Sedimentology of the Narmada Alluvial Fan, Western India

Sedimentary Geology

ELSEVIER 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 occurs through a large number of low-order tributaries connected to the feeder channel of the fan. Alluvial fans owe their existence to several si- Fan deposits are built by rock avalanches, debris- multaneously acting processes. Of prime importance flows, sheet-flows and so forth, and commonly con- is an abrupt change in the regional physiographic tain subangular to angular clasts (Larsen and Steel, setting where the river becomes unconfined (Bull, 1978; Pierson, 1981; Ballance, 1984; Blair, 1987; 1977; Blair and McPherson, 1994a,b). This abrupt McArthur, 1987; Beaty, 1990; DeCelles et al., 1991; change is commonly at a fault that separates a moun- Evans, 1991; Blair and McPherson, 1992, 1994a,b; tainous hinterland from an alluvial plain. The rapid Brierley et al., 1993; Abrahms and Chadwick, 1994; high surface run-off responsible for fan aggradation Kumar et al., 1994). Most fans studied are a few kilometres long. * Corresponding author. The relative roles of climate and tectonic reju-

0037-0738/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SOO37-0738(96)00030-9 venation are much debated. Whereas some work- by a flood discharge of 60.000 m”/s (Kale et al., ers consider coarsening-upward cycles as indicators 1994). The Narmada flows through a basalt domi- of tectonic activity (Mack and Rasmussen, 1984), nated terrain (Sant and Karanth, 1993). Associated others propose a dominant role for climate in fan with these Late Cretaceous basalts (Deccan Traps) aggradation processes (Frostick and Reid, 1989). are outcrops of Cretaceous Bagh sandstones and Blair (1987) has demonstrated that the coarsening- Proterozoic quartzites (Fig. 1C). upward facies put forward as criteria to interpret structural movements may rather be generated by 3. Structural setup climate-controlled debris-flows. In the present study, we document a large alluvial fan of Pleistocene age The geomorphology of the region is controlled from the Narmada valley of western India. The fan mainly by two sets of lineaments related to the Nar- is unusual in its high proportion of subrounded to mada graben (Alavi and Merh, 199 1). A major ENE- rounded clasts, despite its non-conglomeratic prove- WSW and less prominent NNE-SSW and NW-SE nance. The relative roles of tectonics and climate in trends have been identified (Fig. 2). The Narmada the genesis and erosion of the Narmada alluvial fan graben, a deep-seated geofracture (Biswas, 1987), are evaluated. dating back to the Precambrian, extends into Mada- The gravel deposits under consideration have at- gascar (Crawford, 1978). The geofracture, which tracted the attention of many workers (Blanford, acts presently as an intra-continental rift, was an 1869; Zeuner, 1950; Wainwright, 1964; Allchin et inter-continental suture zone in the past around al., 1978) and have been referred to as the ‘Older which the Aravalli-Bundelkhand, Singhbhum and Alluvium’. The discovery of archaeological finds Dharwar crustal blocks were welded (Naqvi and has clarified their Late Pleistocene age (Allchin et Rogers, 1987; Radhakrishna, 1989). The Narmada al., 1978), which is supported by radiocarbon ages graben is a component of an 1800 km long and of 18,000 yr. B.P. for calcareous concretions in the 200 km wide zone termed the ‘SONATA’ zone stratigraphically youngest alluvial deposits (Hegde (Shanker, 1991). The Narmada graben controlled and Switsur, 1973). accumulation of the 800 m thick alluvial sediments through synsedimentary subsidence (Shanker, 1991). 2. Area of study The graben also influences the location of tributaries on the left bank of the Narmada river which flow on The study area is near Tilakwada, in the state of faulted blocks. The Narmada in its boxwork drainage Gujarat, western India (Fig. 1A). The sites (Fig. 1B) pattern reflects the dominant role of pre-existing lin- of detailed work include Nawagam (Site l), Kewadia eaments (Fig. 2). The Narmada-Son fault, a com- Colony (Site 2), Garudeshwar (Site 3), Rampura ponent of the Narmada graben (SONATA zone), (Site 4), Tilakwada (Site 5) and Gamod (Site 6). The provides the physiographic setting (inset in Fig. 1B) region has an average rainfall of 1250 mm (Kale et necessary for the development of an alluvial fan. al., 1994) and falls under a semi-arid to dry sub- humid climatic regime (Singh et al., 1991). 4. Sedimentary facies The area is transected by one of the major rivers of India: the Narmada, which debouches into the The deposits comprise five lithofacies. Their dom- Gulf of Cambay. The river follows the trend of a inance varies at each site (Fig. 3). The facies coding major geofracture known as the Narmada-Son fault, scheme (modified after Miall, 1985) used in this which causes the river to flow westwards, opposite study is shown in Table 1. to the regional slope. The Narmada river originates at Amarkantak and its catchment covers an area of 4.1. Gravel-sheet facies (Gms) 98,796 km*. During monsoonal floods, discharges range from 10,000 m3/s to 60,000 m3/s. The linear Description. The facies consists of laterally un- relationship between discharge and maximum veloc- interrupted matrix-supported gravels and is observed ity implies that a velocity of 5.5 m/s is accompanied 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

v v V I::‘.‘I Quoternary vvvvvv aDeccan Trap Basalts v v V vvvv - v v rnCLP,, o m @ Cretaceous Rocks v v v -= v ” v v v v a Precambrian Rocks v v 1); v v u v v v vv vv -i

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) sheets. The sheets vary in thickness from 0.5 to 2 and essentially polymodal. Typically 25-35% sand- m. The clasts. are discoidal, cuboidal, tabular and mud matrix is present. The larger clasts are restricted spheroidal. Wherever there is a predominance of to the upper margin of each unit. The upper and discoidal clasts, a crude imbrication towards the lower bounding surfaces are gently convex upward southeast is observed implying northwest-directed and non-erosive. However, the upper bounding sur- palaeocurrents. The clasts show a high degree of face shows greater convexity imparting the unit a rounding. The maximum clast size decreases in a convex lens morphology in cross-section. The gravel systematic manner towards the northwest from 150 unit occurs as solitary sheets and also as compos- cm to 10 cm. Another feature is the clustering of ite units built up of two or more vertically stacked 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 cate accumulation due to obstruction by a larger clast as ‘nesting’ (Allen, 1981). The basal gravels in all (Ballance, 1984) or a tendency for clasts to migrate sections are strongly cemented by calcium carbonate towards regions of least internal shear (Allen, 198 1). (case hardening of Lattman, 1973). Interpretation. The inverse grading of clasts along 4.2. Gravel/sand-couplet facies (GSh) with large clast size and protrusion of clasts at the upper bounding surface of each unit suggests that the Description. The facies consists of horizontally facies represent viscous debris-flows. Similar facies stratified gravel-sand couplets of 10 cm to 15 cm thick have been reported extensively from other alluvial (Fig. 6). The sand component of each couplet overlies fan deposits (Larsen and Steel, 1978; Pierson, 1981; the coarser fraction. The clasts are pebbles and cob- Blair, 1987; Beaty, 1990; DeCelles et al., 1991; bles of spheroidal, discoidal and cuboidal basalt. The Evans, 1991; Blair and McPherson, 1992, 1994a,b; clasts are subrounded to rounded and no distinct im- Brierley et al., 1993; Kumar et al., 1994). The main brication is observed. In the sand units pebbles are less mechanism involved for large-clast entrainment is abundant and cobbles are totally absent. The sands are a combination of dispersive pressure and buoyancy discontinuous in some cases and individual units bi- (Costa, 1984). This is reflected in the segregation furcate laterally. The gravels are unsorted, polymodal of the larger clasts within each unit away from the and lack internal stratification. The gravel/sand units lower bounding surface, a feature also recognized occur as up to seven vertically stacked cycles. by Hubert and Filipov (1989). The high clay con- Interpretation. The suite of characters shown by tent of 3% (Chamyal et al., 1994) and the clast this facies suggest that the gravel/sand couplets re- size governed the mobility of the debris-flows, the sulted from sheet-flow processes. The deposition former providing cohesive strength (by reducing per- of sand subsequent to gravel deposition implies a meability and increasing pore pressure) and the latter suspension fall-out. The couplets also suggest low determining the structural framework (Costa, 1984). turbulence in the flows that enabled the step-wise Clustering of clasts in nests of up to five may indi- deposition of gravel and sand with fall in the flow a Gravel m Sand Plol-lor Fl Cross-Stratification &7J Trough Cross-Stratification m Paloeocurrent Direction m lmbricotion Direction

Gms

-Gms @ - Gms 0 P 4

7 Im -Gms llll 1 lm I I TILAKWADA SECTION 1 TILAKWADA SECTION 2 GAMOD GARUDESWAR SITE :6 SITE: 3 SITE : 5 SITE : 5

RAMPURA SECTION SITE:4

Fig. 3. Lithologs measured at various sites showing palaeocurrent and imbrication directions (5-10 counts per level), maximum clast size and stratification types 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 1.56m. 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

Gms Inversely graded cobbly to bouldery Debris flow deposits 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.

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

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

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

Gp2 Planar cross-stratified gravel with lensoidal Rechannelized flows geometries. Normally graded and associated genetically related intimately with the Gms facies 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 1.5m

capacity. Similar couplets have been described by (Blair, 1987). The association of the Gp2 facies (see Blair and McPherson (1994a), who interpret the cou- below) and sand-sheets points to the prevalence of plets as due to changes in flow hydraulics, initiated confined flows subsequent to the deposition of the by flow expansion and decrease in slope gradient. sand sheets. Equivalent controls seem to have been responsible for the genesis of the couplets in the present case. 4.4. Planar cross-struti$ed gravel fucies (Gp)

4.3. Sund-sheet facies (Sm) The cross-stratified gravel facies occurs at two scales. At the smaller scale it contributes to the fan Description. This facies comprises laterally con- architecture as lenses, whereas at the larger scale, tinuous, internally unstratified, sheets of sand. The it occurs as pervasive, laterally continuous gravel units are usually 0.5 m thick and bounded by pla- ribbons. These two subfacies have different origins nar, non-erosive surfaces. Associated with the sheets and are described separately. are occasional stringers of pebbly gravel. The facies also occurs as sand lenses within the massive gravel 4.5. Large-scale planar cross-stratijed gravel facies deposits (Gms). (GPI) Interpretation. The sand-sheets point to sheet- flood events of low turbulence that led to the sepa- Description. The facies (Fig. 7) is bounded by ration of the suspended sand-load and its deposition subhorizontal planar surfaces. Locally these surfaces further down the lobe. Deposition resulted owing to may also be erosive. The gravel units are commonly reduction in flood velocity of the unchannelized flow 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 1.5m. erally over a distance of -50 m. The foresets dip the clasts are normally graded. The planar cross- consistently at an high average angle of 30” and have stratified beds occur as cosets as well as solitary concave-upward surfaces. Within each foreset cycle 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. 2 Interpretation. These cross-stratified gravels are a result of the downstream migration of mid-channel 3 bedforms, the downstream accretion element (DA) of Miall (1985). The high-angle foresets are the result of avalanching slip faces at the leading edge of 4 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 5a plain. Identical facies have been attributed to deposition on longitudinal gravel bars (DeCelles et al., 6b- 1991).

@ GmsFacie* 4.6. Small-scale planar cross-stratijed gravel facies 10 m Gsh Fac~es ((7~2) El Sm Fac~es t Om 10 20m = Gq FXIS Description. These pebbly, clast-supported grav- m G? Facies els are characterised by their lensoidal geometry. The lenses are distributed as solitary units within the 0 Not exposed ubiquitous Gms facies. They are usually less than Fig. 8. Schematic cross-sections of exposures showing the rel- 0.5 m thick and show an average foreset dip of 25”. ative dominance of facies. Note that the Gms facies dominates At some sites these gravels are more sandy and oc- (>60%) the alluvial architecture at all sites. cur in close association with the sand-sheet facies described earlier in the text. Interpretation. The intimate association of the 5. Local occurrences of facies and facies small-scale clast-supported gravels with the Gms associations and Sm facies suggests a genetic link. These gravel bedforms must have formed immediately after a At the sites studied, the abundance of each facies debris-flow event, during a period when the gravel- type varies. However, the Gms facies dominates the loaded flows were rechannelized as small streams. alluvial architecture as can be clearly seen from Fig. 8 and Table 2. 4.7. Trough cross-strati$ed sand facies (St) Site 1. This site is located at the geomorphic divide between the mountainous hinterland and the Description. Trough cross-stratified sand was ob- adjacent alluvial plain (Fig. 9A). No exposures are served at only one site. The unit is -2 m thick. present, but the surface is littered with cobbles and The normally graded foresets are large, concave up, boulders of basalts and sandstones that should be and tangential, dipping at an average angle of 25” considered as remnants of the Gms facies. towards the north. At the base, the unit begins with Site 2. The surface in this area is covered by small-scale sandy bedforms which are draped over a high concentration of cobbly and pebbly gravels. the underlying cobbly clasts. These gravels (Gms remnants) form also part of the Interpretation. The trough cross-stratified facies topsoil. The only section exposes a 10 m succession represents the channel fill elements of rivers (Brierley of vertically stacked gravels of the Gms facies. The et al., 1993) that dominated the fan surface during in- gravels are dominantly basaltic, with an admixture tervening quiet periods between successive episodes of Cretaceous Bagh sandstone, found in particular of fan aggradation. They are comparable with the Gpi around the topographic highs formed by the steeply facies in their mode and time of formation. dipping, highly fractured Bagh sandstones. L.S. Chamyal et al./Sedimentary Geology 107 (1997) 263-279 213

Table 2 Exposure summary

Site Distance from Area of Facies percentage

IlO. exposure apex Gms Sm GSh GPI GP? St (km) ’ (m*) b (%) (%) (%) (%) (%) (%)

2 2.6 125 100 _ _ 3 7.0 130 100C _ _ _ _ 4 12.7 512 68 _ 25 _ 7 5a 16.5 174 64 36 _ - 5b 16.5 829 91 4.5 1.5 3 _ 5c 16.5 422 100 _ _ _ _ 6a 23.0 128 89 11 6b 23.0 136 100 _ _ -~ ~ 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 and 8; Table 2). The succession begins with the the left bank of the Narmada is dominated by the Gms facies, showing cyclic aggradation. Along with Gms facies (Figs. 4A and 8; Table 2), interlayered basalts, a subordinate amount of quartzites, stroma- with friable sands whose origin cannot be ascertained tolitic limestones, microfolded ferruginous banded- because of weathering and covers of reworked sedi- quartzites and breccia is noticed. The coarse fraction ments. The matrix content of the gravel is high. Ero- has a maximum clast size between -50 cm and 45 sion by the Narmada river has led to the formation of cm (based on twenty counts for each layer). Where a bank attached bouldery lag deposit (Fig. 9B). Clast the clasts are tabular to discoidal, imbrication (to- contact is poor, and nesting is absent. The maximum wards the southeast) is present. Nesting of clasts clast size of the lag deposit is -150 cm. occurs. The Gms facies passes upwards into the GSh Site 4. Located 10 km northwest of site 2, the facies via an interlayered St facies. cliff section of site 4 is a 15 m thick multi-storey Site 5. The Tilakwada section illustrates the vari- amalgamation of various sedimentary facies (Figs. 5 ous characteristics of facies developments, both spa-

Fig. 9. (A) Fan apex (shown by arrow) of the Narmada alluvial fan Location is site 1. (B) Bank attached bouldery lag deposit at site 3. Maximum length of clast in foreground is 150 cm. 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-: jectio nal lobate geomc:tries of the Gms facies are indicated by dashed lines. Section is 24 m thick. (B) Amalgamation of vertically stack :ed G ;ms facies showi] ng lobate geometries of the debris-flow units. The cross-sectional lobate geometries of the Gms facies are indic :ated bY dashed I lines. Location is site 5. Length of stick is 1 .S m. L.S. Chamyl et al. /Sedimentary Geology 107 (1997) 263-279 275 tially and temporally (Figs. 8 and 10; Table 2). 30 jor role, with a minor contribution by sheet-floods. m thick bank scarps expose a succession of a ubiq- Debris-flow deposits (Gms facies) make up over 70% uitous packet of Gms facies. The facies is present of the alluvial architecture. In this aspect the Narmada at the base of all exposures in the area. These grav- alluvial fan may be classified as a type 1 fan (Blair and els have lobate geometries, contained within which McPherson, 1994a,b). Debris-flows aggraded the fan are lenses of the Gp? facies. The clasts have thin at both proximal and distal ends. The maximum clast white veneers of calcite, which has also cemented size progressively decreases down-fan, in agreement the gravel. The clasts are basaltic and some show with the expected fall in flood velocity. remnants of pre-depositional spheroidal weathering. Evidence of intervening quiescent periods be- Nesting of clasts is present. The Gpi facies occurs tween fan aggradation events is present in the form intercalated between Gms gravels, and progressively of large-scale planar cross-stratified gravel (Gpt) and becomes more prominent towards the top of the sec- trough cross-stratified sand (St) facies. Braided rivers tion. The alluvial architecture, when traced over long with longitudinal gravel bars dominated the surface exposures using panoramic photographs, reveals the of the fan during these phases. major contribution of the Gms facies (Fig. 10). A major deviation from the norm is observed in Site 6. Along the banks of the river Aswan the 15 the clast roundness of the debris-flow deposits. Most m thick succession of Gms facies (Figs. 8 and 11; of alluvial-fan deposits (Larsen and Steel, 1978; Bal- Table 2) is characterized by convex upward surfaces. lance, 1984; Costa, 1984; Blair, 1987; Hubert and A major bounding surface separates two vertically Filipov, 1989; Beaty, 1990; DeCelles et al., 1991; stacked packages, each containing four cycles of the Blair and McPherson, 1992; Brierley et al., 1993; Gms facies. At the base of the succession a sand- Kumar et al., 1994) are recognized by their angular sheet deposit (Sm facies) separates the underlying to sub-angular nature. Angularity of clasts has been sheet of the Gms facies from the eight cycles of stressed by Blair and McPherson (1994a), and the debris-flows. The dominant clast size is 10 cm. De- only exception they accommodate is a conglomeratic viation from this size occurs in the form of -16 provenance. In tropical alluvial fan settings, however, cm tabular blocky clasts. No St and Gp:! facies are sphericity is also attained by initial spheroidal weath- observed at this locality. ering and subsequent abrasive rounding on river beds (Evans, 1991). Fans formed under these conditions 6. Discussion are as a rule dominated by stream-flow processes and abundant organic detritus. Both are absent in The present data characterize the deposits previ- the Narmada-fan deposits, which coupled with cal- ously termed the ‘Older Alluvium’ (Allchin et al., cretization (case hardening), indicates that the de- 1978). Dated younger deposits indicate that the grav- posits were formed under semi-arid conditions. If els have formed during the Late Pleistocene (Hegde this is true, then rounding of clasts may be affected and Switsur, 1973). The gravel facies was deposited also by other parameters. In the present case we in- in an alluvial fan, whose apex is at Nawagam (Site voke the greatly elongated catchment area upstream 1). The geomorphic divide between mountainous of Nawagam (fan apex) as a determinant. Rounding hinterland and plains was provided by a segment of of clasts took place when the angular fragments were the Narmada-Son fault. A tentative boundary of the transported as bed load along the lengthy course Narmada fan is drawn (Fig. 1B) based on the farthest of the Narmada (feeder channel), to be temporarily exposures of the Gms facies, and the generally lobe- arrested as channel bars upstream of the fan apex, like planimetric geometry (Blair and McPherson, Hemispheroidal, discoidal clasts also attest to such a 1994a,b), so common in alluvial fans. The inferred mechanism. The flat base suggests that these clasts axial length of the fan is 23 km. rested on a stream bed while the exposed surface The alluvial architecture was constructed by both of the clast was modified to its present shape by confined and unconfined flows. Of the primary depo- the stream flow. These subrounded clasts were then sitional processes that directly contributed to aggra- eroded from the bars, remobilized and entrained in dation of the fan, viscous debris-flows played a ma- 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 Fig. 12. Simplified model invoked to explain the formation and and fissures (Bedi and Vaidyanadhan, 1983). These erosion of the Narmada alluvial fan. (A) Formation of the fan data in conjunction with escarpment-like banks are due to flow expansion. The elongated lobe of the fan reflects suggestive of a major neotectonic event in the area. the shape of a structural depression shown by horizontal lines. Up to 10-m vertical scarps in alluvial fan successions Diagonal lines represent highlands. (B) Generation of fractures along river banks are taken to indicate earth move- due to the northward drift of the Indian plate. The fractures follow ENE-WSW, NNE-SSW and NW-SE trends. This stage ments (Jackson and Leeder, 1994). It is proposed is accompanied by vertical uplift. (C) Re-confinement of the here that, due to the northward migration of the In- feeder channel leading to loss in flow expansion and consequent dian plate, fractures along the Narmada-Son fault erosion of the fan along the rejuvenated fractures. This leads to were re-activated during the Late Pleistocene. Linea- the occurrence of vertical bank cliffs. ment rosettes prepared for the path of the Narmada and for the area covered by the fan show great similarity. This supports the view that these fracture sets content due to weathering of bedrock results from served as conduits along which the Narmada was fluctuations in climate. A relatively moister cli- reconfined and now flows into the Gulf of Cambay mate aided by a vegetational abundance favours the (Figs. 2 and 12). breakdown of basalt into clays. The transition from Climatic fluctuations played a significant role in debris-flows to sheet-flows consequently documents the facies variations observed at all sites. The inter- a trend towards aridity. A similar tendency has been vening periods between successive debris-flows are recorded from mainland Gujarat (Khadkikar et al., represented by riverine sediments which document 1996), northwest of the present study area. How- time spans during which flood magnitudes were ever, the dominance of debris-flows in these deposits less. The role of climate in determining the alluvial is attributed to a long-term semi-arid climate. No architecture is exemplified at site 4, where a tran- signature of a clear role of tectonics in generating sition from debris-flows to sheet-flows is observed. debris-flow events was observed. We thus agree with The dominance of debris-flow over sheet-flow pro- Frostick and Reid (1989) that since there is no record cesses is due to a fall in the clay content, all other of a direct observation of a debris-flow triggered by parameters remaining constant (Blair and McPher- earth movements, whereas flood generated debris- son, 1994a). In the present case, since the basaltic flow events have been documented, it is climate that provenance remains unchanged, the variation in clay controls the formation of debris-flow deposits. 278 L.S. Champ1 et nl./Sedimentq Geology 107 (1997) 263-279

7. Conclusions 683-698. Allchin, B., Goudie, A.S. and Hegde, K.T.M.. 1978. Prehistory The present study has important bearing on both, and Palaeogeography of the Great Indian Desert. Academic the local stratigraphy and the alluvial fan deposi- Press London, 370 pp. Allen, P.A.. 1981. Sediments and processes on a small stream- tional environment. Our results indicate: flow dominated, Devonian alluvial fan. Shetland Islands. Sedi- (1) The deposits referred to as the ‘Older Allu- ment. Geol.. 29: 3 l-66. vium’ are alluvial-fan deposits. These deposits are Ballance, P.F., 1984. Sheet-flow dominated gravel fans of the unrelated to the present-day processes on the Nar- non-marine Middle Cenozoic Simmler Formation. Central Cal- mada river. ifornia. Sediment. Geol., 38: 337-359. Beaty, C.B., 1990. Anatomy of a White Mountain debris-llow- (2) The alluvial fan is very large, with an axial the making of an alluvial fan. In: A.H. Rachocki and M. length of 23 km. The alluvial architecture is dom- Church (Editors), Alluvial Fans-A Field Approach. Wiley, inated by debris-flow deposits, which classifies the New York, pp. 69-90. Narmada fan as a type 1 fan (Blair and McPherson, Bedi, N. and Vaidyanadhan, R., 1983. Effects of neotectonics 1994a,b). The dominance of debris-flow deposits on the morphology of the Narmada river in Gujarat. western India. Z. Geomorphol., 26: 87~102. indicates that the Narmada fan formed under the B&as, S.K., 1987. Regional tectonic framework, structure and semi-arid conditions that prevailed during the Pleis- evolution of the western marginal basins of India. Tectono- tocene. physics. 135: 307-327. (3) A major deviation from the norm is the de- Blair, T.C., 1987. Sedimentary processes, vertical stratification gree of rounding observed in the gravel clasts. This sequences and geomorphology of the Roaring River alluvial fan, Rocky Mountain National Park. Colorado. J. Sediment. rounding is ascribed to the elongated catchment area Petrol., 57: 845-862. of the Narmada river basin upstream of the fan apex. Blair, T.C. and McPherson, J.G., 1992. The Trollheim alluvial (4) The erosion of the alluvial fan was due to fan and facies model revisited. Geol. Sot. Am. Bull.. 104: a major episode of tectonic re-activation of pre- 762-769. existing lineaments. This led to the re-confinement of Blair, T.C. and McPherson, J.G., 1994a. Alluvial fans and their the feeder channel, which prevents fan aggradation natural distinction from rivers based on morphology. hydraulic processes. sedimentary processes, and facies assemblage<. J. in spite of high discharges. Sediment. Res., A64: 450489. (5) While tectonics was responsible for providing Blair, T.C. and McPherson. J.G.. 1994b. Allu\lal fan processes the basin depression and the geomorphic contrast and forms. In: A.D. Abrahams and A.J. Parsons (Editors). necessary for flow expansion, climate controlled the Geomorphology of Desert Environments. Chapman and Hall. debris-flow, sheet-flow and stream-flow processes London, pp. 354302. Blanford, W.T.. 1869. Geology of the area between Tapti and that built the alluvial architecture of the fan. Narmada valley and the adjoining districts of Malwa and Gujarat. Geol. Surv. Ind. Mem., 6, 222 pp. Acknowledgements Brierley, G.J.. Liu, K. and Crook, K.A.W., 1993. Sedimentology of coarse-&rained alluvial fans in the Markham Valley. Papua The authors would like to thank Prof. S.S. Merh New Guinea. Sediment. Geol.. 86: 297-323. Bull, W.W., 1977. The alluvial fan environment. Prog. Phys. for helpful suggestions and discussions. Mr. K.M. Geogr., I: 222-270. Makwana and Mr. N. Vankar helped in the field. The Chamyal, L.S., Sharma, B., Merh. S.S. and Karami, H.. 1994. authors are thankful to A.J. Van Loon, G. Brierley Significance of bank material at Tilakwada in Lower Narmada and an anonymous referee for their detailed construc- valley. Current Sci., 66: 306-307. tive reviews. Financial assistance through DST grant Costa, J.E., 1984. Physical geomorphology of debris-flows. In: J.E. Costa and I?J. Fleisher (Editors), Developments and No. ESS/044/0 12/90 is gratefully acknowledged. Applications of Geomorphology. Springer-Verlag. Berlin, pp. 268-3 17. References Crawford, A.R., 1978. Narmada-Son lineament of India traced into Madagascar. J. Geol. Sot. India, 19: 143-153. Abrahms, M.J. and Chadwick, O.H., 1994. Tectonic and climatic DeCelles, P.G., Gray, M.B., Ridgway, K.D.. Cole, R.B., Pivnik, implications of alluvial fan sequences along the Batinah coast, D.A.. Pequera, N. and Srivastava, P., 1991. Controls on Oman. J. Geol. Sot. London, 151: 51-58. synorogemc alluvIaI-fan architecture, Beartooth conglomer- Alavi, S.A. and Merh, S.S., 1991. Morphotectonic analysis of ate (Paleocene), Wyoming and Montana. Sedimentology, 38: south Gujarat landscape. Proc. Indian Natl. Sci. Acad.. 57A: 567-590. L.S. Chamyal et al. /Sedimentary Geology 107 (1997) 263-279 279

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