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Recognition of Paleosols and Their Geochemical

Characteristics in Lower Mahadek Sediments at Wahkyn,

West Khasi Hills District, MeghalayaK. K. Sinha1, Ajoy K. Padhi2, B. K. Tripathi1, S. N. Saini3 &

K. Umamaheswar4

Atomic Minerals Directorate for Exploration & Research

1West block-VII, R.K. Puram, New Delhi-110 066; email: sinhakk@gmail.com

2 AMD Complex, Sector-5 extn. Pratap Nagar (Sanganer), Jaipur-302 033;

3 AMD Complx, Nongmynsong, P.O. Assam Rifles, Shillong-793 011;

4

AMD Complex, 1-10-153/156, Begumpet, Hyderabad-500 016.

Abstract

Five ill to moderately developed paleosol horizons from fluvial Lower Mahadek sediments of Upper

Cretaceous have been recognised at different levels in four drill- core boreholes of Wahkyn area, West

Khasi Hills District, Meghalaya. The Paleosols are distinctly characterized by biogenic activities such as

rhizoliths, rhizocretions and root petrification. They also exhibit textures like mottling, grain coating,

glaebules, non-tectonic slickensides and jigsaw fitting brecciation, all characteristics to soil. The

geochemical data also corroborates the presence of paleosols. The WPS are exceptionally high in Al2O3

content (13.94 to 43.12 wt%, mean = 28.01% and median = 27.77%) in relation to the Lower Mahadek

Sediments (LMS), the parent rock from which it is derived and also there is strong positive correlation

between Al2O3 and TiO2 (r =0.80). They show variable degree of depletion of majority of elements except

highly immobile elements like Al and Ti in comparison to the parent rock. LFS elements like K, Rb and Ba

show strong depletion, whereas HFS elements such as Th, Nb, Ce, Zr and Y show mild to moderate

depletion. High CIA index of 81 to 98 coupled with their plots close to A-vertex along A-K join on A-CN-

K diagram strongly suggest that the paleosols have undergone an intense degree of chemical weathering in

relation to LMS. Kaolinite is the dominant clay mineral in paleosoles similar to the majority of modern

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soils and paleosols. In contrast, dominant clay mineral in the parent rocks (LMS) is found to be illite. This

fact is also reflected by their respective K2O/ Al2O3 ratios (0.01 to 0.19 for the paleosol and 0.019 to 0.40

for LMS).

Key words: Paleosol, Geochemistry, Mahadek, Cretaceous, Meghalaya.

INTRODUCTION

Paleosols have been reported from a variety of geologic environments since

Proterozoic to Recent but it is most commonly found in alluvial deposits. In a fluvial

aggradational environment, paleosol reflects a complex interplay among sedimentation,

erosion, and non-deposition (Kraus, 1999). In such systems, variations in rate of

sedimentation and sediment influx often lead to periodic subaerial exposure of overbank

sediments thus setting pedogenic process typically on the top and it marks a hiatus in

deposition (Kraus, 1999 and 2002). If erosion is insignificant and sedimentation is rapid,

weakly developed profiles of paleosols result. Such poorly developed paleosols are very

difficult to identify, especially, in a siliciclastic sequence of rocks (Retallack and Wright

1990). The present work reports for the first time such ill to moderately developed

paleosol horizons from the fluvial Lower Mahadek sediments belonging to Mahadek

Formation of Upper Cretaceous from Wahkyn area, West Khasi Hills district, Meghalaya

and deals in their petrogeochemical aspects.

GEOLOGY

The Mahadek Formation of Upper Cretaceous age mainly occurs along the

southern fringe of the Meghalaya plateau, mostly in West and East Khasi Hills districts

of Meghalaya (Fig.1). The Lower Mahadek Member is essentially fluviatile, whereas the

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WAHKYN

JADUKATARIVER

PLU

NALA

WABLEI

R.

KYN

SHIR.

LOSTOIN

25

20'

25

20'

25

17'30"

25

17'30"

91 05'

91 05'

STUDY AREA

I N D E X

TERTIARY SEDIMENTS

UPPER MAHADEK SEDIMENTS

LOWER MAHADEK SEDIMENTS

BASEMENT GRANITE/GNEISS

Upper Mahadek Member was deposited in a marine to marginal marine environment (Ali

and Singh 1982; Kak and Mohammad 1979). Wahkyn area lies

Fig.1: Geological map of Wahkyn area, West Khasi Hills District, Meghalaya

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Fig. 2.The spatial distribution of boreholesfrom which samples were drawn alongwith

schematic disposition of recognized

paleosol horizons.

W K N - 6 2 B

293 . 59

W K N -6 6

3 68.26 m

W K N - 5 628 7 .78m

W K N - 6 8277.5 m

m100 1 0 5 0m3 0

2 0

4 0

6 0m

N

in the western part of Mahadek basin where Lower Mahadek sediments unconformably

overly the crystalline basement with an irregularly developed conglomerate horizon that

in turn is overlain by Upper Mahadek sediments. The Lower Mahadek sediments are

predominantly composed of coarse-grained, feldspathic sandstone to arkose with thin

intercalations of siltstone, shale and wacke which are often tuffaceous. It is deposited in a

reducing environment marked by its gray colouration and abundance of carbonaceous

matter and pyrite (Dhanaraju et al., 1989; Kaul and Verma 1990; Gupta et al., 1994;

DCruz et al., 1996; Sen et al., 2002). In general, Lower Mahadek sediments are

immature, moderate to ill-sorted and often exhibit fining upward sequence, trough,

epsilon and tabular cross bedding. The Upper Mahadek sediments are purple coloured,

oxidized, poorly compacted and highly immature

feldspathic sandstone to wacke.

SAMPLING AND

METHDOLOGY

In the course of exploratory drilling by Atomic

Minerals Directorate for Exploration & Research

(AMD) for uranium at Wahkyn, different

paleosol horizons have been recognized in core

samples of four boreholes. While the presence of

paleosols was inferred on the basis of pedogenic

characters like grain coatings, colour mottlings,

illuviation and non-tectonic slickensides

(Retallack, 1988 and 1991; Kraus, 1999 and

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2002, Cox et al., 2001), the same was corroborated through petrographic observations

and geochemical signatures.

A total of 18 samples from paleosol zones and 17 fresh sediment samples belonging to

Lower Mahadek Member were systematically collected from four boreholes, namely

WKN-56, 62B, 66 and 68 (Fig.2). Petrographic studies on these samples were carried out

and also data on major oxides and select trace elements were generated through WD-

XRF.

PETROGRAPHY

Lower Mahadek Sediments (LMS)

The Lower Mahadek sediments of wahkyn area are comprised of quartz, feldspars

(dominantly microcline with subordinate orthoclase, perthite and plagioclase) and lithic

fragments (granites and gneisses) as essential framework components with minor

accessory minerals such as chert, micas, sphene, zircon, pyrite, apatite, magnetite,

monazite and anatase. Quartz is predominantly monocrystalline of plutonic and volcanic

origin. Volcanic quartz is characterized by their typical sickle-and wedge shapes as well

as embayed and corroded grain margins. Feldspars show varying degrees of alteration to

kaolinite and illite . The matrix comprises mainly of clays with subordinate amounts of

quartz and other minerals. Cementing materials include clay, limonite, carbonaceous

matter as well as chert.

Paleosols

The Wahkyn palaeosols (WPS) developed at different levels within LMS are

represented by fine to medium grained, ill-sorted, highly altered sandstones of variegated

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colours. The main constituents are monocrystalline quartz with occasional polycrystalline

one, altered feldspars, devitrified and altered glasses, lithic fragments and carbonaceous

matter with minor rutile, zircon and leucoxene. In some palaeosols, limonite and goethite

are present as a major phase. The matrix is almost entirely comprised of kaolinite with

minor illite and chlorite.The pedogenic characters recognized in these paleosols are as

follows.

Mottles: Colour motteling is a common and characteristic feature of modern as well as

ancient soils. The Wahkyn paleosols (WPS) show yellowish red to brown, buff and gray

mottles, whereas purple, purplish green, brownis and gray are dominating colors

exhibited by the matrix (Fig.3 & 4).

Slickensides: Occasionally, some horizons of Wahkyn paleosols show non-tectonic

slickensides characterized by a striated shiny surface (Fig.5).

Coated grains: Clays, organic matter and iron-oxides are the common grain coating and

even act as binding materials sometimes forming grain aggregates (Fig.6). Grain coatings

are observable on submicroscopic to microscopic scale. There are two types of such

aggregates bound together with clays are observed in some of the palaeosol horizons:

(i) aggregate of organic matter, clays, limonite, goethite and chert and (ii) aggregate of

organic matter, goethite, clays and micas.

Rhizoliths: Remains of plant root traces or rhizoliths are one of strong indicators of

paleosols. Klappa (1980) has defined rhizoliths as organosedimentary structures,

resulting in the preservation of roots of higher plants, or remains thereof, in mineral

matter. In case of WPS, two types of rhizoliths have distinctly been identified, i.e.

rhizocretions and root petrification under optical microscope. Rhizocretions are

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characterized by large plates and flakes of kaolinite accordions accumulated around plant

roots. They clearly show impacts of forceful radial push away from the root centre (Fig.7

& 8).

In WPS, three types of root petrifications can be recognized : i). The cross-

sections of petrified roots are conspicuously marked by their circular features with fine

radial and anastomosing hair-like structures bordered by limonitic materials in which

partial replacement of the organic matter of roots by later minerals has occurred. The

central part is comprised of aggregate of yellowish brown translucent organic matter,

clays (mainly kaolinite), calcite, hydrous mica and minor chert which have been intensely

impregnated with limonite subsequently. The minerals have also delicately replaced root

hairs preserving their essential structures (Fig.8). ii) Another type of root petrification is

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Fig. 3: Gray (thick arrows) and buff coloured clay- illuviation (thin arrows) in a brownish matrix in WPS;

diameter of the core 42mm. Fig.4: Gray (thick arrows) and buff coloured clay- illuviation (thin arrows) in

a purplish matrix in WPS; diameter of the core 42mm Fig.5: Slickensides with striations on a shiny surface

in core sample; diameter of the core 42mm. Fig. 6: Photomicrographshowing grain aggregates bound and

coated by clays and carbonaceous matters which is heavily stained with reddish brown iron oxides; strip

scale: 0.1mm. Fig. 7: Photomicrograph showing transverse sections of petrified roots (short, broad headed

arrows) and rhizocretions represented by kaolinite flakes (long, narrow headed arrows); the root is replaced

by secondary minerals (mainly clays) which is extensively impregnated and masked by ferrugineous and

transluscent organic matters; the border is marked by ferrugineous matter; strip scale: 0.5mm. Fig. 8:

Photomicrograph: a magnified view of a part of fig.7 showing petrified roots and rhizocretions; observe

the preserved delicate radial and anstomosing hair-like structures of roots and warping of kaolinite flakes

around the root indicating the growth of the root that pushes out the clay minerals developed around it; strip

scale: 0.2mm. Fig. 9: Photomicrograph displaying root petrification (outlined tubular structure), the

original root is replaced by fine aggregates of secondary quartz and clay minerals, longitudinal growth of

quartz resulted into elongated grains; also observe transversally grown very fine quartz representing the

root hairs (arrow); strip scale: 0.5mm. Fig. 10: Magnified view of the Fig. 9, the photomicrographexihibits

manypreserved features of the roots; strip scale: 0.2mm. Fig. 11: Photomicrograph of a ferrugineous

glaebule having a concentric structure with a central void; strip scale: 0.1mm. Fig. 12: Photomicrograph

showing in situ, jigsaw fitting brecciation of a quartz grain might be caused by root growth; strip scale:

0.2mm. (All photomicrographs in plane polarized light).

characterized by filling-in clay minerals and fine secondary quartz, grown longitudinally

along the root traces. Root hairs are represented by transversally grown very fine quartz

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and clays (Fig.9 & 10). iii) Apart from these, a few root traces are also preserved as thin

carbonaceous films.

Glaebules: Theyare another distinctive soil structure and occur as common constituents

of majority of soils. It is recognized by its localized occurrence and distinct concentration

of certain constituents than the surrounding matrix (Goldstein, 1988). In case of WPS,

glaebules are scarce and ferruginous in composition in general. Sometimes they show

concentric patterns with central void (Fig. 11).

Brecciation: In situ brecciation with jig-saw fitting fabric is very common in WPS

(Fig.12). Brecciated fragments of individual clasts are separated by matrix. In situ

bercciation have been noted from LMS of areas adjoining to Wahkyn by other workers

too (Dhanaraju et al., 1989). Such brecciation in paleosols may be caused by cracking

resulted by desiccation and root penetrations (Goldstein, 1988). Brecciation of mineral

grains might have been facilitated by the production of organic acids due to biogenic

activities.

GEOCHEMISTRYThe chemical data of WPS (n=18) and parent LMS (n= 17) is given in table 1 & 2

respectively. The most striking geochemical parameter of WPS is its very high Al2O3

(13.94 to 43.12 wt%, mean = 28.01% and median = 27.77%) and low to high SiO2 (17.93

to 79.81 wt %, mean = 58.19% and median = 58.17%) in contrast to the parent LMS

(3.03% to 12.90% Al2O3, mean = 6.97% and median = 6.14%) and (79.41 to 93.14%

SiO2, mean = 86.53% and median = 88.16%). Further, Al2O3 shows strong positive

correlation with TiO2 (r = 0.80) in WPS.

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MnO, CaO and Na2O contents are negligible to very low in samples of paleosols as well

as LMS. In general, FeO (total) for WPS ranges from 0.87% to 7.49% with an average

5.13% and median = 1.97%. However, two samples (W62B/10 & 11) contain

exceptionally high values of 41.46% and 13.60% (Table 1).

DISCUSSION

Paleosols are recognized on the basis of evidences of biogenic activities, textural and

stratigraphic features (Retallack, 1988 and 1991; Kraus, 1999 and 2002; Goldstein,

1988). In addition, paleosols also possess characteristic geochemical signatures distinct

from their parent rocks (Nesbitt and Young 1989; Gay and Grandstaff 1980; Rye and

Holland 2000). WPS show biogenic activities represented by various types of rhizoliths

such as rhizocretions, root petrification and root imprints preserved as carbonaceous

films.

The clay accumulations representing rhizocretions might have developed while the plant

was alive and growing. The warping of kaolinite accordions around the root outline is

indicative of postdate growth of the root and hence the plants.

At the same time, it exhibits various textural features such as colour mottlings, grain

coatings, slickensides and glaebules, which are characteristics to most of modern as well

ancient soils. The mottles show variable redox conditions of the paleosols in response to

fluctuations in soil saturations. The yellowish to brownish mottles represent deposition of

leached iron in a more oxidizing and unsaturated soil. Gray and bleached mottles are

produced in a reduced and saturated soil, whereas buff and earthy mottles are results of

downward percolation of soil clays in the profile. Characteristics features related to the

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process of oxidation such as restricted occurrence of, reddish to purplish colouration

displayed by matrix yellowish red mottles and clay-illuviation within a reducing

sequence of Lower Mahadek is a strong evidence of subaerial exposures of these

sediments. It is further supported by anomalous iron concentration shown by two samples

of WPS (W62B/10 & 11) which is attributed to accumulation of iron in the horizon in a

strong oxidizing condition (The two samples are drawn from the same horizon). The

horizon might represent a ferricrete.

The LMS were deposited in an arid climate and witnessed periodic catastrophic

events causing sedimentations followed by a dryer period with cessation of influx

(DCruz et al., 1996). Such periodic seasonal wet and dry depositional conditions lead to

development of slickensides in paleosols (Goldbery, 1982; Kraus, 2002) as observed in

WPS.

The WPS also have geochemical imprints that are distinct from LMS from which

it has been derived. The geochemical behaviours of WPS are very similar to that of

intensely chemically weathered profile that is modern as well as ancient soils. The

geochemical characteristics such as very high Al2O3 and a strong positive correlation

between Al2O3 and TiO2 shown by WPS strongly suggest them to be paleosols (Sreenivas

and Srinivasan 1994).

Further, the pedogenic process is often marked by the enrichment and depletion of

certain elements in relation to the parent rocks. High field strength elements (HFSE) such

as Ti, Nb, Zr, Y along with Al and P are immobile and they remain conserved in soil

profile during the process of chemical weathering. However, it is not necessary that all of

these elements remain immobile always but may vary depending upon different physico-

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0.00 10.00 20.00 30.00 40.00 50.00

Al2O3

0

10

20

30

40

50

60

Nb

0.00 10.00 20.00 30.00 40.00 50.00

Al2O3

0

50

100

150

Y

0.00 10.00 20.00 30.00 40.00 50.00

Al2O3

0.00

1.00

2.00

3.00

TiO2

0 10 20 30 40 50 60

Nb

0

50

100

150

Y

0.00 1.00 2.00 3.00

TiO2

0

10

20

30

40

50

60

Nb

0.00 1.00 2.00 3.00

TiO2

0

50

100

150

Y

0.00 1.00 2.00 3.00

TiO2

0

200

400

600

800

1000

Zr

0 200 400 600 800 1000

Zr

0

10

20

30

40

50

60

Nb

0 200 400 600 800 1000

Zr

0

50

100

150

Y

Fig. 13: Binary plots of immobile-immobile elements.

remained conserved. Transition element Ni shows a strong depletion but Cr moderate.

The elemental depletion in WPS is in accordance with the behaviours of these elements

in modern soil (Sposito, 1989). A more disperse pattern of the plot for WPS in relation to

LMS may be the reflection of varying degrees of chemical weathering of the sediments as

well as different paleosols horizons.

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The WPS and LMS have very low Na2O and the Na2O/K2O ratio for WPS is low (mean=

0.30, median=0.02). The low ratio values reflect either a physico-chemical condition

causing an intense chemical weathering of plagioclase than K-feldspars or scarcity of

basic plagioclase in source region or some post-depositional processes resulting in

addition of K such as K-metasomatism (Nesbitt & Young 1989; Fedo et al., 1995). The

post-depositional addition of K is not supported by A-CN-K plot (Fig.15). However,

petrographic studies indicate that the WPS as well as LMS contain an appreciable amount

of K-feldspars (microcline and perthite) and very low content of plagioclase and hence

the low values for the Na2O/K2O ratios reflects the scarcity of basic plagioclase in source

region.

K2O/Al2O3 gives important clues about the degree of alteration for alkali feldspars (Cox

et al., 1995). The ratios vary significantly for different K-Al-bearing minerals, such as

alkali feldspar, the ratio is ~ 0.4 1, for illite : ~ 0.3 and for kaolinite it tends to 0. For

WPS, the K2O/ Al2O3 ratio ranges from 0.01 to 0.19. Such a low value for WPS strongly

indicates a very high degree of chemical weathering and suggests dominance of clay

minerals such as kaolinite and illite. Presence of dominant kaolinite in WPS and illite in

LMS confirmed by XRD studies.

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MULTIELEMENTALPLOTFORLMS

0.01

0.1

1

10

100

Sr K2O Rb Ba Th Nb Ce P2O5 Zr TiO2 Y Ni Cr

(Sample/Al2O

MULTIELEMENTAL PLOT FOR WPS

0.01

0.1

1

10

Sr K2O Rb Ba Th Nb Ce P2O5 Zr TiO2 Y Ni Cr

(Sample

/Al2O3)/(AUC/Al

MULTIELEMENTAL PLOTS FOR MEDIANS OF WPS & LM

0.1

1

10

Sr

K2O

Rb

Ba

Th

Nb

Ce

P2O5 Z

r

TiO2 Y N

iCr

(Sample/Al2O3)/(AUC/Al2O3)

WPS

LMS

LOWER MAHADEK SEDIMENTS (LMS)

WAHKYN PALEOSOLS (WPS)

MEDIANS PLOT OF LMS AND WPS

Fig. 14: Multielemental Plots for Lower Mahadek sediments, the Parent Rocks

(LMS), the Wahkyn Paleosols (WPS) and their medians showing elemental

distribution patterns in the two units.

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Table 1: Major, minor and trace elements data of Wahkyn Paleosols (WPS) from Wahkyn area, West Khasi Hills distt., Meghalaya (n=18).

Samples W66/5 W66/8 W66/L/3 W66/14 W66/19 W62B/1 W62B/3 W62B/8 W62B/10 W62B/11 W62B/12 W68/11 W68/14 W68/20 W56/13 W56/20 W56/21B W56/24 Median Mean Std.D

Depth 140.6 142.5 146.6 159.45 1 70.65 57.35 67.15 99.8 114.65 115.55 117 42.05 48 67 52.3 76.25 79.55 87.8

Al2O3 14.88 21.66 17.05 24 .01 28 .04 16 .52 27 .54 40 .19 13 .94 41.46 28 32.51 20.71 38 .63 22 .48 37.48 43.12 35 .97 27.77 28 .01 9.72

SiO2 79.81 69.79 54.6 67 .32 64 .06 74 .7 64 .64 53 .31 17 .93 39.07 46 .68 59.53 71.81 56 .81 67 .42 56.08 50.52 53 .35 58.17 58 .19 14.45

TiO2 0.52 0.68 1.07 1.52 1.81 1.33 1.46 2.14 0.1 3.12 0.11 1.9 1.64 2.31 1.41 2.12 2.29 2.29 1.58 1.55 0.82

FeO (T) 1.02 2.42 nd 1.29 1.01 1.09 2.2 1.92 41.46 13.6 7.49 2.01 0.87 2.78 1.42 1.66 1.28 6.6 1.92 5.30 9.89

MgO 0.44 0.49 0.87 0.19 0.09 0.43 0.4 0.61 0.91 1.78 1.05 0.58 0.11 0.44 0.25 0.23 0.49 0.53 0.47 0.55 0.41MnO 0.04 bd 0.02 bd bd 0.01 bd bd 0.7 0.19 0.03 bd bd bd bd bd bd bd 0.04 0.17 0.27

CaO 0.52 bd 0.11 bd bd 0.05 bd bd 0.18 bd 0.1 bd bd bd bd bd bd bd 0.11 0.19 0.19

Na2O bd 0.04 0.82 0.06 bd 0.2 0.19 0.02 0.74 bd 0.61 0.22 0.05 bd bd bd 0.33 0.02 0.20 0.28 0.29

K2O 2.58 3.19 3.18 3.05 2.19 1.96 2.94 1.39 0.18 0.21 1.68 3.02 2.69 1.9 2.98 1.62 1.55 0.59 2.08 2.05 1.00

P2O5 0.06 0.03 0.06 0.03 0.09 0.08 0.03 0.1 1.7 bd 0.21 0.07 0.15 0.08 0.06 0.07 0.04 0.02 0.07 0.17 0.40

V bd bd 112 bd bd bd bd bd 410 bd bd bd bd bd bd bd bd 289 289.00 270.33 149.8

Cr 49 63 88 93 100 84 96 161 186 183 168 125 94 170 86 171 184 171 112.50 126.22 47.02

Co 60 22 15 45 24 111 123 30 56 34 94 15 27 10 59 17 16 21 28.50 43.28 34.47

Ni 34 18 28 44 38 97 92 50 108 99 97 37 43 42 67 37 45 101 44.50 59.83 30.2

Cu 18 14 19 25 40 40 45 24 31 78 49 34 29 16 25 16 19 33 27.00 30.83 15.76

Zn 16 28 77 40 118 20 69 61 164 100 105 38 56 54 43 45 83 96 58.50 67.39 38.37

Ga 13 23 nd 24 27 20 27 41 23 28 18 32 18 34 20 31 48 26 26.00 26.65 8.75

As bd bd nd bd bd bd bd 18 bd bd bd bd 13 bd bd bd bd bd 15.50 15.50 3.54

Rb 111 114 197 124 109 86 123 94 37 57 59 129 102 99 116 84 105 72 103.50 101.00 34.97

Sr 125 97 81 151 127 101 121 257 26 68 54 158 124 172 128 184 155 117 124.50 124.78 52.73

Y 57 35 59 98 86 89 91 123 21 11 bd 107 53 170 119 153 117 81 89.00 86.47 43.8

Zr 394 387 366 739 806 841 613 880 107 146 77 755 764 976 728 882 625 203 676.50 571.61 296.6

Nb 14 16 35 31 35 29 26 46 bd 9 6 38 34 45 29 41 56 27 31.00 30.41 13.47

Ba 1143 543 732 994 603 418 941 590 243 172 314 914 716 692 1186 616 612 403 614.00 657.33 292.7

Ce 114 430 nd 214 466 391 190 466 132 75 234 133 151 652 330 411 241 970 241.00 329.41 229.4

Pb 46 42 nd 39 37 27 30 51 45 44 46 35 41 42 30 34 43 42 42.00 39.65 6.59

Th 41 74 bd 41 54 63 49 61 bd bd bd 35 59 44 55 39 67 bd 54.00 52.46 12.05

U bd 10 bd bd 14 bd bd 24 bd 22 21 9 bd 5 38 48 69 37 22.00 27.00 19.40

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Table 2: Major, minor and trace elements data of Lower Mahadek Sediments (LMS) from Wahkyn area, West Khasi Hills Distt., Meghalaya (n=17).

Samples W66/12 W66/13 W66/16 W66/17 W66/21 W66/22 W62B/4 W62B/7 W62B/9 W68/9 W68/16 W68/19 W56/10 W56/14 W56/15 W56/19 W56/22 Median Mean Std.Dev.

Depth 153.6 156.4 161.65 167.7 180 189.3 68.35 73.5 105 36.85 56.65 63 43.1 53.45 58.25 73.25 85.7

Al2O3 12.12 12.9 5.95 6.97 3.98 4.46 8.98 3.6 4.57 9.85 7.36 3.03 3.48 6.14 11.36 7.67 6 6.14 6.966 3.13

SiO2 81 .34 79 .41 89 .65 86.8 92.47 90.46 84.44 90.05 88.96 81 .72 84.41 93 .14 89 .92 89.09 80.12 88.16 80.94 88 .16 86.53 4 .51

TiO2 0.51 0.81 0.06 0.6 0.06 0.49 1.27 0.2 0.34 0.17 0.15 0.1 0.8 0.25 1.06 0.59 0.21 0.34 0.451 0.36

FeO (T) 1.56 2.64 0.68 1.54 0.76 1.14 1.43 0.66 2.17 1.34 3.15 0.61 1.64 1.18 1.52 1.99 5.68 1.52 1.746 1.23

MgO 0.3 0.83 0.78 0.58 1 0.99 0.57 0.72 0.86 0.43 0.9 0.1 0.7 0.68 0.53 0.77 0.65 0.7 0.67 0.24

MnO 0.01 bd bd 0.01 bd 0.01 bd 0.05 bd 0.09 0.01 0.01 bd 0.02 bd 0.01 0.02 0.01 0.024 0.03

CaO bd bd bd bd bd 0.02 bd 1.14 bd 0.12 bd 0.08 bd 0.28 bd bd 0.64 0.2 0.38 0.43

Na2O bd bd bd bd bd 0.06 bd bd 0.11 bd 0.07 0.13 bd 0.15 bd 0.01 bd 0.09 0.088 0.06

K2O 2.32 2.43 2.35 1.68 1.42 1.55 1.92 1.41 1.09 2.33 1.56 1.2 0.9 2.08 2.4 1.52 1.53 1.56 1.746 0.49

P2O5 0.03 0.03 0.01 0.03 0.02 0.01 0.03 0.04 0.03 0.02 0.01 0.11 0.02 0.01 0.03 0.02 0.01 0.02 0.027 0.02

V bd bd 86 bd 184 88 bd 62 141 53 300 66 bd 50 bd 52 254 86 121.5 87.95

Cr 41 62 59 45 74 67 58 46 63 50 54 62 56 75 66 90 82 62 61.76 13.21

Co 32 27 64 57 56 58 36 26 64 30 36 75 40 44 39 33 32 39 44.06 15.12

Ni 50 36 59 76 102 67 57 48 59 36 55 102 71 56 59 56 44 57 60.76 18.82

Cu 15 19 21 23 29 25 21 19 23 16 15 30 20 21 19 17 13 20 20.35 4.69

Zn 14 34 21 85 15 27 34 14 53 35 41 176 21 33 40 44 77 34 44.94 39.27

Ga 12 15 11 17 15 17 17 14 14 16 20 21 12 16 19 14 18 16 15.76 2.82

As bd bd bd bd bd bd bd bd bd bd 10 13 bd bd bd 10 bd 10 11 1.73

Rb 83 101 89 88 69 79 79 66 56 73 63 62 66 89 97 73 57 73 75.88 13.71

Sr 45 81 49 45 49 37 82 47 60 64 51 62 29 44 97 73 68 51 57.82 17.95

Y 29 52 17 47 bd 35 69 17 13 18 80 bd 56 25 73 27 15 29 38.2 22.94Zr 275 457 161 310 155 386 943 200 321 90 199 144 709 189 529 360 193 275 330.6 224.55

Nb 18 16 bd 17 bd 15 20 bd 11 10 bd 5 8 10 14 12 bd 13 13 4.43

Ba 437 433 404 330 210 299 410 290 230 485 297 261 266 372 537 301 293 301 344.4 92.92

Ce 213 103 61 253 bd 158 141 bd 224 bd bd bd 172 186 216 145 bd 172 170.2 56.58

Pb 51 44 26 47 34 43 37 44 42 36 39 37 43 28 29 36 40 39 38.59 6.79

Th 33 31 bd 26 bd 27 50 5 bd bd bd 47 28 6 50 35 bd 31 30.73 15.32

U bd bd 21 bd bd 7 7 10 18 24 bd 51 bd bd 46 5 25 19.5 21.4 16.08

. bd- below detection limit, nd- not determined (for certain plots and parameter calculation, bd is replaced by random values.)

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C

I

A

90

80

70

60

100

Fig.15: A-CN-K plot for WPS (hollow circles) and LMS

(solid circles).

Al2O3 /SiO2 ratio (0.19 to 1.06) strongly indicates significant enrichment of

alumina as a result of clay formations that in turn points to intense chemical weathering

and hence pedogenic process.

Al2O3 / (MgO+CaO+K2O+Na2O) ratio, which is also referred as base-loss index,

provides a measure for the degree of leaching of total bases in relation to immobile

alumina during the process of weathering (Retallack and Wright 1990). The ratios for

WPS and LMS range from 3.42 to 31.42 and 1.10 to 4.61 respectively. Such a high value

for WPS in comparison to LMS strongly points intense leaching of bases for WPS on

account of chemical weathering.

The CIA index (given by Al2O3 / (Al2O3 + CaO+Na2O+K2O)x 100) is also a

potential tool for evaluating the degree of weathering as it reflects the removal of mobile

elements like Ca, K and Na in comparison to immobile elements such as Al and Ti

(Nesbitt and Young 1989; Fedo et al., 1995). The CIA index for WPS and LMS range

from 81 to 98 and 71 to 84 respectively

indicating a very high degree of

chemical weathering for WPS. The

chemical composition of

weathering profiles resulting from

chemical weathering follow a

systematic and predictable path which

can be evaluated with the help of A-

CN-K system (Nesbitt and Young 1989;

Fedo et al., 1995).The A-CN-K

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plots for WPS and LMS are shown in Fig. 15.

In the plot, LMS (solid circles) plots either on A-K join or close to it

but away from the A-apex indicating the weathering trend of the source materials, which

is represented by the only sample plotted on feldspars join. The plot of LMS on A-CN-K

system aptly represents the preponderance of illite. The WPS, on the other hand, plots

very close to Al2O3 apex, i.e. in kaolinite field in A-CN-K space (hollow circles). The

position of the WPS composition occupied in A-CN-K space is the result of advance

weathering under which the LMS underwent when they were subjected to subaerial

exposure for a substantial period of time and pedogenic processes set in to give rise

paleosol (WPS). The weathering trend evolved towards Al2O3 apex of A-CN-K system

due to preferable removal of K in comparision to Al (Nesbitt and Young 1989). Further,

sample falling on the feldspars join on the plot clearly points towards a granitic source

composition for the Lower Mahadek sediments.

CONCLUSION

The present study conclusively indicates the presence of at least five paleosol

horizons in Lower Mahadek sediments of Upper Cretaceous age in Wahkyn area of West

Khasi Hills, Meghalaya These horizons appear to be discontinuous and of limited

dimension laterally as well as vertically. The different evidences that support the

presence of paleosols are a) biogenic activities represented by the presence of various

forms of rhizoliths such as rhizocretions and root petrification, b) pedogenic textural

features observed include mottlings, grain coatings, galaebules, slickensides and in-situ

brecciations, and c) geochemical: (i) very high Al2O3 content and a strong correlation

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between Al2O3 -TiO2 in comparison to the parent Lower Mahadek Sediments, (ii)

depletion of majority of elements but conservation of Al and Ti, (iii) very low Na2O/K2O,

Na2O/K2O and K2O/Al2O3 , (iv) high Al2O3 /SiO2 and Al2O3 / (MgO+CaO+K2O+Na2O),

(v) very high CIA Index (81 to 98) and (v) their position very close to Al2O3 apex on A-K

join in A-CN-K system. Further, the plot also points towards a granitic source

composition for the Lower Mahadek sediments.

Acknowledgments

The authors are grateful to Shri Anjan Chaki, Director, Atomic Minerals Directorate for Exploration and

Research (AMD), for the kind permission to publish the data. The authors are greatly thankful to Dr. Minati

Roy and Dr. T. S. Sunil Kumar for their valuable suggestions and support extended during the study and

preparation of the manuscript. Authors are also thankful to S/sh Naresh Gautam and R. Rana for their

supports in drawing figures.

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