GEOLOGY OF TH~ DELW REGION

GEOLOGY OF DELHI REGION

2.1. INTRODUCTION

National Capital Territory ofDelhi occupies 28 24'-28 53' N latitude and 76 50'-77 20'

E longitude falling on the toposheets of 53 H/2 and 53 H/3. The south and south-eastern

parts ofDelhi are a plateau of250-300 meters height, rising about 100 meters above the

surrounding area is known as the famous Delhi ridge. The area provides an interesting

place to study the important surface geochemical processes, because it occurs at the

triple junction of Aravalli mountain, Thar desert and Indo-Gangetic plain and it has

possible some subsurface connection with the Himalayas. The extension of the Delhi

ridge to the base of Himalayas is known as Delhi-Haradwar ridge.

2.1.1 Physiography and soil

Delhi ridge constitutes northernmost extension of the Aravalli range in the form of two

ridges, i.e. Sohna ridge in Harayana, nearly 45 km from Delhi, and west of it is

Harachandpur ridge also known as Delhi ridge, which has become famous for its

environmental importance to this region. Physiographically the north western part of the

India covers deserts of the Rajasthan and Harayana, Aravalli ranges and Indo-Gangetic

alluvium. The Aravalli mountains constitute remnant monuments of Precambrian times,

whereas Thar desert and alluvium are Quaternary features formed by aeolian and

alluvial processes. Four major geomorphic landforms have been identified in this

Geology of the area

region (Thussu et al., 1992). These are (1) structural cum denudational hills and valleys,

(2) Rewari older alluvium surface, (3) aeolian sandy surface, and (4) Yamuna flood

plain surface. The structural cum denudational hills consist of linear ridges with

rounded crest and wide valleys, isolated hillocks and inselberg. These extends

southward from Delhi to Faridabad, Gurgaon, Mahendragarh and Bhiwani districts.

Large numbers of ephemeral streams originate along the edges of these hills with

piedmonts and hill wash deposits at the foot of the hillslopes. These streams are mostly

controlled by joints.

Rewari older alluvial surface represents the surface formed by the earlier river

system of the southern bank of Indogangetic basin. It is featureless plain with

palaeochannels, palaeolakes and topographic depressions. It is mostly used for

cultivation. Aeolian sandy surface is well developed in the districts of Mahendragarh,

Gurgaon and Hissar districts of Harayana, adjacent to Delhi. These are represented by

ergs (dunal flat) and dunes. The Delhi-Harachandpur ridge makes an abatement to the

river Yam una and thereafter submerges into the alluvium northward.

Soils of Delhi have been grouped into 15 soil series by Chibbar (1985). About

15% of this area is affected by salinity or alkalinity and about 64% of the total area is

irrigated. Soils of Delhi are generally low in available nitrogen, low to medium in

phosphorus, medium to high in potassium, adequate in calcium, magnesium and

sulphur. Zinc deficiency has been noted in coarse textured soils. The soil type on Delhi

ridge has been reported as sandy loam to loam (Chibber, 1985).

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Geology of the area

2.1.2 Climate

Delhi, like the adjoining plains of Haryana, has a semi-arid climate. Three different

vegetational seasons are recognised, a dry hot summer from March to June, a wet

monsoon from July to September and a dry cold winter from October to February. The

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summers are very hot with temperature reaching as high as 43 C-45 C. The winters are

severe with minimum temperature varying from 6°C to 8.5°C, occasionally dropping to

near zero. The average rainfall of the of Delhi is about 710 mm, 80% of which is

received during July to September.

2.1.3 Vegetation

Hardy evergreen, and spinuous xerophytic trees and shrubs characteristic of arid climate

are most common vegetation of the ridge. They are Prosopis spicigera L., Acacia

arabica wild, Balanites roxburghi, Butea monosperma, Anogeissus pendula, Cassia

fistula, Albizia lebbec etc. Capparis sepiaria L. is common among larger thorny shrubs.

Other shrubs are undershrubs are Grevia tenax andAdhatoda vasica.

In the alluvial plains, the only timber tree is Dalbergia sisoo. Other trees such as

Acasia arabica, Ficus benghalensis, Prosopis juliflora, Eucalyptus sp. are planted

along the roadsides. In the soils of the recent alluvium trees like Salvadora persica and

Eruca sativa and shrubs like Calotropis procera and grasses like Erianthus ravennae

and Saccharum spontaneum are commonly observed.

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Geology of the area

2.2 PREVIOUS STUDIES

The Delhi region has not been given much importance to the detailed geological,

geochemical studies. Most of the studies on this area cover geological and

geomorphological mappmg and classification, groundwater study and their

hydrochemistry.

Earlier Racket (1881) and Heron (1917) have given rocks of Delhi state as

belonging to the Alwar series. Heron (1917) explained that the pegmatites intruding

into the quartzites are neither foliated nor crushed, showing that they were intruded

when folding had ceased. He made passing references to the clay deposits of Delhi

region. Bhola (1965) has reported the occurrence of radioactive and rare earth minerals

in the pegmatites of South Delhi. He suggested that the feldspar of pegmatites

undergone deep-seated weathering. Srivastava et al. (1975) investigated the area for

geological and geomorphological mapping. Srivastava et al. (1974) have given the

detailed report on the occurrences of china clay in some part of the Delhi state. Tyagi

(1980) has worked on mineralogical and geochemical changes leading to the formation

of clay deposits in the Mehrauli area of South Delhi. She has shown that the pegmatites

are of 850±5 Ma old, which intruded the quartzites of Delhi group in the second phase

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of deformation at a temperature range between 495 C to 540 C. She has suggested that

these pegmatites were altered by the late hydrothermal activity associated with them to

give rise china clay deposits. Roonwal (1985) compared the weathering of pegmatites of

Delhi to the granite weathering of other climatic regions. Rakesh Kumar (1989) has

suggested that the late hydrothermal activity associated with pegmatite intrusion in the

8

Geology of the area

regiOn is responsible for the destruction of interlocking texture of quartzites and

formation of Badarpur sand.

Prasad and Awasthi (1992) have given a note on the sedimentary structures in

the Alwar quartzites of Gurgaon district of Haryana. Awasthi and Prasad (1992) and

Singh and Jain (1989) have reported stratified tuff beds and volcanic fragments within

the quartzites, and suggested penecontemporaneous volcanism during the quartzite

deposition. Kalia et al. (1992) have reported trace fossils from the quartzites of Delhi

ridge.

Vasudev and Chattetjee (1974) reported that, throughout the Delhi region, illite

is the abundant clay mineral but smectite ( 1 0-20%) is mostly present in western and

north western part. Soils of Delhi have been classified into 13 series by National Bureau

of Soil Survey and Land use Planing (1978) into 13 series, whereas Chibbar (1985) has

classified Delhi into four major physiographic zone and 15 soil series.

Sheo Prasad et al. (1985) mapped the areas of Haryana adjacent to Delhi to

demarcate the boundaries of Alwar and Ajabgarh group and to correlate them with the

equivalents in the Rajasthan. Thussu (1995) studied the lithostratigraphy of Haryana

including Delhi on the basis of bore hole data and classified the rocks and Quaternary

sediments into (1) Precambrian Delhi rocks, (2) Rewari older alluvial plain, (3) aeolian

sandy deposits (4) Yam una flood plain.

Das et al. (1988) suggested that Delhi ridge act as ground water divide in the

region and the high salinity of ground water is due to the high rate of evaporation,

recycling of irrigation water, and dissolution of precipitated minerals by monsoon

recharge alongwith non flushing deeper waters. Ericcson (1976) has found that around

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Geology of the area

3 ton!Km2 of airborne chloride is deposited every year and attributed high salinity of

ground water of Delhi to this atmospheric process. Kakar (1985) has indicated that

ground water in Delhi area is saline only in selected patches and pockets. Datta and

Tyagi (1994) investigated this region for major ion chemistry, to identify the source of

major ions in terms of chemical weathering processes. They found that chemistry of

ground water was dominated by carbonate weathering, and the excess of sodium ion

over chloride was attributed to the silicate weathering.

2.3 REGIONAL GEOLOGY

The present area of study constitutes the northernmost tip of the Aravalli ranges. The

Aravalli mountains extending from Gulf of Cambay in the south-west to Delhi in the

north-east constitute an important orographic feature of north-western Peninsula. The

four fold classification of Precambrian rocks of Aravalli ranges (Heron, 1953) envisages

the geological evolution of this terrain through three major orogenic cycles now

represented by the rocks of Banded gneissic complex (B.G.C.)/ Mewar gneiss (older

than 2500 Ma), Aravalli Supergroup (2500 to 2000 Ma) and Delhi Supergroup (2000 to

740 Ma) (Gupta et al., 1980). Although their are certain revisions in the local

stratigraphic sequence and regional correlation due to later investigations, this sequence

remains the basic frame work ofreference (Choudhary et al., 1984; Roy, 1988). On the

basis of tectono-lithostratigraphic characteristics Deb and Sarkar (1990) divide.d the

region into six domains, (1) the B.G.C. and other older granites, (2) The Bhilwara belt,

(3) the Aravalli- Jharol belt, (4) the North Delhi belt (5) the South Delhi belt, and (6)

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Geology of the area

the Vindhayan basin.

The B.G.C. comprises granodioritic gne1ss, migmatites and intrusions of

gneissose granite with enclaves of amphibolite and mica schists. The Sm-Nd isochron

data suggest that the B.G.C. contains rocks as old as 3500 Ma (Macdougall, 1983). The

Bhilwara belt lies between the B.G.C. in the west and the Vindhayan basin rocks in the

east. It comprises of several subparallel linear metasedimentary zones separated from

each other by tracts of migmatised B.G.C. or soil cover. These metasedimentary rocks

are intruded by Berach granites (Sinha Roy, 1985) in the eastern part. A Rb-Sr (whole

rock) isochron age of 2600 Ma has been reported for this granite (Crawford, 1970).

Structural and stratigraphic studies by Roy et al. (1981) and lead isotope data by Debet

al. (1989) have reaffirmed that the rocks of Bhilwara belt are contemporaneous with

Aravalli belt of Heron (1935). The Aravalli-Jharol belt is characterised by distinctive

lithologies but they represent two segments of a single basin, share the same

deformational history and have similar tectonic trends (Deb and Sarkar, 1990). The .,

rocks in the type area around the Udaipur consist of two contrasting facies, a carbonate

bearing shelf facies (constituting the Aravalli belt) and carbonate free deep sea facies

(the Jharol belt), (Roy et al. 1984).

The Delhi belt extends from Gujrat in the south to the Delhi in the north. Heron

(1935) classified the rocks of this belt into two units, The Alwar Series and Ajabgarh

Series (now Groups). This simple classification has been questioned by several workers

(e.g., Sen, 1981), who considered the above units to be time transgressive. The Alwar

Group consists of predominanlly arenaceous facies rocks and the Ajabgarh group

composed of argillaceous -calcareous slates and schists. In Alwar area, the two groups

ll

Geology of the area

of rock formations are separated by an intervening calcareous succession known as

Kushalgarh limestone. Further, the rocks of the Delhi belt to the North and south of

Ajmer show a marked difference in the volcanic/sediment ratio, nature of mafic­

ultramafic rocks, base metal mineralisation and related sulphur and lead isotope ratios,

etc. (Nayak:, 1993). These contrasts reflect that the rocks of this belt in the tw-o parts

may not belong to the same stratigraphic sequence and may have undergone a different

geotectonic evolution. Therefore, they may be named (1) North Delhi belt, and (2)

South Delhi belt, (Sinha Roy, 19.88; Deb and Sarkar, 1990). However, some workers

claim that continuity of Delhi Supergroup of rocks on the basis of a continuous

lithostratigraphy, similar structural history of all the rocks (Naha et al., 19.84) and the

similarity in metamorphic imprints (Sharma, 19.88).

North Delhi belt consists of three sedimentological domains, which are from

west to east, the Khetri sub-basin, the A.lwar sub-basin, and the Bayana-Lalsot sub­

basin (Singh, 19.88). These sub-basins developed as grabens in a gneissic basement and

seJimentation was largely controlled by vertical tectonism and they differ from one

another significantly in their stratigraphic development. Granitic rocks intrusive into the

metasediments are a common feature in the North Delhi belt. Choudhary et al. (1984)

gave their (whole rock) age in the range 1700- 1500 Ma, which indicates the closing

phase of Delhi orogeny. The South Delhi belt extends from north of the Ajmer to south

of the Abu road and conforms to the main Delhi synclinorium of Heron (1953). This

belt is characterised by extensive development of the carbonate facies, the wide

occurrence of mafic-ultramafic metavolcanics and felsic plutonism in the time range

850-750 Ma. The Malani rhyolites (745±20 Ma) coeval with Erinpura granite (800± 50

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Geology of the area

Ma) (Srivastava, 1988, Rathore et al., 1996), also reported from North Delhi belt.

Malani has been reported from Tosham area of Harayana (Kocbhar, 1985) and

pegmatites related to Erinpura oecur in Harayana and Delhi (Heron, 1917; Tyagi, 1980).

Revised lithostratigraphic succession of the pre-Vindhayan rocks after Roy (1988) is

given in table 2.1. The regional geological map simplified after Deb and Sarkar (1990)

is also shown in figure 2.1.

Table 2.1. Lithostratigraphic succession of pre-Vindhyan rocks of Aravalli region (After

Roy, 1988)

Malani volcanics

Erinpura granite

Mafic and ultramafic intrusion in the Delhi fold belt

Champaner Group

Sirohi Group

Ajabgarh Group

Delhi Supergroup AlwarGroup

(2000-740 Ma) Raynhala Group

(Heron's Raialo series in the Alwar district)

Upper Aravalli Group

Aravalli Supergroup (Debari and its equivalents

(2500-2000 Ma) conglomerates at the base)

Lower Aravalli Group

Older than 2500 Ma Mewar Gneiss

13

6

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Km.

N 0 E X

VINOHYAN BASIN

SOUTH DELHI BELT

NORTH DELHI BELT

JHAROL ARAVALLI BHILWARA BELTS

I 3 1 I 2 I ._I· __ __. § Bf.RACII GRANITE

B BAI'IDED GNEISSIC COMPLEX

Figure 2.1: Simplified geological map of Aravalli-Delhi orogenic belt (after Deb and Sarkar, 1990).

Geology of the area

2.4 LOCAL GEOLOGY

In Harayana and Delhi region quartzites are exposed as NE-SW trending ridges amidst

the alluvial and aeolian cover. Sohna-Ferozpur Jhiraka ridge runs from Nowganawa in

Rajasthan to Bhundsi a place about 45 km south of Delhi. Northeast of the Sohna is a

broad Harachandpur ridge, which extends up to Delhi, where it is known as famous

Delhi ridge. These two ridges consist of thickly bedded quartzites with minor schist

(Sheo Prasad et al., 1993; Mangala Prasad and Awasthi, 1992). The quartzites are

reported to exhibit sedimentary structures like ripple marks, current bedding, mud

cracks, flute cast and certain depositional features (Mangala Prasad and Awasthi, 1992).

Volcanic fragments and bands within the Alwar quartzites in and around Sohna

(Awasthi and Mangla Prasad, 1992) and stratified tuffbeds in Badkhal-Surajkund area

have also been reported (Singh and Jain, 1989), indicating penecontemporaneous

volcanic activity in the area.

In the South Delhi and adjacent part of Harayana the country rock i. e. Alwar

quartzites of Delhi Super group has been intruded by pegmatites and quartz veins

representing the acid igneous activity of Post Delhi age (Heron, 1917; Heron, 1935;

Tyagi, 1980). The minor pegmatites occurring as dyke swarms and vein quartz in this

region are the evidences of hydrothermal activity in this region.

Regarding the time of emplacement of these pegmatites it appears that these

igneous intrusions belong to the regional activity in Rajasthan i. e. Erinpura granite

emplacement (Srivastava, 1988). It is also possible that these pegmatites may be

connected even to regional acid volcanic activity represented by Malani rhyolites.

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Geology of the area

Crawford and Composton (1970) have assigned an age of 745± 10 to the Malani

volcanics. The Tosham volcanics of Harayana and associated granites gave Rb-Sr

isochron age 745±20 Ma (Kochhar et al., 1985). Isochron age of the Malani rhyolite is

the same as that of the Mount Abu granite (735 rna) (Crawford, 1975) and similar to

that of the Erinpura granite (830 Ma) suggesting that the Malani volcanics are coeval

with the Erinpura type of granites (Srivastava, 1988, Rathore, 1996). Absolute age

determination of Rajasthan pegmatites have been attempted by Pb-U, K-Ar, Rb-Sr, and

fission track methods (Holmes et al., 1949, Aswathanarayana, 1959, and Nandlal et al.

1976). The available data show that out of 34 determinations 20 give dates between 657

and 940 Ma (Bhattacaharya and Chaudhary, 1988). The most important magmatic

activity of the region known as the Erinpura granite and Godhara granite belong to the

age range of740-955 Ma (Gupta et al., 1981, Chaudhary et al., 1984).

Tyagi (1980) has reported dates of quartz, muscovite and tourmaline of the

pegmatites of South Delhi region using Ar 40 I Ar39 dating method. The ages of quartz

muscovite and tourmaline are 802.63. 915.3+20.9 and 811.5 Ma respectively. These

results conform to the age of Erinpura granite and suggest a temporal relationship

between the Erinpura granite and pegmatites of this region.

The Alwar rocks in this region form basement for the alluvium and aeolian

sediments. Rewari older alluvium directly overlains the Alwar group of rocks and its

thickness increases from south in Harayana to the north towards Delhi. The younger

aeolian sediments are deposited on the older alluvium in Gurgaon and Mahendragarh

districts do not exceed 50 meters in thickness. The Newer alluvium comprising older

:flood plain alluvium of Yamuna and younger alluvium of Krishnavati and Sahibi rivers

16

Geology of the area

are deposited over Rewari Older alluvium or directly over Alwar group of rocks.

The present research is mainly concentrated on the Delhi quartzites, weathered

products of quartzites & pegmatites and sediments deposited on the topographic

depressions on the quartzite ridges. Some of the interesting field features are shown in

the plates 2.1 and 2.2.

The Lithostratigraphic sequence of Delhi and adjacent part of Harayana after

Thussu et al. (1992) is given in table 2.2.

Table 2.2: Lithostratigraphic sequence of Delhi area.

Holocene

Delhi Supergroup

Precambrian

Y amuna/Sahibi/ Krishnavati newer alluvium

Aeolian sediments

Rewari older alluvium

Post Delhi intrusives

AlwarGroup

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PLATE 2.1 j

PLATE 2.2

Plate 2.2: (a) Weathering profile on the quartzite showing unweathered,

white and red zones. (b) Recenly opened quartzite showing

pyrite dissolution and formation of ferric hydroxide to form

goethite minerals. (c) China clay (kaolinite) deposit and (d)

Silica sand in altered pegmatite and adjacent quartzite.

c d