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

Catena 46 (2002) 243 – 259
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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
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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 VlietLanoë 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.
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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.
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