Polygenetic Soils of the North-Central Part of the Gangetic Plains: A Micromorphological Approach

Catena 46 (2002) 243 – 259 www.elsevier.com/locate/catena

Polygenetic Soils of the North-Central Part of the Gangetic Plains: A Micromorphological Approach P. Srivastava *, B. Parkash Department of Earth Sciences, University of Roorkee, Roorkee 247 667, India Received 18 October 2000; received in revised form 29 May 2001; accepted 12 June 2001

Abstract Micromorphology indicates that soils of the central part of the Gangetic Plains are polygenetic. They occur on surfaces originating at 13 500, 8000, 2500, > 500 and < 500 BP (QGH5 to QGH1, respectively). The QGH5 soils on upland interfluves show degraded illuvial clay pedofeatures of an early humid phase (13 500 – 11 000 BP) and thick (150 – 200 mm) microlaminated clay pedofeatures of a later humid phase (6500 – 4000 BP). The earlier clay pedofeatures were degraded by bleaching, loss of preferred orientation, development of a coarse speckled appearance and fragmentation, whereas those of the later phase are thick, smooth and strongly birefringent microlaminated clay pedofeatures. The illuviation was more extensive during the later phase, as indicated by enrichment of groundmass as discrete pedofeatures of clay intercalations. Pedogenic carbonate was formed during the intervening dry phase from the early Holocene to 6500 BP. It forms irregularly shaped nodules of micrite and diffuse needles with inclusions of soil constituents. The subsequent change to wetter conditions caused dissolution – reprecipitation, which resulted in partial to complete removal of carbonate from soils over large areas. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Micromorphology; Mineral weathering; Polygenesis; Clay illuviation; Carbonate nodules

1. Introduction The Gangetic Plains, lying between the tectonically active Himalayas to the north and Peninsular India to the south, have various soils formed on stable land surfaces during the Quaternary Period. The Plains are also tectonically active because of the northward push of the Indian Plate at the rate of 2– 5 cm year 1 (Parkash et al., 1980; Demets et al., * Corresponding author. Present address: Division of Soil Resource Studies, National Bureau of Soil Survey and Land Use Planning, Amravati Road, Nagpur 440 010, India. E-mail address: [email protected] (P. Srivastava).

0341-8162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 ( 0 1 ) 0 0 1 7 2 - 2

244

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

1990). The north-central part, between the Ramganga and Rapti Rivers, is an important segment of the most extensive fluvial plains of the world. Soil development in the area was influenced by climate change and neotectonism (Srivastava et al., 1994). Spasmodic neotectonism led to unstable and stable phases, the latter leading to soil development (Srivastava et al., 1994; Kumar et al., 1996; Parkash et al., 2000). Frequent climatic changes occurred during the Quaternary (Ritter, 1996). This palaeoclimatic record has been documented from the NW and SW parts of India (Singh et al., 1972, 1974, 1990; Hashmi and Nair, 1986; Agarwal, 1992). Climatic variation has also been inferred from Holocene soils (Srivastava et al., 1998), which show the formation of trioctahedral vermiculite and smectite from biotite in a cold arid to semiarid climate in the early Holocene and, during a subsequent humid climate after 6500 BP, the smectite was transformed to interstratified smectite – kaolinite (Sm/K). It is difficult to distinguish pedogenic features of different climatic origins in soils developed in fluvial deposits, especially those disturbed tectonically (Brewer, 1964; Chartres, 1984). However, micromorphology can be used to infer palaeoenvironments from soils (Fedoroff et al., 1990; Kemp and Derbyshire, 1998). It can be used to reconstruct the history of polygenetic soils from their fabric and pedogenic features (Brewer and Sleeman, 1969; Mucher and Morozova, 1983; Chartres, 1984; Fedoroff et al., 1990; Bronger et al., 1994; Muggler and Buurman, 1997). We used micromorphological, physical, chemical, and mineralogical analyses to determine the polygenetic nature of soils on the Gangetic Plains. Micromorphology helped to distinguish pedogenic and nonpedogenic features, older and younger clay pedofeatures, the extent of degradation of pedofeatures in subsequent periods, and the effects of weathering and neoformation.

2. Materials and methods The area studied is in north-central India between the Ramganga and Rapti Rivers (lat 26 to 2930VN and long 78 to 82E) (Fig. 1a). The climate is subhumid. The Himalayas to the north influence the climate, as the temperature in summer may remain below 40 C near the mountains (e.g. at Gonda), but well above 40 C in the southern part (at Allahabad and Kanpur). Monsoonal rainfall occurs from July to August and accounts for most of the annual rainfall. The monsoonal and mean annual rainfall decrease westwards and southwards (Faizabad 1008 mm, Allahabad 975 mm, Lucknow 940 mm). Soil-geomorphic units from the study area were identified using topographic maps and false colour composites. In the field, 47 soil profiles were studied along three transects using the methods of Soil Survey Staff (1966). Particle size distribution was determined after removal of organic matter, calcium carbonate and free iron oxide. Sand (2000 –50 mm), silt (50 –2 mm), and total clay ( < 2 mm) were separated according to the procedure of Jackson (1979) and pH was determined by the method of Richards (1954). For micromorphological studies, oriented and undisturbed soil samples were collected in metal boxes (9  6  6 cm) from different horizons of the pedons as described by Murphy (1986). The samples were air-dried and impregnated with crystic resin (Jongerius and Heintzberger, 1963). Large thin sections (8  6 cm) were described

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

245

Fig. 1. (a) Location and drainage pattern of the north-central Gangetic Plains; (b) soil chrono-associations (QGH1 – QGH5) and soil-geomorphic units of the same area.

246

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

according to the terminology of Bullock et al. (1985). Sand-sized primary minerals from the Bt and BC horizons were examined by petrographic microscope and then fixed on an aluminium stub and coated with gold for study with a Philips Scanning Electron Microscope (SEM).

3. Geomorphology and soils of the area The Indo-Gangetic basin constitutes a major geomorphic unit lying between Peninsular India and the extra-Peninsular region. To the north, it is limited by the outermost Himalayan Ranges (Siwalik Hills) and, to the south, by outcrops of Precambrian rocks. The basin resulted from collision of the Indian and Chinese Plates during the Middle Miocene (Parkash et al., 1980). The Indian Plate is moving N/NE at 2– 5 cm year 1 (Demets et al., 1990; Srivastava et al., 1994; Kumar et al., 1996). The basin is an asymmetric trough with a maximum thickness (about 10 km) in the north (Rao, 1973; Raiverman et al., 1983). The main sedimentation in the area is by rivers debouching onto the Plains from the Himalayas to form megacones (Geddes, 1960). The Gangetic Plains are a productive province supporting about one-third of the population of India. The area studied is a monotonous plain with negligible relief and includes much of the eastern Upper Ganga Plain and part of the Middle Ganga Plain (Singh and Singh, 1971). Topographically, the most significant feature is a submontane piedmont belt (‘‘Bhabhar’’ locally) striking east – west along the Siwalik foothills. This belt is 20 – 30 km wide, has a southerly regional slope (3 –5 m km 1) and a dense parallel to subparallel drainage pattern. To the south of this piedmont zone, the alluvial plains of the rivers constitute a second important landform. The plains are marked by paleochannels and entrenched river courses, with extensive interfluves, which are the highest parts of the Gangetic Plains. Based on soil characteristics, the following soil-geomorphic units were recognized. (1) Piedmonts: the Kosi-Gola Piedmont (KGPD), Gholia-Dhobania Piedmont (GDPD), Young Sihali-Kandra Piedmont (YSKPD), and Old Sihali-Kandra Piedmont (OSKPD). (2) Plains: the Upper Kosi-Gola Plain (UKGP), Lower Kosi-Gola Plain (LKGP), Young Ghaghra Plain (YGP), Old Ghaghra Plain (OGP), Ghaghra Floodplain (GFP), and Rapti Floodplain (RFP). (3) Interfluves: the Upper Deoha/Ganga-Ghaghra Interfluve (UDGGHIF), Middle Deoha/Ganga-Ghaghra Interfluve (MDGGHIF), Lower Deoha/Ganga-Ghaghra Interfluve (LDGGHIF), Upper Rapti-Ghaghra Interfluve (URGHIF), and Lower Rapti-Ghaghra Interfluve (LRGHIF). The Gangetic Plains are known to be tectonically active (Mohindra et al., 1992; Srivastava et al., 1994; Kumar et al., 1996; Singh et al., 1998), and tectonic movement of blocks bounded by faults has helped to preserve the climatic records of different periods. Uplift of blocks above the general level of rivers in the region led to the termination of sedimentation and initiation of pedogenic activity. Soils formed on the uplifted blocks, therefore, record the climate prevailing at that time, and uplift of different blocks at different times provides a sequence of soils showing the imprints of a sequence of climates,

Table 1 Physical and chemical properties of soils General features

Representative pedon Soil-geomorphic unit

Horizonation (B2 horizon thickness)

Remarks

QGHI < 500 BP

Floodplains and Piedmonts (GFP, RFP, KGPD, GDPD, YSKPD)

A/C

Sedimentation and rarely 15 – 20 cm soils

QGH2 500 – 2500 BP

Remnant Piedmont and Young River Plains (UKGP, OSKPD, YGP)

A/B/C (21 – 57 cm)

Weakly developed soils

QGH3 2500 – 5000 BP

Old River Plains and Interfluves (LKGP, OGP, URGHIF, UDGGHIF)

A/B/C (52 – 85 cm)

Moderately developed soils

QGH4 8000 BP

Interfluve (LDGGHIF)a

A/B/C (72 cm)

Strongly developed soils in lower part of the largest interfluve adjacent to MDGGHIF in the east

QGH5 13 500 BP

Interfluve (MDGGHIF, LRGHIF)

A/B/C (93 – 97 cm in salt affected and 105 – 137 cm in non salt affected parts)

Very strongly developed soils of ancient floodplains and paleochannels

a

Depth (cm)

Horizon

Colour

pH (1:2) soil water

CaCO3 in < 2 mm (%)

Sand (%)

Silt (%)

Clay (%)

Pedon PA3, 0 – 30 30 – 60 60 – 90

Typic Ustorthent Ap 2.5Y 3/2 C1 2.5Y 4/4 C2 2.5Y 5/4

7.6 7.5 7.8

– – 2.4

28.1 30.6 18.1

57.8 56.9 73.6

14.1 12.5 8.2

Pedon PA5, 0 – 16 16 – 28 28 – 53 53 – 77 77 – 105

Typic Ustochrept Ap 2.5Y Bw1 2.5Y Bw2 2.5Y Bw3 2.5Y BC 2.5Y

5/3 5/2 5/2 5/3 5/4

8.1 8.2 8.2 8.4 8.7

– 1.0 1.0 2.5 1.5

5.8 3.3 7.9 2.8 6.2

72.3 73.4 68.9 67.5 69.6

21.9 23.3 23.0 29.6 24.7

Pedon PB5, 0 – 15 15 – 40 40 – 68 68 – 95 95 – 130

Typic Haplustalf Ap 2.5Y Bw1 2.5Y Bt1 2.5Y Bt2 2.5Y BC 2.5Y

5/3 4/3 4/2 4/2 4/3

8.3 8.4 8.4 8.6 8.8

2.4 4.2 3.4 6.8 14.3

22.4 10.8 10.2 11.3 18.8

60.2 65.6 62.3 66.3 68.7

17.4 23.6 27.5 22.4 12.5

Pedon RC2, 0 – 16 16 – 28 28 – 48 48 – 72 72 – 95 95 – 120

Typic Haplustalf Ap 2.5Y AB 2.5Y Bt1 2.5Y Bt2 2.5Y Bt3 2.5Y C 2.5Y

5/5 5/4 4.5/4 4.5/4 5/4 6/4

– – – – – –

– – – – – –

19.5 17.3 20.3 20.1 15.8 17.7

58.7 58.5 54.2 53.5 59.6 59.7

21.8 24.2 25.5 26.6 24.6 22.8

Pedon PC6, 0 – 22 22 – 42 42 – 60 60 – 86 86 – 145 145 +

Typic Haplustalf Ap 2.5Y AB 2.5Y Bt1 2.5Y Bt2 2.5Y Bt3 2.5Y BC 2.5Y

5/4 4.5/4 4/4 4/4 4/4 5/4

7.9 7.8 7.9 8.2 8.3 8.8

– – 1.2 4.6 3.4 –

15.6 12.3 10.9 10.6 12.5 10.5

63.5 64.6 60.8 61.5 61.3 70.9

20.9 23.1 28.3 27.9 26.2 18.6

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

Member age (BP)

After Mohindra et al. (1992).

247

248

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

the extent of soil development being mainly a function of the length of period elapsed since uplift. However, if the climate changes, older soils are modified by the later climate and show polygenetic characteristics. The soils of the area we studied were grouped into five members (QGH1 to QGH5) forming a soil chrono-association on the basis of degree of development (Fig. 1b), as indicated by B-horizon thickness, clay accumulation index (Levine and Ciolkosz, 1983), degree of pedality (Bullock et al., 1985) and the thickness and nature of clay pedofeatures and plasma separations. About 50% of the area studied is marked by stable uplifted blocks covered by QGH5 soils on the MDGGHIF and LRGHIF units. Soils started developing on these blocks after their uplift at about 13 500 BP. Later tectonic movements uplifted other blocks and younger soils (QGH1 –QGH4) developed on them. From QGH1 to QGH5, the soils show systematic variations of morphology, texture, chemical composition, and pedofeatures, with maximum development in the QGH5 soils. Table 1 gives physical and chemical properties of one representative pedon from each member. Each member was dated by radiocarbon and thermoluminescence (TL) methods, historical evidence and relative degree of soil development (Srivastava et al., 1994, 1998). Radiocarbon dates from the most strongly developed soils of QGH5 range from 11 000 to 9000 BP (Rajagopalan, 1992). Two 14C dates for calcrete from LDGGHIF and LGYIF of QGH4 soils are 6500 and 7000 BP, respectively (Mohindra et al., 1992; Kumar et al., 1996). TL dates of soils from MDGGHIF and LGYIF are 13 600 and 8300 BP, respectively (Das, 1993). These reflect the start of pedogenesis and can be regarded as the maximum age of the soils (Singh et al., 1998). The QGH3 soils overlap in character the QGD3 and QGD4 soils occurring on the adjoining Gandak Megafan (Mohindra et al., 1992). The Gandak River has shifted eastward due to tilting of its megafan (Mohindra et al., 1992). Historical evidence (Mathur, 1969, p. 214) suggests that during the time of Buddha (2500 BP), the Gandak flowed close to the town of Kushinagar (presently called Kasia). Based on the historically recorded positions of the River Gandak during its eastward shift by over 80 km in the last 5000 years, QGD3 and QGD4 soils were assigned ages of 2500 and 5000 BP, respectively, (Mohindra et al., 1992), so the QGH3 soils are dated to 2500 – 5000 BP. For the soils of QGH1 and QGH2, tentative ages are given as < 500 and >500 BP, respectively.

4. Results Thin sections of A, B, and C horizons of representative soils from each soil-geomorphic unit (Table 2) were described according to Bullock et al., (1985). Important micromorphological features of each member are described below. 4.1. QGH1 soils These are poorly developed soils occurring on piedmonts (KGPD, YSKPD, GDPD) and floodplains (GFP and RFP). These usually have A/C profiles rarely with thin (15 –

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

249

20 cm) B horizons. They are mainly apedal and massive, rarely with very weakly developed subangular blocky structure (Fig. 2a). The coarse/fine (C/F, 20 mm limit) ratio for these varies from 80/20 to 60/40. Floodplains show gefuric, chitonic and monic related distribution patterns (rdp) and, in the piedmonts, the rdp varies from enaulic to close porphyric. The nature and distribution of sand and silt indicate sedimentary layering and very weak pedogenic alteration. Most of the minerals and lithorelicts of shale and schist do not show any postdepositional alteration. The voids are mainly interconnected rough surfaced channels and vughs. Intergrain and bridged grain microstructures are more common in the floodplain soils than in those of the piedmont. There has been too little pedogenic activity for any plasma separation, and most soils have an undifferentiated b fabric. However, in some soils from the piedmonts, parallel orientation of silt-sized mica flakes is inherited from the original sediment. Development of secondary calcium carbonate in lower horizons is common in the piedmont soils, but is a rare feature in the floodplain soils. 4.2. QGH2 soils These are weakly-to-moderately developed soils on remnant piedmont (OSKPD) and river plains of the Kosi-Gola (UKGP) and Ghaghra Rivers (YGP). In these soils, the B2 horizon is 25 –45 cm thick and shows weakly-to-moderately developed subangular blocky structure. The peds are partially separated by channel and vughy microstructures. The upper horizons show enaulic and porphyric rdp and the B horizons are predominantly porphyric. Thin (20 – 30 mm) illuvial clay pedofeatures along the voids are common in the B horizons (Fig. 2b). These horizons are also characterized by moderately developed cross-striated b fabric resulting from plasma separation. Most of the primary minerals show some alteration. Micas show bleaching and exfoliation and feldspars show etch pits and formation of clay minerals. Isotubules and fabric features of reworked soil as excremental aggregates indicate strong animal activity in these soils. These are possibly produced by earthworms (Bullock et al., 1985). Secondary calcium carbonate is a common feature of these soils (Fig. 2c). It forms sparitic and prismatic calcite nodules and coatings along voids. In lower horizons, these are marked by extensive dissolution – precipitation features and there is a crystallitic plasmic b fabric because of fine carbonates in the groundmass. 4.3. QGH3 soils These are moderately-to-strongly developed soils occurring on lower parts of the Kosi-Gola Plain (LKGP) and upper parts of upland interfluves (UDGGHIF and URGHIF). The B2 horizons have a strongly developed subangular blocky structure and are 55 – 80 cm thick. The peds are accommodating and partially separable. Total clay ( < 2 mm) shows an appreciable increase with depth into the Bt horizons, which have C/F ratios of 30/70 – 40/60 and a predominantly porphyric rdp. A strongly developed crossstriated b fabric indicates strong plasma separation. Illuvial pedofeatures constitute >1% of the thin sections; they are 50– 60-mm-thick, microlaminated clay coatings along voids (Fig. 2d), composed of yellowish brown limpid to dusty clay. Primary minerals are

250

Table 2 Micromorphological properties of the soils Pedality

Surface of voids

C/F related distribution

b fabric

Clay pedofeatures

CaCO3

Remarks

QGH1 Kosi-Gola Piedmont (KGPD)

Apedal

Rough channels and vughs

Gefuric

–

–

Gholia-Dhobania Piedmont (GDPD)

Apedal massive

Rough channels and vughs

Enaulic and porphyric

Undifferentiated and stipple speckled Undifferentiated and weakly cross-striated

–

Hypocoatings and nodules of sparite

Rapti Flood Plain (RFP)

Apedal massive

Rough channels and vesicles

Monic and Gefuric

–

Young Sihalikandra Piedmont (YSKGP)

Apedal

Rough channels and vughs

Gefuric

Undifferentiated and parallel striated Undifferentiated

–

Hypocoatings and nodules of sparite –

Ghaghra Flood Plain (GFP)

Apedal

Rough simple packing

Monic and Gefuric

Undifferentiated and parallel striated

–

–

Primary minerals are unaltered, compact grain microstructure Fresh to weakly weathered minerals with vughy and channel microstructure Fresh to weakly altered minerals with compact grain microstructure Fresh minerals and compact grain microstructure Fresh to weakly altered minerals and intergrain microstructure

QGH2 Upper Kosi-Gola Plain (UKGP)

Weak subangular blocky

Rough channels and vughs

Enaulic and porphyric

Moderate stipple speckled and cross-striated

Few 30 – 40 mm clay coatings

Old Sihali-Kandra Piedmont (OSKPD)

Weak subangular blocky

Partially smooth channels and vughs

Enaulic and porphyric

Stipple speckled

Few 30 – 40 mm coatings

Hypocoatings of micrite and sparite and nodules –

Young Ghaghra Plain (YGP)

Apedal to weak subangular blocky

Rough to partially smooth channels

Monic and Gefuric

Weak to moderate poro and cross striated

Few 20 – 30 mm clay coatings

Micrite hypocoatings and nodules

Weakly to moderately altered minerals with channel microstructure Weakly altered minerals with channel and vughy microstructure Fresh to weakly altered minerals with intergrain channel and vughy microstructure

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

Soil-geomorphic unit

Moderate to strong subangular blocky

Smooth channels and vughs

Porphyric

Moderate cross and reticulate striated

Old Ghaghra Plain (OGP)

Moderate subangular blocky

Smooth channels and vughs

Porphyric

Moderate to strong cross and reticulate striated

Upper Deoha/ Ganga-Ghaghra Interfluve (UDGGHIF)

Moderate to strong subangular blocky

Smooth channels

Porphyric

Upper Rapti-Ghaghra Interfluve (URGHIF)

Moderate subangular blocky

Smooth channels and vughs

Porphyric

QGH4 Lower Deoha/ Ganga-Ghaghra Interfluve (LDGGHIF)a

Strong subangular blocky

Smooth channels

QGH5 Middle Deoha/ Ganga-Ghaghra Interfluve (MDGGHIF)

Very strong subangular blocky

Lower Rapti-Ghaghra Interfluve (LRGHIF)

Very strong subangular blocky

a

Common 50 – 60 mm microlaminated clay coatings Common 60 – 70 mm microlaminated clay coatings

Impure sparitic nodules

Moderate to strong poro, cross and reticulate striated Moderate stipple speckled and cross striated

Common 50 – 60 mm microlaminated clay coatings

Impure sparitic nodules

Common 50 – 60 mm microlaminated clay coatings

–

Moderately to strongly altered minerals with channel microstructure

Porphyric

Strong cross, poro and reticulate striated

Common 80 – 100 mm microlaminated clay coatings

Impure micrite nodules

Strongly altered minerals with channel microstructure

Smooth channels

Porphyric

Strong cross and reticulate striated

Nodules of impure micrite and diffused needles

Strongly to completely altered minerals with channel microstructure

Smooth channels

Porphyric

Strong cross and reticulate striated

Common 150 – 200 mm microlaminated clay coatings and disrupted clay pedofeatures Common 150 – 200 mm microlaminated clay coatings and disrupted clay pedofeatures

Nodules and disseminated CaCO3

Moderately to very strongly altered minerals with channel microstructure

–

Moderately to strongly altered minerals with channel microstructure Moderately to strongly altered minerals with channel and vughy microstructure Moderately to strongly altered minerals with channel microstructure

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

QGH3 Lower Kosi-Gola Plain (LKGP)

After Mohindra et al. (1992).

251

252

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

strongly altered: plagioclase feldspars and biotite micas, in particular, show more alteration than in the QGH2 soils. Rubified masking of the soil fabric, Fe– Mn mottles and Fe –Mn concretions commonly occur. Secondary carbonate is rare, occurring only as

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

253

Fig. 2. Micromorphological features of QGH1 – QGH5 soils. (a) Apedal soil of piedmont, Pedon PA3, C Horizon, (KGPD), QGH1; (b) thin (20 – 30 mm) clay pedofeature in Pedon PA6, B horizon, (UKGP), QGH2; (c) secondary calcium carbonate in Pedon PA5, C horizon, (UKGP), QGH2; (d) illuvial clay pedofeature of Pedon PB9, Bt horizon, (OGP), QGH3; (e) strongly developed subangular blocky structure in Pedon PB7, (MDGGHIF), QGH5; (f) thick (200 mm) microlaminated clay pedofeature of limpid to dusty clay, Pedon PC5, Bt horizon, (MDGGHIF), QGH5; (g) strongly oriented yellowish brown clay pedofeature of Pedon PB7, (MDGGHIF), QGH5; (h) discrete clay pedofeature as clay intercalations in the groundmass of Pedon PC5, Bt horizon, (MDGGHIF), QGH5; (i) degraded clay pedofeature showing fragmentation and loss of orientation in Pedon PC5, Bt horizon, (MDGGHIF), QGH5; (j) degraded clay pedofeature showing loss of birefringence in Pedon PC5, Bt horizon, (MDGGHIF), QGH5; (k) degraded clay pedofeature showing disruption from voids and loss of orientation in Pedon PC5, Bt horizon, (MDGGHIF), QGH5; (l) thin clay coatings on secondary calcium carbonate nodules (marked by arrows) in Pedon PC5, Bt horizon, (MDGGHIF), QGH5; (m) secondary calcium carbonate of Pedon PC1, Bt horizon, (MDGGHIF), QGH5. All between crossed polarisers.

impure, coarse irregularly shaped nodules in the northern part of the Lower Kosi-Gola Plain (LKGP) and in the southern part of the Upper Deoha/Ganga-Ghaghra interfluve (UDGGHIF).

254

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

4.4. QGH4 soils These soils occur in lower parts of Ganga-Ghaghra Interfluve in the east of the Middle Deoha/Ganga-Ghaghra Interfluve (Mohindra et al., 1992). This upland unit is marked by patchy occurrence of salt in the surface layer. Rivers draining the centre of this unit have narrow incised channels. The soils are very mature with 50 – 75-cm-thick Bt horizons. Well-developed subangular blocky peds are separable and accommodating. The C/F ratio is usually 40/60 and the rdp is porphyric. Micas and feldspars show strong alteration. Large domains of welldeveloped cross and reticulate striated b fabric indicate strong plasma separation. Illuvial clay pedofeatures constitute >1% of thin sections from the Bt horizons; they are 80 –100mm-thick, microlaminated, strongly oriented, pale yellow to deep yellowish brown clay coatings along voids. Some of the disrupted clay pedofeatures are speckled and fragmented. Pedogenic carbonate formation is an important feature of these soils. It occurs as hypocoatings and nodules of dense impure micrite with dissolution –reprecipitation features. 4.5. QGH5 soils These are the most strongly developed soils covering >50% of the upland interfluves of Ganga-Ghaghra (MDDGHIF) and Rapti-Ghaghra (LRGHIF). The surface layer shows elongated patches of salt efflorescence. Areas without salt efflorescence are slightly raised (15 – 20 cm) and have 120– 130-cm-thick B2 horizons, whereas areas with salt efflorescence have 90 – 95-cm-thick B2 horizons. Subangular blocky peds are very strongly expressed in these soils and often show complete separation (Fig. 2e). Channel microstructures with smooth surfaces are very common, the C/F ratio is usually 30/70 – 40/60 and the rdp is dominantly open porphyric. Biotite shows bleaching, exfoliation, curling and partial dissolution. Feldspars are also strongly altered with dissolution features and neoformation of clay minerals. Strongly developed cross and reticulate striated b fabric is extensive in thin sections of the Bt horizons. There is a substantial increase in total clay ( < 2 mm) with depth. Thin sections from the Bt horizons of QGH5 soils indicate formation of more than one type of clay pedofeature by different phases of illuviation activity. These include (i) deep yellow microlaminated 150 –200-mm-thick coatings of limpid clay (Fig. 2f), (ii) strongly oriented yellowish brown clay pedofeatures of 100 – 150 mm thickness along channel voids (Fig. 2g), (iii) discrete fabric pedofeatures as clay intercalations resulting from enrichment of the groundmass by illuviated clay (Fig. 2h), (iv) disrupted clay pedofeatures along voids showing bleaching and fragmentation (Fig. 2i, j), (v) large domains of fragmented clay pedofeatures showing distinct microlamination and rubification, but separated from voids (Fig. 2k), and (vi) thin (20 – 30 mm) impure clay coatings on secondary carbonates (Fig. 2l). Clay coatings are perhaps the most striking pedofeatures reported in soils of polygenetic origin (Kemp, 1999). The paleoenvironmental significance of disrupted illuvial clay pedofeatures was discussed by van VlietLanoe¨ et al. (1992, 1995) and Kemp (1999); they were interpreted as remnant illuvial clay pedofeatures of past climates.

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

255

Secondary calcium carbonate occurs throughout the pedons in areas with salt efflorescence; however, it is confined to B2 or B3 horizons of the pedons in areas without salts (Fig. 2m). The carbonates are thin filaments to irregularly shaped nodules made of impure micrite and microsparite with inclusions of soil constituents. SEM study showed dense micrite with diffuse needles and inclusion of quartz, mica and feldspar. Within the carbonate, these materials show more weathering than those in the adjacent groundmass. The sequence of pedogenic development established from the thin sections indicates that the degraded illuvial clay pedofeatures were the first to develop, followed by the formation of pedogenic calcrete and lastly the nondegraded clay pedofeatures.

5. Discussion 5.1. Systematic pedogenic changes and weathering Soils developed in the fluvial deposits of the Gangetic Plains show systematic variation with increasing age from very weakly altered sediments in QGH1 to the most strongly developed pedofeatures in QGH5. Peds have developed progressively from the nonaggregated parent material, and the surrounding voids and extent of separation define the degree of pedality (Bullock et al., 1985). The soils of QGH1 are mainly apedal with massive structure. QGH2 to QGH5 soils show greater pedality from weakly to very strongly developed subangular blocky structure. The void pattern also changes: in QGH1 the voids are mainly of the simple packing type; however, in QGH5 soils, they are of the intrapedal accommodating type with smooth surfaced channels. In Ap horizons, the structure may be modified by cultivation (Curmi et al., 1994; Hall, 1994), and clods in the Ap horizons can be attributed to this; however, in lower horizons, well-developed peds must have resulted from natural pedogenic processes. Soil evolution can be assessed by comparing the properties resulting from pedological changes with those of the parent material. The related distribution pattern (rdp) of plasma and skeleton grains has evolved from chitonic and enaulic to open porphyric in the more strongly developed soils. The weathering processes in these soils are characterized by dissolution of plagioclase and transformation of biotite into clay minerals (Srivastava et al., 1994, 1998). The petrographic study shows more strongly weathered minerals in the progressively older soils, but does not indicate a polyphase alteration pattern (Fedoroff et al., 1990; Delvigne, 1994). In the youngest soils (QGH1), all the minerals are fresh to weakly weathered; however, in QGH5 soils, the plagioclase and biotite alteration patterns correspond to classes 3 and 4 of Stoops et al., (1979). The plagioclase grains show increased alteration along cleavage/twinning planes with increasing age of the soils (Fig. 3a, b, c, d). Orthoclase feldspar showed less alteration, though microcline grains are strongly weathered in the QGH5 soils. Biotite decreases in abundance with increasing age of soils. In the QGH5 soils, many of the biotite flakes are completely altered and show exfoliation, curling and bleaching features (Fig. 3g, h). Release of Fe from biotite has produced micas resembling muscovite in character (Bisdom et al., 1982). Muscovite is the predominant mica, but shows no weathering in most of the soils (Fig. 3e, f ); however, in QGH5, it shows a little exfoliation along the edges.

256

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

Fig. 3. Photomicrographs showing weathering features of minerals. (a) Unaltered plagioclase (P) and biotite (B) from QGH1 soil in thin section; (b) the same plagioclase under SEM; (c) strongly altered plagioclase (P) in QGH5 soil in thin section; (d) the same under SEM; (e) unaltered muscovite (M) in QGH5 soil in thin section; (f) the same under SEM; (g) strongly altered biotite (B) in QGH5 soil in thin section; (h) the same under SEM. All optical photomicrographs between crossed polarisers.

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

257

Table 3 Role of climate on pedogenic processes Period

Climate

Pedogenic processes and pedogenic features

Effect of climate on pedofeatures

13 500 – 11 000 BP

Humid

–

11 000 – 6500 BP

Semiarid

Illuviation and weathering of minerals in QGH5 soils Calcification or formation of pedogenic calcrete

6500 – 4000 BP

Humid

Decalcification and second phase of illuviation

Degradation of earlier clay pedofeatures and retardation of feldspar and biotite weathering Dissolution of calcrete and degradation of earlier clay pedofeatures

5.2. Role of climate in polygenesis Micromorphological observations of the soils on stable upland interfluves (MDGGHIF and LRGHIF) clearly demonstrate the influence of climatic changes in their evolution. Formation of these soils began only after the end of a cold arid period during the last glaciation. The degraded thick illuvial clay pedofeatures record the earliest phase of pedogenesis in humid conditions. The climate then shifted to semiarid conditions favouring formation of pedogenic calcium carbonate. TL and radiocarbon dates indicate formation of the degraded illuvial clay pedofeatures before 11 000 BP and pedogenic carbonate between 11 000 – 6500 BP. The episode of pedogenic carbonate formation was followed by a wetter phase, in which further clay illuviation occurred. These later clay pedofeatures show strong orientation and continuity along voids unlike the degraded clay pedofeatures. During this humid phase, the pedofeatures of earlier phases were also affected: pedogenic carbonate was partially dissolved and reprecipitated in lower horizons. Extensive dissolution –reprecipitation features indicate this phase. However, some of the calcium carbonate nodules escaped dissolution and were coated with thin illuvial clay. This phase of increased rainfall seems to have continued until 4000 BP and since then present climatic conditions have continued with minor modifications (Table 3).

6. Conclusions Our work demonstrates that some of the soils (QGH3– QGH5) developed in the alluvium of the Gangetic Plains are polygenetic. Degraded illuvial clay pedofeatures, pedogenic calcrete, unaltered thick illuvial clay pedofeatures, and weathered minerals constitute a set of polygenetic features in soils that developed after 13 500 BP. This shows that micromorphology can be effectively used in detailing polygenesis induced by climate changes in soils of the Gangetic Plains.

References Agarwal, D.P., 1992. Man and Environments in India Through Ages. Books and Books Publishers, New Delhi.

258

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

Bisdom, E.B.A., Stoops, G., Delvigne, J., Curmi, P., Altemuller, H.J., 1982. Micromorphology of weathering biotite and its secondary products. Pedologie 32, 225 – 252. Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Wiley, New York. Brewer, R., Sleeman, J.R., 1969. The arrangement of constituents in Quaternary soils. Soil Sci. 107, 435 – 441. Bronger, A., Bruhn-Lobin, N., Heinkele, T., 1994. Micromorphology of paleosols—genetic and paleoenvironmental deductions: case studies from central China, south India, NW Morocco and Great Plains of the USA. In: Ringrose-Voase, A.J., Humphreys, G.S. (Eds.), Soil Micromorphology: Studies in Management and Genesis. Elsevier, Amsterdam, pp. 187 – 206. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T., 1985. Handbook of Soil Thin Section Description. Waine Research Publications, Wolverhampton. Chartres, C.J., 1984. The micromorphology of quaternary river terrace deposits in the Kennet Valley Berkshire, England. Earth Surf. Process. Landforms 9, 343 – 355. Curmi, P., Kertzman, F.F., Neto, J.P.Q., 1994. Degradation of structure and hydraulic properties in an Oxisol under cultivation (Brazil). In: Ringrose-Voase, A.J., Humphreys, G.S. (Eds.), Soil Micromorphology: Studies in Management and Genesis. Elsevier, Amsterdam, pp. 569 – 580. Das, P., 1993. Thermoluminescence dating of soils of the Western Gangetic Plains. MTech thesis, University of Roorkee, India. Delvigne, J.E., 1994. Proposals for classifying and describing secondary micro-structures observed within completely weathered minerals. In: Ringrose-Voase, A.J., Humphreys, G.S. (Eds.), Soil Micromorphology: Studies in Management and Genesis. Elsevier, Amsterdam, pp. 333 – 342. Demets, C., Gorden, R.G., Argus, D.P., Stein, S., 1990. Current plate motions. Geophys. J. Int. 101, 425 – 478. Fedoroff, N., Courty, M.A., Thompson, M.L., 1990. Micromorphological evidence of paleoenvironmental change in Pleistocene and Holocene paleosols. In: Douglas, L.A. (Ed.), Soil Micromorphology: A Basic and Applied Science. Elsevier, Amsterdam, pp. 653 – 665. Geddes, A., 1960. Alluvial morphology of the Indo-Gangetic Plains: its mapping and geographical significance. Trans. Inst. Br. Geogr. 28, 253 – 276. Hall, N.W., 1994. Soil structure transformations over the growing season—a micromorphological approach. In: Ringrose-Voase, A.J., Humphreys, G.S. (Eds.), Soil Micromorphology: Studies in Management and Genesis. Elsevier, Amsterdam, pp. 659 – 668. Hashmi, N.H., Nair, R.R., 1986. Climatic aridity over India 11000 years ago: evidence from feldspar distribution in shelf sediments. Palaeogeogr., Palaeoclimatol., Palaeoecol. 53, 309 – 319. Jackson, M.L., 1979. Soil Chemical Analysis—Advanced Course. Published by the author, Madison, WI, USA. Jongerius, A., Heintzberger, G., 1963. Methods in Soil Micromorphology: A Technique for the Preparation of Large Thin Sections. Soil Survey Paper 10. Netherlands Soil Survey Institute, Wageningen. Kemp, R.A., 1999. Micromorphology of loess-paleosol sequences: a record of paleoenvironmental change. Catena 35, 179 – 196. Kemp, R.A., Derbyshire, E., 1998. The loess of China as records of climatic change. Eur. J. Soil Sci. 49, 525 – 539. Kumar, S., Parkash, B., Manchanda, M.L., Singhvi, A.K., Srivastava, P., 1996. Holocene landform and soil evolution of the Western Gangetic Plains: implications of neotectonics and climate. Z. Geomorphol. N.F. 103, 283 – 812. Levine, E.L., Ciolkosz, E.J., 1983. Soil development in tillites of various ages in northern Pennsylvania. Quat. Res. 19, 85 – 99. Mathur, V.K., 1969. Aithasic Sthanavali. Scientific and Technological Terminology Commission, Education Ministry, New Delhi (in Hindi). Mohindra, R., Parkash, B., Prasad, J., 1992. Historical geomorphology of Gandak Megafan, Middle Gangetic Plain, India. Earth Surf. Process. Landforms 17, 643 – 662. Mucher, H.J., Morozova, T.D., 1983. The application of soil micromorphology in quaternary geology and geomorphology. In: Bullock, P., Murphy, C. (Eds.), Soil Micromorphology. Academic Press, Berkhamsted, U.K., pp. 151 – 194. Muggler, C.C., Buurman, P., 1997. Micromorphological aspects of polygenetic soils developed on phyllitic rocks in Minas Gerais, Brazil. In: Shoba, S., Gerasimova, M., Miedema, R. (Eds.), Soil Micromorphology: Studies on Soil Diversity, Diagnostic, Dynamics. Proceedings 10th International Working Meeting on Soil Micromorphology, Moscow, 1996, International Society of Soil Science, Wageningen, pp. 129 – 138.

P. Srivastava, B. Parkash / Catena 46 (2002) 243–259

259

Murphy, C.P., 1986. Thin Section Preparation of Soils and Sediments. AB Academic Publishers, U.K. Parkash, B., Sharma, R.P., Roy, A.K., 1980. The Siwalik Group (Molasse) — sediments shed by collision of continental plates. Sediment. Geol. 25, 127 – 159. Parkash, B., Kumar, S., Rao, M.S., Kumar, C.S., Gupta, S., Srivastava, P., 2000. Holocene tectonic movements and stress field in the western Gangetic Plains. Curr. Sci. 79, 438 – 449. Raiverman, V., Kunte, S.V., Mukherji, A., 1983. Basin geometry, Cenozoic sedimentation and hydrocarbon prospects in northwestern Himalaya and Indo-Gangetic Plains. Petrol. Asia 6, 67 – 97. Rajagopalan, G., 1992. Radiocarbon ages of carbonate materials from Gangetic alluvium. In: Singh, I. (Ed.), Gangetic Plain: Terra Incognita. Lucknow University, Lucknow, pp. 45 – 48. Rao, M.B.R., 1973. The sub-surface geology of Indo-Gangetic Plains. J. Geol. Soc. India 14, 217 – 242. Richards, L.A., 1954. Diagnosis and Improvement of Saline and Alkaline Soils. Handbook No. 60. U.S. Department of Agriculture, Washington, DC. Ritter, D.F., 1996. Is Quaternary geology ready for the future? Geomorphology 16, 273 – 276. Singh, G., Joshi, R.D., Singh, A.B., 1972. Stratigraphic and radiocarbon evidence for the age and development of the three salt lake deposits in the Rajasthan, India. Quat. Res. 2, 496 – 505. Singh, G., Joshi, R.D., Chopra, S.K., Singh, A.B., 1974. Late Quaternary history of vegetation and climate of the Rajasthan desert, India. Philos. Trans. R. Soc. London 267, 467 – 501. Singh, G., Wasson, R.J., Agarwal, D.P., 1990. Vegetation and seasonal climatic changes since the last full glacial in the Thar desert, NW India. Rev. Paleobot. Palynol. 64, 351 – 358. Singh, L.P., Parkash, B., Singhvi, A.K., 1998. Evolution of the Lower Gangetic Plain landforms and soils in West Bengal, India. Catena 33, 75 – 104. Singh, R.L., Singh, K.N., 1971. Upper Ganga Plain. In: Singh, R.L. (Ed.), India—A Regional Geography. National Geographic Society of India, Varanasi. Soil Survey Staff, 1966. Soil Survey Manual. Handbook No. 18. Oxford and IBH, New Delhi. Srivastava, P., Parkash, B.P., Sehgal, J.L., Kumar, S., 1994. Role of neotectonics and climate in development of the Holocene geomorphology and soils of the Gangetic Plains between Ramganga and Rapti rivers. Sed. Geol. 94, 129 – 151. Srivastava, P., Parkash, B., Pal, D.K., 1998. Clay minerals in soils as evidence of Holocene climatic change, Central Indo-Gangetic Plains, North-Central India. Quat. Res. 50, 230 – 239. Stoops, G., Altemuller, H.J., Bisdom, E.B.A., Delvigne, J., Doborovolsky, V.V., Fitzpatrick, E.A., Paneque, G., Sleeman, J., 1979. Guidelines for description of mineral alteration in soil micromorphology. Pedologie 29, 121 – 135. van Vliet-Lanoe¨, B., Fragnart, J.P., Langohr, R., Munault, A., 1992. Importance de la succession des phases e´cologiques anciennes et actuelles dans la diffe´rentiation des sols lessive´s de la couverture loessique d’ Europe occidentale: argumentation stratigraphique et arche´ologique. Sci. Sol 30, 75 – 93. van Vliet-Lanoe¨, B., Pellerin, J., Helluin, M., 1995. Morphogenese-pedogenese: le heritages du dernier cycle glaciaire en foret de Fouge`res (IIIe et Vilaine France). Z. Geomorphol. 39, 489 – 510.