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Sedimentary Geology

Sedimentary Geology 107 (1997) 263-279

Sedimentology of the Narmada alluvial fan, western India

L.S. Chamyal *, A.S. Khadkikar, J.N. Malik, D.M. Maurya

Department of Geology, Fuculr) of Science, MS. University of Baroda, Baroda, 390 002, India

Received 20 June 1995; accepted 29 April 1996

Abstract

The Narmada alluvial fan is one of the world’s largest, with an axial length of 23 km. The architecture is dominated by debris-flow deposits (Gms facies). Matrix support, a clay content of 3% and clast contact indicate that the clast-support mechanism resulted from a combination of buoyancy and dispersive pressure. The other facies include gravel/sand-couplet facies (GSh), planar cross-stratified gravel facies (Gp, and Gpz), sand-sheet facies (Sm), and trough cross-stratified sand facies (St). Gms, GSh and Sm facies are debris-flow and sheet-flow deposits that aggraded the fan, whereas Gp, and St are channel bars and channel fills that dominated the fan between major flood events. The fan is characterised by subrounded to rounded clasts. The rounding is due to the elongated catchment area upstream of the fan apex, as clasts are rounded during prolonged bed load transport and are temporarily arrested upstream of the fan apex as channel bars. These clasts are remobilized and entrained in debris-flows on the fan during events of anomalous discharge (storm events). The basalt clasts show a progressive fall in maximum clast size from 150 cm to 10 cm away from the fan apex.

The Narmada river exhibits discharges of up to 60,000 m3/s, but, due to reconfinement of the feeder channel resulting from tectonic reactivation of pre-existing lineaments during the Late Pleistocene, this does not aggrade the fan. Tectonism has influenced the location of the depositional site, has provided the necessary physiographic contrast, and has played an important role in the erosion of the fan, whereas climate-controlled primary and secondary processes have determined the nature of alluvial architecture.

Keywords: alluvial fan; river; Quatemary; sedimentology; India

1. Introduction

Alluvial fans owe their existence to several si- multaneously acting processes. Of prime importance is an abrupt change in the regional physiographic setting where the river becomes unconfined (Bull, 1977; Blair and McPherson, 1994a,b). This abrupt change is commonly at a fault that separates a moun- tainous hinterland from an alluvial plain. The rapid high surface run-off responsible for fan aggradation

* Corresponding author.

occurs through a large number of low-order tribu- taries connected to the feeder channel of the fan. Fan deposits are built by rock avalanches, debris- flows, sheet-flows and so forth, and commonly con- tain subangular to angular clasts (Larsen and Steel, 1978; Pierson, 1981; Ballance, 1984; Blair, 1987; McArthur, 1987; Beaty, 1990; DeCelles et al., 1991; Evans, 1991; Blair and McPherson, 1992, 1994a,b; Brierley et al., 1993; Abrahms and Chadwick, 1994; Kumar et al., 1994). Most fans studied are a few kilometres long.

The relative roles of climate and tectonic reju-

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venation are much debated. Whereas some work- ers consider coarsening-upward cycles as indicators of tectonic activity (Mack and Rasmussen, 1984), others propose a dominant role for climate in fan aggradation processes (Frostick and Reid, 1989). Blair (1987) has demonstrated that the coarsening- upward facies put forward as criteria to interpret structural movements may rather be generated by climate-controlled debris-flows. In the present study, we document a large alluvial fan of Pleistocene age from the Narmada valley of western India. The fan is unusual in its high proportion of subrounded to rounded clasts, despite its non-conglomeratic prove- nance. The relative roles of tectonics and climate in the genesis and erosion of the Narmada alluvial fan are evaluated.

The gravel deposits under consideration have at- tracted the attention of many workers (Blanford, 1869; Zeuner, 1950; Wainwright, 1964; Allchin et al., 1978) and have been referred to as the ‘Older Alluvium’. The discovery of archaeological finds has clarified their Late Pleistocene age (Allchin et al., 1978), which is supported by radiocarbon ages of 18,000 yr. B.P. for calcareous concretions in the stratigraphically youngest alluvial deposits (Hegde and Switsur, 1973).

2. Area of study

The study area is near Tilakwada, in the state of Gujarat, western India (Fig. 1A). The sites (Fig. 1B) of detailed work include Nawagam (Site l), Kewadia Colony (Site 2), Garudeshwar (Site 3), Rampura (Site 4), Tilakwada (Site 5) and Gamod (Site 6). The region has an average rainfall of 1250 mm (Kale et al., 1994) and falls under a semi-arid to dry sub- humid climatic regime (Singh et al., 1991).

The area is transected by one of the major rivers of India: the Narmada, which debouches into the Gulf of Cambay. The river follows the trend of a major geofracture known as the Narmada-Son fault, which causes the river to flow westwards, opposite to the regional slope. The Narmada river originates at Amarkantak and its catchment covers an area of 98,796 km*. During monsoonal floods, discharges range from 10,000 m3/s to 60,000 m3/s. The linear relationship between discharge and maximum veloc- ity implies that a velocity of 5.5 m/s is accompanied

by a flood discharge of 60.000 m”/s (Kale et al., 1994). The Narmada flows through a basalt domi- nated terrain (Sant and Karanth, 1993). Associated with these Late Cretaceous basalts (Deccan Traps) are outcrops of Cretaceous Bagh sandstones and Proterozoic quartzites (Fig. 1C).

3. Structural setup

The geomorphology of the region is controlled mainly by two sets of lineaments related to the Nar- mada graben (Alavi and Merh, 199 1). A major ENE- WSW and less prominent NNE-SSW and NW-SE trends have been identified (Fig. 2). The Narmada graben, a deep-seated geofracture (Biswas, 1987), dating back to the Precambrian, extends into Mada- gascar (Crawford, 1978). The geofracture, which acts presently as an intra-continental rift, was an inter-continental suture zone in the past around which the Aravalli-Bundelkhand, Singhbhum and Dharwar crustal blocks were welded (Naqvi and Rogers, 1987; Radhakrishna, 1989). The Narmada graben is a component of an 1800 km long and 200 km wide zone termed the ‘SONATA’ zone (Shanker, 1991). The Narmada graben controlled accumulation of the 800 m thick alluvial sediments through synsedimentary subsidence (Shanker, 1991). The graben also influences the location of tributaries on the left bank of the Narmada river which flow on faulted blocks. The Narmada in its boxwork drainage pattern reflects the dominant role of pre-existing lin- eaments (Fig. 2). The Narmada-Son fault, a com- ponent of the Narmada graben (SONATA zone), provides the physiographic setting (inset in Fig. 1B) necessary for the development of an alluvial fan.

4. Sedimentary facies

The deposits comprise five lithofacies. Their dom- inance varies at each site (Fig. 3). The facies coding scheme (modified after Miall, 1985) used in this study is shown in Table 1.

4.1. Gravel-sheet facies (Gms)

Description. The facies consists of laterally un- interrupted matrix-supported gravels and is observed in varying proportions, at all localities. The gravels

L.S. Chamyal et al. /Sedimentary Geology 107 (1997) 263-279 265

GUJARAT

a Study Area

_

NSFzNarmada-Son Fault 0

I::‘.‘I Quoternary aDeccan Trap Basalts

m Cretaceous Rocks

a Precambrian Rocks

v v

v v

v v

v v

v v v v u v v

V vvvvvv V vvvv - v v rnCLP,, @

v -= v ”

-i

o

v v 1); v vv vv

Fig. 1, (A) Location map of the study area. (B) Map of the area showing the Narmada river and its tributaries. Locality names of the sites are given in the text. The area demarcated by broken lines represents the inferred spatial extent of the Narmada alluvial fan. Inset shows a longitudinal profile showing the topographic contrast between the mountainous hinterland and the alluvial plain. (C) Geological map of the catchment area illustrating the distribution of various stratigraphic units and dominance of Deccan Trap basalts. Tilakwada represents site 5 shown in (B).

are inversely graded, poorly sorted (Figs. 4 and 5) and essentially polymodal. Typically 25-35% sand- mud matrix is present. The larger clasts are restricted to the upper margin of each unit. The upper and lower bounding surfaces are gently convex upward and non-erosive. However, the upper bounding sur- face shows greater convexity imparting the unit a convex lens morphology in cross-section. The gravel unit occurs as solitary sheets and also as compos- ite units built up of two or more vertically stacked

sheets. The sheets vary in thickness from 0.5 to 2 m. The clasts. are discoidal, cuboidal, tabular and spheroidal. Wherever there is a predominance of discoidal clasts, a crude imbrication towards the southeast is observed implying northwest-directed palaeocurrents. The clasts show a high degree of rounding. The maximum clast size decreases in a systematic manner towards the northwest from 150 cm to 10 cm. Another feature is the clustering of clasts in groups of three to five. A similar organiza-

266 L.S. Chamyal et al. /Sedimentap Geology 107 (1997) 263-279

Fig. 2. (A) Lineament map of the area. (B) Directional rosette of the lineaments, showing the dominant ENE-WSW, NNE-SSW and NV-SE trends. (C) Directional rosette of the Iineaments controlling the path of the river Narmada from Jabalpur to its confluence with the Gulf of Cambay. Note the similarity between the two rosettes

tion of clasts in conglomerates has been described as ‘nesting’ (Allen, 1981). The basal gravels in all sections are strongly cemented by calcium carbonate (case hardening of Lattman, 1973).

Interpretation. The inverse grading of clasts along with large clast size and protrusion of clasts at the upper bounding surface of each unit suggests that the facies represent viscous debris-flows. Similar facies have been reported extensively from other alluvial fan deposits (Larsen and Steel, 1978; Pierson, 1981; Blair, 1987; Beaty, 1990; DeCelles et al., 1991; Evans, 1991; Blair and McPherson, 1992, 1994a,b; Brierley et al., 1993; Kumar et al., 1994). The main mechanism involved for large-clast entrainment is a combination of dispersive pressure and buoyancy (Costa, 1984). This is reflected in the segregation of the larger clasts within each unit away from the lower bounding surface, a feature also recognized by Hubert and Filipov (1989). The high clay con- tent of 3% (Chamyal et al., 1994) and the clast size governed the mobility of the debris-flows, the former providing cohesive strength (by reducing per- meability and increasing pore pressure) and the latter determining the structural framework (Costa, 1984). Clustering of clasts in nests of up to five may indi-

cate accumulation due to obstruction by a larger clast (Ballance, 1984) or a tendency for clasts to migrate towards regions of least internal shear (Allen, 198 1).

4.2. Gravel/sand-couplet facies (GSh)

Description. The facies consists of horizontally stratified gravel-sand couplets of 10 cm to 15 cm thick (Fig. 6). The sand component of each couplet overlies the coarser fraction. The clasts are pebbles and cob- bles of spheroidal, discoidal and cuboidal basalt. The clasts are subrounded to rounded and no distinct im- brication is observed. In the sand units pebbles are less abundant and cobbles are totally absent. The sands are discontinuous in some cases and individual units bi- furcate laterally. The gravels are unsorted, polymodal and lack internal stratification. The gravel/sand units occur as up to seven vertically stacked cycles.

Interpretation. The suite of characters shown by this facies suggest that the gravel/sand couplets re- sulted from sheet-flow processes. The deposition of sand subsequent to gravel deposition implies a suspension fall-out. The couplets also suggest low turbulence in the flows that enabled the step-wise deposition of gravel and sand with fall in the flow

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268 L.S. Chamyul et al. /Sedimenrav Geology IO7 (I 997) 263-279

Fig. 4. Gravel-sheet facies (Gms). (A) Location is site 3. Length of stick is 1.5 m; circled areas show nesting of clasts. (B) Location is site 5. Height of field assistant 1.56 m.

L.S. Chamyal et al. /Sedimentary Geology 107 (1997) 263-279 269

Fig. 5. Gravel-sheet facies (Gms) showing four cycles of aggradation demarcated by black broken lines. Location is site 4. Length of stick is 1.5 m.

Table 1 Facies coding scheme modified after Miall (1985)

Facies code Description Interpretation

GSh Sheet flow deposits

Sm

GPI

Gms Inversely graded cobbly to bouldery gravels, having cross-sectional lobate geometry. Maximum clast size up to 150 cm. Large clasts within each unit appear to ‘float’. Clasts usually of subrounded basalt.

Debris flow deposits

Gravel-sand couplets, stratified, but no internal stratification. Sandier units contain pebbles but cobbles absent. Clasts usually of subrounded basalt.

Sand sheets, massive with lobate flow deposits cross-sectional geometry, no visible internal stratification.

Sheet flow deposits

Planar cross-stratified gravel, may occur as solitary set or coset, shows normal grading with clasts of subrounded basalt. At times a cobbly basal lag deposit present.

Longitudinal gravel bars

Gp2 Planar cross-stratified gravel with lensoidal geometries. Normally graded and associated intimately with the Gms facies

Rechannelized flows genetically related to debris-flow events

St Trough cross-stratified sands Channel-fill element

270 L.S. Char@ et al. /Sedimentary Geolo,~ 107 (1997) 263-279

Fig. 6. Gravel/sand couplet facies (GSh). Location is site 4. Length ot stick IS 1.5 m

capacity. Similar couplets have been described by Blair and McPherson (1994a), who interpret the cou- plets as due to changes in flow hydraulics, initiated by flow expansion and decrease in slope gradient. Equivalent controls seem to have been responsible for the genesis of the couplets in the present case.

4.3. Sund-sheet facies (Sm)

Description. This facies comprises laterally con- tinuous, internally unstratified, sheets of sand. The units are usually 0.5 m thick and bounded by pla- nar, non-erosive surfaces. Associated with the sheets are occasional stringers of pebbly gravel. The facies also occurs as sand lenses within the massive gravel deposits (Gms).

Interpretation. The sand-sheets point to sheet- flood events of low turbulence that led to the sepa- ration of the suspended sand-load and its deposition further down the lobe. Deposition resulted owing to reduction in flood velocity of the unchannelized flow

(Blair, 1987). The association of the Gp2 facies (see below) and sand-sheets points to the prevalence of confined flows subsequent to the deposition of the sand sheets.

4.4. Planar cross-struti$ed gravel fucies (Gp)

The cross-stratified gravel facies occurs at two scales. At the smaller scale it contributes to the fan architecture as lenses, whereas at the larger scale, it occurs as pervasive, laterally continuous gravel ribbons. These two subfacies have different origins and are described separately.

4.5. Large-scale planar cross-stratijed gravel facies (GPI)

Description. The facies (Fig. 7) is bounded by subhorizontal planar surfaces. Locally these surfaces may also be erosive. The gravel units are commonly 1.4 m in thickness, with no significant change lat-

L.S. Chamyal et al. /Sedimentary Geology 107 (1997) 263-279 271

Fig. 7. Succession of interlayered gravel-sheet facies (Gms) and large-scale planar cross-stratified gravel facies (Gpl). Location is site 5. Length of stick is 1.5 m.

erally over a distance of -50 m. The foresets dip consistently at an high average angle of 30” and have concave-upward surfaces. Within each foreset cycle

the clasts are normally graded. The planar cross- stratified beds occur as cosets as well as solitary units. The clasts, which consist of granules and peb-

272 L.S. Chamyal et al. /Sedimmtaty Geology 107 (1997) 263-279

bles of basalt, are subrounded. A basal lag deposit of pebbles and cobbles is observed in each set.

Interpretation. These cross-stratified gravels are a result of the downstream migration of mid-channel bedforms, the downstream accretion element (DA) of Miall (1985). The high-angle foresets are the re- sult of avalanching slip faces at the leading edge of the gravel bar (Smith, 1990). The clast size indicates derivation by surface winnowing of the debris-flow units (Gms) by channelized flows over the alluvial plain. Identical facies have been attributed to depo- sition on longitudinal gravel bars (DeCelles et al., 1991).

4.6. Small-scale planar cross-stratijed gravel facies ((7~2)

Description. These pebbly, clast-supported grav- els are characterised by their lensoidal geometry. The lenses are distributed as solitary units within the ubiquitous Gms facies. They are usually less than 0.5 m thick and show an average foreset dip of 25”. At some sites these gravels are more sandy and oc- cur in close association with the sand-sheet facies described earlier in the text.

Interpretation. The intimate association of the small-scale clast-supported gravels with the Gms and Sm facies suggests a genetic link. These gravel bedforms must have formed immediately after a debris-flow event, during a period when the gravel- loaded flows were rechannelized as small streams.

4.7. Trough cross-strati$ed sand facies (St)

Description. Trough cross-stratified sand was ob- served at only one site. The unit is -2 m thick. The normally graded foresets are large, concave up, and tangential, dipping at an average angle of 25” towards the north. At the base, the unit begins with small-scale sandy bedforms which are draped over the underlying cobbly clasts.

Interpretation. The trough cross-stratified facies represents the channel fill elements of rivers (Brierley et al., 1993) that dominated the fan surface during in- tervening quiet periods between successive episodes of fan aggradation. They are comparable with the Gpi facies in their mode and time of formation.

2

3

4

5a

6b-

t

@ Gms Facie*

10 m Gsh Fac~es

El Sm Fac~es Om 10 20m = Gq FXIS

m G? Facies

0 Not exposed

Fig. 8. Schematic cross-sections of exposures showing the rel- ative dominance of facies. Note that the Gms facies dominates (>60%) the alluvial architecture at all sites.

5. Local occurrences of facies and facies associations

At the sites studied, the abundance of each facies type varies. However, the Gms facies dominates the alluvial architecture as can be clearly seen from Fig. 8 and Table 2.

Site 1. This site is located at the geomorphic divide between the mountainous hinterland and the adjacent alluvial plain (Fig. 9A). No exposures are present, but the surface is littered with cobbles and boulders of basalts and sandstones that should be considered as remnants of the Gms facies.

Site 2. The surface in this area is covered by a high concentration of cobbly and pebbly gravels. These gravels (Gms remnants) form also part of the topsoil. The only section exposes a 10 m succession of vertically stacked gravels of the Gms facies. The gravels are dominantly basaltic, with an admixture of Cretaceous Bagh sandstone, found in particular around the topographic highs formed by the steeply dipping, highly fractured Bagh sandstones.

L.S. Chamyal et al./Sedimentary Geology 107 (1997) 263-279 213

Table 2 Exposure summary

Site IlO.

Distance from

apex (km) ’

Area of exposure

(m*) b

Facies percentage

Gms Sm

(%) (%)

GSh

(%) GPI GP? St (%) (%) (%)

2 2.6 125 3 7.0 130 4 12.7 512 5a 16.5 174 5b 16.5 829 5c 16.5 422 6a 23.0 128 6b 23.0 136

100 _ _

100C _ _ _ _

68 _ 25 _ 7 64 36 _ - 91 4.5 1.5 3 _

100 _ _ _ _

89 11 100 _ _

-~ ~ a Linear distance. b Length x height. c Primary depositional characteristics in 40% of area destroyed by erosion

Site 3. A well developed, 10 m thick section on the left bank of the Narmada is dominated by the Gms facies (Figs. 4A and 8; Table 2), interlayered with friable sands whose origin cannot be ascertained because of weathering and covers of reworked sedi- ments. The matrix content of the gravel is high. Ero- sion by the Narmada river has led to the formation of a bank attached bouldery lag deposit (Fig. 9B). Clast contact is poor, and nesting is absent. The maximum clast size of the lag deposit is -150 cm.

Site 4. Located 10 km northwest of site 2, the cliff section of site 4 is a 15 m thick multi-storey amalgamation of various sedimentary facies (Figs. 5

and 8; Table 2). The succession begins with the Gms facies, showing cyclic aggradation. Along with basalts, a subordinate amount of quartzites, stroma- tolitic limestones, microfolded ferruginous banded- quartzites and breccia is noticed. The coarse fraction has a maximum clast size between -50 cm and 45 cm (based on twenty counts for each layer). Where the clasts are tabular to discoidal, imbrication (to- wards the southeast) is present. Nesting of clasts occurs. The Gms facies passes upwards into the GSh facies via an interlayered St facies.

Site 5. The Tilakwada section illustrates the vari- ous characteristics of facies developments, both spa-

Fig. 9. (A) Fan apex (shown by arrow) of the Narmada alluvial fan Maximum length of clast in foreground is 150 cm.

Location is site 1. (B) Bank attached bouldery lag deposit at site 3.

274 L.S. Chamyal et al. /Sedimentary Geology 107 (1997) 263-279

Fig. lr 3. (4 Multi-storey architecture at site 5, showing interlayered Gms, Gpl and Sm facies, shown by arrows. The cross-: lobate geomc :tries of the Gms facies are indicated by dashed lines. Section is 24 m thick. (B) Amalgamation of vertically stack facies showi] ng lobate geometries of the debris-flow units. The cross-sectional lobate geometries of the Gms facies are indic dashed I lines. Location is site 5. Length of stick is 1 .S m.

jectio nal :ed G ;ms :ated bY

L.S. Chamyl et al. /Sedimentary Geology 107 (1997) 263-279 275

tially and temporally (Figs. 8 and 10; Table 2). 30 m thick bank scarps expose a succession of a ubiq- uitous packet of Gms facies. The facies is present at the base of all exposures in the area. These grav- els have lobate geometries, contained within which are lenses of the Gp? facies. The clasts have thin white veneers of calcite, which has also cemented the gravel. The clasts are basaltic and some show remnants of pre-depositional spheroidal weathering. Nesting of clasts is present. The Gpi facies occurs intercalated between Gms gravels, and progressively becomes more prominent towards the top of the sec- tion. The alluvial architecture, when traced over long exposures using panoramic photographs, reveals the major contribution of the Gms facies (Fig. 10).

Site 6. Along the banks of the river Aswan the 15 m thick succession of Gms facies (Figs. 8 and 11; Table 2) is characterized by convex upward surfaces. A major bounding surface separates two vertically stacked packages, each containing four cycles of the Gms facies. At the base of the succession a sand- sheet deposit (Sm facies) separates the underlying sheet of the Gms facies from the eight cycles of debris-flows. The dominant clast size is 10 cm. De- viation from this size occurs in the form of -16 cm tabular blocky clasts. No St and Gp:! facies are observed at this locality.

6. Discussion

The present data characterize the deposits previ- ously termed the ‘Older Alluvium’ (Allchin et al., 1978). Dated younger deposits indicate that the grav- els have formed during the Late Pleistocene (Hegde and Switsur, 1973). The gravel facies was deposited in an alluvial fan, whose apex is at Nawagam (Site 1). The geomorphic divide between mountainous hinterland and plains was provided by a segment of the Narmada-Son fault. A tentative boundary of the Narmada fan is drawn (Fig. 1B) based on the farthest exposures of the Gms facies, and the generally lobe- like planimetric geometry (Blair and McPherson, 1994a,b), so common in alluvial fans. The inferred axial length of the fan is 23 km.

The alluvial architecture was constructed by both confined and unconfined flows. Of the primary depo- sitional processes that directly contributed to aggra- dation of the fan, viscous debris-flows played a ma-

jor role, with a minor contribution by sheet-floods. Debris-flow deposits (Gms facies) make up over 70% of the alluvial architecture. In this aspect the Narmada alluvial fan may be classified as a type 1 fan (Blair and McPherson, 1994a,b). Debris-flows aggraded the fan at both proximal and distal ends. The maximum clast size progressively decreases down-fan, in agreement with the expected fall in flood velocity.

Evidence of intervening quiescent periods be- tween fan aggradation events is present in the form of large-scale planar cross-stratified gravel (Gpt) and trough cross-stratified sand (St) facies. Braided rivers with longitudinal gravel bars dominated the surface of the fan during these phases.

A major deviation from the norm is observed in the clast roundness of the debris-flow deposits. Most of alluvial-fan deposits (Larsen and Steel, 1978; Bal- lance, 1984; Costa, 1984; Blair, 1987; Hubert and Filipov, 1989; Beaty, 1990; DeCelles et al., 1991; Blair and McPherson, 1992; Brierley et al., 1993; Kumar et al., 1994) are recognized by their angular to sub-angular nature. Angularity of clasts has been stressed by Blair and McPherson (1994a), and the only exception they accommodate is a conglomeratic provenance. In tropical alluvial fan settings, however, sphericity is also attained by initial spheroidal weath- ering and subsequent abrasive rounding on river beds (Evans, 1991). Fans formed under these conditions are as a rule dominated by stream-flow processes and abundant organic detritus. Both are absent in the Narmada-fan deposits, which coupled with cal- cretization (case hardening), indicates that the de- posits were formed under semi-arid conditions. If this is true, then rounding of clasts may be affected also by other parameters. In the present case we in- voke the greatly elongated catchment area upstream of Nawagam (fan apex) as a determinant. Rounding of clasts took place when the angular fragments were transported as bed load along the lengthy course of the Narmada (feeder channel), to be temporarily arrested as channel bars upstream of the fan apex, Hemispheroidal, discoidal clasts also attest to such a mechanism. The flat base suggests that these clasts rested on a stream bed while the exposed surface of the clast was modified to its present shape by the stream flow. These subrounded clasts were then eroded from the bars, remobilized and entrained in viscous debris-flows during flash floods.

L.S. Chamyal et al. /Sedimentary Geology 107 (19971263-279

L.S. Chamyal et al./Sedimentary Geology 107 (1997) 263-279

6.1. Role of tectonics and climate in the history of the fan

Alluvial fans form primarily because the feeder channel experiences a loss in confinement, as it emerges from the mountains onto the plains. Such an abrupt loss in confinement is provided mostly by a regional fault (Bull, 1977; Blair and McPherson, 1994a,b). Such a physiographic setting is provided by the Narmada-Son fault. The inferred extent of the fan (its elongated nature in particular) is considered as evidence of the role of tectonics in providing a structural depression in the area. The debris-flows were thus not totally unconfined, but the magnitude of confinement was so low that it did not inhibit flow expansion. Although the Narmada experiences aperiodic floods of large magnitudes (Kale et al., 1994), fan aggradation is absent. The region has wit- nessed a lot of structural disturbances represented by slickensides, asymmetrical terraces, fault breccias and fissures (Bedi and Vaidyanadhan, 1983). These data in conjunction with escarpment-like banks are suggestive of a major neotectonic event in the area. Up to 10-m vertical scarps in alluvial fan successions along river banks are taken to indicate earth move- ments (Jackson and Leeder, 1994). It is proposed here that, due to the northward migration of the In- dian plate, fractures along the Narmada-Son fault were re-activated during the Late Pleistocene. Linea- ment rosettes prepared for the path of the Narmada and for the area covered by the fan show great simi- larity. This supports the view that these fracture sets served as conduits along which the Narmada was reconfined and now flows into the Gulf of Cambay (Figs. 2 and 12).

Climatic fluctuations played a significant role in the facies variations observed at all sites. The inter- vening periods between successive debris-flows are represented by riverine sediments which document time spans during which flood magnitudes were less. The role of climate in determining the alluvial architecture is exemplified at site 4, where a tran- sition from debris-flows to sheet-flows is observed. The dominance of debris-flow over sheet-flow pro- cesses is due to a fall in the clay content, all other parameters remaining constant (Blair and McPher- son, 1994a). In the present case, since the basaltic provenance remains unchanged, the variation in clay

Fig. 12. Simplified model invoked to explain the formation and erosion of the Narmada alluvial fan. (A) Formation of the fan due to flow expansion. The elongated lobe of the fan reflects the shape of a structural depression shown by horizontal lines. Diagonal lines represent highlands. (B) Generation of fractures due to the northward drift of the Indian plate. The fractures follow ENE-WSW, NNE-SSW and NW-SE trends. This stage is accompanied by vertical uplift. (C) Re-confinement of the feeder channel leading to loss in flow expansion and consequent erosion of the fan along the rejuvenated fractures. This leads to the occurrence of vertical bank cliffs.

content due to weathering of bedrock results from fluctuations in climate. A relatively moister cli- mate aided by a vegetational abundance favours the breakdown of basalt into clays. The transition from debris-flows to sheet-flows consequently documents a trend towards aridity. A similar tendency has been recorded from mainland Gujarat (Khadkikar et al., 1996), northwest of the present study area. How- ever, the dominance of debris-flows in these deposits is attributed to a long-term semi-arid climate. No signature of a clear role of tectonics in generating debris-flow events was observed. We thus agree with Frostick and Reid (1989) that since there is no record of a direct observation of a debris-flow triggered by earth movements, whereas flood generated debris- flow events have been documented, it is climate that controls the formation of debris-flow deposits.

278 L.S. Champ1 et nl./Sedimentq Geology 107 (1997) 263-279

7. Conclusions 683-698.

The present study has important bearing on both, the local stratigraphy and the alluvial fan deposi- tional environment. Our results indicate:

Allchin, B., Goudie, A.S. and Hegde, K.T.M.. 1978. Prehistory and Palaeogeography of the Great Indian Desert. Academic Press London, 370 pp.

(1) The deposits referred to as the ‘Older Allu- vium’ are alluvial-fan deposits. These deposits are unrelated to the present-day processes on the Nar- mada river.

Allen, P.A.. 1981. Sediments and processes on a small stream- flow dominated, Devonian alluvial fan. Shetland Islands. Sedi- ment. Geol.. 29: 3 l-66.

(2) The alluvial fan is very large, with an axial length of 23 km. The alluvial architecture is dom- inated by debris-flow deposits, which classifies the Narmada fan as a type 1 fan (Blair and McPherson, 1994a,b). The dominance of debris-flow deposits indicates that the Narmada fan formed under the semi-arid conditions that prevailed during the Pleis- tocene.

Ballance, P.F., 1984. Sheet-flow dominated gravel fans of the non-marine Middle Cenozoic Simmler Formation. Central Cal- ifornia. Sediment. Geol., 38: 337-359.

Beaty, C.B., 1990. Anatomy of a White Mountain debris-llow- the making of an alluvial fan. In: A.H. Rachocki and M. Church (Editors), Alluvial Fans-A Field Approach. Wiley, New York, pp. 69-90.

Bedi, N. and Vaidyanadhan, R., 1983. Effects of neotectonics on the morphology of the Narmada river in Gujarat. western India. Z. Geomorphol., 26: 87~102.

B&as, S.K., 1987. Regional tectonic framework, structure and evolution of the western marginal basins of India. Tectono- physics. 135: 307-327.

(3) A major deviation from the norm is the de- gree of rounding observed in the gravel clasts. This rounding is ascribed to the elongated catchment area of the Narmada river basin upstream of the fan apex.

(4) The erosion of the alluvial fan was due to a major episode of tectonic re-activation of pre- existing lineaments. This led to the re-confinement of the feeder channel, which prevents fan aggradation in spite of high discharges.

Blair, T.C., 1987. Sedimentary processes, vertical stratification sequences and geomorphology of the Roaring River alluvial fan, Rocky Mountain National Park. Colorado. J. Sediment. Petrol., 57: 845-862.

Blair, T.C. and McPherson, J.G., 1992. The Trollheim alluvial fan and facies model revisited. Geol. Sot. Am. Bull.. 104: 762-769.

(5) While tectonics was responsible for providing the basin depression and the geomorphic contrast necessary for flow expansion, climate controlled the debris-flow, sheet-flow and stream-flow processes that built the alluvial architecture of the fan.

Blair, T.C. and McPherson, J.G., 1994a. Alluvial fans and their natural distinction from rivers based on morphology. hydraulic processes. sedimentary processes, and facies assemblage<. J. Sediment. Res., A64: 450489.

Blair, T.C. and McPherson. J.G.. 1994b. Allu\lal fan processes and forms. In: A.D. Abrahams and A.J. Parsons (Editors). Geomorphology of Desert Environments. Chapman and Hall. London, pp. 354302.

Acknowledgements

The authors would like to thank Prof. S.S. Merh for helpful suggestions and discussions. Mr. K.M. Makwana and Mr. N. Vankar helped in the field. The authors are thankful to A.J. Van Loon, G. Brierley and an anonymous referee for their detailed construc- tive reviews. Financial assistance through DST grant No. ESS/044/0 12/90 is gratefully acknowledged.

Blanford, W.T.. 1869. Geology of the area between Tapti and Narmada valley and the adjoining districts of Malwa and Gujarat. Geol. Surv. Ind. Mem., 6, 222 pp.

Brierley, G.J.. Liu, K. and Crook, K.A.W., 1993. Sedimentology of coarse-&rained alluvial fans in the Markham Valley. Papua New Guinea. Sediment. Geol.. 86: 297-323.

Bull, W.W., 1977. The alluvial fan environment. Prog. Phys. Geogr., I: 222-270.

Chamyal, L.S., Sharma, B., Merh. S.S. and Karami, H.. 1994. Significance of bank material at Tilakwada in Lower Narmada valley. Current Sci., 66: 306-307.

Costa, J.E., 1984. Physical geomorphology of debris-flows. In: J.E. Costa and I?J. Fleisher (Editors), Developments and Applications of Geomorphology. Springer-Verlag. Berlin, pp. 268-3 17.

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