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
9
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)
10
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
12
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
0 ~0
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.
15
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
17
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