Southard, 1984; Southard & Boguchwal, 1990; Best, 1996). Nevertheless, the lack of experimental data for gravel has ensured that diagrams of bedform existence-fields have traditionally shown dunes to ‘pinch-out’ in coarse sand as flow strength increases (Allen, 1984; Southard 81 Boguchwal, 19901, being replaced by plane-beds or antidunes in gravels beneath shallow high-velocity flows. The depth-limitation, common to both field and laboratory, may explain why lunate pebblebedforms (Galay, 1967; Galay & Neill, 1967; Gustavson, 1978) do not attain the steepness 64 7
648 P. A. Carling
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Fig. 1. Location of Katun River, Kuray Basin (K) and Chuja Basin (C) in southern Siberia.
Lake depths and volumes, flood depths and maximum discharges have been reported elsewhere (Baker et al., 1993; Rudoy & Baker, 1993). However, the history of lake drainage is complex and many diluvial features may not be contemporaneous. Interpretation of dunefields should prove of sedimentological interest and aid reconstruction of the complex palaeogeography. This paper reports the results of fieldwork in the Katun River basin (Fig. 1) which demonstrates that six ‘rippled’ tracts are dunefields. Dune notation is given in Fig. 2, bedform nomenclature follows Allen (1968) and particle grade terms are those of Wentworth (Pettijohn, 1975). Carling (1996) provides reconstructed palaeohydraulic data for the Kuray dunefield described here.
General description of dunefields typical of 3-D dunes in sand (Ashley, 1990). However, where depths permit, a variety of coarse, steep flow-transverse structures have been documented from flume studies (Hubbel et al., 1987) and less frequently from nature, but usually these consist of sand/granules (Wells & Dohrenwend, 1985; Whiting et al., 1988; Ditchfield & Best, 1992), sandlpebble mixtures (Dinehart, 1992a,b) or pebbles (Baker, 1984; Allen, 1993). Steep cobble bedforms are associated with late-Pleistocene deep floods in the western USA (Pardee, 1942; Bretz et al., 1956; Bretz, 1959; Baker, 1973; Fisher & Spooner, 1994) whilst modern examples are also associated with highenergy floods (Theil, 1932; Fahnestock & Bradley, 1973; Gustavson, 1978; Baker, 1984; Baker & Kochel, 1988).
Geographical perspective Speranski (1937) reported ‘rippled’ plains in south Siberia and argued, from the basis of orientated ‘erratic’ blocks and presumed flood marks, that these landforms could only be explained by powerful fluvial currents. Subsequently, in the 1980s, inspection of previously classified airphotography showed that tracts of ‘ripples’ were numerous in the mountainous areas of south Siberia. Rudoy (1988a, 1990) and Rudoy et al. (1989) established that deep intraglacial lakes had formed in large intermontane basins as a result of damming by Quaternary ice-sheets; ‘ripples’ and other landforms in the Altai Mountains were attributed to breaching of the glacial dams and rapid draining of the lakes (Rudoy, 1984, 1988b; Rudoy & Kirjanova, 1990).
Platovo dunefield Extensive fields of transverse 2-D dunes occur on a terrace, either side, and about 2 0 m above the Katun River south-east of the town of Biysk (Fig. 1). The age of this dunefield is uncertain. A sand-silt lens in one dune has been TL-dated to 36 kyr ( i4 kyr). However, radio-carbon assay of organic remains and marl in higher terraces (40-60 m) provides four dates ranging between 28 730yr BP ( & 9 9 5 ) and 20500yr BP ( & 2 4 0 ) (Maloletko & Paniechev, 1991).The bearing of the dunefields is %15-20” west of north in accord with the modern drainage. The plan-view is difficult to determine owing to cultivation and dense forest obscuring detail. Nevertheless, between the villages of Platovo and Podgornoje, the flowparallel profile of six dunes is displayed by lateral cutting by the River Katun. In addition, during 1993, building-construction resulted in excellent exposures of both lee and stoss-slope deposits. The dunes are underlain by at least 11m of terrace gravels which often consist of simple cross-bedded units up to 4 m thick (Maloletko, 1980).
Little Jaloman dunefield Between the villages of Little Jaloman and Inya in the Katun River valley, a dunefield occurs close to a giant flood bar. The proximal end of the field is a gravel layer, some 2 m thick, deformed into low-amplitude gravel trochoidal bedforms (whalebacks sensu Allen, 1968) overlying a terrace surface armoured by small boulders.
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The whalebacks are interspersed by a train of large blocks (c5 m), leading from a boulder berm (sensu Carling, 1989) across the terrace surface. Excavation has determined that flood vortex scour hollows, now filled by aeolian deposits, occur around the flanks of the boulders. Near the apex of the valley bend, the whalebacks develop into isolated barchans or linked catenary dunes although the topography is subdued by loess infilling the dune troughs. Two TL-dates from the same sample indicate the loess has an age of 7.4 kyr ( f 0.8 kyr) to 6.2 kyr ( f 0.7 kyr).
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Length
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Fig. 2. Definition diagram in section (above) and plan (below) of Siberian dunes: modified from Allen (1968). Dash-dot lines delimiting wings of dune in plan indicate saddle junctures with span-wise neighbours.
Kuray dunefield Trains of low-amplitude gravel whalebacks and transverse 2-D dunes occur patchily over a 24-km tract fronting the Northern Chuja Alps; from Kara Kjol to near the village of Kuray (Fig. 3). The most westerly 20 km, falling steeply to the east, appears to be a flood-scoured ‘scabland’ (sensu Bretz, 1923) consisting of water-scoured hills often apparently ‘plucked’ on the upstream sides with gravel bars deposited in sheltered locations, and scattered groups of starved dunes. The lack of
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Fig. 3. General location of Kara Kjol, Akturu (A) and Kuray (B) dunefields in the Kuray Basin. Location C is the hill shown in Fig. 5.
Fig. 4. Oblique view to south-east across the Kuray dune field from hill location d (see Fig. 5). Flow right to left.
substantial dunefields in this area probably is owing to the evacuation of available material by strong palaeocurrents flowing towards the east. In contrast, the easterly tract consists of a plain (former lake bed) upon which sediment has accumulated as two well-defined dune-fields (A & B in Fig. 3). The Kuray field was investigated in detail and extends ~ 3 . km 3 in the direction of the palaeoflow with a span of 2-4km (Figs 3-5). Radio-carbon assays of organics from fossil pingos demonstrate the lake-bed was dry by 3810 yr BP ( f 105) to 3200 yr BP ( f 600) (Rudoy, 1988~).The Akturu dunefield consists of more than 70 arcuate subparallel 2-D dunes (indicative of an expanding flow from the west)
developed over a 1.5-km tract below and to the east of a pronounced valley constriction associated with a rock-step. Aerial photographs also reveal two smaller dunefields to the immediate west and north of the constriction.
Kara Kjol dunefield The dunes at Kara Kjol (Fig. 3) are the most westerly example of the dune-trains found immediately north of the Chuja Alps. The dunefield on the southern side of the basin is 0.5km in length and appears to be orientated W-E. Here the palaeoflow direction needs clarification but less than 0.5km to the north the orientation of the
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Large gravel dunes 651
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Fig 5. Vertical air photograph of Kuiray dunefield. Scale z 1:30000. POC;k marks are pin-pricks.
Fig. 6. View (towards the south-west) of the lee slope of the largest Kuray dune, showing three distinct lobes. Track in middle foreground runs along the toe. Lower stoss of adjacent downstream dune is visible to the left. Truck on crest is 2.8 m high.
dunes is clearly E-W towards a small spillway (Fig. 3). A further distinct field (Lower Kara Kjol) consists of 2-D dunes orientated E-W and extends for 0.5 km across a shallow hollow.
Dune morphology Transverse dunes The morphology of 2-D flow-transverse dunes is best exemplified by the Kuray system (Fig. 4). Dunes at Kuray have straight or slightly sinuous crests with saddles spaced typically every 60-120 m (range 30-150 m) dipping 1-2 m below
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the intervening lobes (Figs 5 and 6). When comparing adjacent crest-lines in plan, saddles are in phase or exhibit a phase-shift up to 0.33 owing to veering of the palaeoflow from west-east to northeast. There are no spurs on lee or stoss slopes. Dune lengths vary from 15 to 30 m on the southside of the field where dunes are 1-2 m high to a maximum of 200 m in the north-west where the dunes are 10-16 m high. The reduction in length southwards along the span entails an increase in the number of dune junctures. These are of the simple zigzag type (Allen, 1968) involving a bifurcation in the troughline. Open junctures
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(Allen, 1968) are more common in the smaller dunes to the south. Dunes in the south-west have straight crestlines, whilst further north-east sinuosity increases locally to linguoid form. The increase in height, length and change in duneform on a south to north traverse towards the centre of the basin (where any flood would have been at its deepest) may be indicative of bedforms developing in progressively deeper and faster flow (Harms & Fahnstock, 1965; Allen, 1968). In contrast, a recent study of ripple development (Baas, 1994) has shown that 2-D bedforms in uniform flow grow in height and evolve into 3-D bedforms given sufficient time. However, is not possible to isolate the time-factor at Kuray, as both velocity and the period of dune growth can be expected to have been greatest where the flood waters were deepest. Despite this, the regularity of form over most of the field indicates a fairly steady current and a large water depth relative to the dune heights. The dunes diverge around an isolated hill
Fig. 7. Longitudinal profiles of Kuray dunefield data. Vertical exaggeration x 30.
in the north (C in Fig. 5) where stalled ‘climbing’ dunes (cf. Tsoar & Greeley, 1980; Breed et a!., 1984) occur against the upstream flank whilst immediately to the east no dunes developed owing to the ‘shadow-effect’ of the hill. Wellrounded water-transported boulders are found scattered over hill (c) and (d) to a height of c. 40 m providing some evidence for flood depth (Carling, 1996).
Dunes rapidly reduce in height and length over the first kilometre as the surface underlying the dunes rises 16 m (Fig. 7). Beyond this point there is no systematic change in dune geometry. The dune furthest to the west (Fig. 8a) has a low length to height ratio (L/H=12) which falls just above the LIHcurve proposed by Ashley (1990) to describe equilibrium dunes (see Palaeohydraulic significance). Approximately 1 km to the east, duneform may represent a local equilibrium with the flow, in that LIH ratios (20-40: Figs 7 and 9) define a trend subparallel to Ashley’s curve albeit
01996 International Association of Sedimentologists,
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Large gravel dunes 653 a 15
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Fig. 10. Dune asymmetry does not vary with dune steepness. Examples from 0 =Kuray; + =Kara Kjol; JC =L. Jaloman; 0=Platovo.
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Fig. 8. Longitudinal dune centreline sections: (a) highest dune at Kuray; (b) Kara Kjol; (c) traverse at Platovo; (d) first upstream dune at Platovo. Flow right to left.
with dunes about 2 m lower in height. Many of these latter dunes are trochoidal but still have similar steepness ratios (LIH) when compared with strongly asymmetrical dunes (Fig. 10). Asymmetry is represented by the alb ratio (Fig. 2) which for all the Kuray dunes ranged between 0.48 and 9.0 (average=1.70; standard error for 50 observations=0-18). On the smaller dunes, lee slopes can be as little as 3" whilst large dunes have angles of 17-19".
20
Stoss slopes are typically 3-10', with the steeper gradients occurring towards the base of the slope which subsequently decreases smoothly in angle towards the summit point. However, there are several exceptions displaying well-formed crestal platforms which are nearly horizontal (2 m) of these blocks are orientated transverse to the palaeoflow. The intermediate-axes dip up-current at an angle greater than the stoss slopes. In some cases, imbricated stacks of two or three slabs are found. The largest (cubiform) block in the Kuray field measures 3 m and is immediately downstream of the brink of a 7-m-high dune. A distinct horseshoe scour hollow occurs around the upstream edge and flanks of this block, but for smaller blocks scour is not evident. An explanation may lie in the proximity of the large block to the flowseparation point on the dune crest where velocities are high, whilst most blocks stabilized within the boundary-layer of the stoss slopes. However, flow over blocks is complex (Hunt et al., 1978), the erosional and depositional pattern largely being conditional on the protrusion of the blocks such that, smaller, less exposed blocks promote little upstream scour (Iversen et al., 1991). Large blocks, with no preferred orientation, are frequent in the Kuray basin in areas devoid of dunes, and are believed to be dropstones delivered to the original lake bed by direct settling in quiescent water from melting ice-floes. However, the absence of dropstones in the dune troughs, the preferred orientation and the horseshoe scour indicate that the dropstones on the dunefield have been transported by the flood which formed the dunefield. It follows that the maximum size of the well-rounded gravel (
