A reconnaissance study of the mineralogy of sand fractions from East Pakistan sediments and soils

Geoderma Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

A RECONNAISSANCE STUDY OF THE MINERALOGY OF SAND FRACTIONS FROM EAST PAKISTAN SEDIMENTS AND SOILS H.G.J. HUIZING

Soil Survey Project of Pakistan, Dacca (East Pakistan) (Received August 31, 1970) ABSTRACT Huizing, H.G.J., 1971. A reconnaissance study of the mineralogy of sand fractions from East Pakistan sediments and softs. Geoderma, 6: 109-133. Two main sand provinces comprising nearly all the sediments of East Pakistan can be recognized on the basis of their sand mineralogy. One province contains most of the folded Tertiary sedimentary formations bordering East Pakistan in the north, east and southeast as well as old clays, "Madhupur clay", which occupy extensive terrace-like areas in the centre and west. These sediments are characterized by low contents of feldspars and micas and by high contents of epidote, zircon, tourmaline, kyanite, staurolite and sillimanite in the heavy sand fraction. Except perhaps for the youngest sedimentary formations of this group which are of Pliocene-Early Pleistocene age, the sediments of this main province probably mainly originated from the crystalline rocks of the old Indian peninsula. The second main sand province comprises the recent and subrecent floodplain deposits of the Brahmaputra, Ganges, Meghna, Tista and some smaller rivers. The sand fractions from this unit are characterized by relatively high contents of feldspars, micas and amphiboles. They have probably mainly been derived from crystalline rocks in the Himalayas. This sand province can be subdivided into a number of smaller provinces on the basis of the relative importance of different heavy minerals. Each of these sub-provinces generally corresponds with one or more physiographic units recognized during soil surveys carried out in East Pakistan (Brammer, in preparation). Soil formation and weathering have had little influence on the composition of the sand fraction of East Pakistan soils. Even in the oldest softs sampled which axe young, latosolic soils developed in Madhupur clay deposits, relatively easily weatherable minerals such as feldspars occur in similar amounts as in the unweathered parent material. Biotite is the only mineral which shows clear signs of weathering in most soils. Rapid weathering resulting in the complete destruction of this mineral appears to take place in seasonally flooded or puddled plough layers of both floodplain and terrace soils used for rice cultivation. Similar weathering processes may have taken place in some bleached subsoil horizons developed in poorly drained Madhupur clay deposits.

INTRODUCTION

East Pakistan occupies the greater part of the Ganges-Brahmaputra delta, but also includes part of the Himalayan piedmont plain and the low western ranges of the Assam hills (Fig. 1). It is entirely underlain by sedimentary formations. Recent and subrecent floodplain deposits of the Brahmaputra, the Ganges and some other smaller rivers occupy about 70% of the Province. Unconsolidated clays which are probably of Tertiary age occupy terrace-like areas in the centre and the west. Steeply folded and faulted Tertiary

Geoderma, 6 (1971) 109-133

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STUDY OF SAND FRACTIONS FROM EAST PAKISTAN SEDIMENTS AND SOILS

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samples analysed cover the major floodplain, terrace and hill areas of East Pakistan. Most samples have been taken within 1.25 m from the surface. Raw alluvium of the major rivers was collected under water from the river bed. GEOLOGYAND PHYSIOGRAPHY The Chittagong, Tripura and Sylhet hills in the southeast, east and northeast o f East Pakistan respectively consist of folded and faulted, little consolidated and unconsolidated, Tertiary and Early Pleistocene sediments. They form hill ranges between 15 and 900 m high. Much of this hilly land has steep slopes and hill tops mostly have sharp or rounded summits. Hills and hill ranges isolated4rom the main hills are found west of Comilla (Lalmai hills) and east and south of Sylhet. Piedmont alluvium covers narrow strips at the foot of the hills. Unconsolidated Tertiary or Pleistocene clays, probably of marine origin, which are found in the central and western parts of the Province are locally called "Madhupur clay". These clays occupy uplifted and slightly tilted terrace-like areas which are about 2 - 6 m above the adjoining recent floodplain north of Dacca, but as much as 15-30 m higher near the Indo-Pakistan border in the west of East Pakistan. Large areas, especially in the part west of the Jamuna river, "Barind tract", are nearly level. The landscape north of Dacca, "Madhupur tract", is more complex: some extensive level areas occur here beside closely dissected landscapes with both narrow and broad valleys. The southeastern parts of both the Madhupur and Barind tracts dip down and are cove~ed by floodplain sediments. Three main areas of recent and subrecent deposition can be recognized in East Pakistan: an old Himalayan alluvial fan in the northwest of the Province; meander floodplains occupying extensive areas in the northeast and centre; and estuarine and tidal floodplains covering the southern part. The subrecent piedmont alluvium in the northwest has probably been deposited by braided river systems of the former Tista and of other rivers draining the adjacent Himalayas. It is predominantly sandy and generally occupies broad ridges and depressions. Part of it is believed to be of Late Pleistocene age. The meander floodplains of the Brahmaputra, Ganges, younger Tista, middle Meghna and Surma rivers occupy a usually complex landscape, comprising relatively high floodplain ridges and ridge complexes, ridge-basin landscapes and some extensive deep basin areas. The estuarine floodplain of the lower Meghna and the Ganges tidal floodplain have an almost level topography. Sediments of both the meander floodplains and the estuarine and tidal floodplains are predominantly silty and clayey. The floodplain areas have a complex sedimentary history due to the many changes in river courses that have taken place. The Brahmaputra, for instance, left its former floodplain in Mymensingh and Dacca districts and changed into its present Jamuna channel west of the Madhupur tract, and the Tista channel shifted eastward to its present confluence with the Brahmaputra in the north of the'province. 1 Both changes have taken place since 1770, the time of Rennell's surveys (Morgan and Mclntire, 1959). Fig. 2 shows the distribution of the major physiographic units recognized in East I Since October, 1968, the Tista has shown signs of resuming its pre-Rennell course down its Atrai distributary.

STUDY OF SAND FRACTIONS FROM EAST PAKISTANSEDIMENTSAND SOILS

113

Pakistan. A detailed account of the geology and physiography can be found in "Softs of East Pakistan" chapters I and 2 (Brammer, in press). SOILS Most of the hill softs occupy steep slopes. They closely reflect the nature of their parent material. They are classified mainly as Dystrochrepts in the USDA soil classification system (7th Approxamation). A wide variety of softs has been developed in the Madhupur clay. Grey, poorly drained, softs occupy the largest areas, especially in the Barind tract. A great part of them has light coloured, silty, upper horizons which possibly have been formed by destruction of clay due to alternate seasonal reduction and oxidation and the accompanying leaching of cations under hydromorphic conditions (Brinkman, 1970). These softs are provisionally classified as Haplaquepts but may need reclassification when the process producing the albic material is better understood. On better drained sites, red, latosolic softs have been developed mainly in areas where the Madhupur clay has undergone deep weathering. These soils are mainly Dystrochrepts, but some of them are marginal to Palaeustults. Softs of the floodplains are generally young. However, soft formation appears to be very rapid and most of the softs have developed sufficient structure and brown mottling in the subsoil to be classified as Haplaquepts in the 7th Approximation. A typical characteristic of most floodplain and Madhupur clay softs is the presence of a topsoil 6 - 1 0 cm thick which becomes seasonally strongly reduced due to flooding or puddling for rice cultivation. In non-calcareous softs, these topsoils are near-neutral in reaction in the seasonally reduced condition but become medium to strongly acid when dry (see also Ponnamperuma, 1964). On the Brahmaputra and Tista floodplains and on the Madhupur clay, topsoils of relatively older soils are usually significantly lighter textured than the underlying subsoils. It is believed that these more silty topsoils have been developed by the same process of clay destruction as mentioned above for the poorly drained Barind tract soils. METHODS All the samples were air dried and ground. After shaking with Calgon, the greater part of the clay and silt was removed by decantation. The sandy residue was treated with hot sodium dithionite buffered at pH 7.3 for removal of iron oxide coatings. The 50-500/a fraction was obtained by wet sieving. The pre-treated sand was mounted on a slide in Canada balsam. The heavy minerals were separated in bromoform (s.g. 2.81) and similarly mounted in Canada balsam. The approximate percentage of the different minerals was determined under a microscope by line counts of one hundred grains. A micrometer was used to determine the influence of grain size on the mineralogical composition of the heavy fraction for one sample from each of the main sedimentary provinces. The absence of cleavage and twinning characteristics in most plagioclase grains as well as their usually relatively unweathered appearance made it difficult to distinguish plagioclases of oligoclase composition from quartz. Therefore, the total feldspar/quartz and Geoderma, 6 (1971) 109-133

114

H.G.J. HUIZING

alkali feldspar/quartz ratios were determined after staining of the grains. After exposure to hydrofluoric acid vapour, one part of the sample was treated with sodium cobaltinitrite for staining of the alkali feldspars (Chayes, 1952) and another part with aluminon for staining of all feldspar minerals (Donker, 1970). The etching and staining techniques applied are those described by Van der Plas (1966). Because a high iron content prevented proper staining of some samples, only the non-magnetic fraction was used for samples containing high amounts of small iron concretions and biotite. The stained samples were examined under a microscope using incident light. The approximate alkali feldspar/quartz and total feldspar/quartz ratios were determined by line counts of about 400 grains. Biotite/muscovite ratios were determined by counts of a total of about 50 mica grains. A good agreement was found between the percentage of alkali feldspars determined tentatively by their optical properties and the percentage of grains stained lemon-yellow by cobaltinitrite. A check on the reliability of the brick-red aluminon stain could not be carried out, but the small amount of imperfectly stained grains indicates that probably all the aluminium-containing minerals, including perhaps some of the non-magnetic heavy minerals, become stained. PRESENTATION OF THE DATA Most of the sands analysed appear to have been derived from a wide variety of rocks and, in consequence, a large number of different minerals is found in their heavy fractions. During the line counts, all minerals identified were noted, where possible, under their proper name. The result was a large number of mineral species with low percentages. Therefore, the data regarding the heavy fraction presented in this paper have been simplified. Different mineral species have been grouped together to form, as far as possible, natural mineral groups. This could be done, in most cases, without much loss of information. A short description of the groups of heavy minerals recognized is given below. Non-transparent group The non-transparent group of minerals includes opaque ore minerals; opaque and nearly opaque iron oxide concretions not dissolved by the pre-treatment with sodium dithionite; strongly weathered grains of which the identity could not be established; and some aggregates, usually containing nearly opaque inclusions. Classification of these grains in different groups appeared not to be useful because of the large number of intergrades which were difficult to classify properly. The percentage of this group has been noted as percent of the total heavy fraction. Biotite group The biotite group includes brown and green biotites as well as some chlorite grains. Its percentage has been noted as percent of the total heavy fraction and not as percent of the diagnostic group of clear grains. This has been done because the separation of biotite by bromoform appeared not to be complete, part of the grains remaining in the light fraction.

STUDY OF SAND FRACTIONSFROM EAST PAKISTAN SEDIMENTSAND SOILS

115

Furthermore, the biotite content of most sediments is, extremely variable and probably mainly depends on small changes of environment during the deposition. This can be seen in finely stratified floodplain sediments in which thin layers, often less than 1 turn thick, are darker coloured due to a high concentration of dark-coloured micas. Zircon and tourmaline group The zircon and tourmaline group includes both zircon and tourmaline grains. Zircon is usually colourless. A zoned variety of zircon, often pink coloured, frequently occurs in Tertiary sediments and in the Madhupur clay. Tourmaline grains with green, brown and blue types of pleochroism are found. Zircon and tourmaline have been grouped together because of their relatively constant ratio which appears not to depend on the total percentage of these grains. The zircon/tourmaline ratio is between 1/1 and 2/1 in most samples. Slightly lower ratios are found in relatively coarse grained sands. Titaniferous group The titaniferous group includes anatase, rutile, sphene and some rare grains o f brookite. The proportion of this group is generally below 10%, but higher contents (mainly of rutile and anatase) are found in some heavy sand fractions from Tertiary formations and from the Madhupur clay. Sphene is the dominant titaniferous mineral in recent and subrecent floodplain deposits of the main rivers. Kyanite group The kyanite group includes kyanite, staurolite and in some samples a few grains of andalusite. These highly metamorphic minerals are grouped together because, independent on their total percentage, their ratio appears to be relatively constant. Sillimanite Sillimanite includes both single sillimanite grains and fibrous sillimanite aggregates. Amphibole group The amphibole group includes all amphiboles, from almost coloudess grains to dark coloured grains with a strong pleochroism. Green amphiboles predominate, but a few brown amphiboles are also found in some samples. Amphiboles with a bluish pleochroism occur, but are rare. Carbonate group The carbonate group mainly includes dolomite. A few grains of siderite and calcite may also occur in the heavy fraction and have been included in this group. Geoderma, 6 (1971) 109-133

116

H.G.J. HUIZING

Miscellaneous This group includes accessory minerals like apatite, cassiterite, corundum, monazite (in some Ganges sediments and in beach sands south of Chittagong), olivine, spinel and grains which could not be identified with certainty. THE COMPOSITIONOF THE HEAVY FRACTION The samples used for analysis of the heavy fraction have mainly been taken f r o m the substratum of soils, but some subsoil samples have also been included. The possible effect of soil formation on the heavy mineral composition has not been considered but is believed to be negligible in most soils. Although the data given below are based on analysis of the 50-500/a sand fraction, only those samples have been used for comparison and classification of sediments which are predominantly composed of grains smaller than 200/a. This has been done because in the coarser fractions some minerals are more frequent than in the fine fractions (see Fig. 3) which makes comparison between coarser and finer grained samples difficult in m a n y cases. Moreover, relatively coarse sands are rather exceptional in East Pakistan and not available in each sedimentological unit. Except for carbonate minerals in Ganges alluvium, no minerals were found that could be used as "key minerals" for characterization of a certain group of sediments. Classification of the sediments into sand associations, therefore, has mai~y been based on the relative importance of different heavy minerals. One or more of these associations, each usually depending on the grain size of the sediment, generally characterizes a sand province, i.e., a group of sediments with common origin. These sand provinces closely correspond to the main physiographic units recognized during soil surveys in East Pakistan (Brammer, in preparation). The composition of the heavy mineral fraction from the major groups of sediments in East Pakistan is discussed below.

Tertiary and Early Pleistocene formations in the hills and associated piedmont deposits Most of the investigated samples from Tertiary and Early Pleistocene sediments have been taken from the usually gravelly C horizons of soils developed in unconsolidated and little consolidated shales and sandstones. The heavy mineral fractionfrom these samples is generally dominated by one or more of the following minerals: zircon, tourmaline, kyanite, staurolite and epidote (Table I). The total proportion of these minerals is, in most samples, more than 80% of the transparent grains. The individual percentages o f these minerals, however, vary from sample to sample. This appears partly to be due to variations in grain size composition. Most of the sand fractions examined were poorly sorted and contained, besides a bulk of very fine and free sand, a varying (in some samples relatively high) amount of grains coarser than 200/a. An example of the influence that grain size can have on the composition of the heavy minerals is shown in Fig. 3 for a weathered sandstone sample from hills north of Mymensingh. In this sample, zircon, tourmaline and epidote, predominate in the finest fractions, whereas kyanite and staurolite are relatively concentrated in the coarsest fraction.

STUDY OF SAND FRACTIONS FROM EAST PAKISTAN SEDIMENTS AND SOILS

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TABLE I The heavy mineral composition of sand fractions from the substratum of soils developed in TertiaryEarly Pleistocene sandstones and shales and associated piedmont deposits Heavy mineral association t

Nature and location of samples and age of associated rock formations 2

No. of samples investigated

Main components 1 (%)

Other components 1,3 (%)

AE

unconsolidated Pleistocene sand; isolated low hills near Sylhet

1

A 55 E30

035 K 8 B 4

EA

weathered Pliocene-Pleistocene sands/sandstones; hills north-west of Mymensingh and Lalmai hills

2

E 45--60 A15--30

piedmont deposits near Comilla and Chittagong

2

O B K S G Z

A 55--70 Z10--30

O 40-80 E 4-10 G 0-8 B 3-6 Ti 176

E 30--55 Z10=25 K 10--45

O 45-70 A 2-14 Ti 1 - 9 B 0-3 (11 in piedmont deposit north of Mymensingh)

Z 25-45 E 15-25 K 15-30

O 65-90 Ti 7 - 1 3 S 0-5 B 0-2

Z 35-65 Til0-15 K 5-30

O 75-80 E 4-8 S 0-7 A 1-6

AZ

sediments of the Someswari river and associated piedmont deposits (north of Mymensingh)

EZK

weathered Miocene-Pliocene shales and sandstones; Chittagong hill tracts and hills north of Mymensingh

4

unweathered, unconsolidated, Miocene-Pliocene sand; Chittagong coastal area

1

piedmont deposits north of Mymensingh and south of Sylhet

2

ZEK

red, unconsolidated, Miocene-Pliocene 1 deposits; n o r t h of Mymensingh piedmont deposits north of Mymensingh

ZTiK

red-coloured, weathered material from sandstones and shales; Chittagong hill tracts and north Mymensingh piedmont deposit north of Mymensingh; derived from Oligocene formations?

1

30-50 5-15 5-15 1-6 0-6 0-5

I Legend of symbols used: A = amphibole group; B ~ biotite group; D = dolomite; E = epidote group; G = garnet; K = kyanite and staurolite group; 0 -- opaque group;P -- pyroxene group; S = siUimanite; Ti = titaniferous group; and Z = zircon and tourmaline group. 2 Age of associated rocks as shown on geological map of lndia (Krishnan, 1956). 3 Components exceeding 5% in one or more samples and non-diagnostic minerals (symbols B and O).

STUDY OF SAND FRACTIONS FROM EAST PAKISTANSEDIMENTSAND SOILS

119

The weathered and unweathered Tertiary and Early Pleistocene sediments o f which the heavy mineral composition is shown in Table I can be roughly divided into two groups: one containing high amounts of epidote and relatively low amounts of zircon and tourmaline; and another with low epidote and high zircon and tourmaline contents. The difference in composition between these groups cannot be explained by differences in grain size. Neither could any correlation be found between the composition of the samples and the geological formations to which they belong according to the geological map of India (Krishnan, 1956). A close relation exists, however, between the colour of the deposits and their zircon+tourmaline/epidote ratio. All samples taken from redcoloured Chorizons have very high zircon and tourmaline contents; and all samples from unweathered sediments and yellow-brown C horizons have relatively low zircon and tourmaline and high epidote contents. The relative concentration of zircon and tourmaline in the red Chorizons has apparently taken place by weathering of the epidote minerals. However, it is not yet known whether the red-coloured C horizons have been formed in beds which were deposited as red sediments or by weathering of the sediments after their deposition. The occurrence of relatively unweathered red sandstones under one of the Chorizons might suggest that part of the materials investigated were already red at the time of deposition. Relatively high amphibole and biotite contents are found in samples from hills northwest of Mymensingh, east of Sylhet and west of Comilla. The deposits forming these hills belong to the Dupi Tila and Diking formations which are of Upper Miocene to Early Pleistocene age (Krishnan, 1956). They probably represent the youngest deposits of the Tertiary and Pleistocene group of sediments analysed. Piedmont deposits at the foot of the hills contain the same heavy mineral associations as are found in the Tertiary and Early Pleistocene rocks. High zircon contents are found in piedmont deposits at the foot of hills north of Mymensingh and high amounts of epidote in piedmont deposits at the foot of the Tripura hills and in piedmont deposits near Chittagong. However, deposits of the Someswari river and associated piedmont deposits (north Mymensingh) have heavy fractions which are dominated by amphibole. Deposits of this river and probably also of other rivers which penetrate deeply in the crystalline centre of the Shillong plateau are characterized by an amphibole-zircon association.

Madhupur clay The Madhupur clay can be divided into an eastern and western mineralogical province, each containing different heavy mineral associations. The eastern area includes the whole of the Madhupur tract and the eastern part of the Barind tract (see Fig. 2). It contains heavy mineral associations similar to those of the Tertiary sediments described above. The total percentage of zircon, tourmaline, kyanite, staurolite, sillimanite and epidote is more than 80% in all the samples analyzed from this area (Table II). Zircon+tourmaline/ epidote ratios are very low in most samples from this area but rise.to about 1 in most of the samples from red-coloured, deeply weathered, Madhupur clay deposits. The highest zircon+tourmaline/epidote ratio is, however, found in a sample taken from relatively unweathered Madhupur clay. Most of the samples analysed are not better sorted than the samples from the Tertiary

Geoderma, 6 (1971) 109-133

120

H.G.J. HUIZING

TABLE II The heavy mineral composition of sand fractions from the substratum of soils developed in t h e Madhupur clay and from sandy deposits below the Madhupur clay Heavy mineral association 1

Location and nature of the samples

No. of samples investigated

EZK

Madhupur tract and eastern part of 8 the Barind tract; relatively unweathered substratum of soils

Main components 1 (%)

Other components 1,2 (%)

E 35-60 Z 8-20 K10-20

O S A G Ti B

30-90 3-12 2-12 1-8 1-6 0-3

E 20-35 Z 20-35 S 10-25 K 15-25

O Ti A B

50-70 2-9 2-6 0-5

E 25-35 K 10-30

O 30-60 S 5-9 (33 in 1 sample) Z 1-7 A 2-5 (30 in 1 sample) B 1-2

deeply weathered substratum

2

Madhupur tract and eastern part of Barind tract; relatively unweathered substratum of soils

1

deeply weathered substratum

5

ESK EK EA

Madhupur tract; deep sandy layers underlying Madhupur clay

3

EGSK

western part of Barind tract; relatively unweathered Madhupur clay

E G S K

15-35 20-25 10-20 10-25

O 30-40 Z 2-10 A 4-9 B 0-4

SZK

western part of Barind tract; relatively unweathered Madhupur clay

S 30-55 Z10-15

O 40-55 G 1-15 E 3-9 Ti 1-7 B 3-5

EZSK

K 25

GS

Barind tract; deep sandy layers underlying Madhupur clay

1

G 60 S 20

O 10 K 10 B 7 A 6

i For legend of symbols, see Table I. 2 Components exceeding 5% in One or more samples and non-diagnostic minerals (symbols B and O). and Early Pleistocene formations; almost all o f them contain an important a m o u n t o f sand coarser than 200/a, beside a bulk o f very free material. As in the Tertiary d e p o s i t s , the highest amounts o f kyanite and staurolite are found in the coarsest fraction ( F i g . 3). Sfllimanite also occurs in significantly higher amounts in the coarser fractions. The western area has other heavy mineral associations. In the southern part o f t h i s area,

STUDY OF SAND FRACTIONSFROM EAST PAKISTAN SEDIMENTSAND SOILS

121

relatively high percentages of garnet and sillimanite are found besides a rather high epidote content; further to the north, in Dinajpur district and in the western part of Rangpur district, the Barind deposits contain only small amounts of epidote and have sillimanite as the dominant heavy mineral. In all of these deposits, a relatively high amount of kyanite and staurolite is found. Little is known about the substratum of the Madhupur clay but the information available suggests that the clay is generally underlain by more silty deposits at a depth of 6-15 m. These again are underlain by layers of fine and coarse sand which at depths between 30 and 100 m contain pebble beds. Two samples taken at depths of 15 and 100 m below the surface of the Madhupur clay north of Dacca have a heavy mineral composition similar to that of the surface deposits. A third sample taken 30 m below the surface in the same locality contains a high amount of amphiboles (about 30%). A coarse sand sample taken from a depth of about 35 m from a deep tube-well boring south of Dinajpur town in the extreme north of the Barind tract contains a garnet-sillimanite heavy mineral association which correlates satisfactorily with surface deposits from the western part of the Barind tract. Floodplain sediments

Common characteristics of the floodplain and river-bed deposits of the major rivers are a high amphibole content and a variable, but usually high, biotite content in the heavy sand fraction. The influence of grain size on the heavy mineral composition of the sand (see Fig. 3) is less striking in these deposits than in the older formations described above, but the same trends can be recognized: higher contents of zircon+tourmaline and epidote in the finer fractions and the highest amount of kyanite, staurolite and sillimanite in the coarsest fraction. Amphiboles do not show a significant preference for a particular grain size. Garnet is more abundant in the coarser fractions. Table III shows the composition of heavy sand fractions from Brahmaputra, Ganges and Tista floodplain sediments and from floodplain deposits of smaller rivers. Sediments with approximately the same composition are grouped together in sand associations. One or more of these sand associations form a sand province, i.e., a group of sediments with common origin. These sand provinces are discussed below and are correlated with the main physiographic units recognized in East Pakistan. Brahmaputra - Meghna province

The Brahmaputra-Meghna province comprises young Brahmaputra (including Jamuna) floodplain sediments; old Brahmaputra sediments; old Meghna estuarine sediments; middle Meghna sediments; lower Meghna river and estuarine sediments; beach sand deposits from the southern shore of Patuakhali; and at least part of the Karatoya floodplain sediments. This province is characterized by an amphibole-epidote heavy mineral association. Amphibole is the dominant mineral and occupies about 45-70% of the heavy fraction. The percentages of epidote range from 15 to 30. In all Brahmaputra and Jamuna samples analysed, the epidote/garnet ratio is higher than 1. Geoderma, 6 (1971) 109-133

122

H.G.J. HUIZING

TABLE III The heavy mineral composition of sand fractions from floodplain deposits Heavy mineral associations 1

Floodplain

No. of samples investigated

Main components 1 (%)

Other c o m p o n e n t s 1,2 (%)

AE

young Brahmaputra and Jamuna

8

Karatoya

1

A 50-70 E 15-30

beach sand, Patuakhali district

1

old Brahmaputra and old Meghna estuarine

12

O 10-30 B 5-30 G 1-14 Z 1-8 K 1-8 Ti 1 - 7

middle Meghna

3

lower Meghna

3

Ganges (east of Jamuna river)

1

A 45-60 E 15-25

B 5-60 O 5-30 G 6-14 Z 5-10 K 0-8 D in 3 samples

AGd

Ganges

15

A 25-40 G 20-40

O 10-30 B 7-25 K 5-12 E 5-12 Z 1-9 P 1-8 D 1-8 Ti 1 - 7 S 1-5

AGE(d)

Ganges (part of floodplain in Kushtia district)

3

young Mahananda (Rajshahi district)

1

A 30-40 G 1 5 - 25 E 15-20

B 5-.40 O 15-30 K 5-10 D 1-10 P 0-9 Z 2-7 Ti 2 - 9

AGEZ

old Mahananda (Rajshahi district)

1

A 20 E 20 G 25 Z15

O 30 B 10 K 9

AGS(d)

Tista (fine sand) Atrai

9 2

A 25-45 G 15-25 S 10-25

Ganges, (part of floodplain in Ku shtia district)

2

B 45-75 (10-30% in samples from n o r t h w e s t e r n area) O 10-20 E 5-13 K 2-12 Z 1-10 P 1-9 Ti 3 - 7 D in samples of A t r a i and Ganges alluvium

AE(d)

STUDY OF SAND FRACTIONS FROM EAST PAKISTAN SEDIMENTS AND SOILS

123

TABLE III (continued) No. of samples investigated

Main components 1 (%)

Other components 1,2

Tista (very fine sand)

2

A 40-50 S 15-20

B K G Z E O

EZG

eastern Surma and Karnaphuli

2

E 25-30 Z 20 G 11-15

O 30-65 A 8-12 Ti 7 - 9 K 6-8 B 2-7

AEGK

Matamuhari and beach sands south of Chittagong (fine to medium sands)

2

A 30-40 E 15-30 G15 K 10-20

O 20-35 Z 3-6 B 0-1

AE&EA western Surma and Sylhet basin

6

A 25-55 E 20-40

O 15-80 Z 5-15 K 2-11 G 2-11 B 1-10 Ti 1-6

AEGD

Indus (West Pakistan)

1

A 45 D15 E 15

O B P G

AGPEd

Seti (Nepal)

1

A 20 P 20 G 20 El0

O 25 B 20 D 8 Z 7 Ti 6 K 5

Heavy mineral associations 1

Floodplain

ASd

(%)

55-70 9-14 4-10 5-8 4-8 2-5

20 12 7 4

I For legend of symbols, see Table I. 2 Components exceeding 5% in one or more samples and non-diagnostic minerals (symbols B and O). d = small amounts of dolomite in all samples examined; (d) = small amounts of dolomite present in most of the samples examined.

Deposits of the middle Meghna floodplain are strikingly similar to old Brahmaputra sediments in heavy mineral composition. This confirms field evidence that deposits of this river are mainly reworked old Brahmaputra sediments. A sample from the Karatoya floodplain in Pabna district is also similar to old Brahmaputra alluvium in mineralogical composition. Part of the sediments of this floodplain were Geoderma, 6 (1971) 109-133

124

H.G.J. HUIZING

probably deposited by distributaries of the old Brahmaputra river before this river was diverted into its present Jamuna course. The lower Meghna river and estuarine floodplain deposits are very silty and usually contain only a very small amount of sand. The sand fraction from these sediments is usually dominated by micas. The three samples examined all contain an amphiboleepidote heavy mineral association. Two of the three samples contain a few grains of dolomite. The relatively high epidote/garnet ratio of the samples indicates that the major part of the sand is probably of Brahmaputra origin, and the presence of dolomite that some Ganges sediment has been admixed. A beach sand deposit from the southern shore of Patuakhali district also contains an amphibole-epidote heavy mineral association indicating that the sand is mainly of Brahmaputra (Megtma) origin.

Ganges province The Ganges province includes Ganges floodplain sediments (except for some deposits in Kushtia and Jessore districts) and Mahananda alluvium. The Ganges province is characterized by amphibole-garnet and amphibole-gametepidote heavy mineral associations. Diagnostic is the presence of small amounts of dolomite in all unweathered and part of the weathered samples. The amphibole content ranges from 25 to 40%; the garnet content is usually higher than 20%, also in relatively fine-grained samples. The epidote content is less than 10% in most samples, but higher contents (up to about 20%) are found in some old Ganges deposits in Kushtia and Jessore districts. Ganges deposits can be differentiated from Brahmaputra deposits by their lower content of amphiboles, their generally low epidote/garnet ratio (less than 1 in all samples examined) and by the presence of carbonate minerals. Relatively high contents of sillimanite and biotite are found in some old sandy deposits in Kushtia district. These non-calcareous sands may be Tista alluvium deposited at a time when the Ganges flowed west of the present Indo-Pakistan border, possibly in the Late Pleistocene. In Table III, these deposits are grouped with the heavy mineral associations characteristic for the Tista. Sediments containing carbonates but with an epidote/garnet ratio higher than 1 are found east of the Ganges-Jamuna confluence. This composition may indicate that these sediments are of mixed Ganges~ old Brahmaputra and possibly Tista origin, which confirms field evidence. Young floodplain deposits of the Mahananda river in Rajshahi district contain an amphibole-garnet-epidote heavy mineral association similar to the one found in some old Ganges deposits in Kushtia district. Older deposits of the same river differ from the younger deposits in their relatively high zircon content (about 15%). The Mahananda floodplain sediments are probably mixed and inter-stratified Ganges and Tista deposits. The clay and fine silt sediments of the tidal floodplain in the southern and southwestern part of East Pakistan are presumably mainly Gangetic deposits. The sand content of these sediments, however, is so low that analysis of representative sand fractions was not possible.

STUDY OF SANDFRACTIONSFROM EAST PAKISTANSEDIMENTSAND SOILS

125

Tista province The Tista province includes old Himalayan piedmont, Tista and Atrai sediments and some sandy deposits from the Ganges floodplain in Kushtia district. It is characterized by an amphibole-garnet-sillimanite association for the fine sand and an amphibolesillimanite association for the very fine sand samples. Sediments of the Tista and Atrai have very high contents of biotite in the heavy fraction (45-75%). Old Himalayan piedmont deposits contain less biotite (10-30%). Some samples of Tista and Atrai alluvium contain a few grains of dolomite. Sediments from the Tista province can be differentiated from Ganges alluvium by their higher content of sillimanite and usually very high content of biotite in the heavy fraction; from Brahrnaputra alluvium by their low epidote and usually very high biotite content.

Eastern Surma-Karnaphuli province The sediments of this province have been derived from Tertiary and Early Pleistocene formations in the upper courses of these rivers. On entering East Pakistan, the Surma river has relatively high contents of both zircon and epidote and a rather high garnet content. This composition is rather similar to that of the Tertiary deposits described above, the garnets probably being derived from Eocene limestones in Assam. The heavy fraction of the Karnaphuli river sand is also dominated by epidote and zircon indicating that the sand has been derived from Tertiary formations. The heavy fractions of sands from the Matamuhari river (which drains the southern part of the Chittagong hill tracts) and beach sand deposits south of Chittagong contain relatively high amounts of amphibole, epidote, garnet, kyanite and staurolite. These deposits also, are probably mainly derived from Tertiary formations.

Western Surma-Sylhet basin province The western Surma-Sylhet basin province is characterized by amphibole-epidote and epidote-amphibole heavy mineral associations. The percentage of amphibole ranges from 25 to 55%; the epidote content from 20 to 40%. Epidote and zircon contents in most of the analysed samples are significantly higher than in old Brahmaputra alluvium. The origin of the western Surma and Sylhet basin deposits is not certain. They may be partly reworked old Brahmaputra sediments; or they may be a mixture of reworked old Brahmaputra sediments, Surma sediments and deposits from rivers such as the Someswari which penetrate deeply in the Shillong plateau and contain relatively high amounts of amphibole and zircon (see Table I, amphibole-zircon association). THE COMPOSITIONOF THE TOTAL SAND FRACTION The sand fractions from the Tertiary formations and their associated piedmont deposits are rich in quartz and relatively poor in feldspars and micas. The quartz content ranges from 70 to 90% in most samples, the feldspar (mainly alkali feldspar) content ranges from 1 to about 10%. Muscovite and biotite are present in most samples but their percentage

Geoderma, 6 (1971) 109-133

126

H.G.J. HUIZING

generally does not exceed 5. Heavy mineral contents in all samples investigated are lower than 3%. High contents of nearly opaque aggregates, probably with an iron oxide cement, are found in some samples. The total content of relatively easily weatherable minerals (feldspars and biotite) is generally lower than 10%. Higher feldspar contents (up to about 15%) are found in samples from isolated hills near Sylhet and in piedmont deposits near Chittagong. The total percentage of relatively easily weatherable minerals exceeds 15% in these samples. Floodplain deposits of the Surma and Karnaphuli rivers have feldspar and mica contents similar to those of the Tertiary formations. Their content of easily weatherable minerals is about 10%. Most sand fractions from Madhupur clay deposits are similar in composition to the sand fraction of the Tertiary rocks, except that slightly higher feldspar and mica contents are found in the western part of the Barind tract. The total content of easily weatherable minerals ranges from 4 to 9% in sand fractions from the Madhupur tract and the eastern part of the Barind tract, from 8 to 14% in samples from the western part of the Barind tract. Floodplain deposits of the main rivers are relatively rich in feldspars and micas. Their sand fraction usually contains between 15 and 30% feldspars. Alkali feldspars and plagioclases occur in most samples in about equal amounts. The mica content is variable and depends mainly on the grain size of the sediments. Most samples contain between 5 and 30% micas, but in some silty deposits the mica content of the sand fraction can be as high as 80%. In most Ganges deposits, about 45-60% of the mica grains are biotites; in Brahmaputra and Tista deposits, biotite grains prevail over muscovite and occupy about 65-75% of the total mica fraction. The heavy mineral content of most floodplain sediments ranges from 2 to 9%. Aggregates are more abundant in coarser sands. The content of relatively easily weatherable minerals ranges from about 25-40% in most samples. SOIL FORMATION AND COMPOSITIONOF THE SAND FRACTION In none of the soils investigated do weathering and soil formation seem to have reached the stage where they have appreciably affected the feldspars and most of the heavy minerals. Even in young latosolic soils developed in some areas where the Madhupur clay has undergone deep weathering, relatively unweathered alkali feldspar and plagioclase grains are found. The total feldspar contents are low in these soils, but not significantly lower than in the relatively unweathered Madhupur clay deposits. Low epidote contents in the heavy sand fraction of most of the deeply weathered Madhupur clay samples are probably due to weathering. However, it could not be established whether this weathering took place after the deposition of the clay or was caused by a local supply of more strongly weathered sediment at the time of deposition. The low epidote content in one sample from relatively unweathered Madhupur clay might favour the latter suggestion. Both in Madhuptrr clay and floodplain soils, variations in feldspar content and in composition of the heavy minerals within the same profile are believed to be due to sedimentological variations rather than to weathering. Biotite is probably the only mineral which has been appreciably affected by weathering in some soils. In some topsoils and subsoils of relatively old floodplain soils, all stages of weathering of this mineral can be observed: from fresh, transparent, brown and

STUDY OF SAND FRACTIONSFROM EAST PAKISTAN SEDIMENTSAND SOILS

127

greenish brown grains to weathered, nearly opaque, dark reddish brown grains. Biotite grains in an intermediate stage of weathering often show signs of bleaching and liberation of reddish brown iron oxides within the same grain. Weathered biotite is found in the sand fraction of all soils and parent materials investigated. Small amounts of weathered grains usually occur in raw river alluvium and in young floodplain soils. Higher amounts are found in topsoils and subsoils of some older floodplain soils. The biotite content is generally very low in sand fractions from soils and relatively unweathered parent materials of the Tertiary formations and the Madhupur clay, but grains present appear to be considerably weathered in most samples. To investigate the possible effect of weathering on the biotite content, several soil profiles and parent materials have been analysed to determine biotite/total mica ratios (Tables IV and V). In contrast to the absolute biotite content, these ratios appear to be relatively constant in unweathered parent materials: about 0.45-0.60 in Ganges alluvium; and about 0.65-0.75 in Tista and Brahmaputra alluvium. In most topsoils, however, biotite/total mica ratios are significantly lower. The lowest ratios are found in topsoils of relatively old, non-calcareous, floodplain soils which are seasonally flooded and puddled for rice cultivation. This indicates a considerable, relatively rapid, weathering which probably has taken place in only several hundreds of years. The same process of weathering may also have taken place in some bleached subsoil horizons of soils developed in the Madhupur clay, but because of the very low biotite content of the unweathered parent material this could not be confirmed. DISCUSSION OF THE RESULTS Several important sand provinces could be recognized mainly on the basis of the relative importance of different heavy minerals. These sand provinces closely correspond to the main physiographic units recognized during soil surveys (Brammer, in preparation). Two main provinces can be recognized on the basis of the mineralogical composition of the sand fraction. (1) A province comprising sediments with a heavy fraction dominated by one or more of the following heavy minerals: zircon, tourmaline, kyanite, staurolite, sillimanite and epidote. Most sediments of this main province have low feldspar and mica contents. Their content of relatively easily weatherable minerals (feldspars and biotite) is usually less than 10%. They include the Tertiary and Early Pleistocene formations, the piedmont and floodplain deposits derived from these formations, and the Madhupur clay. (2) A main province comprising sediments with a heavy sand fraction dominated by amphibole and/or garnet. Sediments of this main province usually have relatively high contents of feldspars and micas. Their content of easily weatherable minerals ranges from about 25 to more than 40% of the total sand fraction. They include the recent and subrecent floodplain deposits of the Brahmaputra, Ganges, Tista, Meghna and some other smaller rivers. Variations in the composition of the heavy sand fraction within the first main sand province are due to variations in grain size distribution of the sediments, differences in age of the formations to which the sediments belong and possibly to the degree of weathering of the sand. Very t'me and fine sands are dominated by epidote and zircon in Geoderma, 6 (1971) 109-133

128

H.G.J. H U I Z I N G

TABLE IV The total mineralogical c o m p o s i t i o n of sand fractions from different sediments Sediment

Percentage +

'~

Miocene sandstone

17

Upper Miocene-Pliocene sand Pleistocene sand P i e d m o n t deposits n o r t h Mymensingh S o m e s w a r i r i v e r sand

7

1

3

I1

1

3

+

6

18

2

2

3

11

2 i1

2 3

1 +

-

3 1

-

4

I0

9

3

-

"~1

0.04

O

"~

*

.

69

-

4

2

77

-

0.07

*

6

3

61

-

0.18

•

16

1 -

91 85

-

0.04 0.01

* *

4 1

8

2

67

-

0.13

0.90

19

P i e d m o n t deposit near Chittagong

7

1

1

2

12

4

73

-

0.18

•

18

Eastern Surma f l o o d p l a i n

9

4

3

5

6

3

70

-

0.11

*

1:1.

Kar naphuli floodplain

7

1

+

2

8

1

81

-

0.10

*

9

Beach sand south of Chittagong

18

4

13

1

7

2

55

-

0.09

*

7

Madhupur clay (Madhupur tract and eastern part Barind tract)

9 20 9 5

1 -

1 + 2 2

2 1 + 3

3 5 3 6

2 1 2 3

83 72 84 81

-

0.06 0.05 0.06 0.10

* * <0.1 *

5 6 5 9

Madhupur clay (western part B a r i n d t r a c t )

8 2

1

2 3

2 5

5 8

3 4

80 77

-

0.09 0.13

0.20

Young B r a h m a p u t r a floodplain

7 3

7 2

9 2

8 34

10 7

13 6

46 46

-

0.33 0.22

0.75 0.70

29 37

Old B r a h m a p u t r a a n d lower M e g h n a e s t u a r i n e f loodp lains

6 6 5

2 1 2

6 2 9

12 12 3

10 16 16

10 13 13

54 50 52

-

0.27 0.37 0.36

0.65 0.75 0.70

28 38 31

Ganges floodplain

6 6 6

10 I 4

5 2 5

6 28 9

7 8 8

11 13 9

53 34 58

2 8 1

0.25 0.38 0.23

0.50 0.45 0.45

25 42 22

Young Tista flo odplain Old Tista floodplain

1 8

6 8

5 4

16 12

9 12

9 11

54 45

-

0.25 0.34

0.80 0.75

31 32

Lower Meghna f loodplain

4 5

1 1

2 4

71 82

2 1

3 1

17 6

+ -

0.25 0.25

0.55 0.60

52 53

Beach sand (Patuakhali)

6

1

7

14

8

6

56

2

0.20

0.60

25

Sylhet basin

9

3

4

14

9

9

52

-

0.26

0.70

28

8 14

STUDY OF SAND FRACTIONS FROM EAST PAKISTAN SEDIMENTS AND SOILS

129

TABLE IV (continued) Sediment

Percentage +

~ 0

~tj

z < Indus floodplain (West Pakistan)

5

Bagmati river sand (Nepal) Setiriver sand (Nepal)

1 6 4 10

7

.-

E

~ 7

~ 10

- 44 12 21

~

-~

~

~

7

~

~ &

12 45

22 5 7 16

22 21

~.

~

~

~ 2~ 7

o

~ j

~ 0

~

~_

".=

~

.~

0.30

0.30

29

- 0.55 9 0.43

0.70 0.70

58 48

+ = present; * = ratio not calculated because of very low mica content. "Weatherable minerals" only include relatively easily weatherable minerals such as biotite and feldspars. the heavy fraction; sands coarser than about 200/a by kyanite and staurolite. Relatively high amounts of feldspars, amphibole and biotite are found in deposits which are probably of Pliocene and Early Pleistocene age. The heavy mineral composition of most of the Tertiary sediments is totally different from that of the recent floodplain deposits. This, and the low content of weatherable minerals, probably indicates that these sediments, except perhaps the youngest ones, are not of Himalayan origin but originated from the old crystalline massifs of the Indian peninsula. The sediments may be pre-Himalayan (deposited before the main upheaval and dissection of the Himalayas), but they may also be younger. In the latter case, they probably have been derived from the Rajmahal hills and the Shillong plateau which may once have been a continuous crystalline massif. There are indications that this massif was broken up later by a subsiding fault trough between the Madhupur tract and the Barind tract which made areas south of the massif a part of the Himalayan drainage system (Morgan and McIntire, 1959). Of the two sand provinces recognized in the Madhupur clay, the eastern one has a composition which is similar to that of the older Tertiary rocks discussed above. This is in agreement with the geological map of Pakistan (Geological Survey of Pakistan, 1964) which tentatively correlates the Madhupur clay with the Dupi Tila formation of MiocenePliocene age. The western sand province has relatively higher contents of feldspars and micas. Sand from the southern part of this province which contains relatively high amounts of garnet in the heavy fraction may have been derived for a great part from metamorphic carbonaceous shales intercalated in basic traps of the adjacent Rajmahal hills. The high sillimanite content of samples taken further to the north may have come from metamorphic siUimanite-containing Archaean rocks in eastern Nepal and Sikkim (Krishnan, 1956). The relatively low content of weatherable minerals, the clay fraction which contains appreciable amounts of kaolinite (J.M. Coleman, personal communication, 1970) and the uniformity of the deposits over long distances suggest that the area from which the Madhupur clay deposits were derived must have been a rather level landscape Geoderma, 6 (1971) 109-133

1 4 1 2 6 8 8 1 2

4 4 8 3 6 3 5 7 4 1 4 5

Ap E** C

Dhamrai series; Ap young Brahmaputra B alluvium (Mymen singh) C

Sherpur series; older Ap Brahmaputra alluvium B (Mymensingh) C

Lokdeo series; old Ap Brahmaputra alluvium B (Mymensingh) C

Amnura series; Madhupur clay (Rajshahi)

2

12 9

B C

Tejgaon series; deeply weathered Madhupur clay (Dacca)

1 -

Ap E** C

Chhiata series; Madhupur clay (Dacca)

5 4 9

Ap C

Demra series; Madhu, pur clay (Dacca)

-

o

Percentage

5 9

Soil series, parent material and location

5 8 7

8 6 8

5 4 2

3 1 2

2

3 1 2

2 1

9 4 6

12 8 15

21 37 34

5 4 2

1

1 + +

1 2

nd nd nd

nd nd nd

8 8 7

6 4 5

4 2

3 3 3

3 3

nd nd nd

nd nd nd

6 4 6

2 1 3

1

1 2

1 2

84 84 80

69 71 65

67 52 59

88 90 88

84 90

91 94 89

92 88

nd nd nd

-

-

nd

-

-

nd nd

-

0.23 0.22

0.09 0.06 0.09

0.21

0.05 0.03

-

-

0.03 0.04 0.06

-

-

0.04 0.06

-

The mineralogical composition of sand fractions from different soil horizons

TAB LE V

light coloured, silty, topsoil and subsoil grading into relatively unweathered Madhupur clay, poorly drained, Haplaquept (Pseudogley) young, latosolic, clay loam soil developed in deeply weathered Madhupur clay, well drained, Dystrochrept as Chhiata series

grey, mottled brown, silty clay loam soil with prismatic structure overlying a silt loam substratum, poorly drained, Haplaquept brown silt loam soil, moderately well drained, Eutrochrept

3 4 5 4 4 8 5 8 25 36 37 nd nd nd nd nd nd

0.I0 >0.1 0.20 0.50 0.65 0.70 0.65 0.70 0.70 0.35 0.75 0.65

dark grey, mottled brown, silty clay loam soil with prismatic structure, poorly drained, Haplaquept

light coloured, silty, topsoil overlying relatively unweathered Madhupur clay, poorly drained, Haplaquept (shallow Pseudogley)

O

o

4 5 >0.1 >0.1 >0.1

*-,

.2

Soil series descriptions

7~

N

I

',O

O

Ap B C

Ap B C

Ap B C

Sara series; old Gangeticalluvium (Rajshahi)

Ishurdi series; old Gangeticalluvium (Rajshahi)

Kaunia series; young Tistaalluvium (Rangpur)

2 8 1

3 5 2

4 2

5 8 5

1 -

1 1 1

1 1 2

1 2

7 2 5

1 2 2

2 2 4

6 6 9

21 18 50

32 22 34

11 14 10

16 21 3

nd nd nd

nd nd nd

nd nd nd

10 12 16

nd nd nd

nd nd nd

nd nd nd

12 8 13

69 72 44

63 68 52

77 76 75

73 64 81

-

+ 2 9

5 7 7

-

nd nd nd

nd nd nd

nd nd nd

0.30 0.31 0.36

0.45 0.65 0.65

0.45 0.65 0.50

0.65 0.50 0.60

0.10 0.50 0.70

nd nd nd

nd nd nd

nd nd nd

24 31 31

brown silty clay loam soil, poorly drained, Haplaquept

brown, calcareous, silty clay soil, poorly drained, Haplaquept

brown, calcareous, silt loam soil, imperfectly drained, Haplaquept

light coloured, silty, topsoil overlying a dark grey, mottled brown, silty clay loam subsoil with strong prismatic and blocky structure, poorly drained, Haplaquept

no = not determined; * = ratio not calculated because of very low mica content; ** = light coloured silty subsoil horizon which is possibly formed by destruction of clay under hydromorphic conditions. "Weatherable minerals" only include relatively easily weatherable minerals such as biotite and feldspars.

Ap B C

Ghatail series; old Brahmaputraalluvium (Mymensingh)

t"

Z

Z

o~

Z

~O

©

Z r~

>

> Z

0

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H.G.J. HUIZING

probably with mature, deeply developed, soils. This landscape may have been a part of the extensive Late Tertiary peneplain of central India, large parts of which were covered by a thick laterite cap (Krishnan, 1956). The sand from deposits of the main rivers is probably mainly derived from crystalline rocks in the Himalayas. This is suggested by the relatively high amount o f weatherable minerals such as feldspars and micas. High amphibole contents in the heavy fraction, which are characteristic of the floodplain deposits in East Pakistan, are also found in other rivers originating in the Himalayas like the Indus and the Seti river in Nepal (see Table III). Although southern tributaries of the Ganges drain a part of the Deccan trap formations in central India, no definite indication could be found that basic minerals derived from these formations contributed to the composition of Ganges sand in East Pakistan. Olivine is a rare mineral in the Ganges sediments analysed, and augite and plagioclase do not occur in significantly higher amounts than, for instance, in Brahmaputra and Tista alluvium. The relatively high amount of augite and plagioclase in sand from the Seti river in Nepal (see Tables 1II and IV) indicates that these minerals can also have been derived from basic traps interbedded in folded rocks of the Himalayas. Differences in heavy mineral composition of the major floodplain deposits in East Pakistan are believed to be caused by small differences in the geology of the catchment area of the rivers by which they have been deposited. The characteristically high epidote content of Brahmaputra sand, for instance, may be due to the contribution of sand from Tertiary rocks in Assam. The relatively high amount of sillimanite in the heavy sand fraction of most of the Tista deposits has probably been derived from Archaean sillimanite-bearing schists in eastern Nepal and Sikkim (Krishnan, 1956). The origin of the carbonate minerals in Ganges alluvium is not known. They may have been derived from Pleistocene softs with a zone of carbonate accumulation (kanker) in northern India, but also from limestones in the Himalayas (see high carbonate content of Seti river sand, Nepal) and on the old Indian peninsula. Weathering and soil formation have not affected the composition of the sand fraction in most of the soils investigated. The relatively low epidote content in the heavy sand fraction of some deeply weathered Madhupur clay sediments has probably been caused by weathering but it is not certain if this weathering took place before or after the deposition of the clay. Biotite is the only mineral which shows clear signs of weathering in most of the older soils. Rapid weathering resulting in the complete destruction of this mineral appears to take place in seasonally flooded or puddled, non-calcareous, topsoils. This rapid weathering of biotite is believed to be caused by the seasonal alternation of strong reduction and following oxidation resulting in the liberation of iron in the ferrous form and precipitation of free iron oxides. A relatively slower weathering of biotite may take place in better drained soils but this needs further investigation. ACKNOWLEDGEMENTS The author is grateful to Mr. H. Brammer for his encouragement during the work and the critical reading of the text; to the field officers of the Directorate of Soil Survey,

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Dacca, for providing most of the samples; and to Dr. H.J. MOiler for his support during experiments on staining of feldspars. This paper is published with the permission of the Food and Agriculture Organization of the United Nations. REFERENCES Brammer, H., in prep. Climate, Geology, Physiography and Hydrology, 1. Soils, 2. In: M.A. Islam (Editor), Soils ofEast Pakistan. Government of East Pakistan, Dacca. Brinkman, R., 1970. Ferrolysis, a hydromorphic soil forming process. Geoderma, 3: 199-206; Chayes, F., 1952. Notes on the staining of potash feldspar with sodium cobaltinitrite in thin section. Am. Mineralogist, 37: 337. Donker, N.H.W., 1970. A note concerning a staining method for feldspars. In: From fieM to laboratory. Fys. Geogr. Bodemk. Lab., Univ. Amsterdam, Publ., 16: 21-22. Geological Survey of Pakistan, 1964. GeologicalMap of Pakistan. Quetta, West Pakistan. Krishnan, M.S., 1956. Geology oflndia and Burma. Higginbothams, Madras, 555 pp. Morgan, J.P. and Mclntire, W.G., 1959. Quarternary geology of the Bengal Basin, East Pakistan and India. Bull. Geol. Soc. Am., 70: 329. Ponnamperuma, F.N., 1964. Dynamic aspects of flooded soil and the nutrition of the rice plant. Symp. Mineral nutrition of the rice plant., Intern. Rice Res. Inst., Los Banos, Philippines, 293- 328. Van Der Plas, L., 1966. The Identification of Detrital Feldspars. Elsevier, Amsterdam, 305 pp. Wadia, D.N., 1957. Geology of India. MacMillan, London, 3rd. ed. 531 pp.

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