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Mica in the Indus River: An Indicator of Changes inthe Depositional Environment

NAJEEB RASUL and ALI SAID BASAHAM1

Geological and Geophysical Systems,Mississauga, Ontario, Canada; and

1Faculty of Marine Science, King Abdulaziz University,Jeddah, Saudi Arabia

ABSTRACT. The abundance of mica has been used to determine theenergy levels of the depositional environments of the Indus River/Estuary in terms of the interactions between the river currents andtidal, wind and wave induced currents. Three distinct zones have beenidentified on the basis of sediment texture and mica content. Based ontextural class, the Indus River/Estuary along the 40 km stretch up-stream is subdivided into three distinct classes: sandy silt, silty clayand sand silt clay. The textural data demonstrate the hydraulic associ-ation of mica with finer sediments. The excess presence of mica in ar-eas of fine sediment and its relative abundance in coarser sedimentshave been used to identify the boundaries between the three zoneswhich are classified into environments of high-medium-low energy.Marine processes in the lower reaches (mouth) of the Indus River areresponsible for winnowing of finer sediments, leaving coarser materi-als as lag deposits. However, the presence of coarser sediments in theupper limits shows the incompetency of the river flow to transportmaterial and the waning effects of wave and wind generated currentson the depositional regime. The presence of abundant mica withinmud is attributed to flocculation induced by the mixing of fresh andsaline waters and the limit of tidal intrusion. Mica dominates in themud and mimics the depositional characteristics of clay. The distribu-tion of mica and sediment texture suggest that sedimentation is con-trolled primarily by different energy levels, flocculation, and by prov-enance (metamorphic rocks in the Himalayan range).

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J. KAU: Mar. Sci., vol. 13, pp. 77-91 (1422 A.H. / 2002 A.D.)

Najeeb Rasul and Ali Said Basaham78

Introduction

The presence of abundant mica within the Indus River sediment was reported asearly as 1893 (Oldham, 1893). Karachi, located northwest of the river mouth isconsidered a predominantly mica beach with about one half as much mica in thedunes. The coastal dunes are formed by the wind blown mica of Indus origin(Shepard and Young, 1961). Mica flakes have been used for the determinationof palaeoenvironment. The relative abundance of mica in a depositional envi-ronment has also been used to characterize areas of high and low energy (Parkand Pilkey, 1981). Large quantities of mica in a depositional basin are regardedas an indicator of a low energy environment suggesting that there is little win-nowing or by-passing of finer material, reinforcing the importance of energyfluctuation (Doyle et al., 1968; 1979; 1983; Adegobe and Stanley, 1972; Hashi-mi et al., 1978; Rothwell, 1989; Rasul, 1992; 2002). Because of the flaky natureof mica, fine sand-sized mica (125-250 µm) is considered as the hydraulicequivalent of the mud fraction of other minerals including clay minerals (whichhave a similar platy form to micas). Laboratory experiments using a holograph-ic micro-velocimeter developed by Carder (1978), Carder and Meyers (1979)and the techniques applied by Doyle et al. (1983) on silt to fine sand-size micahave shown that mica is the hydraulic equivalent of quartz grains 4 to 12 timessmaller. The depositional and transportational characteristics of fine sand-sizemica are similar to finer particles (Neiheisel, 1965). For example, fine to medi-um size sand size mica can remain permanently in suspension under turbulentflow conditions mimicing the behaviour of fine-grained sediments (Pomeranc-blum, 1966). In this paper, we investigate the distribution of mica in the lowerreaches of the Indus River/Estuary and relate it to the energy levels based on thesediment texture and distribution of mica.

Study Area

The Indus River is 2,900 km long and drains the arid to semi-arid mountainslopes of Pakistan before issuing into the Arabian Sea (Fig. 1). The river flow var-ies from year to year and month to month because of the variability in both snow-melt and rainfall, depending on the geographical position, monsoon season andman-made structures including dams, barrages and irrigational channels. TheMangla Dam, built on the Jhelum River in 1967, the Tarbela Dam on the IndusRiver near Darband, built in 1974, and a considerable number of barrages and irri-gational channels have diverted most of the water for agricultural purposes, thusgreatly influencing the sediment yield to the Arabian Sea. During the last 60 yearsthere has been a considerable decrease in the sediment load of the Indus River thathas dropped from 675 million tonnes to less than 50 million tonnes per annum(Milliman et al. 1984; Ittekot and Arain, 1986). There is an appreciable inflow of

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FIG. 1. The Indus River and its tributaries. The Mangla and the Tarbela Dams have reduced thesediment yield since their inception.


sediment-laden water to the Arabian Sea when the snow melts in the southwestmonsoon, in the upper reaches of the Indus River valley in the Himalayas. The riv-er flow is generally low in November to mid spring while the discharge is high inJuly and August. In general the water supply is low rest of the year.

The geology of the catchment area of the Indus River consists of a terrain ofintricately folded and faulted Palaeozoic to Tertiary sediments (Arain and Khu-hawar, 1982), together with a wide variety of both igneous and metamorphicrocks (Gansser, 1964; Ahmad et al. 1976; Desio, 1979 and Sillitoe, 1979), most-ly of Paleozoic age (?) (Ahmad et al.,1976). In addition, there are both large andsmall scale outcrops of intrusive, acid and intermediate and ultra mafic rocks(Ahmad et al., 1976). The abundance of metamorphic minerals has been report-ed by Chaudri (1971 and 1972), Raju (1972) and Abid et al. (1983) in the Indussource area but not in quantitative terms. Detrital input to the drainage basin isderived from the metamorphic-igneous rock exposures of the Himalayan range.

Methodology

A total of 19 surficial sediments were recovered using a Petersen grab samplerdeployed from aboard RB Mumtaz, along a section of the Indus River/Estuarywith an average water depth of 2.5 m. Samples were taken at roughly 2 km inter-vals from various sub-environments stretching from the mouth of the river/estuary to approximately 40 km upstream (Fig. 2). The procedure for particlesize analyses involved pre-treatment and dispersion of the samples as describedin British Standard 1377 (BSI, 1975). Sand (≥ 63 µm) was mechanically sievedin accordance with the method adopted by Folk (1980). Grain size of silt (lessthan 63 µm) and clay (less than 4 (m) was carried out by Sedigraph. Detrital ma-terials were identified under binocular and polarizing microscopes. Percentagesof mica and other detrital materials including the most common light mineralsquartz and feldspar were calculated on the basis of 500 grain counts. Both spe-cies of mica namely biotite and muscovite were put together for convenience.Samples where necessary, were treated with acetic acid (CH3COOH) and hydro-gen peroxide (H2O2) to remove excessive carbonate and organic material.

Results and Discussion

Initial sediment sampling of the river bed gave indications of the relationshipbetween texture and mineralogy, particularly the abundance of mica and its dis-tribution pattern with respect to the hydrodynamic regime.

On the basis of texture and mica content the 40 km reach of the river/estuaryhas been classified into Zones A, B and C. Three textural types have been iden-tified: sandy silt, silty clay and sand silt clay in Zones A, B, C respectively (Ta-ble 1; Fig. 3).

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FIG. 2. The sampling stations and the zonations of the Indus River/Estuary based on the sedimenttexture and mica content.


TABLE 1. Particle size distribution along the study reach of the Indus River.

Zone

Sand Silt Clay Texture (%) (%) (%)

A 33-41 38-54 6-29 sandy silt(38) (47) (15)

B 6-12 30-51 37-62 silty clay(9) (40) (51)

C 23-26 46-47 27-30 sandy silt clay(25) (47) (28)

Range values are given with averages in parentheses.

FIG. 3. The sediment texture in Zones A, B and C in the study reach of the Indus River.

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The ratios between sand/mud and silt/clay and mean grain size are presentedin Table 2. The distribution clearly shows the abundance of fine sediments inZone B. Mean grain size is a function of the size range of available materialsupplied to the environment of deposition and the amount of energy impartedon the sediment by the velocity, or turbulence, of the transporting medium(Folk, 1980). These data indicate that the Indus River deposits are anomalous.They differ in textural distribution from those expected from a normally flow-ing river that would have more regularity in particle size (a decreasing trenddownstream) if the discharge had been normal. The percentage of mica and mudand the ratios in the three zones are presented in Table 3 and Fig. 4.

TABLE 2. Sand/mud and silt/clay ratios and mean grain size along thestudy reach of the Indus River.

Zone Sand/mud Silt/clay Mean grain size

A 1.46-1.99 0.10-0.78 3.12-5.05(1.63) (0.32) (3.98)

B 7.73-16.27 0.73-2.04 4.64-6.30(9.64) (1.25) (5.45)

C 2.78-3.26 0.58-0.63 3.79-5.84(2.96) (0.60) (5.13)


TABLE 3. Distribution of mica in fine sand and mud:mica ratios in sed-iment along the study reach of the Indus River.

Zone Mica Mud

Mud:mica(%) (%)

A 13 62 0.21

B 36 91 0.40

C 18 75 0.25

A detailed compositional study was carried out on the sand fraction in orderto achieve the objective of relating the sediment texture and presence of micawith the energy levels in the river (Table 4). The results suggest that shell frag-ments are derived from the offshore; the plant materials are derived from thenearby land whereas mica, quartz and feldspar including the heavy minerals arethe products of the Himalayas. Although the detrital materials are originally theproduct of the Indus River, some of the minerals are derived from the reworkingof deposits on the Indus shelf and transported into the river/estuary during flood


FIG. 4. The relationship between mica and mud in Zones A, B and C of the study reach showingthe average percentages of mud and mica in the samples and the mud:mica ratios in thethree zones.

tide. Shell debris was used as an indicator of the marine environment and plantmaterial to determine the proximity of land (Table 4). The intermixing of sedi-ment from two sources is very prominent especially in Zone B where the shellfragments from the Indus shelf and detrital materials from the mountains con-verge (Rasul, 2002). Biotite and muscovite rich metamorphic rocks at thesource are the main supplier of mica to the Indus River (Ahmad et al., 1976;Ahmad 1978).

The concentration of mica in the Indus River/Estuary is greater in the 125-250 µm (fine sand) fraction than the other sand fractions. Similar findings wereobserved elsewhere by Doyle et al. (1968). The mineralogy of the fine sand inthe sample shows the abundance of detrital material (Table 5).

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In Zone B there is a strong relationship between the abundance of mud andmica (Fig. 5). Abundant mica is found within high mud concentration areas.Three distinct zones can be identified on the basis of total mica content. Theaverage amount of mica in the Indus River is 22% and the average mud/micaratio is 0.29 along the 40 km reach sampled. Therefore the presence of abundantmica in mud is significant, indicating the strong relationship between the twoand that mica content could be used as a proxy indicator of the hydrological re-gime.

The majority of the fluvial material carried by the Indus River is in suspen-sion. Sand is transported as bed-load, and forms a relatively small part of thesurficial sediment because of the incompetency of the river to transport coarsesediment and also due to the decline in sediment input caused by damming up-stream. Thus, silt and clay dominate the river bed landwards from the mouthbecause of the waning effects of currents, resulting in the deposition of mud in

TABLE 4. Mineralogy and composition of sand (≥ 63 µm) along the study reach of the Indus River.

Shell PlantMica Quartz Feldspar Others

Zone debris material(%) (%) (%) (%)

(%) (%)

A 6-9 1-2 5-16 52-68 10-12 8-11(8) (1) (11) (59) (11) (10)

B 0-8 0-9 29-46 33-39 9-14 8-16(1) (2) (36) (37) (11) (13)

C 0 0-1 18-29 56-61 9-14 4-9(23) (58) (12) (6)


TABLE 5. Mineralogy of fine sand along the study reach of the Indus River.

Zone

Mica Quartz Feldspar Others (%) (%) (%) (%)

A 7-16 57-71 14-14 8-12(13) (63) (14) (10)

B 27-45 37-46 9-18 10-18(36) (39) (13) (12)

C 15-20 60-65 10-14 8-8(18) (62) (12) (8)



quiescent conditions particularly during lag time and at the turn of the tide. Thethree zones reveal a marked difference in sediment texture indicating the vary-ing energy levels in the 40 km stretch. The varying amounts of mica and thethree distinct textural classes show a strong relationship between the sedimentveneer and the hydraulic regime. The importance of mica as an environment in-dicator and its association with finer particles in Zone B show a close relation-ship (Fig. 6). The absence of mica at the mouth of the river at station 1 in ZoneA is because of high energy levels that winnow the mica and fine sediments(Fig. 7). Similar pattern of sediment transport at the mouth of Mono Estuary isalso witnessed (Anthony et al., 1996).

In Zone B, at about 8 km upstream from the mouth of the river and furtherlandwards the sediments begin to get finer (silty clay). The decrease in the grainsize of the sediment is accompanied by an increase in mica content (averaging36% in Zone B) as compared to Zones A (averaging 11%) and C (averaging23%) in the total sand fraction.

The low values of mica in Zone A compared to Zone B do not result from adecrease in input from the source area but from high energy levels. The strongcurrents at the mouth of the estuary tend to winnow the fine material and redis-tribute the sediments either on the northwest and southeast continental shelf de-pending on the monsoonal winds (Rasul, 1992).

FIG. 5. The relationship between mud % and mica % in the three zones of the study reach of theIndus River.

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FIG. 6. Mineral assemblage from Zone B in the study reach of the Indus River. Mica and fine tex-ture indicate low energy environment.

FIG. 7. Mineral assemblage at the mouth of the Indus River. Coarser sediment indicates high ener-gy environment, winnowing of the fine sediment and mica by tidal, wind and wave gener-ated currents.


Flocculation is suggested as one of the most important factors for the highermica contents in Zone B where mud dominates. Flocculation is caused by theinteraction of saline and fresh water that takes place in Zone B. The flocs con-taining mica settle out during the turn of the tide when the current velocitydrops to zero and there is sufficient time for the flocs to settle out through thewater column onto the river bed. According to Postma (1967), during lag timesimilar processes are active in most estuarine environments and play an impor-tant role in the deposition of fine materials. Such a mechanism is believed to beoperative in the Indus River/Estuary and is used to explain the deposition ofabundant mud and mica in Zone B. The presence of excess mud and mica inZone B is thus attributed to an area of low energy and rapid deposition withminimal effect of tide and wind generated currents particularly during slackwater. The amount of mica and the sediment texture suggest that sedimentationis controlled primarily by energy levels and flocculation and secondarily by theprovenance of source material. The depletion of mica in the finer fractions sug-gests that the winnowing of fines is an important process. Conversely, the abun-dance of mica in the same size range is indicative of an environment character-ized by lower energy and recent sediment deposition.

Conclusions

Mica dominates in the mud fraction of the 40 km long, lower reach of the In-dus River and mimics the depositional characteristics of clay. The distributionof mica and sediment texture suggests that sedimentation is controlled primarilyby wave and tidal actions (different energy levels), flocculation, and by prove-nance (metamorphic rocks of the Himalayan range). The three Zones A, B andC reflect sediment deposition under different energy levels forming zones ofwinnowing (high energy), deposition-winnowing (low-medium energy) anddeposition (low energy). The greatest sedimentation in the estuary is within sev-eral kilometers of the mouth (up to 25 km upstream).

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