Sedimentary Geology, 85 (1993) 53-62 53

Elsevier Science Publishers B.V., Amsterdam

Sedimentary effects of cataclysmic late Pleistocene glacial outburst flooding, Altay Mountains, Siberia

A . N . R u d o y a and V . R . B a k e r b

a Geography Department, Tomsk State Pedological Institute, Tomsk, Russia h Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA

Received June 30, 1992; revised version accepted November 30, 1992

ABSTRACT

Rudoy, A.N. and Baker, V.R., 1993. Sedimentary effects of cataclysmic late Pleistocene glacial outburst flooding, Altay Mountains, Siberia. In: C.R. Fielding (Editor), Current Research in Fluvial Sedimentology. Sediment. Geol., 85: 53-62.

Pleistocene glacial outburst floods were released from ice-dammed lakes of the Altay Mountains, south-central Siberia. The Kuray-Chuja lake system yielded peak floods in excess of 1×106 m 3 s 1 and as great as 18×106 m 3 s -I. The phenomenally high bed shear stresses and stream powers generated in these flows produced a main-channel, coarse-grained facies of coarse gravel in (1) foreset-bedded bars as much as 200 m high and several kilometers long, and (2) degradational, boulder-capped river terraces. Giant current ripples, 50 to 150 m in spacing, composed of pebble and cobble gravel, are locally abundant. The whole sedimentary assemblage is very similar to that of the Channeled Scabland, produced by the Pleistocene Missoula Floods of western North America.

Introduction

In July 1991, the authors investigated evidence for Pleistocene cataclysmic flooding in the Altay Mountain region of south-central Siberia. The purpose was to compare the recently documented diluvial morphogenesis of the Altay Mountains (Rudoy, 1988a, 1990; Rudoy et al., 1989) with established work on the Channeled Scabland re- gion of North America, which was formed by cataclysmic erosion and sedimentation associated with outbursts from Pleistocene Lake Missoula (Bretz et al., 1956; Baker, 1973, 1978a; Baker et al., 1987).

In both Eurasia and North America, Pleis- tocene glaciations are accompanied by the incep- tion and growth of gigantic ice-dammed lakes

Correspondence to: A.N. Rudoy, Geography Department,

Tomsk State Pedological Institute, Tomsk, Russia.

(Elson, 1992). Recent work at the margins of the former North American ice sheets (Baker and Bunker, 1985; Kehew and Lord, 1987; Teller, 1987) has documented pervasive evidence of Pleistocene cataclysmic outburst floods (j6kulh- laups). Such floods, which are a direct conse- quence of deglaciation, may have exerted major short-term influences on global fluxes of water and sediment (Baker, 1993). Shaw (1989) has even proposed that subglacial meltwater may be a major contributor to cataclysmic flooding, al- though his evidence of fluvially eroded drumlins (Shaw et al., 1989) is disputed by many glaciolo- gists (e.g., Muller and Pair, 1992).

In one reconstruction of the late Pleistocene Eurasian ice sheets, Grosswald (1980) envisions spectacular damming of the great north-flowing Siberian rivers with associated spillovers and drainage diversions (Fig. 1). An alternative recon- struction involves much less ponding and diver- sion (Velichko et al., 1984), but incomplete field

0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

54 A.N. RUDOY AND V.R. BAKER

I . ~ . . . ~ - ~ ' v ~ . I / . ' , . - >'~,/ ~ ° 80o100 ° ~2~ - '1 \vE3~J-~, ' ~ - / . .. % / , . "

,w , ~-~" \~o~5U_/" S" /"~=J . . . . "+- ,%,~-

• ~ 70°

STUDY i ->/J Glacial Boundaries I AREA

~ Ice-Dammed Lakes, i Other Freshwater

. 40° Basins

60'

-•L¢'• Proglaeial Drainage, Spillways; CataOysm~c

[ - - • Floods

Fig. 1. Late Pleistocene Eurasian Ice Sheet and related drainage features (Grosswald, 1980), showing location of Ahay Mountain study area. Glacial spillways lead to Pleistocene predecessors of the Aral Sea (A) , Caspian Sea (C), and Black Sea (B). The

western outlet of the system was the English Channel (C), for which cataclysmic flood landforms are described by Smith (1085).

mapping and geochronology precludes scientific preference for either of these hypothetical alter- natives. Nevertheless, it is clear that the phe- nomenon of massive, regionally extensive, high- energy flooding was an important feature of global Pleistocene glaciation. Evidence of such phenom- ena is to be expected in pre-Quaternary glacial sediments.

Physical setting and flood processes

The Altay Mountain study area lies in the upper Ob River basin, upstream of the Pleis- tocene network of Siberian glacial lakes and di- versions (Fig. 1). The intermontane depressions of the Altay have long been known to have con- tained Pleistocene glacial lakes. The two largest Altay Pleistocene lakes occupied the Kuray and Chuja depressions in the headwaters of the Chuja River (Fig. 2). Together these interconnected lakes held at least 1000 km 3 during the latest Pleistocene glaciation (Rudoy, 1988a). Water depths reached 600 m with a 900 m stage during an earlier Pleistocene phase (Rudoy et al., 1989). The lake dimensions are comparable to those of

Pleistocene Lake Missoula, which held about 2500 km 3 at a maximum depth of 635 m at its ice dam (Baker and Bunker, 1985).

~ - ' i ~ _ ~ "~t, 3256

-- ~ ~ 1 ~

~_ ; " K I " " ~ o ~

:;HUJSKYA ' ' RASIN

37~A. ~ Mountain Range va dora P, , : , , ~ , L _ / ~ Peak Etev (Meters) / ~ , , / .,/V

i ilt%~ Giant Current ~"~' ' , _ Ripples

~-~.. ~ F,ood F,ow ~ { ~ G,acia, r- Isteep ~, Direction __ _ Moraines L '_ J' 8topes ~ "~

Fig, 2. Generalized map of the Kuray and westcrn Chuja Basins showing cataclysmia flood features. Glacial ice occu- pied the preglacial course of the Chuja River at Aktash, diverting floodwater into a fracture-controlled route several kilometers to the south. A late-Pleistocene ice dam at the Maseu-Chuja River junction probably created the j6kulhlaup setting as did another ice dam between Kuray and Chuja

Basins.

EFFECTS OF CATACLYSMIC LATE PLEISTOCENE GLACIAL OUTBI3RST FLOODING 55

Paleoflood discharges and related hydraulics

can be estimated by three forms of scientific procedure. First, a theoretical simulation model

can be formulated from the assumed physics of ice-dammed lakes. Using a computer model of this type, Clarke et al. (1984) deduced outflow hydrographs for glacial Lake Missoula. A second approach to the problem is inductive. Empirical data on ice-dammed lake volumes are compared statistically to peak outburst flood (j6kulhlaup) discharges associated with lakes of those volumes. Formulae applicable to modern ice-dammed lakes include the following:

Qm = 0.0075VIL667 (1)

Qm = (t.0065V°'~'~ (2)

Qm = 0.0113V°'°l (3)

where Qm is the peak outburst flood discharge in m s s-~ and V is the ice-dammed lake volume in m s according to, respectively, (1) Clague and Mathews (1973), (2) Beget (1986), and (3) Costa (1988).

Using eqs. 1 through 3, the Pleistocene Kuray-Chuja lake system is estimated to yield flood discharges of between 4 and 9 X l0 s m 3

s-~. Large as these values are, they probably err on the low side. The error is caused by the fact that the modern ice-dammed lakes, from which the equations were derived, were much smaller than the Pleistocene Altay-region lakes. The largest modern lake had a volume of less than 2 km 3 and a flood peak of 1 × 104 m 3 s -~.

Because of problems with both the theoretical- deductive and the inductive-empirical procedures for estimating Missoula flood paleodischarges, O'Connor and Baker (1992) emphasized the use of a third procedure: the retroductive fit of a hydraulic flow model to the field evidence of high-water marks. Baker et al. (1993) use the same procedure to infer the peak outflow from the Kuray-Chuja paleolake system. The peak of 18 X 106 m 3 s i is comparable to the 17 × 10 c' m s s J estimated by O'Connor and Baker (1992) for the glacial Lake Missoula outflow. Both these values seem to scale to a cataclysmic mode of ice-dam failure, in contrast to the ice-tunneling model of j6kulhlaup release that is (a) assumed

by the theoretical models of Clarke et al. (1984) and Waitt (1985), and (b) characteristic of the

empirical data set on modern j6kulhlaups (Clague

and Mathews, 1973; Beget, 1986; Costa, 1988). From the Kuray-Chuja peak paleodischarge

and extant paleochannel geometries it is possible to estimate several parameters of importance to understanding depositional landforms and their sedimentology. The applicable relationships are:

r b = y d S (4)

~,QS w - y d S V = % 9 (5)

W

where ~o is the power per unit surface area, W is flow width, y is the specific weight of the fluid, d is the flow depth, S is the energy slope, Q is the discharge, r h is the bed shear stress, and V is the mean flow velocity. These simple parameters de- scribe the most important aspects of geological work performed by cataclysmic floods (Baker and Costa, 1987). They describe the enormous amounts of geological work performed by glacial floods in exceptionally short periods of time. They also allow flood processes to be compared to other cataclysmic planetary phenomena in terms of scaling relationships (Baker, 1983) and power per unit area (Baker, 1985). The Kuray-Chuja flows of depths 400 to 500 m achieved peak velocities of 20 m s-I to 40 m s-I bed shear stress of 5 x 103 to 5 x 104 N m -2 and stream power per unit area of l0 s to 1(} ~ W m - 2 (Baker

et al., 1993).

Cataclysmic flood sediments

As in the Channeled Scabland (Baker, 1973), the cataclysmic flood sediments of the Altay re- gion occur in two general facies: (1) coarse- grained channel deposits associated with the high-velocity (high-energy) flood waters of the main channelways, and (2) finer-grained slackwa- ter deposits, which accumulated in low-velocity (low-energy) regions marginal to the main flood channelways. A typical association occurs imme- diately downstream of the Chuja and Katun River junction (Fig. 3), located about 70 km west-north- west of the village of Kuray (Fig. 2).

5(3 A.N. RUDO~ A N D ~ R , I ] A K L R

,e i i Scale in km

Fig. 3. Generalized map of flood features near the Katun- Chuja River junction. Giant bars occur at Big Jaloman (B), Litt le Jaloman (L), ]nya ( / ) , and the Katun-Chuja junction (KC). An eddy bar (E), slackwater sedimentation area (S),

and giant current ripple area (B) are also shown.

Coarse-grained facies

The coarse-grained flood deposits comprise a series of longitudinal bars (Fig. 4), which rise to about 200 m above the modern river level. These bars are about 2 to 5 km in length and occupy alternating positions along the flood channelway, conforming to valley meander bends of the Chuja and Katun Rivers. It is clear that the Inya and Little Jaloman bars formerly blocked the valley's tributary to the Katun Valley (Fig. 3). The tribu- taries subsequently incised deep valleys to breach

i

. . . . . . . . . . . . . J

Fig. 4, View of Inya Bar, Katun River Valley (Fig. 3). The bar rises I00 m above the prominent terrace in the foreground, which is 100 m above the Katun River level. Flood-trans-

ported boulders tie in the foreground.

.... ~ r ~ o

Fig. 5. Giant current ripples, Little Jaloman (R in Fig. 3).

the blockade, leaving split remnants of the origi- nal bar form (Fig. 4). The high bars are composed of foreset-bedded gravel with prominent, long slip-faces. These are characteristic for bar forma- tion in narrow, deep flood channels (Baker. 1984) and distinguish their deposits from those of wide. shallow braided rivers (Rust, 1975).

At about 100 m above modern river level, a series of terraces is developed. Although once ascribed to proglacial aggradation and degrada- tion (Okishev, 1974), the local occurrence of giant current ripples (Fig. 5) indicates association with cataclysmic flood flows. It was a similar discovely in the Channeled Scabland (Bretz et al.. 1956) that falsified Flint's (1938) nonflood hypothesis for scabland genesis and provided the basis for modern diluvial explanations of the phenomena (Baker, 1978b; Baker and Bunker, 1985). Most of the terraces, such as those at the Katun-Chuja River junction (Fig. 6), have capping boulder layers about 1 to 2 m thick overlying units of foreset bedded gravel, 5 to 10 m thick. The boul- der caps may be lag units created by fluvial inci- sion into the unusually thick foreset gravels, which likely resulted from the late Pleistocene flooding. The incision phases probably resulted from the degradation of post-flood streams, but further work is obviously required to understand these interesting relationships.

Fine-grained facies

The Little Jaloman River affords exposures of the giant bar blocking its mouth (Fig. 7) and of

EFFECTS OF CATACLYSMIC LATE PLEISTOCENE GLACIAL O U T B U R S T FLOODING 57

Fig. 6. Giant bar (KC in Fig. 3) at the junction of Chuja River (foreground) and Katun River (out of view in background). In front of the 200-m high bar are several boulder-capped degra-

dational terraces.

the slackwater sediments (SWS) that were em-

placed by backflooding from the main floodway

down the Katun River. Maximum flood depths

were at least 200 m, based on the heights of the

largest bars (Fig. 4), but SWS units lie about 100

m lower and are inset against the giant bar de-

posits. As in the case of Missoula-Flood slackwa-

ter deposits (Baker and Bunker, 1985; Smith,

1993), fine-grained silt-and-sand layers are in-

terbedded with coarser units proximal to the main

channelways of the flood path. Deposits fine dis-

tally up the tributary valleys and away from the

main flood channels. Gently dipping foresets indicate currents up

the Little Jaloman Valley. Layers of sand and

Fig. 8. Slackwater sediments exposed along Little Jaloman River (S in Fig. 3).

pebble gravel about 25 cm thick alternate with

layers of silt about 10 cm thick. Locally, rip-up silt

clasts occur. A massive, meter-thick unit of

open-work, angular cobble-sized clasts, including

small boulders, disconformably overlies the SWS

sequence (Fig. 8). By analogy to the Missoula-

Flood SWS facies, these coarse units probably

represent deposition by energetic flood surges

flowing up the tributary valleys (Baker, 1973,

Smith, 1993). The powerful upvalley currents

would generate rip-up clasts from previously de-

posited slackwater silt layers.

These very preliminary, observations need to

be followed by more detailed study. The SWS

units could represent either flow pulsation (Baker,

1973) or multiple j6kulhlaups (Waitt, 1985), but

at much lower magnitudes than the great bar-for-

Fig. 7. Giant bar (L in Fig. 3) at the mouth of Little Jaloman River (foreground). Main Katun River Valley is to the right Slackwater deposit exposures (S in Fig. 3) occur at the far left, inset against the bar.

58 A.N. R U D O Y AND V R. BAKER

ming flood(s). The relationships are similar to those observed in the Channeled Scablaad (O'Connor and Baker, 1992).

Giant current ripples

Giant current ripples are extensively devel- oped in the flooded valleys of the Altay and nearby Tuva regions of central Asia (Rudoy, 1988a). Their crests rise to as much as 10 m above their swales, and display an asymmetric stoss and lee morphology (Fig. 9). The spectacu- lar Kuray ripple field (Fig. 10), located east of the Tete River (Fig. 2), displays ripple crest spacings (chords) of 50 to 150 m. The Kuray ripples occur at elevations of 1540 to 1600 m, and the Kuray paleolake had a water level of 2000 m. Thus, the ripples may have developed under water depths as great as 460 m.

Smaller ripple dimensions characterize the Little Jaloman (Fig. 5) and Aktru (Fig. 11) ripple fields. The latter are located in the Kuray Basin east of the Aktru River (Fig. 1). The crest spac- ings (chords) for both these examples average 40 m. As with the Kuray ripples, these bedforms are composed of semi-rounded openwork gravel ranging from 1 to 15 cm in diameter (Fig. t2). Small boulders to 1 m occur on some of the ripple crests. Crude foreset bedding parallel to lee-side slopes can be seen in some exposures. In all these attributes, the bedforms are similar to the giant current ripples of the Lake Missoula floods, discovered by Pardee (1942) and de- scribed by Baker (1973).

The terminology for giant current ripples poses a problem. The term "dune", introduced by Gilbert (1914) to distinguish sand waves from

Fig. 10. Giant current ripples south of Kuray Village and east of Tome River (Fig, 3).

smaller ripple forms, has been applied to them by Alien (1982) and by Waitt (1985). Like fluvial dunes, the giant current ripples probably formed at lower flow regime, with Froude numbers less than 1.0 (Baker, 1973). However, the giant cur- rent ripples are gravel waves and, unlike sand-bed fluvial dunes, they do not have smaller ripples super imposed upon them. Middleton and Southard (1984) note the distinction between small ripples and large ripples, and include in thc latter class megaripples, dunes, and sand waves. They do not believe distinction of various types of large ripples to be practical. More examples of gravel-bed giant current ripples will need to be studied to resolve this issue.

Giant gravel bedforms with crests perpendicu- lar to the flow direction are classified as meso- forms in Jackson's (1975) hierarchical interpreta- tion of fluvial bedforms because of their pre- sumed scaling to flow depth. In conventional bed- form phase plots, the features called "dunes" do not form in sediment coarser than 10 mm (Costello and Southard, 1981; Allen, 1983).

(foreground). Note the human figures and tents on the near- est ripple crest.

Fig. 11. Giant current ripples, partly accentuated by forest cover, in the Kuray Basin east of Aktru River.

EFFECTS OF CATACLYSMIC LATE PLEISTOCENE GLACIAL OUTBURST FLOODING 59

Fig. 12. Openwork pebble and cobble gravel exposed in a pit

excavated into the ripple crest from the Little Jaloman site,

Platovo village (Fig. 9).

Moreover, the commonly used bedform scaling relationship (Yalin, 1964):

A --- 5d (6)

where A is ripple spacing and d is flow depth in the same units, indicates relatively shallow depths over dunes. A 100-m dune spacing would develop in 20 m flow depths. These scaling rules apply loosely to small gravel mesoforms, such as ob-

served by Dinehart (1992), but they differ greatly from indirect flow reconstruction for larger meso- forms associated with highly energetic flows (Ta- ble 1).

A tentative conclusion is that the giant current ripples are bedforms that do not scale directly from observations in flumes and small-scale braided rivers. The observation of transverse "gravel waves" on the floors of submarine canyons, formed at depths of 2000 m (Malinverno et al., 1988), shows that the "dune" categoriza- tion can be misleading. The discovery of the Altay-region giant current ripples shows that this phenomenon is a distinctive feature of cata- clysmic glacial flooding and deserves further study.

Discussion

The central Asian and North American ice- dammed Quaternary lakes and their associated outburst floods have many similarities in their processes and sedimentary consequences. In the Altay region, where landforms and sediments were previously attributed solely to the action of glacial ice (Okishev, 1977), extensive evidence of cataclysmic lake outburst flooding has been found. Despite considerable resistance to these concepts by some Russian geoscientists, they have recently

TABLE 1

Representat ive hydraulic data for coarse-gravel transverse mesoforms

channels

developed in unidirectional water flows in laterally stable

Location North Toutle Medina River, Channeled Scabland

River, Wash. Texas

Flood

References

Bedform chord, A (m)

Bedform height, h (m)

Mean sediment size, Dso (mm)

Coarse sediment size, D84 (mm) Flow depth (m)

Mean velocity, P (m s l )

Bed shear stress, r b (N m 2)

Power per unit area, w (W m 2) Peak discharge, Q (m 3 s 1)

December 1989 August 1978 Pleistocene

Dinehart , 1992 Baker and Baker, 1973,

Kochel, 1988 1978a

6-15 80 120

0.2 3 6

30 - 60

60 100 20(1 1.4-2.4 10 ll)0

2.5 3.5 18

100 300 1800 2.5 × 102 1 x 103 3.2 × 104

175 7 × 103 1 × 107

60 A.N. RUDOY AND V,R. BAKER

been able to be published (Rudoy, 1988, 1990). We suggest that there may be many other regions of sedimentation, which have been incorrectly attributed solely to glacial ice processes, but which rightly owe their origin to cataclysmic flooding. The reasons for this incorrect attribution are related to misinterpretations of meaning of uni- formitarianism (Baker, 1981). For example, Clifton (1988)writes, "Parsimony demands that we attribute phenomena in the sedimentary record to the most probable explanation, and convulsive events are, by nature, improbable." Besides its confusion of the probability of expla- nations (hypotheses) with probabilities in nature, this statement also errs in supposing that Okham's razor (the "Principle of Parsimony") is a general principle of nature rather than one of human reasoning about nature. In any matter of incon- sistency between such principles of reasoning and the facts of nature there can only be one scientifi- cally valid resolution: nature wins on all hands.

The concept of diluvial morpholithogenesis (DM) was formulated by Rudoy (1988b) and Rudoy and Kirjanova (1990). This paper repre- sents its first presentation in the English lan- guage. Briefly stated, DM holds that cataclysmic outburst floods (j6kulhlaups) are a characteristic feature of the ice-dammed lakes that necessarily develop in warm-based glacial complexes. Rela- tively small-scale modern j6kulhtaups are com- mon and widespread phenomena (Costa, 1988), and their magnitudes scale approximately to the associated lake volumes (eqs. 1, 2, and 3). Thus, the much larger Pleistocene ice-dammed lakes (Fig. 1) must have had stupendous discharges, e x c e e d i n g 10 6 m 3 s-1. Where confined to narrow valleys, as in the Altay region, such discharges necessarily produced flow depths of hundreds of meters and velocities of tens of meters per sec- ond. These short-lived super rivers probably de- stroyed the evidence of preflood glaciation along the main valleys of flow. Although occurring above base level, where long-term geological preserva- tion is less likely (Dott, 1983), the floods emptied to both lakes and seas (Fig. 1). Episodic sedimen- tation in such settings (Dott, t987) should be carefully considered as the possible effects of upstream cataclysms.

This study was performed under the auspices of the 1991 Altay Soviet-American Geoecologi- cal Expedition to the headwaters region of the Ob River, Siberia. Field assistance on the expedi- tion was provided by P.M. Baker, T.W. Baker, M. Kirjanova, N. Rudaya, N. Chikunova, I. Kon- draschov, G. Sharm, A, Mizin, E. Olejnik, V. Dorkin, V. Sidorkin, and J. Charchenko. This report is a contribution to the new INQUA Com- mission on Global Continental Paleohydrology. Some of VRB's work was supported by the NASA Planetary Geology and Geophysics Program grant NAGW-285. The report is also contribution num- ber 13 to the Arizona Laboratory for Paleohydro- logical and Hydroclimatological Analysis (AL- PHA).

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