CHAPTER 2

GEOLOGY AND SAMPLING

Geology

The Crescent shaped mid Proterozoic intracratonic Cuddapah Basin of

Peninsular India (Fig. 1) covers an area of 44,500 km2 in parts of Chittoor, Anantapur,

Cuddapah, Kurnool, Mahaboobnagar, Nalgonda, Krishna and Guntur districts of

Andhra Pradesh, India. Its concave eastern margin stretches 440 km long with

maximum basinal width of 145 km in the middle and consists of about 12 km thick

sediments with subordinate volcanics. It consists of two groups of formations namely

Cuddapahs and Kurnools separated by an unconformity with the Kurnool Formations

overlying Cuddapahs. The Kurnools occur in two separate basins, Kurnool Basin in

the southern part and Palnad Basin in the northern part of the Cuddapah Basin. In

geology of India Kurnools and Palnads are considered as the same formations but

occurring as two distinct outliers with in the Cuddapah Basin (Fig.1).

The geology and stratigrphy of the Cuddapah. and Kurnool Groups is discussed

in detail by Nagarajarao et al. (1987). Stratigraphic succession as given by them

includes the lithology, formation names, unconformities and thicknesses of the

different formations (Table . 1).

Lithologically the Cuddaph supergroup is dominantly composed of arenaceous

and argillaceous sequences with subordinate calcareous units along with igneous

intrusives and extrusives in the lower horizons, These are classified into three groups

with 3risailam Quartzite as a unitary formation at the top, which itself is overlain by

the Kurnool Group (Table. 1). The Cuddapah Basin consisting of these formations has

an undeformed western margin and a folded and deformed eastern margin called the

Nallamalai fold belt. The beds in the western part of the Cuddapah Basin and the

Kurnool Formations in the Kurnool and Palnad Basins overlying the Cuddapah

Formations are least disturbed and are nearly horizontal.

The Kurnool formation in the.Kurnoo1 and Palnad Basins (Sub-basins) consists

of conglomerate -quartzite, carbonate and Shale facies. These are recognised as

consisting of six formations (Table.1) and were thought to have formed by continuous

and uninterrupted sedimentation (Nagarajarao et nl., 1987).

Banganapalli Quartzite is the basal unit of the Kurnool Group with a lower

conglomeritic bed. It has a thickness of 10 - 50 m and consists of dark red, Grey or

brown Sandstone which is coarse grained and gritty. The conglomerate consists of

pebbles of variously coloured cherts, jaspers and quartzites. Narji Limestone

conformably overlies Banganapalli Quartzite. The Limestone is pink and purple in

colour. It has a thickness ranging from 100 - 200 m.

The Banganapalli Quartzite and the Narji Limestone have been sampled for

this study. This sampling is for, the palaeomagnetic studies aiming to find whether the

Srisailam reversal continues into the Kurnool Formations.

There are innumerable dyke swarms particularly around the western margin of

the Cuddapah Basin. The dykes intrude the basement of the basin which consists of

granites and gneisses (Murthy, 1995 and Murthy et al. 1987). The dykes occur as

prominent outcrops paralleling each other mostly in an EW direction. The width of

the dykes ranges from a few centimeters to about 100 meters. They are found to be

medium grained and are dark in colour suggesting their mafic nature. Due to the

quarrying of some of the dykes for dimensional stone very good fresh exposures are

available for collection of oriented samples. All dykes are generally doleritic and

tholeiitic in character as they appear in the field.

There is no detailed work on the Kurnools or Palnads except for stratigraphy

and some field relations as described by King (1872), Dutt (1962), Narayanaswamy

(1966), Rchards et a1 (1968), Ramalingaswamy (1974), Natarajan and Rajagopalan

Nair (1977) and Rajurkar and Ramaiingaswamy (1978). Based on these studies a

general description of the Cuddapah Basin including the Kurnool and Palnad sub

basins is given above.

Rb-Sr dating of the Cuddapah traps (Crawford and Compston, 1973) and the

micro biotic characteristics (Viswanathaiah, et al. 1979) indicate that Cuddapah

sedimentation began around 1700 Ma. According to Chalapathi Rao et al. (1999) the

Cuddapah Basin has not been subjecied to tectono-thermal activity after to 1400Ma.

Uranium mineralization in the Vempalle and Tadpatri formations of the Cuddapah

Supergroup is considered to have taken place at 1756k29 Ma based on a Pb-Pb age by

Zachariah et al. (1 999). According to these authors this gives a minimum age for

carbonate sedimentation and dolornitization of the Cuddapah Formations. A similar

conclusion was drawn earlier by Bhaskara Rao et al. (1995) based on an age of

1817rt24 Ma for the Pulivendla mafic sill. Based on carbonaceous impressions and

fossil assemblages fiom Owk Shale formation of the Kurnool Group of the Cuddapah

Basin (Sharma and ShuMa, 1999) and based on chronostratigraphy (Asru K.

Chaudhuri el 01. 1999) it is considered that the Kurnool Group of the Cuddapah Basin

and the Rewa and Bhander Groups of the Vindhyan Supergroup belong to the

Neoproterozoic era. These supposed Neoproterozoic Kurnool Formations are

glauconite bearing and exhibit cross -bedding, ripple marks and mud cracks

(Nagarajarao. et al. 1987) suggesting a near shore to shallow marine environment of

deposition. These authors have also suggested that " The Kumool sub basin has an

eastern tectonic contact and western transgressive contact while the Palnad sub basin

has a southern tectonic contact and northern transgressive contact". They suggest

these basins as half grabens.

Table: 1. Lithostratigraphy of Cuddapah Supergroup and Kurnool Group.

(After Nagarajarao et al, 1987).

Group Formation Thickness (m) Lithology

Nandyal Shale 50-100 Koilkuntla Limestone 15-50

KURNOOL Paniam quartzite 10-35 GROUP Owk Shale 10-15

Narji Limestone 100-200 Banganapalli Quartzite 10-50

Shale Limestone Quartzite Shale- Ocherous Limestone

Conglomerate, Q w i t e

f Srisailan Quartzite 300 Quartzite and Shale

U urnbum (Pullampct) 2000 Cu~ubum: Pliyllilc, Sletc, Formation Quartzite

D dolomite NALLAMALAI Pullampet Shale

D GROUP Dolomite, Quartzite

A Bairenkonda (Nagari) 1500 Bairenkonda Quartzite, Shale Quartzite 4000 Nagari Conglomerate

P Quartzite and Shale with intrusives.

A ------ Angular unconfodty ----

Gandikota Quartzite 300 Quartzite and Shale Tadpatri formatian I 4600 Shalc, ash fall LuKs, Qu;trlzitc,

S \ CHITRAVATI Dolomite with intrusivcs. GROUP

Quartzite 1-75

------- Disconformity

Vempalle ~ o n h t i o n 1900

PAPAGHNJ

GROUP

Gulcheru Quartzite 28-210

Conglomerate and Quartzite

Stromatolitic dolomite, dolomite mud- Stone, Chert breccia and

Quartzite- with basic flows and intrusives

Conglomerate arkose, Quartzite and Shale

------ Non- Conformity --- Archaean and Dharwar.

Mafic dykes intruding the basement granite at the southern margin of the

Cuddapah Basin have been sampled (Fig.5). This area is selected due to availability of

swarms of dykes and good exposures as shown in Fig.1 of Murthy (1995). The

collected dykes are nearly all EW trending and it was difficult to ascertain their

inclinations but they all dip with steep angles. Due to quarrying of some of the dykes

for dimensional stone very good fresh exposures are available for collection of

oriented samples. All the dykes are doleritic and tholeiitic in character.

Sampling

In the Kurnool and Palnad Basins the formation, which was mainly sampled, is

the Banganapalli Quartzite (Fig 2A and 2B), the base of which is conglomeratic and

diamondiferous and it is a well-known formation in geology of India (Krishnan,

1982') This formation is comparatively more ferruginous than the others and

therefore likely to be more suitable for palaeomagnetic research. The more

compelling reason is that the Banganapalli Quartzite sits unconformably on the

Srisailam Quartzite (Nagarajarao et al. 1987), which was found to be reversely

magnetized by earlier workers (Prasad et al., 1987). Therefore to find the

continuation or otherwise of the Srisailam reversal into the younger Kurnool

Formations it was thought to be worthwhile to sample the more ferruginous

Banganapalli Quartzite in detail. A total number of 106 samples of Banganapalli

Quartzite from 9 sites in Kurnool Basin (Fig.3) and 97 samples from I I sites in Palnad

Basin (Fig.4) were collected. The locations of the sampling sites are shown in Fig.3

for the Banganapalli Quartzite from Kurnool Basin and in Fig.4 for Banganapalli

Quartzite and Narji Limestone from Palnad Basin and in Fig.5 for dykes. A table

showing the names of the collected sites, number of samples collected from each site

and the number of specimens obtained by drilling of the samples from each site in

Kurnool and Palnad Basins are shown in Tables 2 and 3 respectively. Geological dip

of the Banganapalli Quartzite and Narji Limestone at different sites is given in

Appendix I. Where the exposures are good and covering wide area, more number of

Fig. 2: (Izttr~.ups rfhngnctapnlfi Qurrrtdtc iiz fIrir~c Pu/t'rttrd busin A) Nfayrirjrinnlinnrftt B) Ethipotf~lit - CChli~if1rfnj~~lk(r t~md Suvmtcmkit sbredinzr.ffmi~ing over Rrmbguntyafiri (111tt~$e

~rrzdjiming nlm'~tr$rJs over Lts eitge.

samples were collected, where as at some sites less number of samples were collected

because of the difficulty in locating insitu outcrop. At such places samples could be

collected only from the firm and massive portions of the outcrop. The Narji

Limestone immediately overlies the Banganapalli Quartzite and is sampled from three

sites (Fig.4) over a length of about 10 Km. This Limestone is ferruginous and brown

in colour. This brown colour is uniform through out the thickness of the formation.

At each of the sites a minimum of three samples are collected (except in the

Lllhipotala site in lllc I'alnad Basin) with each sample measuring at least about 20 x 20

cm with a minimum thickness of about 12 cm. These blocks are oriented with

reference to geographic coordinates before breaking them loose. The orientation of

each sample is carried out by marking a north direction on the top of the specimen

with marking of two level lines on two sides of the sample, which are as nearly

perpendicular to each other,

Seventeen dykes intruding the Archaeans are collected from five sites near the

southern margin of the Cuddapah Basin (Fig.5). In the case of the collection of

oriented samples from dykes care is taken to see that there is no effect of the dyke

rocks on the compass. This is done by determining the north direction by magnetic

compass away from the dyke to avoid local magnetic influences and then aligning that

direction on to the sample by using survey techniques. Level lines are drawn on two

sides of the samples, which are nearly perpendicular to each other. A minimum of

five samples from each dyke are collected. Sampling details of seventeen dykes

collected are given in Fig.5.

The location of the sites and the number of the samples collected at each site

are shown in Fig.5 for dykes, in Fig.3 for the Banganapalli Quartzite from Kurnool

Basin and in Fig.4 for Banganapalli Quartzite and Narji Limestone from Palnad Basin.

LEGEND

(For Fig: 3 - Geological Map of Kurnooi Basin. . . . . . Palaeomagnetic Studies)

i a 1 b : (a) Shale; (b) Limestone (NANDYAL SHALE) I- K -1 r Limestone with shale (KOILKUNTLA LIMESTONE) 81 G

/ Quartzite (PANIAM QUARTZITE) -- 1

i I t Shale (OWK SHALE)

Z i (a) Limestone, Unclassified; (b) Massive Limestone (b) Flaggy Limestone (NARTI LIMESTONE)

17-1 Quartzite with Conglomerate (BANGANAP-ALL1 QUARTZITE) ; I I

7 I Quartzite with Shale (SRISAILAM QUARTZITE)

(a) Shale with Phyllite; (b) Dolomite1 Limestone C (c) Quartzite (CUMBUM FORMATION)

U S (a) Quartzite; (b) Shale (BAIRENKONDA FORMATION) l d i J IB U (a) Quartzite; (b) Shale (GANDIKOTA FORMATION)

D P (a) Shale, Tuff; (b) Dolomite/Limestone; (c) Quartzite (TADIPATRI FORMATION)

/- Quartzite with Conglomerate (PULIVENDLA QUARTZITE) P R

:T/-T (a) Dolomite, Chert, Mudstone; A G a+_,, (b) Quartzite (VEMPALLE FORMATION)

[:$:{:p H R. i:03L?::~Cl:q QuarziteIArkose with Conglomerate (GULCHERU QUARTZITE) L . 2 1 ~ I

Granite, Gneiss, Schist, Amphibolite, I --- Granulite and Associated Basic Intrusive~

No. Of No. Of Samples Specimens

1. CrnADRAGJRI

3. ERRASUPALL1 4. SURENDWAGAR

5. CKITTQOR

LENGTH OFTBE DYKES NOT TO SCALE

Fig. 5: Location map of the sampling sites of the dykes at the Southern margin of Cuddapah basin.

Table 2:Sampling details of Banganapalli Quartzite in Kurnool Basin

Site No. of samples

Nallalingayapalli (NLP) 13 Aluru (ALR) 20 Tallacheruw (TC) 2 Obuleswara Temple (OBT) 5 Nereducherla (NDC) 13 Banganapalli (BNG) 4

Cernentnagar (CNR) 3 Ramallakota (RKT) 19 Yambai (YMB) 27

No.of specimens

TOTAL 106

Table 3:Sampling details of Banganapalli Quartzite and Naji

Limestone in Palnad Basin

S.No Site No. of samples No.of specimens

Banganapalli Quartzite

Jaggayyapeta (JP) 11 Annavaram (ANV) 2 Hemadri (Vedadri)(HMD) 10 Mediacheruvu (MCR & MLC) 14 Kandibanda (KNB) 8 Choutapalli (CTP) 3 Gurrambodu Tanda (GBD) 5 Rajavaram (RIV) 3 Naga rjunasagar (NSR) . 3 6 Ethipotala (ETP) 2 Veerlapalem (VRP) 3

TOTAL 97 173

Narji Limestone 1 1. Veerlapalem (VRP) 11 27 12. , Damaracherla (DMC) 4 9 13. Tallaveerannagudem (TVG) 4 7

TOTAL 19 43