Evolution of the Lower Gangetic Plain landforms and soils in West Bengal, India

Catena 33 Ž1998. 75–104

Evolution of the Lower Gangetic Plain landforms and soils in West Bengal, India Lalan P. Singh a

a,),1

, B. Parkash a , A.K. Singhvi

b

Department of Earth Sciences, UniÕersity of Roorkee, Roorkee, (U.P.)-247 667, India b Physical Research Laboratory, NaÕrangpura, Ahmedabad, 380 009, India Received 14 May 1996; revised 6 May 1998; accepted 6 May 1998

Abstract Three major landforms, Uplands, Old FluvialrDeltaic Plains and Young Fluvial Plains are identified from the Lower Gangetic Plains of West Bengal, India, on the basis of remote sensing and field studies. Morphologic, quantitative and thermoluminescence studies of soils of the study area have been conducted. Two types of Uplands are recognised: Upland in the west overlain by red soils Žautochthonous and allochthonous types. and upland in the north ŽBarind Tract., characterised by three topographic levels. Aspects of the autochthonous Upland Red Soils ŽLower to Middle Pleistocene age. show the ferrugination Žferrisol. phase of development. Soils of the Old FluvialrDeltaic Plains Ž6–3 ka. and Lower Level of the Barind Tract have argillic horizons and exhibit the fersiallitisation phase of development. Development of ferrugination and fersiallitisation phases was favored due to the pre-weathered nature of the parent material. Soils of the Old FluvialrDeltaic Plains, Barind Tract ŽLower Level. and Young Fluvial Plains show effects of hydromorphism due to waterlogging in the form of segregations of Fe–Mn oxidesrgleying and chloritisation, probably due to ferrolysis in the upper horizons of some of these soils. Neotectonics seems to have affected development of landforms and soils significantly. Due to reactivation of some basement faults in the western region, some tectonic blocks subsided, causing transgression during the Early Pleistocene and at ca. 7 ka. Subsequent uplift of these blocks caused regressions and development of soils on the exposed landscapes. Episodic uplift of the Barind Tract in the northern region may have given rise to three topographic levels. Some faults confine the courses of the Damodar, Ganga and Bhagirathi rivers. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Tectonics vs. soil development and landform; Ferrugination; Fersiallitisation; Hydromorphism; Gangetic Plain; India

)

Corresponding author. Fax: 0091-1332-73-560; E-mail: [email protected] Present address—Engineering Geology Division, Geological Survey of India, Western Region, Jaipur302004, India. 1

0341-8162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 4 1 - 8 1 6 2 Ž 9 8 . 0 0 0 6 6 - 6

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1. Introduction The Lower Gangetic Plains of the Indian sub-continent form one of the most extensive fluviordeltaic plains of the world. They conceal the Bengal Sedimentary Basin. Regional studies of landforms and soils of the Upper and Middle Gangetic plains show that climatic changes and neotectonism are significant factors in their evolution ŽMohindra et al., 1992; Srivastava et al., 1994; Kumar et al., 1996.. The proximity of

Fig. 1. Location of study area showing sites of soil profiles Žcircles. examined in the field. Solid circles indicate soil profiles studied in detail.

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this area to the coast line and many transgressions–regressions in the area since the Cretaceous ŽSengupta, 1966; Roybarman, 1983. suggest that the plains may bear the imprints of Quaternary sea level changes. The objectives of our study in the study area ŽFig. 1. were: 1. Identification and mapping of different landforms and soil-geomorphic units in the Lower Gangetic Plains of West Bengal ŽFig. 1., and construction of a soil chrono-association for the area, 2. Deciphering the major soil-forming processes in the area, and 3. Broad elucidation of the roles of sea level changes and neotectonics on the evolution of landforms and soils.

2. Previous work Ball Ž1877. divided the Quaternary deposits of the study area in parts of West Bengal into the Older and Newer Alluvium; however, the basis for this classification was not clearly defined. Mukerji Ž1955, 1958 in Shankamarayana, 1982. described the morphology of the soils of West Bengal on the basis of their occurrence in the following different physiographic regions: Ganga riverine alluvium, Ganga flats, Ganga uplands, Ganga lowlands, Vindhyan riverine lands, Vindhyan flat lands, Vindhyan Highlands and coastal areas. Morgan and McIntire Ž1959. suggested that the ‘Older Alluvium’ of the Bengal Basin ŽWest Bengal, India and Bangladesh. consisted of multiple Pleistocene terraces, which they called the ‘Barind’ and the ‘Madhupur Jungle’. The geomorphology of the Lower Gangetic Plain has been studied by Niyogi et al. Ž1968., Mallick Ž1971., Niyogi Ž1975. and Bhattacharya and Banerjee Ž1979., but these studies did not include examination of soils. The National Bureau of Soil Survey and Land Use Planning ŽNBSS and LUP. published a regional Ž1:250,000 scale. soil resource map of West Bengal State in 1991, but its recognition of major geomorphological units is not appropriate Žsee Section 5.5. and it does not provide reasonable constraints on ages of soils. Bagchi Ž1951., Adhikari Ž1957, 1958., Sarkar and Chatterjee Ž1964., Anjaneyulu et al. Ž1965., Ghosh and Datta Ž1974., Ghosh et al. Ž1976 in Ghosh and Kapoor, 1982. have identified clay minerals in surface soils at depths of less than 50 cm from small areas. Kooistra Ž1982. studied the micromorphology of benchmark soils Žsoils that occupy a key interpretive position in the soil classification and cover a large area, Murthy et al., 1982. from six soil series of the Lower Gangetic Plains. There has been no detailed work on the evolution of landforms in relation to soils in the Lower Gangetic Plains.

3. Methods of investigation Topographic maps, Landsat MSS False Colour Composite ŽFCC. images and field studies were used to identify major landforms, and, within each landform, subunits called soil-geomorphic units were distinguished. Soil-geomorphic units were marked by

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soils with values of properties within narrow ranges. Forty pedons that were well distributed among various soil-geomorphic units were studied in the field. Disturbed Žbulk. and undisturbed soil samples from various horizons were collected for laboratory analyses. Twenty-one pedons were selected for grain-size analysis, clay mineralogy, total major element analysis and micromorphology. Grain size distribution was determined by the method of Galehouse Ž1971.. Clay fractions Ž- 2 mm. were separated and minerals were identified using the procedure described by Wilson Ž1987. and semiquantitatively analysed by the method of Schultz Ž1964.. Major element composition was determined using the two solution method of Shapiro Ž1975.. For micromorphological studies, undisturbed soil samples were collected in metal boxes. The samples were impregnated with crystic resin and large Ž60 = 40 mm. thin-sections were prepared according to the procedure described by Jongerius and Heintzberger Ž1963., FitzPatrick Ž1984. and Murphy Ž1986.. Thin sections were described according to the terminology of Bullock et al. Ž1985.. Eight soil samples from different soil-geomorphic units were dated at Physical Research Laboratory, Ahmedabad by the partial bleached thermoluminescence technique described by Wintle and Huntley Ž1980. and Berger and Lutenauer Ž1987..

4. Climate In general, the region experiences a hot and subhumid monsoonal climate that is mainly controlled by proximity to the Bay of Bengal in the south and the alignment of the Himalaya in the north. The range in annual rainfall Ž1200 mm to ) 4000 mm. reflects the uneven seasonal and spatial distribution characteristic of the monsoonal climate. Nearly 80% of the total annual precipitation is concentrated during the 4 months of the monsoon ŽJune to September.. Owing to the proximity of the Himalaya and the Bay of Bengal, the northern and southern parts of the area experience more annual rain than the central and the western parts. Four well marked seasons, i.e., hot summer, wet summer, pre-winter transition and winter can be recognized. The average maximum temperature in hot summer is between 298C and 338C. and the average winter temperature varies from 178C to 248C. These data indicate a hyperthermic soil temperature regime. Most of the area experiences soil moisture deficiency for 4–5 months in a given year and the data indicate a ustic soil moisture regime ŽSoil Survey Staff, 1992..

5. Major landforms and soil-geomorphic units Using remote sensing techniques followed by direct observations in the field, three major landforms, i.e., Upland Areas, Old Fluvialrdeltaic Plains and Young Fluvial Plains, were recognized and mapped in the study area. Within each landform a number of soil-geomorphic units were recognized ŽFig. 2..

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Fig. 2. Soil-geomorphic map of the study area showing locations of pedons from which samples were collected for TL dating. The boundary of the Moribund Ganga Deltaic Plain with the Bhagirathi Plain is not defined. MGDP—Moribund Ganga Deltaic Plain, RSU—Upland with Red Soils, BAP—Bhagirathi-Ajay Plain, ASP —Ajay-Silai Plain, DDP—Damodar Deltaic Plain, BP—Bhagirathi Plain, OGP—Old Ganga Plain, GFP— Ganga Floodplain and BT—Barind Tract.

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5.1. Upland Areas The Upland Areas occur at the highest elevations Ž20–125 m amsl. and are characterized by dissected topography and several distinct levels. The topographically highest levels have strongly developed, presumably polygenetic, soils. 5.1.1. Barind Tract (BT) The Barind Tract ŽBT. is an uplifted region in the northern part of the study area. It is associated with an east–west trending horst block and the major portion of the Tract is located in Bangladesh. A number of Himalayan rivers flowing southwards cut across this tract and divide it into four parts that trend almost north–south. In India it has an area of about 1200 km2 . The major southerly flowing streams have entrenched floodplains and numerous secondary small, highly sinuous meandering streams that exhibit dendritic patterns drain the upland regions. In the Landsat FCC image, the Barind Tract exhibits grayish brown colour with strong red mottles and fine to medium texture. At least three topographic levels with height ranging from 5 to 25 m above the adjoining major stream floodplains occur on the Barind Tract. The degree of development of associated soils decreases from the highest to lowest level ŽHossain, 1994.. In the study area, the predominant level is the lowest level. 5.1.2. Upland with Red Soils (RSU) The Upland lies east of the Chhotanagpur-Rajmahal Hills at the highest elevation of 35 to 120 m amsl in the study area. The landscape is very gently sloping to undulating and dissected and exhibits red soils. Slope ranges from 1.2 to 1.8 mrkm and the region is subjected to moderate erosion. The drainage pattern is dendritic in general, but in the southern parts the pattern is annular ŽFig. 3.. In the FCC, the unit is marked by reddish brown colour with smooth texture. Sub-dendritic pattern of interfluve ridges with dark shadow zones are very conspicuous. Several large patches of red soils covered by forest are characterised by dull red colour. A number of meander scars and valley-fill deposits close to the active streams showing light grayish tone with medium texture are evident in the Landsat images and in the field. 5.2. Old FluÕialr Deltaic Plain The Old FluvialrDeltaic Plains are level to nearly level Žaverage slope 1.2 mrkm. and are located to the east of the Upland with Red Soils. They have moderately developed soils, and include the Bhagirathi-Ajay Plain ŽBAP., the Ajay-Silai Plain ŽASP. and Damodar Deltaic Plain ŽDDP.. In the FCC, these plains are characterised by light gray colour with very common red mottles, fine texture and high drainage density and paleochannels are marked by light red colour. 5.2.1. Bhagirathi-Ajay Plain (BAP) and Ajay-Silai Plain (ASP) The Bhagirathi-Ajay Plain ŽBAP. and the Ajay-Silai Plain ŽASP. are part of a large plain trending northwest–southeast which in general is level or has a very gentle slope. Most of the rivers originate in the western highlands and follow the regional slope to form a sub-parallel pattern ŽFig. 3.. However, in the southern parts ŽAjay-Silai Plain.,

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Fig. 3. Drainage map of the study area and adjoining region Žmodified after Singh, 1971.. Major faults and tectonic blocks are also shown. 1—Tectonic Shelf Ž1a—Upland with Red Soil, 1b—Bhagirathi-Ajay Plain and Ajay-Sillai Plain., 2—Barind Tract and 3—Ganga Fluvio-deltaic Plain Graben.

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the Dwarkeshwar, Silai and Kasai rivers show an annular drainage pattern ŽFig. 3.. The floodplains of most of the rivers are incised, more so in the western half giving these areas a slightly undulating topography. The Ajay and Morakhi rivers draining to the east currently flow along the southern margins of their floodplains. 5.2.2. Damodar Deltaic Plain (DDP) The Damodar Deltaic Plain ŽDDP. lies between the Damodar river in the west and the Hooghly river in the east. It extends up to 70 km and 40 km in the north–south and east–west directions, respectively. The plain width decreases southwards ŽFig. 2.. The deltaic plain exhibits the two styles of slope patterns—from west to east in the northern part and northwest to southeast in the southern part. According to Deshmukh Ž1973. Žp. 9. the Damodar, which was flowing in an easterly direction to meet the Bhagirathi during the middle of 18th century, has since shifted its mouth 128 km to the south. The plain is characterized by a network of paleochannels with associated natural levees and swamps forming a dichotomic pattern ŽHoward, 1967.. These paleochannels fan out from the point where the rivers turns south from its easterly course, suggesting that the deltaic plain was constructed by the Damodar river some time in the past. The Damodar river has a braided channel in its upper reaches and acquires a low sinuous channel morphology after entering the deltaic plain. 5.3. Young fluÕial plains In satellite images, these plains are identified by their clear association with some active river channels and show bluish grey colour with some red patches. Sands of point and mid-channel bars appear as bright white patches. Oxbow lakes and abandoned channels exhibit dark grey colour. The older plains are distinguished from active floodplains by higher uniformity of soils marked by more uniform red colour and fewer oxbow lakes and abandoned channels. Their slopes range from 0.5 to 0.1 mrkm. They include the Old Ganga Plain, the Ganga Floodplain and the Bhagirathi Plain. 5.3.1. Old Ganga Plain (OGP) and Ganga Floodplain (GFP) The Ganga Plains lie along a fault and separate the north–south trending Upland with Red Soils and Old FluvialrDeltaic Plains from the east–west trending Barind Tract. The rivers coming from the Barind Tract change their courses from southwest to southeast after entering this area. Within these plains, two soil-geomorphic units can be recognized: Old Ganga Plain ŽOGP. and Ganga Floodplain ŽGFP.. The Old Ganga Plain is at a higher topographic level than the Ganga Floodplain and is located on both sides of the Ganga Floodplain. It is marked by numerous paleochannels, oxbow lakes and meander scars. The soils of this Plain show at places a soil with moderately developed subangular structure overlain by a ca. 50 cm thick floodplain deposit Žwith weakly developed soil., probably representing a catastrophic flood. It indicates two episodes of soil development separated by a period of deposition by exceptional floods. The channel associated with the Ganga Floodplain has a meandering pattern, and is marked by oxbow lakes, old abandoned channels and point bars on the northern bank of the river. This suggests that the channel recently shifted south-westerly across the floodplain.

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5.3.2. Bhagirathi Plain (BP) This plain lies to the east of the Old FluvialrDeltaic Plains at an elevation 5–10 m amsl; its slope is less than 0.8 mrkm. The Bhagirathi river, a distributary of the Ganges, leaves the Ganga about 15 km north of Jangipur ŽWest Bengal. and flows in a southerly direction along a north–south oriented fault. Throughout its course it flows in a number of straight stretches punctuated by high sinuosity meanders. Well preserved meander scars, cut offs, abandoned channels and levees are clearly seen in the satellite images. 5.4. Raghunathganj Surfacer Soil The Raghunathganj SurfacerSoil is an undulating buried surface generally occurring at a depth of 1.5–3 m under the Bhagirathi-Ajay and Ajay-Silai Plains, but with a minor surface exposure in the northern part of the Bhagirathi-Ajay Plain. The soil profile has a strongly cemented petrocalcic horizon consisting of very hard calcium carbonate nodules of 2–30 mm diameter. The estimated calcium carbonate content in the soil is about 45%. The thickness of the horizon varies from 20 cm to about 35 cm. A radiocarbon date of 23 ka has been obtained on calcrete in the Raghunathganj soil. As the estimated age of a soil by radiocarbon dating of calcrete may differ significantly from the actual age estimated by geomorphological and soil development evidence and luminescence dating ŽAmundson et al., 1994; Wang et al., 1996., 23 ka may be taken as a probable age of the Raghunathganj soil. Because of its limited areal exposure and probable Late Pleistocene age, the Raghunathganj Soil is considered to be an exhumed one ŽBirkeland, 1984, p. 11., exposed on the surface due to erosion of the overlying soil. This region is also marked by the presence of a number of other calcrete-bearing horizons in the subsurface ŽVaidyanadhan and Ghosh, 1993.. 5.5. Comparison with preÕious units Previously, Niyogi Ž1975. recognized four major units from the present area, i.e., Lateritic Upland, Older Deltaic Plain, Young Deltaic Plain and Recent River Sediments from west to east and assigned them ages of Pleistocene to Recent. Our map is similar to Niyogi’s map. Our Upland with Red Soils is the same as the Lateritic Upland. Our Damodar Deltaic Plain corresponds to Niyogi’s part of the Young Deltaic Plains ŽYDP.. In Fig. 1 of Niyogi Ž1975., YDP also includes vast plains east of the Bhagirathi river. However, they are older than the YDP west of the Bhagirathi River ŽHossain, 1994.. The soil resource map for West Bengal by National Bureau of Soil Survey and Land Use Planning Ž1991. demarcates 115 soil units and is very useful for agricultural purposes, but differs from our map as follows. Ži. The major geomorphic units of the ‘Indogangetic Plain’ Žincluding the Barind Tract of this study. and ‘Bengal Basin’ of National Bureau of Soil Survey and Land Use Planning Ž1991. lie within the Bengal Sedimentary Basin ŽSengupta, 1966; Roybarman, 1983. and the landforms and soils of both of the units are related to the basin tectonics Žas discussed later and Hossain, 1994. and so the two units should have been grouped together.

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Žii. ‘Sub-recent to recent’ ages assigned to soils occurring east of the Upland with Red Soils by NBSS and LUP are imprecise and we give them more probable ages.

6. Depth to ground water The ground water is at the deepest level in the Upland with Red Soils and Barind Tract and occurs at ) 10 m depth from the surface. Soils are well drained in the Upland with Red Soils and imperfectly drained in the Barind Tract. Ground water is at moderate depth Ž6–10 m. in the Old FluvialrDeltaic Plains, except for the Damodar Deltaic Plain, where it is at slightly more than 3 m depth. The soils in these plains are generally poorly to imperfectly drained and are subject to local waterlogging for part of the rainy season. The Bhagirathi Plain and Old Ganga Plain are marked by shallow water table Ž0–3 m.. The floodplains of the Ganga and other small rivers are subject to overflow of rivers and large areas are waterlogged during rainy season. Soils in these plains are moderately well to imperfectly drained.

7. Soil chrono-association Mohindra et al. Ž1992. defined a soil chrono-association as a sequence of soils ranked on the basis of degree of development in a region with varying climate or a climatic gradient and underlain by different parent materials. As the present area is marked by a climatic gradient and varied parent materials ŽSection 9.1., an attempt is made here to develop a soil chrono-association. An integrated approach utilizing soil morphology Žthickness and development of solum, particularly B-horizon., texture Žrelative clay content of B-horizon, clay accumulation index., chemical composition Žmolar ratio SiO 2rFe 2 O 3 q Al 2 O 3 q TiO 2 ., and micromorphology Ždegree of pedality, thickness and nature of clay coatings and development of the fabric. has been used to determine the degree of soil profile development. On the basis of relative degree of development, the soils of different geomorphic units can be classified into five members ŽQGWB1 to QGWB5, youngest to oldest.. QGWB stands for Quaternary soils of the Gangetic Plains of West Bengal. Soils from various units included in different members are: QGWB1— Ganga Floodplain; QGWB2—Bhagirathi Plain and Old Ganga Plain; QGWB3—Barind Tract ŽLower Level. and Damodar Deltaic Plain; QGWB4—Bhagirathi-Ajay Plain and Ajay-Silai Plain and QGWB5—Upland with Red Soils.

8. Variation in soil characteristics among different members of the soil chrono-association There are systematic differences in the soil characteristics of the soil chrono-association members ŽFigs. 5–8..

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8.1. Soil morphology QGWB1 member soils of the active floodplains of the study area have a ArC profiles. Weakly developed ArBrC soils are encountered in the QGWB2 member. Moderately developed ArBrC soils are observed in QGWB3 member. The BA horizon of the Barind Tract soils and lower horizons of soils of the Damodar Deltaic Plain are commonly gleyed Žgleyed horizons are not shown in generalized soils of Figs. 5–8.. Soils of QGWB4 are moderately to strongly developed ArBtrC soils. They have vertical cracks, so are vertic intergrades. The cracks are more common and deeper in the Bhagirathi-Ajay Plain than in the Ajay-Silai Plain. The thickness of the B-horizon increases in general from QGWB2 Ž15–20 cm. to QGWB4 Ž80–110 cm. soils. The Upland in the west is overlain by a well developed red soil over most of the area and the lower slopes of the Upland Žeastern parts. show a compound soil ŽDuchaufour, 1982, p. 144.. The lower soil of the Compound Soil is a well-developed red soil and is overlain by a weakly developed soil with a maximum thickness of 50–200 cm. The parent material of the latter soil is composed of alternating red gravelly sand and mudstone consisting of detrital red soil particles bound by iron oxide ŽFig. 4.. The lower soil of the Compound Red Soil has a thickness of 200–350 cm and shows distinct horizons. It starts with a plastic regolith ŽBC. Žcf. Lelong, 1969 in Duchaufour, 1982, p. 409. at the base and grades into a plinthite horizon ŽBt.. The plastic regolith horizon is less than 1 m thick, mottled, plastic and vermicular and has subangular blocky structure. The plinthite horizon is firm, soft, when fresh and hardens on exposure and it has two distinct units. The lower unit has vermicular structure with hard, irregular red

Fig. 4. C horiozon Žmudstone. of the upper soil of the Compound Red Soils exposed in a canal-cut. The total thickness is ca. 2 m. Slightly coarser thin layers stand out on weathering.

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Fig. 5. Generalized morphological features of the different soil chrono-association members.

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Fig. 6. Variation of total clay and estimated accumulated clay and Clay Accumulation Index Žgiven in parentheses. for soils of the chrono-association members QGWB1-QGWB4. Data for QGWB5 soils are not presented, as these were not analysed for clay content.

iron oxide concretions embedded in a kaolinitic matrix and cemented by sesquioxides ŽFig. 5.. The upper unit is strongly cemented by iron oxide and iron oxide pisolites constitute greater than 50% of the soil mass. Because of the deep red colour, abundant iron oxide pisolites and hardening of the soil on exposure, this horizon has been called ‘LateriterLateritic Soil’ by some earlier workers ŽBhattacharya and Banerjee, 1979; Niyogi, 1975; Vaidyanadhan and Ghosh, 1993.. According to Soil Taxonomy ŽSoil Survey Staff, 1992., most soils of the QGWB1 and QGWB2 would tentatively be classified as Ustifluvents and Typic Ustochrepts, respectively. QGWB3 soils are mostly Typic Haplustalfs and Typic Endoaqualfs in the Damodar Deltaic Plain and Aquic Haplustalfs in Barind Tract. Vertic Haplustalfs and less commonly Typic Endoaqualfs are present in the QGWB4 member soils. The QGWB5 member soils are mostly Haplustalfs. Nodules of both calcium carbonate and Fe–Mn are observed in soils of the study area. Few to common, fine calcium carbonate nodules occur in the Barind Tract ŽLower

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Fig. 7. Variation of the molar ration SiO 2 rR 2 O 3 with depth in the typical soil profiles of chrono-association members QGWB2 to QGWB5. R 2 O 3 s Al 2 O 3 qFe 2 O 3 qTiO 2 .

Level. soils ŽQGWB3.. Small ferro-manganese mottles and nodules of about 1 mm diameter with clear or diffuse boundaries are common in the lower parts of profiles of the QGWB1–QGWB2 soils and in the upper parts of the profiles of QGWB3–QGWB4 soils. Fe–Mn nodules Ž- 4 mm diam.. with sharp boundaries and mottles are common towards the base of the soil profiles in QGWB3 and QGWB4 soils. In general, the Fe–Mn nodules constitute - 5% of the matrix. 8.2. Textural Õariation Field grain size analysis of different chrono-association members indicates that most of the soils are non-gravelly, loams to silty clay loams. Medium to coarse textured

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Fig. 8. Variation of clay mineral type and composition with depth in typical pedons of soil chrono-association members QGWB2 to QGWB5.

Žloam. soils in QGWB1 do not show any significant variation in clay content with depth. Soils of the other members are mainly silty loams and have greater clay content in the B-horizon, suggesting greater soil development. Clay content of the parent materials varies over a wide range of 4 to 37% in QGWB1–QGWB4 soils and up to 13% over several decimeters depth in individual pedons in very weakly developed soils of QGWB1 and QGWB2 soils. Such large variations in clay content are characteristic of fluvial deposits and make the identification of parent-material clay content difficult. Increase in clay content of B-horizon can be a useful indicator of relative age of soils ŽBirkeland, 1984.. QGWB2 to QGWB4 member soils show successively higher amounts of clay in B-horizon, as compared with ArC horizons ŽFig. 6., but no such increase is observed in QGWB1 soils. This increase in clay content in the Bt- horizon multiplied by its thickness is called the Clay Accumulation Index and is suggested as a criterion for inferring relative soil ages by Levine and Ciolkosz Ž1983.. Here a slightly modified version of Clay Accumulation Index is used. The index is calculated by summing thicknesses of B-subhorizons multiplied by respective clay amounts minus the mean

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clay content of the A- and C-horizons. The index values increase from QGWB2 to QGWB4 soils ŽQGWB2: 58–122; QGWB3: 199–545 and QGWB4: 852–1564.. The Red Soils ŽQGWB5. were not analysed for their texture, so no such indices could be calculated for them. Also, the index values could not be calculated for profiles that lacked B-horizon or were not adequately sampled. 8.3. Major element analysis In the hot and subhumid climate with neutral to slightly acidic environment of the present area, weathering of silicate minerals and downward leaching of the silica and bases results in enrichment of less mobile elements Al, Fe and Ti in upper soil horizons. The ratio of more mobile to less mobile elements therefore decreases in B-horizon ŽSmith and Buol, 1968 in Catt, 1990. and with increasing time of soil development ŽHarden, 1987.. Thus molar ratio SiO 2rR 2 O 3 Žwhere, R 2 O 3 s Fe 2 O 3 q Al 2 O 3 q TiO 2 ., suggested as an index of weathering ŽBirkeland, 1984., could be used in the present area for inferring degree of leaching and has been calculated for soils of different chrono-association members. As discussed later, the QGWB1–QGWB4 soils have been subjected to hydromorphism, which leads to formation of Fe–Mn segregations in the top and bottom of the soil profiles and this may modify the above pattern of variation in molar ratio. In the present study, the molar ratio SiO 2rR 2 O 3 commonly varies over a wide range of 3–4.7 for parent materials of soils between different pedons due to variation in parent material composition ŽFig. 7.. In spite of this large variation, a small decrease in the ratio from the A- to the B-horizon can be discerned in profiles of the QGWB2 and QGWB3 soils, except for pedons LA3 and LA1. The vertic intergrade soils of QGWB4 member, however, do not show any systematic change in this ratio in the top 30–35 cm of the profile Ždepth of vertical cracks., indicating homogenization of soil material due to pedoturbation. Below this depth, molar ratio significantly decreases Žnet change as high as 3.5. from the A- to the B-horizon. Thus the use of molar ratio in distinction of different chrono-association member soils is severely limited due to large variations in composition of fluvial parent material. The two pedons of Red Soils show high values of 4.1 to 8.2 and a decrease or a minor variation with depth in the molar ratio, although these soils are enriched in Fe. The observed absence of the effects of hydromorphism on the molar ratio in the QGWB4 soils and the high molar ratio of the Red Soils may reflect chemical analysis of fine earth soil that lacked Fe–Mn nodules that occur in the coarse fraction. 8.4. Clay mineralogy The major clay minerals in the soils are illite, kaolinite, smectite and interstratified minerals like illite–smectite, kaolinite–smectite, illite–chlorite and smectite-chlorite. Small amount of chlorite occur in the northern part Žnorth of the Ganges. of the study area. The apparent variation of clay mineral contents from the typical pedons of different soil-geomorphic units ŽFig. 8. suggests that the major clay minerals of the parent material of soils in the region south of the Ganges are illite, smectite and interstratified

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illite–smectite formed by chemical weathering of the Rajmahal Basaltic Trap and granitic rocks in the west and transported to the east by the easterly flowing rivers. As a part of the clay content of B-horizon owes to illuviation of clay from the A-horizon and because analysis of clay mineral content is semiquantitative in nature, contents of kaoliniterchlorite and interstratified clay minerals and types of interstratified clay minerals in ArB-horizons as compared with those in C-horizon are used for inferring possible clay mineral transformations. In the lower soil of the Compound Red Soils ŽQGWB5. most of smectite and interstratified clay minerals have probably been transformed to kaolinite, iron oxides and minor amounts of gibbsite ŽPadhi, 1995.. In the QGWB4 and QGWB3 ŽDamodar Deltaic Plain., smectite and illite may be transforming to kaolinite through interstratified minerals like illite–smectite and kaolinite–smectite. Also, transformation of illite and smectite to chlorite through interstratified minerals like illite–chlorite and smectite–chlorite may have occurred on a small scale in the surface horizons of QGWB3 member soils. The higher amounts of illite in the most of the Old FluvialrDeltaic Plain ŽQGWB4. Žpedons LB6 and LC2. as compared with the northern parts Žpedon LB2. is probably due to a greater contribution from granitic rocks in addition to the Rajmahal Traps in west. Soils of the QGWB2 and QGWB1 do not show any systematic change in clay mineral composition with depth and large variations in clay mineral composition over several decimeters in these soils largely reflect variations in parent material composition. The presence of smectite in these soils suggests that in addition to the Himalayan source, some clay has come from rivers draining the Rajmahal Basaltic Traps in the west. 8.5. Micromorphology The micromorphology of the undisturbed soil samples indicates that the soils of various units exhibit distinct variations in features such as the grade of pedality, degree of b-fabric development, alteration of primary minerals, formation of sesquioxide nodules, and abundance and thickness of illuvial clay coatings. Soils are apedal in QGWB1 soils and the grade of pedality increases to weakly developed peds in QGWB2 ŽFig. 9a. to the most strongly developed peds in QGWB5. The nature and abundance of voids in different members are closely related to the coarserfine ratio of particle size. Channels, chambers, planar voids and vughs ŽFig. 9b. are more common in fine-grained soil materials whereas intergrain packing voids are more common in coarse materials. Also, the void surfaces have been smoothed in the horizons having clay coatings around voids Že.g., in QGWB3, QGWB4 and most commonly QGWB5.. The finer fraction of the soil material shows different types of b-fabric Žb-fabric or birefringent fabric is the fabric of fine mass observed under crossed polarisers and described by the orientation and distribution of the patterns of interference colours and their nature, Bullock et al., 1985, p. 90.. The degree of development and abundance of b-fabric increases in progressively older soils. In QGWB1 and QGWB2 member soils weakly developed stipple-speckled fabrics are common, parallel striated b-fabric is observed occasionally. The soils of QGWB3 member are marked by weak to moderate cross-and reticulate b-fabrics ŽFig. 9c.. Soils of QGWB4 member show mostly porostri-

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ated and granostriated b-fabric in the upper part but with increasing depth the parallel striated b-fabric dominates. The extensive and frequent wetting of these soils would have resulted in swelling of the soil matrix. This probably led to compression and

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realignment of particles adjacent and parallel to the grains and void surfaces, giving rise to grano- and porostriated b-fabrics. The parallel striated b-fabric in QGWB4 soils might have developed due to the closure of some of the planar voids by the repeated wetting and drying in linear birefringent zones. Red soils of the QGWB5 member show undifferentiated to weak stipple-speckled b-fabric due to the presence of sesquioxides. Micromorphological studies confirm field observations about the presence of two types of Fe–Mn segregations: small nodules Ž- 1 mm. and mottles with diffuse external boundaries in QGWB1–QGWB4 soils, and rounded Fe–Mn nodules with sharp boundaries with the matrix in QGWB3–QGWB5 soils. Fe–Mn concretions and nodules with sharp boundaries are most conspicuous in the lower soil profile of Compound Red Soils of QGWB5 member. Following Brinkman Ž1970. and Kooistra Ž1982., the diffuse nodules are considered to be forming by current pedogenic processes, whereas the nodules with sharp external boundaries are interpreted to have formed due to the past pedogenic processes. Micritic as well as sparitic nodules are observed in thin sections from the petrocalcic horizon of the Raghunathganj Soil. Calcareous coatings, infillings and matrix impregnation features, and micritic nodules have diffuse boundaries. In contrast, the sparitic nodules have sharp external boundaries ŽFig. 9d. and have ferric oxiderhydroxide impregnations. The sharp boundaries of sparitic nodules suggest their formation by past pedogenic processes and a radiocarbon age of late Pleistocene for the calcrete nodules ŽSection 5.4. supports this suggestion. Associated ferric oxiderhydroxide impregnations and diffuse calcium carbonate features indicate that sparitic nodules are being modified by the current pedogenic processes occurring in the overlying QGWB4 soils. Sehgal and Stoops Ž1972., Bal Ž1975. and Kemp Ž1985. suggested that the formation of similar features indicates multicyclic formation in the course of soil development. Impregnated micritic nodules of - 4 mm diameter with diffuse boundaries constitute - 5% area of thin sections from the upper part of the profile in the Barind Tract ŽQGWB3. soils. These are also considered to be forming by current pedogenic processes. Clay coatings are absent in the QGWB1 and QGWB2 soils. The QGWB3 member soils of the Damodar Deltaic Plain have flood-coatings ŽGerrard, 1992, p. 97. together with thin Ž20–30 mm. illuvial clay coatings. Flood-coatings are very weakly oriented

Fig. 9. Ža. Partially accommodating and moderately well separated angular blocky peds, B2 horizon. Pedon LB7, Bhagirathi-Ajay Plain ŽQGWB4., Frame length 2 mm, Ordinary light. Žb. Vughy structure, B1 horizon, Pedon LB3, Ajai-Silai Plain ŽQGWB4.. Frame length—20 mm, Ordinary light. Žc. Cross and reticulate striated b-fabric, B2 horizon. Pedon LA1, Barind Tract ŽQGWB3.. Frame length—0.7 mm, XPL. Žd. Strongly impregnated discrete sparitic nodule with ferruginous impregnation. Raghunathganj Soil. Frame length 2.8 mm, XPL. Že. Stress-oriented clay domain Žarrow., B1 horizon, Pedon LB2. Bhagirathi-Ajai Plain ŽQGWB4.. Frame length 1.4 mm, XPL. Žf. Compound embedded clay coating around an opaque grain, 2Btb-horizon. Pedon LB5, Lower Soil of the Compound Red Soil. Frame length 1.4 mm, XPL. Žg. Micropan consisting of alternating ferruginous silty clay and coarser material, 2Btb1-horizon, Pedon LC1, Lower Soil of the Compound Red Soils, Frame length 1.4 mm, XPL. Žh. Well oriented free grain coating Žarrows. of ferruginous clay, 2Btvb1 horizon. Pedon LB5, Lower Soil of the Compound Red Soil. Frame length 1.3 mm, XPL.

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and contain some silt material, which suggests that their formation might have occurred due to translocation of silt and clay during flooding. They are differentiated from illuvial clay coatings by their very weak orientation and diffuse boundary against the ground mass. The Barind Tract ŽLower Level. soils of QGWB3 members have thin Ž30–40 mm. illuvial clay coatings, flood coatings and deformed and broken clay-features Žpapules.. Vertic intergrade soils of QGWB4 member show papules, slickensided ped faces, stress-oriented coatings ŽFig. 9e. and thin Ž30–60 mm. illuvial clay coatings. The clay coatings in these soils generally consist of limpid and speckled clay. The mudstone in C-horizon of the top profile in the Compound Red Soil contains rounded sesquioxide nodules with sharp boundaries, quartz grains with iron oxide in the cracks Žtermed runiquartz by Eswaran et al., 1975., papules and embedded grain clay coatings ŽFig. 9f.. These features, combined with their occurrence as interbeds with sands, suggest their derivation from the Red Soils occurring in the western region Žallochthonous origin.. The lower soil in the Compound Red Soil exhibits a crumb microstructure and contains microlaminated clay coatings 200–300 mm thick, ) 1 mm thick micropans Ž) 0.5 mm thick horizontal quasi-coatings with varying thickness, Bullock et al., 1985, p. 100. ŽFig. 9g., irregular shaped hematitic nodules, void hypocoatings of ferruginous matter, free-grain clay coatings ŽFig. 9h. and fragments of angular quartz in the iron oxide matrix or in nodules indicating an autochthonous origin. The coarse fraction of this soil has partially weathered and fresh K-feldspars and mica constituting up to 5% of the fractions. Comparison of morphological, textural, chemical and micromorphological data suggest that for the weakly developed QGWB2 soils, total clay content shows a slight increase and molar ratio shows a slight decrease from A- to B-horizon. For QGWB3 and QGWB4 soils similar variation from A- to Bt-horizon in these parameters are more pronounced. Also the Bt-horizon in these soils exhibit better pedality and higher cutan thickness. However, the lower parts of the Bt-horizons have the highest values of totalraccumulated clay content and the lowest molar ratios in some of the pedons ŽLA1, LB6, LC2 and LC3., though middle parts are marked by highest degree of soil development morphologically and micromorphologically. This dichotomy may be due to higher content of clay in the parent rock of the lower part of B-horizon. Kaolinite and kaolinite-interstratified clay minerals occur in higher amounts in solum as compared to C-horizon in QGWB3 and QGWB4 soils and correlation of clay mineral types and contents with other soil parameters is not very strong in QGWB1–QGWB4 soils. 8.6. Dating of the soil chrono-association Soils represent a period of landscape stability and consequently their chronology is cardinal to the understanding of geomorphic evolution of a region. The chronology of soils in the present study has been determined by Thermoluminescence ŽTL. dating technique. The technique dates the sediment burial event that marks the cessation of further exposure to sunlight. Prior to burial, sun-exposure during weathering or transport reduces the previously acquired geological TL to a low value. On burial, the TL starts reaccumulating due to irradiation from the ambient radioactivity caused mainly by the

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presence of K 40 , U 238 and Th232 in trace amounts. Thus, the acquired TL at any time is related to the time elapsed since the burial. The basic TL age equation is, TL Ž age . s

Total acquired TL since burial Rate of TL acquisition Ž TLryear.

Three aspects merit a discussion of the luminescence technique for dating of soils. First is the estimation of TL level at the time of burial, i.e., magnitude of pre-depositional sunexposure. Any error that is introduced in carrying out laboratory zeroing of the TL signal for estimating the pre-burial zeroing level, a significant step in TL dating, can result in erroneous ages. This is particularly important in dating the sediments deposited in fluvial environment, as in the present case, where completeness of zeroing of TL signal cannot be assumed. Minerals during their under-water transport receive far less solar exposure than when they are on the earth surface. This is mainly due to scattering of sunlight by turbulence and suspended sediment in water, and it results in attenuation and reddening of solar spectrum under-water. For dating such sediments, Wintle and Huntley Ž1980. suggested a partial bleach method, in which the age is determined by examining constancy in age for different daylight bleaching durations. Subsequently Berger and Lutenauer Ž1987. advanced this method and suggested that normal daylight bleaching may be replaced by bleaching using an optical source that replicates under-water spectrum. In the present study, however, an alternative approach was taken: the daylight bleaching timings were adjusted in such a way that a known zero age sample from a floodplain gave nearly a zero age. Maintaining the same bleaching timings, the TL ages of all other soil samples were determined. The second aspect is the interpretation of luminescence ages. For A-horizons the pedoturbation during their formation may result in an in situ churning of the material such that a portion of sediments gets exposed to daylight, which further depletes the transiently acquired TL. Thus, a sample from A-horizon may indicate the time of its last exposure to sunlight during pedoturbation. On the other hand, B- or C-horizons do not suffer faunal pedoturbation and as such their luminescence signal could reflect the ages of deposition of the parent material. The third aspect that merits a consideration is the movement of water and siltrclay from A- to B-horizon that can result in disturbance in the radioactive decay chain. The disequilibrium in the decay chain makes the annual doserate a time dependent. In the present case, K was estimated by NalŽTl. t-spectroscopy and U and Th were measured using ZnSŽAg. a-counting. The data suggest that the radioactive disequilibrium may not be a serious problem. It is difficult to ascertain the change in radioactivity of the B- and C-horizons due to leaching from the A-horizon. However, the overall consistency of the radioactivity data and - 10% change in the dose-rate with depth suggest that the change in radioactivity due to leaching in most cases may not be significant. Nonetheless, it must be clarified that these effects could produce finite systematic offsets in the numerical ages. In view of difficulty of any estimate of such a systematic error, we have refrained from estimation of total errors. The measurement errors using the conventional error calculation method in these cases are computed to be 10–12% and a working

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estimate of total error should be taken as "15%. It is suggested that these ages be considered only as estimates of numerical ages. With the above premises, eight soil samples from the base and top of B-horizons of some soils from different soil-geomorphic units were collected for TL dating in tin boxes with due care to eliminate the exposure to sunlight during or after sampling. These samples were pretreated with 1 N HCl and 30% H 2 O 2 for removal of carbonates, iron oxide coatings and organic matter. Polyminerallic Žquartz and feldspar. fine fractions Ž4–11 mm. extracted from the soil samples were used for TL dating. The TL measurements were made using a Daybreak TL System. The TL glow curves were recorded under inert high purity nitrogen atmosphere using EMI 9635 QA photomultiplier tube coupled to a photon counting system. The heating rate was 58Crs. A heat absorbing, Chance–Pilkington HA-3 and a UV transmitting Schott UG-11 filter were interposed between the sample and the photomultiplier tube. The glow curves indicated that the signal was predominantly from rapidly bleaching 3258C peak of quartz. Nature of the TL glow curves indicates that the fluvial parent sediments of the soils were significantly bleached. The low equivalent dose to an extent supports this inference. On the other hand if it is assumed that the soil samples were only partially bleached, then our TL dates will be the upper bounds. Thus, we contend that our broad inferences on ages of QGWB2 to QGWB4 soils in the range of the Mid. to Late Holocene should be reasonable or indicate upper bounds. Detailed and more precise age estimates would have to await the use of the Optically Simulated Luminescence technique, which utilizes signals that can be bleached in just a few seconds of sunlight and also use of a- and t-ray spectrometry for estimating any disequilibrium in the decay chains of U 238 and Th232 . The ages from the basal samples indicate the time of deposition and approximate upper bound on ages of soils. The ages of samples from the top of the B-horizon are discussed later. The notional TL ages estimates obtained from basal samples for the soils of different units are: Ži. Ganga Floodplain: 0.5 ka Žii. Old Ganga Plain: 1.5 ka, Žiii. Bhagirathi Plain: 1 ka, Živ. Damodar Deltaic Plain: 3.6 ka, Žv. Ajay-Silai Plain: 5.4 ka and Žvi. Bhagirathi-Ajay Plain: 6.7 ka. These dates indicate the following tentative ages for the QGWB1 to QGWB4 members: 500 a, 1–1.5 ka, 3–4 ka and 5–6 ka, respectively. The only radiocarbon date of Ž4810 " 120 BP. on a decomposed wood from an unknown depth ŽVaidyanadhan and Ghosh, 1993. from the Damodar Deltaic Plain compares reasonably with our TL estimate of 3600 a. However, four TL dates of soils from the Upper Gangetic Plains were found to be compatible with archaeological evidence and radiocarbon dates ŽKumar et al., 1996.. As alluded to above, our dates though reasonable should be taken as notional estimates, until they are corroborated by other techniques or additional refinement of the TL method for dating soils is achieved. The ‘Lateritic Upland’ ŽUpland with Red Soils of the present study., Old Deltaic Plain and Young Deltaic Plain have been assigned ages of 350–1000, 175–275 and 60–82 ka by Niyogi Ž1975., respectively, by comparison with the Quaternary surfacesrformations of the Gulf Coast area of USA, described by Russell Ž1964.. These surfaces were given ages of Early to Middle Pleistocene, Late Pleistocene to Early Holocene, and Middle Holocene, respectively by Vaidyanadhan and Ghosh Ž1993.. The

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Early to Middle Pleistocene age for the Upland Red Soils given by the above workers is used in our later discussion. Our estimated TL ages for the Old and Young Deltaic Plains are much younger than those assigned by these workers. Comparison of our estimated ages with those published from other parts of the Indogangetic Plains ŽSinghai et al., 1991; Mohindra et al., 1992; Srivastava et al., 1994; Kumar et al., 1996. suggests that the Upland Red Soils are the oldest soils from the Indian part of these plains.

9. Development of landforms and soils 9.1. Pedogenic processes Duchaufour Ž1982. Žpp. 373–425. proposes that in hot, humid climates, an increasing degree of weathering is associated with greater magnitude of alteration of primary minerals, loss of combined silica and an increasing amount of neoformation of clay minerals. With increased weathering the following three phases can be recognized: Ži. Fersiallitisation phase is marked by partially inherited and partially neoformed 2:1 clays and a Bt-horizon. Free iron oxides are formed and soils may be rubified. Žii. Ferrugination phase is characterised by more neoformed 1:1 clays than 2:1 clays, exchange capacity between 16–25 mEqr100 g clay, silt:clay ratio ) 0.2, moderate clay illuviation, and incomplete weathering of feldspars and micas. Two subphases—ferruginous soils and ferrisols—are distinguished on the basis of the absencerpresence of a distinct horizon with large amounts of red, dehydrated ferric iron, respectively Žin Fig. 12.5 of Duchaufour, 1982., Žiii. Ferrallitisation phase shows complete weathering of primary minerals Žexcept quartz., abundant 1:1 neoformed clays, frequent occurrence of gibbsiterhematite, silt: clay ratio - 0.2, exchange capacity less than 15 mEqr100 g clay, and negligible clay illuviation. Characteristic features of the major soils of the area can be explained in terms of phases of weathering ŽDuchaufour, 1982. described above. Because of the presence of partially weathered to fresh grains of feldspars and mica, hematite, silt:clay ratio ) 0.2 ŽSingh, 1995. and preponderance of kaolinite over 2:1 clays, the lower soil of the Compound Upland Red Soils represents in situ weathering to the ferrugination phase Žautochthonous.. The hematitic pisolites constituting ) 50% of soil mass in a distinct horizon suggests development of ferrisol subphase. Ferrugination took place under overall well drained conditions. The parent material for the top part of the Compound Red Soils represents eroded red soil material from the hinterland and its deposition in the topographically lower regions Žallochthonous nature. due to the general tectonic uplift of the area as discussed later. This unit is analogous to a Laterite-Derived-Facies of Valeton and Wilke Ž1993.. Continued pedogenesis has caused induration of these soils. The QGWB3 and QGWB4 soils are marked by clay illuviation, formation of CaCO 3 nodules and transformation of smectite and illite to interstratified clay minerals to kaolinite. The soils of the Bhagirathi-Ajay and Ajay-Silai Plains are marked by the presence of vertic intergrade soils with high proportions of 2:1 interstratified minerals

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and smectite. They are marked by swelling and shrinking pedoturbation leading to formation of slickensides and stress-oriented coatings. The presence of illuviation features and the large amounts of 2:1 swelling clays suggests the fersiallitisation phase of weathering ŽDuchaufour, 1982, p. 375.. Two samples from the base of shrinkage cracks in the vertic intergrade soils gave TL ages of 500 a and 1.5 ka. As these samples show complete TL ’bleaching’ indicating their exposure to the sun for sufficient period in the recent past and they are from the base of pedoturbation zone, these dates probably represent the approximate time of turnover of the soils. The presence of calcium carbonate nodules with diffuse boundaries in the soils of the Barind Tract, with a high annual precipitation, is unexpected in view of our experience from other parts of the Indogangetic Plains. In the Gandak megafan area in the Middle Gangetic Plains with shallow groundwater and similar rainfall, the soil parent material is rich in calcitic silt and calcite is being leached down ŽMohindra et al., 1992.. Only in the Upper Gangetic Plains with much lower rainfall, calcium carbonate nodules are forming in the active floodplains and lower piedmont zones with shallow groundwater ŽSrivastava et al., 1994; Kumar et al., 1996.. In a region marked by fersiallic weathering and a distinct dry season, bases are retained within the profile due to capillary rise during dry season and CaCO 3 is precipitated ŽDuchaufour, 1982, p. 382.. Similar origin is envisaged for CaCO 3 nodules in soils of fersiallic stage development on the Barind Tract, marked by a dry season. Ferrolysis process was first described by Brinkman Ž1970. from the Barind Tract of Bangladesh. Ferrolysis is a hydromorphic process comprising degradation of clays, release of Si and exchange reactions involving iron that result in drastic decrease in a soil’s cation exchange capacity, Also part of the reduced iron gets trapped in the interlayer space of clays and chlorite forms. The process is characteristic of seasonally alternating cycles of reduction Žwet season. and oxidation Ždry season. especially in soils marked by seasonal near-surface waterlogging. We do not have detailed data on soils to establish firmly the precise nature of ferrolysis in the present area. Formation of small amounts of chlorite and chlorite–smectiterillite–chlorite in the surface horizons of QGWB3 soils suggests that illite is being transformed to chlorite through interstratified illite–chlorite in the Barind Tract and smectite to chlorite through chlorite–smectite in the Damodar Deltaic Plain. As hydromorphic conditions due to near-surface waterlogging prevail in the area for part of the rainy season, chloritisation of 2:1 clays in the upper horizons of QGWB3 soils is probably due to ferrolysis. The cause of the observed absence of such chloritisation in other soils of the area is unknown. Development of an argillic horizon in subtropic and tropic climates ŽMcKeague, 1981. and time taken for its development ŽBirkeland, 1984, p. 208–209. are controversial issues. Argillic horizons are commonly described from semiaridic to aridic regions and reported to have developed in recent fluvial sediments and sediments as old as 140,000 a ŽBirkeland, 1984, p. 208–209.. Very few accounts of argillic horizons from humid climates Že.g., Leigh, 1996. are available. Following Birkeland Ž1984. Žp. 45., argillic horizons have been recognized in QGWB3–QGWB4 soils of the present subhumid, subtropic area on the basis of content of clay in B-horizon as compared to that in C-horizon and the presence of illuvial clay coatings. Argillic horizons have developed in the present area only in a few thousand years old soils.

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Fe–Mn segregations develop in QGWB1 soils away from the main channel or on slightly raised QGWB2 soils with shallow water table Ž- 3 m.. Small amounts of Fe and Mn get reduced in the upper horizons due to a shallow water table, move down slightly and get precipitated in the lower horizons as envisaged by Duchaufour Ž1982., p. 353.. Development of Fe–Mn nodules in QGWB3 and QGWB4 soils is considered to have taken place in two phases. In the first phase, Fe–Mn segregations develop in the floodplains as in QGWB1 and QGWB2 soils. These grow in size and number with time. Sometimes this may result in gley properties as observed in the lower horizons of the Damodar Deltaic Plain soils ŽQGWB3.. In the second phase due to uplift of regions of the QGWB3 and QGWB4 soils ŽSection 9.2., water table is at moderate depth. Illuviation of clay produces Bt-horizon. Formation of Bt-horizons along with the swelling nature of soils are conductive to poor to imperfect drainage and near-surface, locally waterlogged conditions during part of the rainy season. Fe–Mn segregations with diffuse boundaries probably form in the upper horizons under these conditions. At times this may lead to gleying as observed in the upper horizons of Barind Tract soils ŽQGWB3.. It suggests that Fe–Mn segregations develop under two distinct types of drainagergroundwater conditions, related to evolution of the landscape and pedogenesis. Thus, effects of hydromorphism are observed in the form of Fe–Mn segregations in QGWB1–QGWB4 soils, chloritisation probably due to ferrolysis in the upper horizons and gleying in some of these soils. The parent materials for the Upland Red Soils and soils of the Old FluvialrDeltaic Plains have been derived from the rocks exposed to the west in the Chhotanagpur Hills, a part of the stable Peninsular Shield. These soils have developed mainly on the floodplain muds with minor intercalations of sand and silt, as indicated by lithology of C-horizon in the present study and earlier study of Bhattacharya and Banerjee Ž1979.. Only a small westernmost part of the area have the red soils developed on the Pliocene boulder conglomerates ŽVaidyanadhan and Ghosh, 1993.. Since the Chhotanagpur hills are underlain mainly by gneissrgranites covered by Haplustalfs, Paleustalfs and Ustochrepts ŽSehgal, 1993., the parent material derived from the hinterland contained significant amounts of kaolinite and kaolinite-interstratified clays. This could have favored the development of fersiallitisation and ferrugination phases in the present area. The parent materials for the Barind Tract, Ganga Plains and Bhagirathi Plain soils were mainly floodplain muds derived from the Himalayan source. These were relatively poor in kaolinite as compared to the parent materials of soils occurring south of the Ganga River. Large variations in clay content, clay mineralogy and chemical composition Žmolar ratio SiO 2rR 2 O 3 . in C-horizons of various pedons and over thicknesses of decimeters within pedons of poorly developed QGWB1–QGWB2 soils suggest that parent materials for soils of the study area varied significantly, a characteristic feature of many types of fluvial deposits. As clay content of the parent material affects pedogenesis significantly, distinction of QGWB2 soils from QGWB1 soils by methods used here is difficult. Slightly higher topographic position of QGWB2 soils relative to active floodplains ŽQGWB1 soils. helps to distinguish between the soils of the two members. A soil ŽQGWB3. that is a few thousand years old can be, however, distinguished from much younger soils on the basis of soil morphology, textural analysis and micromorphology.

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With respect to older soils ŽMiddle to Lower Holocene., experience from the present area and other parts of the Gangetic Plains shows that combined approach of soil morphology, textural analysis and micromorphology and to a small extent clay mineralogy and chemical analysis help to distinguish at least three distinct soil groups of differing ages ŽSinghai et al., 1991; Mohindra et al., 1992; Srivastava et al., 1994; Kumar et al., 1996.. In order to distinguish geomorphic units on the basis of variable degree of soil development, pedons from presentrpast floodplains, which commonly have a siltyrloamy texture and cover more than 85% of the area, were carefully selected to minimize variation in the composition of the parent material. 9.2. Role of tectonics Most of the study area lies within the Bengal Sedimentary Basin, whose contact with the Peninsular rocks is associated with the Chhotanagpur Foothill Fault. Within the Basin, there are three major tectonic units ŽFig. 3.: the Tectonic Shelf in the western part, the Barind Tract Horst in the north and the Ganga Fluvio-deltaic Plain ŽGFDP. Graben in the eastern part. The Tectonic Shelf and Barind Tract Horst are separated by the Ganga–Padma Fault and the Damodar Fault separates the Tectonic Shelf and GFDP Graben ŽFigs. 3 and 10.. The Medinipur–Farakka Fault within the Tectonic Shelf separates the Upland with Red Soils in the west from QGWB4 soils in the east, thus it divides it into two subunits. The Chhotanagpur Foothill, Medinipur–Farakka and Damodar Faults trend roughly north–south, and they are associated with scarps or

Fig. 10. Highly schematic cross-section for the region, trending NWW–SEE Žline AB in Fig. 2..

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landforms that have significant topographic reliefs of 25 m, 20 m and 6 m across them, respectively. The throw of these faults is down to the east. Earlier, Sengupta Ž1966. designated a series of north–south trending basement faults on the Tectonic Shelf as Basin Margin Faults. These faults extend for variable distances into the overlying Cretaceous–Tertiary Sequence. The previously mentioned north–south trending faults cutting through surficial deposits have been formed probably due to reactivation of the basement faults. The study area has been subjected to a number of transgressions and regressions since Cretaceous ŽRoybarman, 1983.. Of particular interest is the transgression at 7–6 ka that swept many parts of the Lower Gangetic Plain ŽBanerjee and Sen, 1987. during which sea level rose to 6–10 m above the present msl ŽMerh, 1992.. As indicated by ages of the Old FluvialrDeltaic Plain soils, starting at ca. 6 ka, the sea began receding from west to east and has returned to the present msl. As no such world-wide eustatic sea-level changes producing such transgressions at this time are known, this transgression–regression may be attributable to regional tectonic uplift and associated subsidence. Some tentative inferences about tectonic control on pedogenesis and sedimentation can be drawn from the above observations. Tectonic processes can directly affect sedimentation by upliftrsubsidence of the different tectonic blocks Žaccompanied by movements of bounding faults. and determining area of deposition. For example, during the Ž?. Early Pleistocene the entire Tectonic Shelf Unit subsided to become the site of deposition. Subsequently its uplift enabled the Red Soils to develop. During the transgression at 7–6 ka, only, the eastern Tectonic Shelf subunit subsided and was subject to a marine transgression. During regression at ca. 6 ka, its uplift along the part of Damodar Fault and subsequent erosion favored development of the Damodar Deltaic Plain at the river terminus. Tectonics seems to have influenced the overall magnitude of pedogenesis indirectly, through uplift of the western and eastern Tectonic Shelf subunits above the zone of sedimentation at about the Early Pleistocene and 6 ka, respectively. This determines the durations of pedogenesis in these subunits during subaerial exposure. Other tectonic controls on geomorphologic features of the area are also observed. The Dwarkeshwar, Silai and Kasai Rivers have an annular drainage pattern, suggesting relatively active domal uplift. Also, all or part of the courses of the rivers Ganga, Damodar and Bhagirathi Rivers are localized along faults. As mentioned earlier the Barind Tract Horst has at least three erosional levels Žwith topographic reliefs of 5 to 20 m above the major channel beds. with the well developed older soils present on higher levels. These characteristics are tentatively considered to suggest at least three probable stages of tectonic uplift.

10. Conclusions Ž1. Three major landforms Uplands, Old FluvialrDeltaic Plains and Young Fluvial Plains are identified from the Lower Gangetic Plains. Two Uplands are recognised: Upland in the west overlain by red soils and that in the north ŽBarind Tract. with three topographic levels.

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Ž2. The Upland Red Soils have a widespread, autochthonous and strongly developed soil. In the eastern, lower part of the Upland, this soil is overlain by a weakly developed red soil Žallochthonous., whose parent material was derived from the autochthonous unit at higher levels in the west. Ž3. Preliminary thermoluminescence dating of a few soils indicates that Old FluvialrDeltaic Plains are 3–6 ka and Young Fluvial Plains are less than 1.5 ka in age. The autochthonous Red Soils have been given an age of Lower to Middle Pleistocene by earlier workers. Ž4. The autochthonous Upland Red Soils exhibit features associated with the ferrugination Žferrisol. phase of Duchaufour Ž1982.. Development of soils possessing argillic horizons on the Old FluvialrDeltaic Plains and Lower Level of the Barind Tract is comparable with the fersiallitisation phase. Ž5. Development of ferrugination and fersiallitisation phases was favored due to pre-weathered nature of parent material. Ž6. Soils of the Old FluvialrDeltaic Plains and Young Fluvial Plains show effects of hydromorphism due to waterlogging in the form of segregations of Fe–Mnrgleying and chloritisation probably due to ferrolysis in upper horizons of some of these soils. Ž7. Neotectonics seems to have directly and indirectly affected development of landforms and soils significantly. Due to reactivation of some basement faults and tectonic subsidence, eastern and western subunits of the Tectonic Shelf in the western region were sites of transgression during Early Pleistocene and at about 7 ka. Uplifts of these subunits at different times that triggered regression in the Holocene have determined the length of time of pedogenesis on the subblocks. Episodic uplift of the Barind Tract Horst in the north may have given rise to three topographic levels. Some faults confine courses of rivers ŽDamodar, Ganga and Bhagirathi..

Acknowledgements Sincere thanks are due to Dr. J.A. Catt for his comments and suggestions for improving the manuscript. We are grateful to Drs. P.W. Birkeland, L.D. McFadden and D.H. Yaalon for their critical reviews of the manuscript.

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