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Detrital Mineral Age, Radiogenic Isotopic Stratigraphy and Tectonic Signifi-cance of the Cuddapah Basin, India

Alan S. Collins, Sarbani Patranabis-Deb, Emma Alexander, Cari N. Bertram,Georgina M. Falster, Ryan J. Gore, Julie Mackintosh, Pratap C. Dhang, DilipSaha, Justin L. Payne, Fred Jourdan, Guillaume Backe, Galen P. Halverson,Benjamin P. Wade

PII: S1342-937X(14)00318-9DOI: doi: 10.1016/j.gr.2014.10.013Reference: GR 1352

To appear in: Gondwana Research

Received date: 4 September 2014Revised date: 22 October 2014Accepted date: 29 October 2014

Please cite this article as: Collins, Alan S., Patranabis-Deb, Sarbani, Alexander, Emma,Bertram, Cari N., Falster, Georgina M., Gore, Ryan J., Mackintosh, Julie, Dhang, PratapC., Saha, Dilip, Payne, Justin L., Jourdan, Fred, Backe, Guillaume, Halverson, GalenP., Wade, Benjamin P., Detrital Mineral Age, Radiogenic Isotopic Stratigraphy andTectonic Significance of the Cuddapah Basin, India, Gondwana Research (2014), doi:10.1016/j.gr.2014.10.013

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Detrital Mineral Age, Radiogenic Isotopic Stratigraphy and Tectonic Significance

of the Cuddapah Basin, India

Alan S. Collinsa*, Sarbani Patranabis-Debb, Emma Alexandera, Cari N. Bertrama, Georgina M.

Falstera, Ryan J. Gorea, Julie Mackintosha, Pratap C. Dhangb, Dilip Sahab, Justin L. Paynea, Fred

Jourdanc, Guillaume Backéa, Galen P. Halversond, Benjamin P. Wadee

aTectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of

Adelaide, SA 5005, Australia.

bGeological Studies Unit, Indian Statistical Institute, Kolkata 700108, India

cDepartment of Applied Geology, Curtin University, GPO Box U1987, Perth, WA, Australia

dDepartment of Earth and Planetary Sciences/Geotop, McGill University, Montréal, H3A 0E8,

Canada.

eTectonics, Resources and Exploration (TRaX), Adelaide Microscopy, University of Adelaide, SA

5005, Australia.

ABSTRACT

The Cuddapah Basin is one of a series of Proterozoic basins that overlie the cratons of India

that, due to limited geochronological and provenance constraints, have remained subject to

speculation as to their time of deposition, sediment source locations, and tectonic/geodynamic

significance.

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Here we present 21 new, stratigraphically constrained, U-Pb detrital zircon samples from all the

main depositional units within the Cuddapah Basin. These data are supported by Hf isotopic

data from 12 of these samples, that also encompass the stratigraphic range, and detrital

muscovite 40Ar/39Ar data from a sample of the Srisailam Formation. Taken together, the data

demonstrate that the Papaghni and lower Chitravati Groups were sourced from the Dharwar

Craton, in what is interpreted to be a rift basin that evolved into a passive margin. The

Nallamalai Group is here constrained to be deposited between 1659 ± 22 Ma and ~1590 Ma. It

was sourced from the coeval Krishna Orogen to the east, and was deposited in its foreland

basin. Nallamalai Group detrital zircon U-Pb and Hf isotope values directly overlap with similar

data from the Ongole Domain metasedimentary rocks. Depositional age constraints on the

Srisailam Formation are permissive with it being coeval with the Nallamalai Group and it was

possibly deposited within the same basin. The Kurnool Group saw a return to Dharwar Craton

derived provenance and is constrained to being Neoproterozoic. It may represent deposition in

a long-wavelength basin forelandward of the Tonian Eastern Ghats Orogeny. Detrital zircons

from the Gandikota Formation, which is traditionally considered a part of the Chitravati Group,

constrain it to being deposited after 1181 ± 29 Ma, more than 700 Ma after the lower Chitravati

Group. It is possible that the Gandikota Formation is correlative with the Kurnool Group.

The new data suggest that the Nallamalai Group correlates temporally and tectonically with the

Somanpalli Group of the Pranhita-Godavari Valley Basin, which is tightly constrained to being

deposited at ~1620 Ma. These syn-orogenic foreland basin deposits firmly link the SE India

Proterozoic basins to their orogenic hinterland with their discovery filling a ‘missing-link’ in the

tectonic development of the region.

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1. Introduction

India has a remarkable record of Proterozoic sedimentation preserved in a sequence of well

exposed and extensive basins that partially cover both major Archaean-Proterozoic cratons (the

northern Bhundelkund craton and the composite southern Dharwar-Bastar-Singbhum craton).

These basins include the Vindhyan, Indravati, Bhima-Kaladki, Kharyar, Pranhita-Godavari,

Chhattisgarh and Cuddapah Basins (Fig. 1 inset illustrates the southern and eastern basins) and

have traditionally been lumped together as the ‘Purana’ basins, considered to comprise part of

an extensive Proterozoic basin system (Chaudhuri et al., 2002; Kale and Phansalkar, 1991).

However, until recently, there has been very little geochronological and sedimentological data

available to test this hypothesis. Recent work in the Chhattisgarh and the Pranhita-Godavari

Basins has demonstrated that significant age differences occur in different ‘Purana’ Basins. In

the Chhattisgarh Basin, much of the succession was deposited between ~1.4 and 1.0 Ga

(Bickford et al., 2011a; Bickford et al., 2011b; Patranabis-Deb et al., 2007), with a younger,

presumably Neoproterozoic, succession unconformably overlying the Mesoproterozoic. In the

Pranhita-Godavari Basin, ages from detrital zircons and authigenic glauconite (Amarasinghe et

al., 2014; Conrad et al., 2011) from low in the basin succession (the Somanpalli Group)

demonstrate that early deposition occurred at ~1620 Ma. The upper part of the basin includes

the Sullavai Group, which contains many Tonian detrital zircons constraining it to being

deposited after this time (Amarasinghe et al., 2014).

The Cuddapah Basin is one of the largest of the Indian cratonic basins, covering 46,000km2 of

the Eastern Dharwar Craton, and reaching depths of over 5 km towards its eastern margin

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(Kalia et al. 1987). Until now, very little has been known about the ages of the voluminous

sedimentary rocks within the basin, the provenance of the original sediments and, particularly,

the change of provenance through time. Because of this, the existing basin evolution models

lack essential constraints and, therefore, the significance of this basin for the tectonic evolution

of Proterozoic India is unknown.

Here we present detrital zircon U-Pb (LA-ICP-MS) data on 21 samples throughout the

succession, Hf isotope data on a subset (12) of the detrital zircon samples, and detrital

muscovite 40Ar/39Ar ages from one key sample. These data are the basis of a new

tectonostratigraphic model for the Cuddapah Basin and revised correlations with the other

Purana basins.

2. Geological setting

The Cuddapah Basin was first mapped in the 19th century (Ball, 1877; King, 1872), but gained

significant attention only during the mid-20th century. The majority of the studies were focused

on the classification of the Cuddapah succession and reconstruction of the stratigraphy (King,

1872; Meijerink et al., 1984; Nagaraja Rao et al., 1987; Patranabis-Deb et al., 2012; Rajurkar and

Ramalingaswami, 1975; Ramakrishnan and Vaidyanadhan, 2008; Saha et al., 2009; Sen and

Narasimha Rao, 1967) (Table 1). The outcrops of the basin-fill successions cover an area of

about 46 000km2 on the eastern part of the Eastern Dharwar Craton (Figure 1). Nagaraja Rao et

al. (1987) suggested that the Cuddapah Basin is a composite of four subbasins, the Papaghni,

Kurnool, Srisailam and Palnad sub-basins (Fig. 1).

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The Papaghni sub-basin has an arcuate western boundary, which is primarily depositional, and

is bordered on the south and the west by granites and gneisses of the basement complex

(Peninsular Gneiss), which includes slivers of Archaean greenstone belts. The fill of the sub-

basin is represented by the Papaghni Group and the Chitravati Group (Fig. 2), which are

separated by an unconformity (Chaudhuri et al., 2002; Lakshminarayana et al., 2001; Saha and

Tripathy, 2012). The age of sedimentation for the Papaghni Group is constrained by 1891-1883

Ma volcanic rocks and dolerite dykes that are interbedded with, and cut through, the group

(Anand et al., 2003; Bhaskar Rao et al., 1995; French et al., 2008). The intensely deformed

Nallamalai Group has long been considered to be a part of the Cuddapah Supergroup (King,

1872; Lakshminarayana et al., 2001; Meijerink et al., 1984; Narayanswami, 1966). However,

recent studies demonstrate that a major thrust—the Maidukuru Thrust (the Rudravaram line of

Saha et al., 2010)—at the base of the Nallamalai Group has juxtaposed it against the Kurnool

and Papaghni-Chitravati Groups (Saha et al., 2010; Saha and Chakraborty, 2003). The

complexity of the lithostratigraphy of the Cuddapah Basin is reflected in widely divergent

stratigraphic classifications that have been proposed so far and are summarized in Patranabis-

Deb et al. (2012). We use the stratigraphy proposed by Saha and Tripathy (2012), which is

presented in Figure 2.

3. Methodology

3.1 U/Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS)

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Zircons were separated from a crushate by standard flotation and magnetic techniques, then

mounted on epoxy discs and imaged using a Gatan cathodoluminescence analyser attached to a

Phillips XL20 scanning electron microscope. U-Pb zircon geochronology was undertaken using

LA-ICP-MS at the University of Adelaide following the methods of Payne et al. (2010). Zircons

were ablated with a New Wave Research UP-213 laser using a spot size of 30µm, frequency of

5Hz and intensity at 75%. Isotopes (206Pb/238U, 207Pb/235U, 207Pb/206Pb and 208Pb/232Th) were

measured with the attached Agilent 7500 series Inductively Coupled Plasma Mass

Spectrometer. GEMOC GJ-1 zircon with TIMS normalising data 207Pb/206Pb = 607.7 ± 4.3 Ma,

206Pb/238U = 600.7 ± 1.1 Ma and 207Pb/235U = 602.0 ± 1.0 Ma (Jackson et al., 2004) was used to

correct for the U-Pb fractionation. The Plešovice zircon internal standard (ID TIMS 206Pb/238U

age = 337.13 ± 0.37 Ma, Sláma et al., 2008), was used to assess accuracy before and during the

analysis of unknowns. Over the course of the laser sessions, a total of 171 Plešovice internal

standard analyses were made, and gave a weighted average 207Pb/206Pb age of 335.7 ± 4.7 Ma

(MSWD = 0.95), and a weighted average 206Pb/238U age of 343.6 ± 1.9 Ma (MSWD = 7.5).

Data were collected, corrected and filtered in the GLITTER version 3.0 (Van Achterbergh et al.,

2001) software package. Concordia diagrams were created and weighted averages calculated

using ISOPLOT 4.11 for Excel (Ludwig, 2009), whilst probability density distributions and kernel

distributions were created using DensityPlotter 2.4 (Vermeesch, 2012). Ages of single analyses

are quoted at the 1 sigma level, whereas weighted averages of ages and chord intercepts are

quoted at the 2 sigma level and indicated as such.

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3.2 Lu/Hf isotopes by laser ablation multicollector inductively coupled plasma mass

spectrometry (LA-MC-ICPMS)

The zircon mounts prepared for U/Pb LA-ICPMS analysis were also used for Lu/Hf isotopic

studies undertaken with LA-MC-ICPMS at Waite (CSIRO) campus, South Australia. Only grains

with U/Pb LA-ICPMS analysis greater than 90% concordance were analysed for Hf with the LA-

MC-ICPMS. Laser spots were placed as close as possible to concordant U/Pb LA-ICPMS spots,

within the same CL zone. Zircons were ablated with a New Wave UP-193 Excimer laser (193nm)

using a spot size of 50 µm, frequency of 5 Hz, 4 ns pulse length and an intensity of ~8-10 J/cm2.

Zircons were ablated in a helium atmosphere, which was then mixed with argon upstream of

the ablation cell. The attached Thermo-Scientific Neptune Multi Collector ICP-MS measured

171Yb, 173Yb, 175Lu, 176Hf, 177Hf, 178Hf, 179Hf and 180Hf on Faraday detectors with 1011Ω amplifiers.

A 0.232 second integration time was used with total analysis time of 1-3 minutes. Hf mass bias

was corrected using an exponential fractionation law with a stable 179Hf/177Hf ratio of 0.7325.

Yb and Lu isobaric interferences on 176Hf were corrected by using the methods of Woodhead et

al. (2004). 176Yb interference on 176HF was corrected for by direct measurement of Yb

fractionation using measured 171Yb/173Yb with the Yb isotopic values of Segal et al. (2003). The

applicability of these values were verified by analysing JMC 475 Hf solutions doped with varying

levels of Yb with interferences up to 176Yb/177Hf = ~0.5. Lu isobaric interference on 176Hf

corrected using a 176Lu/175Lu ratio of 0.02655 (Vervoort et al., 2004) assuming the same mass

bias behaviour of as Yb.

Set-up of the system prior to ablation sessions was conducted using analysis of JMC475 Hf

solution and an AMES Hf solution. Before and during the analysis of unknowns, zircon standards

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Mudtank and Plešovice were analysed to check instrument performance and stability. ΕHf and

TDM crustal model ages were calculated using 176Lu decay constant after Scherer et al. (2001). TDM

crustal was calculated using the methods of Griffin et al. (2000) with an average crustal

composition of 176Lu/177Hf=0.015.

3.3. Muscovite 40Ar/39Ar geochronology

We selected a fresh sample from the Srisailam Formation for 40Ar/39Ar dating and extracted 5

unaltered detrital muscovite grains of ~150 microns in diameter. These minerals were hand-

picked under a binocular microscope. The muscovite crystals were rinsed with distilled water in

an ultrasonic cleaner.

Samples were loaded into a small well of one 1.9 cm diameter and 0.3 cm depth aluminum disc.

This well was bracketed by small wells that included Fish Canyon sanidine (FCs) used as a

neutron fluence monitor for which an age of 28.294 ± 0.036 Ma (1σ error) was adopted (Renne

et al., 2011) and an excellent grain-to-grain reproducibility was demonstrated (Jourdan and

Renne, 2007). The discs were Cd-shielded (to minimize undesirable nuclear interference

reactions) and irradiated for 40 hours in the US Geological Survey nuclear reactor (Denver, USA)

in central position. The mean J-value computed from standard grains within the small pits is

0.008815 ± 0.000022 (0.25%) determined as the average and standard deviation of J-values of

the small wells. Mass discrimination was monitored using an automated air pipette and

provided a mean value of 1.00537 (± 0.27%) per dalton (atomic mass unit) relative to an air

ratio of 298.56 ± 0.31 (Lee et al., 2006). The correction factors for interfering isotopes were

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(39Ar/37Ar)Ca = 7.30 x 10-4 (± 11%), (36Ar/37Ar)Ca = 2.82 x 10-4 (± 1%) and (40Ar/39Ar)K = 6.76 x 10-4

(± 32%).

The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at

Curtin University. The crystals were fused in a single step using a 110 W Spectron Laser

Systems, with a continuous Nd-YAG (IR; 1064 nm) laser rastered over the sample during 1

minute to ensure that all the gas has been extracted from the sample. The gas was purified in a

stainless steel extraction line using two SAES AP10 getters and a GP50 getter. Ar isotopes were

measured in static mode using a MAP 215-50 mass spectrometer (resolution of ~450; sensitivity

of 4x10-14 mol/V) with a Balzers SEV 217 electron multiplier using 9 to 10 cycles of peak-

hopping. The data acquisition was performed with the Argus program written by M.O.

McWilliams and ran under a LabView environment. The raw data were processed using the

ArArCALC software (Koppers, 2002) and the ages were calculated using the decay constants

recommended by Renne et al. (2011). Blanks were monitored every 3 to 4 steps and typical 40Ar

blanks range from 1 x 10-16 to 2 x 10-16 mol. Ar isotopic data corrected for blank, mass

discrimination and radioactive decay are given in the supplementary publication.

4. Results

4.1. Gulcheru Formation

The Gulcheru Formation of the basal ~2000m thick Papaghni Group (Patranabis-Deb et al.,

2012) is the basal formation of the Cuddapah Supergroup and nonconformably overlies

Neoarchaean granitoids of the Eastern Dharwar craton (Dasgupta and Biswas, 2006)(Fig. 3a).

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The formation consists of conglomerates and sandstones (Fig. 3b) that were deposited within a

series of fan deltas and shelf bar sequences (Dasgupta and Biswas, 2006; Patranabis-Deb et al.,

2012).

One hundred and forty six zircon grains were analysed for U-Pb from two samples (CU10-01,

GF-01). Of these, only 36 yielded results that were within 10% of concordance (Table 1; Figs.

4a,b & 5; see also supplementary publication) with an age maxima at ~2520 Ma. Appreciable

numbers of ≤10% discordant zircons yielded ages between ~2900 and ~3450 Ma (Fig. 5). The

youngest ≤10% discordant 207Pb-206Pb ages retrieved from CU10-01 and GF-01 are 2490 ± 19

Ma and 2502 ± 17 Ma respectively (Table 1). Twenty zircons from sample GF-01 were also

analysed for Hf isotopes. These yielded ϵHf values between 3.1 ± 2.1 and -4.0 ± 1.2 (1σ errors).

When corrected for U-Pb age, these demonstrated that zircon older than ~3.1 Ga plotted

between CHUR and DM (Fig. 6a), whereas the ~3.0 Ga analyses were uniformly more evolved

than CHUR and probably indicate appreciable amounts of pre-existing continental crust in the

source magma (Fig. 6a). Latest Neoarchaean zircon yielded the widest range of ϵHf values,

suggesting a combination of mantle-derived magma and intra-crustal sources (Fig. 6a).

4.2. Vempalle Formation

The carbonate-dominated Vempalle Formation gradationally overlies the underlying Gulcheru

Formation. It contains shales with abundant desiccation cracks and halite pseudomorphs

passing up into shallow-water stromatolitic carbonate platforms (Dasgupta and Biswas, 2006;

Patranabis-Deb et al., 2012).

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Ninety nine detrital zircon grains were analysed for U-Pb geochronology from a siliciclastic

horizon (Fig. 3c) ~100m above the base of the formation (Sample GF-14). Fifty nine zircons

yielded ≤10% discordant data that define a prominent age maxima at ~2510 Ma (Table 1, Figs.

4c & 5, see also supplementary publication). Rare analyses yield ages as old as ~2765 Ma. The

youngest ≤10% discordant analysis yielded a 207Pb/206Pb age of 2422 ± 17 Ma (Table 1). Sixteen

of the zircons analysed for U-Pb geochronology were also analysed for Hf isotopes. One early

Neoarchaean analysis yielded a negative ϵHf value of -10.5 ± 2.1 (1σ error), whereas the ~2510

Ma analyses yielded a range of predominantly juvenile ϵHf values between -1.5 ± 2.3 to 7.0 ± 1.7

(1σ errors)(Fig. 6a).

4.3. Chitravati Group

The ~5000 m thick Chitravati Group overlies the Papaghni Group (Fig. 2). The basal Pulivendla

Formation is dominantly a quartz-arenite (Meijerink et al., 1984; Patranabis-Deb et al.,

2012)(Fig. 3e). It is overlain by mixed shales and stromatolitic carbonates of the Tadpatri

Formation (Fig. 3d). These, in turn, are overlain by quartz-arenites of the Gandikota Formation

(Fig. 2).

Two samples of the Chitravati Group were analysed for detrital zircon U-Pb age determinations;

one from the basal Pulivendla Formation (CU10-19), and a second from the uppermost

Gandikota Formation (GF06). Only eight of the 50 analyses undertaken from the Pulivendla

Formation yielded ≤10% discordant data, whereas 27 of the 96 analyses from the Gandikota

Formation sample were ≤10% discordant (Table 1). As a result of the common discordance,

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both samples are plotted together in Figure 5, but the eight near concordant analyses from the

Pulivendla Formation sample (CU10-19) are also plotted as a white probability density

distribution to highlight the sample differences.

Zircons from both samples yielded 207Pb/206Pb ≤10% discordant ages that range from 1207 ± 22

Ma to 3038 ± 18 Ma. The youngest ages were extracted from the Gandikota Formation sample

where five analyses define a chord with an upper intercept close to the age of the Earth and

consistent with common Pb contamination and a lower intercept of 1181 ± 29 Ma (2σ error,

MSWD = 0.16)(Fig. 4q). The youngest ≤10% discordant age extracted from the Pulivendla

Formation was 1899 ± 19 Ma (Table 1). Together, ≤10% discordant age concentrations from

both samples occur at ~1940 Ma and ~2500 Ma (Fig. 5).

Sixteen zircons from GF06 (the Gandikota Formation sample) were analysed for Hf isotopes.

Two analyses of zircons older than 2900 Ma yielded evolved negative ϵHf values, whilst the

~2500 Ma population spanned a wide range of ϵHf values from -8.7 ± 5.2 to +4.2 ± 2.3, similar to

the coeval analyses from the Papaghni Group (Fig. 6a). Limited mid-late Palaeoproterozoic

zircons yielded negative to near zero ϵHf values, whereas two analyses of the two youngest

(Mesoproterozoic) concordant zircons yielded values of ~+3, demonstrating the juvenile nature

of their source region (Fig. 6a).

4.4. Nallamalai Group

The Nallamalai Group is thrust over the lowest two groups of the Cuddapah Basin, the

uppermost Kurnool Group, and the Srisailam Formation (Saha et al., 2010; Saha and

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Chakraborty, 2003) along a major thrust known as the Maidukuru Thrust (Patranabis-Deb et al.,

2012), or Rudavarum line (Saha et al., 2010)(Fig. 1). Lithologically, the group is composed of

arenites, shales and dolomitic carbonates, which are found in the east of the outcrop. The

group is often divided into two formations, a lower, arenite-dominated, formation known as

the Bairenkonda Formation, and an upper formation, found more in the east, dominated by

shales and carbonates and known as the Cumbum Formation (Saha and Tripathy, 2012). The

Nallamalai Group is folded (Fig. 3f) and internally thrust imbricated and it is not clear whether

the two ‘formations’ are distinct stratigraphic units, or lateral facies changes coupled with

internal imbrication. As such, we have combined the eight samples studied here for detrital

zircon analysis and discuss them together (Fig. 5).

Four hundred and sixteen U-Pb analyses of the eight samples (CU10-09, CU10-10, EA01, EA04,

EA05, EA06, EA07 and EA08) yielded age data ≤10% discordant (Figs. 4e,f,g,h,I,j,k,l & 5, see also

the supplementary publication). The 207Pb/206Pb ages of these data range nearly continuously

from ~2700-1700 Ma, with the youngest analysis at 1661 ± 20 Ma (Table 1). More sporadic

results stretch out to ~3665 Ma (Fig. 5). The most prominent peak in the age profile occurs at

~2510 Ma, with a second major peak at ~1850 Ma and two less prominent peaks at ~2335 Ma

and 2680 Ma (Fig. 5).

Hafnium isotopic data were collected from previously dated zircons from five samples (CU10-

09, EA01, EA04, EA05 and EA08). These data yielded ϵHf values from <-25 to >+10 with

Neoarchaean to earliest Palaeoproterozoic zircons yielding the most juvenile results and a

general decrease in mean ϵHf value with decreasing age (Fig. 6b). An exception to this trend are

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the low negative to near zero ϵHf values in some of the ~1800-2000 Ma analyses that suggest

the involvement of mantle-derived magma in the source region at this time (Fig. 6b).

4.5. Srisailam Formation

The Srisailam Formation nonconformably overlies the Dharwar Craton on its northern margin

and has a tectonic contact with the Nallamalai Group in the south and east (Fig. 1). It consists of

>600m of well-sorted quartz-arenites (Fig. 3g) with prominent m-scale cross-bedding. The

formation is interpreted as preserving tidal bundles and is thought to be a marine shallow shelf

deposit (Patranabis-Deb et al., 2012).

Four samples of Srisailam Formation quartz arenite were collected for detrital U-Pb analysis

(RG01, RG02, RG04, RG15)(Table 1, Fig. 4m,n,o,p & 5, see also supplementary publication).

These yielded 130 ≤10% discordant analyses that concentrate at a ~2510 Ma peak (Fig. 5).

Older, near concordant data stretch back to ~3800 Ma. A minor age peak occurs at ~2775 Ma

and rare younger analyses stretch down into the late Palaeoproterozoic, with the youngest

≤10% discordant analysis yielding a 207Pb-206Pb age of 1787 ± 22 Ma (Table 1, Figs. 4m & 5).

Zircons from two of these samples (RG-01, RG-04) were analysed for Hf isotopes (Fig. 6c). The

Neoarchean to earliest Palaeoproterozoic data yielded ϵHf values ranging from <-10 to values

equivalent to that of Depleted Mantle (or even higher)(Fig. 6c, see also supplementary

publication). This suggests that igneous rocks from the source region(s) included both pre-

existing continental crust and new juvenile material in their magmatic precursors. Younger

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Palaeoproterozoic zircons yielded largely negative ϵHf values suggesting the involvement of

Archaean crust in their source (Fig. 6c).

Five detrital muscovites from an arenite collected near the top of the exposed succession (RG-

02) were also analysed using the 40Ar/39Ar single grain total fusion technique (Fig. 7). The

sample was completely unmetamorphosed, yet the five analyses yielded ages within error of

each other (Fig. 7) that provided a weighted mean of 1773 ± 18 Ma (2σ error, MSWD = 0.99, P =

0.41). This suggests that the source region cooled below the closure temperature for Ar in

muscovite (~350˚C) at about the same time that the youngest near-concordant detrital zircon

was crystallizing, suggesting very fast exhumation of the source region.

4.6. Kurnool Group

The Kurnool Group unconformably overlies rocks of the Cuddapah Supergroup (both the

Papaghni and Chitravati Groups), and the Srisailam Formation (Figs. 1, 2 & 3e). It is overthrust

by the Nallamalai Group on its eastern margin along the, Neoproterozoic or younger,

Maidukuru Thrust. The Kurnool Group consists of >500 m of undeformed mixed siliciclastics and

carbonates that pass from the basal detrital diamond-bearing Banganapalle Formation

(conglomerates and coarse arenites)(Joy et al., 2012), through micritic carbonates of the Narji

Limestone Formation, the Owk Shale Formation, the Panium Formation (quartz arenites)(Fig.

3h), the Koilkuntala Formation (limestones) to the uppermost Nandyal Shale Formation (Fig. 2).

Detrital zircons were separated from both the basal Banganapalle Formation (samples CU10-05,

CU10-06) and the Panium Formation (CU10-21, CU10-22). Fifty four ≤10% discordant analyses

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were retrieved from the four samples (Table 1), with zircons being particularly hard to find in

the extremely pure quartz arenite of the Panium Formation (Fig. 3h). Zircon 207Pb/206Pb ages of

the near concordant data range from ~3400 Ma to ~900 Ma, with a distinct age peak at ~2500

Ma and an appreciable number of analyses yielding ages between 3000 and 3400 Ma (Figs.

4r,s,t,u & 5, see also supplementary publication). The youngest near concordant age from the

Banganapalle Formation was 2516 ± 19 Ma, whereas the Panium Formation yielded a number

of Palaeoproterozoic ages and a single Neoproterozoic analysis at 913 ± 11 Ma (the 206Pb/238U

age is quoted because of its increased precision with <1000 Ma ages with respect to the

207Pb/206Pb age). This lone analysis is considerably younger than the next youngest near-

concordant analyses (1717 ± 20 Ma)(Fig. 5) and therefore must be treated with caution. Eight

late Palaeoproterozoic analyses can be used to define a discordia chord with lower and upper

intercepts at 264 ± 230 Ma and 1763 ± 33 Ma respectively (2σ errors, MSWD = 1.5)(Fig. 4t).

Hafnium isotope data were collected from previously dated zircons from both the Banganapalle

Formation (sample CU10-05) and the Panium Formation (CU10-22)(Fig. 6d). The data for the

earlier Archaean samples plot with positive to low negative ϵHf values, whereas the data from

the ~2500 Ma zircons demonstrate a similar range to those seen in the older rocks—with ϵHf

varying from ~-12 to +10 (Fig. 6d). Two analyses from Palaeoproterozoic zircons plotted with

near zero ϵHf values.

5. Discussion

5.1. Depositional age constraints of Cuddapah Basin formations

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The youngest ≤10% discordant detrital zircon analysis recovered from a formation is here

interpreted to provide a maximum age constraint on the age of deposition of the formation in

question. These ages are listed in Table 2. However, as much of the detritus is sourced from

cratonic domains, these maximum depositional ages do not always approach the real

depositional age. In addition, in a number of cases, the youngest detrital zircon is considerably

younger than the next youngest age recovered, casting doubt on the reliability of the age

implication of that zircon. In some cases, less concordant data can be used to argue a more

robust and/or precise maximum depositional age, whereas complementary data, such as

detrital muscovite ages, can assist in age interpretation. Below, we discuss new and published

data from all the formations investigated in the context of constraining the depositional age of

the formation as far as possible.

The oldest formations examined belong to the Papaghni Group, the stratigraphically lowest

group in the Cuddapah Basin (Fig. 2). The youngest ≤10% discordant zircon with the basal

Gulcheru Formation yields a 207Pb/206Pb age of 2490 ± 19 Ma. A weighted mean of the 17

youngest ≤10% discordant analyses provides a more precise estimate of 2524 ± 9 Ma (2σ error,

MSWD = 1.04), which lies within the two standard deviation error of the youngest analysis and

is here taken as the best estimate of the maximum depositional age. The youngest ≤10%

discordant zircon within the overlying Vempalle Formation yielded a 207Pb/206Pb age of 2422 ±

17 Ma. Although forming a prominent peak in the probability density distribution (Fig. 5), the

data do not form a statistically robust single age. We therefore take the youngest near

concordant analysis as the best estimate of the maximum depositional age.

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The two samples from the unconformably overlying Chitravati Group come from the basal

Pulivendla Formation (CU10-19) and the top-most formation, the Gandikota Formation (GF06).

The youngest near-concordant zircon in the Pulivendla Formation yielded a 207Pb/206Pb age of

1899 ± 19 Ma. This analysis is one of five ≤10% discordant results that yield a weighted mean of

1931 ± 17 Ma (2σ error, MSWD = 1.07). The same five analyses define a discordia chord with a

zero lower intercept and an upper intercept of 1923 ± 22 Ma (2σ error, MSWD = 1.3). When

strongly discordant data are included, 13 analyses can be used to define a discordia chord with

a lower and upper concordia intercepts of 39 ± 34 Ma and 1924 ± 25 Ma respectively (2σ errors,

MSWD = 1.8)(Fig. 4d). These near equivalent upper intercepts bolster the 1923 ± 22 Ma age as a

reliable maximum depositional age from the Pulivendla Formation.

The youngest near-concordant zircon from the Gandikota Formation yielded a 207Pb/206Pb age

of 1207 ± 22 Ma. This relatively young age is supported by an array of five analyses that define a

chord with a ‘common Pb’ upper intercept of approximately the age of the Earth, and a lower

intercept of 1181 ± 29 Ma (2σ error, MSWD = 0.16)(Fig. 4q). This latter age is interpreted as the

maximum depositional age of the Gandikota Formation.

The shale-dominated Tadpatri Formation separates the Pulivendla and Gandikota Formations,

which also contains appreciable quantities of extrusive volcanic rocks and shallow sills. One of

these sills has been dated using the 40Ar/39Ar technique on phlogopites (1899 ± 20 Ma,Anand et

al., 2003) and U-Pb thermal ionization mass spectrometry of baddeleyite (1885 ± 3 Ma,French

et al., 2008) providing an excellent minimum depositional age of the lower part of the Chitravati

Group, and tightly constraining the deposition age of the Pulivendla and Tadpatri Formations to

between ~1920 Ma and ~1882 Ma. Our data suggest that the Gandikota Formation is separated

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from the lower Chitravati Group by a significant disconformity that represents at least ~700

million years. Most workers have considered the boundary between the Tadpatri Formation

and the Gandikota Formation to be broadly conformable (Nagaraja Rao et al., 1987; Patranabis-

Deb et al., 2012; Saha et al., 2009), but Dasgupta & Biswas (2006) mention the presence of an

unconformable contact at the base of the Gandikota Formation.

The youngest ≤10% discordant zircon from the eight samples from the tectonically bound

Nallamalai Group has a 207Pb/206Pb age of 1659 ± 22 Ma. Two other samples have their

youngest near-concordant zircons with ages >1700 Ma, whilst the youngest grains in the rest

are all >2000 Ma (Table 1). The minimum age constraints on the age of deposition of the

Nallamalai Group are the age of the crosscutting Chelima lamproite and Racherla alkali syenite,

which are constrained to ~1350 Ma (Chalapathi Rao et al., 1999; Chalapathi Rao et al., 2012;

Kumar et al., 2001)(location on Fig. 1). Close to the eastern border of the Nallamalai Group with

rocks of the Krishna Orogen (Dobmeier and Raith, 2003), lie a series of alkaline granites that

post-date most of the deformation in both the Krishna Orogen and the Nallamalai Group

(Dobmeier et al., 2006; Saha and Chakraborty, 2003). The Vinukonda Granite crops out only 4

kilometers from the boundary with the Nallamalai Group and crystallised at ~1590 Ma

(Dobmeier et al., 2006)(Fig. 1). Although not directly crosscutting the Nallamalai Group, its

proximity, and lack of pervasive deformation, suggests that this age provides a minimum age of

deposition. This interpretation is also supported by the adjacent Vellaturu Granite that does

intrude the Nallamalai Group (Saha and Chakraborty, 2003)(Fig. 1), and has a Rb-Sr model age

of ~1575 Ma (Crawford and Compston, 1973). The Nallamalai Group, therefore, is interpreted

to have been deposited between 1659 ± 22 Ma and ~1590 Ma.

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Quartz arenites of the Srisailam Formation preserve a number of Palaeoproterozoic detrital

zircons, with the youngest ≤10% discordant zircon yielding a 207Pb/206Pb age of 1787 ± 22 Ma

(Table 1). Five detrital muscovites from sample RG-02 yielded a weighted mean of 1773 ± 18

Ma (2σ error, MSWD = 0.99)(Fig. 7), which is taken as the maximum depositional age of the

formation.

The Kurnool Group unconformably overlies all previously described formations (Fig. 3e), with

the exception of the Gandikota Formation (where no direct contact is mapped), and the

Nallamalai Group (which is thrust over the Kurnool Group along most of its outcrop). The lone

near-concordant analysis of 913 ± 11 Ma is considered the maximum depositional age because,

even though it is much younger than any other zircons, it is consistent with independent

geological interpretations. However, we cannot completely exclude the possibility of

contamination. The next youngest age of 1717 ± 20 Ma, is one of a cluster of similar ages.

5.2. Provenance of Cuddapah Basin formations

All 21 samples looked at in this study have appreciable detrital zircons that yield late

Neoarchaean to earliest Palaeoproterozoic ages (Fig. 5). When all data are combined, 411 ≤10%

discordant zircon 207Pb/206Pb ages between 2610 Ma and 2405 Ma (ages chosen by identifying

minima or inflection points in the probability density distribution) yielded an error weighted

mean of 2516 ± 4 Ma (2σ error, MSWD = 4.6). The large MSWD demonstrates that there are

real differences in age within this population, as would be expected in detrital zircons from a

large source area. The age, although probably not accurate within uncertainty, nevertheless

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demonstrates that the dominant source of detritus throughout the Cuddapah Basin is latest

Neoarchaean. This supports the study of Bickford et al. (2013), where detrital grains within the

Owk Shale Formation yielded similar Neoarchaean detritus.

The Cuddapah Basin unconformably overlies the Dharwar Craton, which has long been

subdivided into a Palaeoarchaean-Mesoarchaean Western Dharwar Craton and a Neoarchaean

Eastern Dharwar Craton (Chadwick et al., 2000; Jayananda et al., 2000; Swami Nath et al.,

1976). The oldest components of the Western Dharwar Craton are the >3.0 Ga Peninsular

Gneiss orthogneisses and ~3.3 Ga Sargur Group greenstone belts that are overlain by ~2.8-2.6

Ga greenstone belts (the Dharwar Supergroup) and intruded by granitoids up until ~2.61 Ga

(Jayananda et al., 2006). Recently, Peucat et al. (2013) subdivided the Eastern Dharwar Craton

into two provinces—an Eastern and a Central Dharwar Province (Fig. 1). The Eastern Dharwar

Province consists of ~2.7–2.6 Ga tonalities intruded by a phase of 2.55–2.53 Ga juvenile

granitoids followed by orogenesis and migmatitic granitoids at ~2.52–2.51 Ga (Friend and

Nutman, 1991; Mohan et al., 2014; Sarma et al., 2012). Their Central Dharwar Province,

however, contains pre 3.0 Ga crust that was extensively deformed and metamorphosed

between 2.56-2.51 Ga. The present western margin of the Cuddapah Basin lies exclusively in

the Eastern Dharwar Province as defined by Peucat et al. (2013)(Fig. 1). However, at its closest,

the Gulcheru Formation (the basal Cuddapah Basin unit) is only a few kilometers from the

Anantapur shear zone that marks the boundary between the two provinces (Fig. 1).

Surprisingly, the dominant ~2516 Ma detrital zircon age peak in the Cuddapah units is younger

than most U-Pb zircon ages so far obtained from the eastern or central Dharwar Craton (see

Table 5 in Mohan et al., 2014). However, it matches the age of the youngest magmatism in the

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region, which includes the well-known Closepet Granite (Friend and Nutman, 1991),

metamorphic zircons analysed from the Eastern Dharwar Craton (see summary in Glorie et al.,

2014) and gold mineralization in the craton (Sarma et al., 2011). Although the error weighted

mean age is ~2516 Ma, many zircons stretch back through the earlier Neoarchean and are likely

sourced from the older rocks presently exposed in the Dharwar Craton. Hafnium isotope values

from the Neoarchean Cuddapah detrital zircons range from approximately -15 to +15 (Fig. 6a).

There are not many Hf results published from zircons from within the Dharwar Craton, but

those that exist show a similar range in ϵHf values, from strongly evolved to juvenile (Glorie et

al., 2014; Mohan et al., 2014; Sarma et al., 2012).

Major components of detrital zircons that are older than 3.0 Ga are present in the

stratigraphically oldest formation, the Gulcheru Formation, and also from the youngest unit,

the Kurnool Group. Rare analyses of this antiquity also occur in the Srisailam Formation, the

Nallamalai Group samples, and the Gandikota Formation. Zircons of this age are reported from

the Central Dharwar Province and from the Western Dharwar Craton, but not from the Eastern

Dharwar Province (see Peucat et al., 2013 for summary). The Hf isotopes reported in this study

show that zircons older than 3.1 Ga have ϵHf values between CHUR and DM, suggesting a

juvenile origin (Fig. 6a), whereas the zircons at ~3.0 Ga are more evolved, with ϵHf values

stretching down to -10. Published data of this antiquity from the Dharwar Craton (Sarma et al.,

2012) also show similar ϵHf patterns. In fact, when published Archaean detrital zircon U-Pb and

Hf data are plotted with detrital zircon data from the Gulcheru Formation and Kurnool Group

obtained in this study (Fig. 8a) the similarity is extraordinary, strongly suggesting that these

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Cuddapah Basin zircons are broadly sourced from the present-day west—from the Dharwar

Craton.

The near continuous Neoarchaean and Palaeoproterozoic U-Pb age spectra for the Nallamalai

Group demonstrate that zircons were sourced from a terrane that contained considerably

younger felsic source rocks than that found in the Dharwar Craton (Glorie et al., 2014; Peucat et

al., 2013). The Nallamalai Group is deformed into a classic fold-and-thrust belt (Saha et al.,

2010) in the foreland of the Krishna Orogen (Dobmeier and Raith, 2003)—an orogen that

contains a hinterland core of granulite-facies metasedimentary and metaigneous rocks (the

Ongole Domain of Dobmeier and Raith, 2003). U-Pb and Hf isotopic values from zircons in the

Nallamalai Group (this study) are very similar to those reported in the Ongole Domain

(Henderson et al. 2014)(Fig. 8b). The Ongole Domain was metamorphosed between ~1.68-1.60

Ga (Henderson et al., 2014), coeval with the constraints on the timing of deposition of the

Nallamalai Group (Table 2). We therefore interpret the Nallamalai Group as syn-orogenic,

deposited in the foreland of the evolving Krishna Orogen and containing a large detrital

component recycled from the rocks of the Krishna Orogen. The ultimate origin of these zircons,

found as detritus within the Ongole Domain metasedimentary rocks, is uncertain, but

Henderson et al. (2014) suggested that they may originate from the Napier Complex of

Antarctica, or even originally from Proterozoic Australia.

The constraints on the depositional age of the Srisailam Formation are looser than those on the

Nallamalai Group. However, it is possible that they both were deposited at broadly the same

time (Table 2), with the Srisailam Formation representing the undeformed edge of the foreland

basin fill, west of the orogenic front. The Srisailam Formation also has a number of mid to late

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Palaeoproterozoic detrital zircons that may have been derived from the developing Krishna

Orogen. Detrital muscovite total fusion 40Ar/39Ar data from five grains suggests that the source

area cooled below the closure temperature of argon in muscovite by ~1775 Ma (Fig. 7). It is

possible that this represents cooling and exhumation of the Dharwar Craton after the ~1.89 Ga

magmatic event that has been interpreted as a large igneous province by French et al. (2008).

Alternatively, it may represent erosion from high levels of the evolving Krishna Orogen, where

similar-aged metamorphism and magmatism has been associated with a continental margin arc

(Bose et al., 2011; Dasgupta et al., 2013). The vast amount of detritus, however, is ~2500 Ma

(Figs. 5 & 6c) and likely sourced from the Dharwar Craton.

Notably, the Srisailam Formation is the only unit where a near-concordant Eoarchean zircon has

been found (this study). This suggests a possible source link with the ~1620 Ma Somanpalli

Group in the Pranhita-Godavari Basin found <100 km to the north (Amarasinghe et al., 2014).

Amarasinghe et al. (2014) argued that abundant Palaeoarchaean–Eoarchaean detritus was

derived either from the Bastar Craton, further north, or from the Napier Complex of Antarctica.

Similarities in the detrital zircon record of the Neoproterozoic Kurnool Group, and the

Palaeoproterozoic Gulcheru Formation, have already been noted above (see also results in

Bickford et al., 2013). The Palaeoproterozoic detritus in the Kurnool Group may record limited

erosion of the Nallamalai Group, or a minor source from the eastern Krishna Orogen. However,

the Kurnool Group otherwise appears to reflect a source area from the Dharwar Craton to the

west, consistent with palaeocurrent data from the basal detrital diamond-bearing Banganapalle

Formation that varies from ENE and SE (Sivaji and Rao, 1989) to S-SW (Patranabis-Deb et al.,

2012). This is permissive with the most likely source of the diamonds—the ~1.1 Ga Wajrakarur

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and Narayanpet kimberlite fields of the Eastern Dharwar Craton (Chalapathi Rao et al., 2013;

Gopalan and Kumar, 2008; Kumar et al., 1993)(Fig. 1). Note that this indirectly suggests that the

Kurnool Group was deposited after ~1.1 Ga.

Finally, the identification here that the Gandikota Formation is no older than Stenian, suggests

that it may be coeval with the Kurnool Group and not belong genetically to the Chitravati

Group. This is corroborated by the lack of any direct field relationship described between the

two successions. The Stenian detrital zircons within the Gandikota Formation are similar in age

to some zircons reported from within the Eastern Ghats Province far to the northeast (Dasgupta

et al., 2013; Dobmeier and Raith, 2003) and may indicate far-transported sediment derived

from this orogen in the latest Mesoproterozoic, or early Neoproterozoic.

5.3. Basin formation and evolution

The emerging picture of the evolution of the diverse sequences within the Cuddapah Basin is

one where early basin formation (pre ~1885 Ma) involved clastic sediments derived exclusively

from the Dharwar Craton. Early topography, represented by the fans of the Gulcheru Formation

(Dasgupta and Biswas, 2006; Patranabis-Deb et al., 2012), was soon eroded into a tectonically

quiet, most probably marine, environment represented by the extensive stromatolite-bearing

carbonates of the Vempalle Formation, the shelf-arenites of the Pulivendla Formation and the

shales of the Tadpatri Formation. We interpret this early phase of basin development to

represent initial rifting and evolution into a marine passive margin (Fig. 9), similar to the model

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proposed by Ravikant (2010). The volcanism and plutonism within the Tadpatri Formation is

interpreted to reflect plume-related magmatism, as proposed by French et al. (2008).

The development of the Krishna Orogeny at ~1.68-1.6 Ga (Henderson et al., 2014; Sarkar et al.,

in press), by collision of cratonic southern India with the Enderby Land in Antarctica (Henderson

et al., 2014), interrupted this plume-modified passive margin environment with the influx of

considerable sediment derived from the evolving orogen in the east. Voluminous clastics of the

Nallamalai Group and, most likely, the Srisailam Formation were deposited at this time as the

original passive margin was inverted into a foreland basin that overstepped the margin of the

original basin in the north-east (Fig. 9).

A major period of erosion followed, which denuded the highlands of the Krishna Orogen. This

lasted until the Neoproterozoic when the Kurnool Group and the Gandikota Formation were

deposited in a fluvial to epicontinental sea environment (Patranabis-Deb et al., 2012) with

major river systems that transported clastic sediment from the eroding Dharwar Craton (Fig. 9).

No diamictites or other ‘characteristic’ rocks of the Cryogenian have been reported in the

Kurnool Group, implying a Tonian age. Helically-coiled microfossils (Obruchevella sp.) have been

reported from the Owk Shale Formation (Sharma and Shukla, 2012); but the age significance of

these is not clear. Sharma and Shukla (2012) suggest that these are Ediacaran; however,

Obruchevella are reported globally as found in Neoproterozoic and Palaeozoic rocks

(Mankiewicz, 1992). No truly diagnostic Ediacaran fossils have been recovered from the Kurnool

Group, though, and we suggest that the Kurnool Group may represent earlier, Tonian, basin

formation, possibly due to far-field tectonic warping in the peripheral bulge area of the Tonian

Eastern Ghats/Rayner Orogeny found to the north-east and in Antarctica (Dasgupta et al., 2013;

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Gupta, 2012; Korhonen et al., 2013; Korhonen et al., 2011). As well as forming the basin that

the Kurnool Group was deposited within, the far field effects of this orogeny are interpreted to

have caused a ‘rhyolite flare up’ producing ~1000 Ma tuffs in the Chhattisgarh and Indravati

Basins to the north (Bickford et al. 2014), and may be the trigger for the intrusion of many of

the kimberlites in the Dharwar Craton.

The present topography in the Nallamalai Hills is interpreted to be the result of renewed

contraction in the late Neoproterozoic/Cambrian (Dobmeier et al., 2006) due to the collision

with the Mawson Continent in Antarctica, NE India and far SW Australia (Collins, 2003; Collins

and Pisarevsky, 2005; Fitzsimons, 2000; Kelsey et al., 2008; Yin et al., 2010). This is also when

the Maidukuru Thrust is interpreted to have formed, thrusting the Nallamalai Group over the

Kurnool Group (Fig. 9).

5.4. Regional correlations and significance

Saha and Patranabis-Deb’s (2014) recent overview of the age constraints, correlations and

tectonic significance of the Proterozoic basins that overlie the Dharwar and Baster Cratons

demonstrate the diversity, as well as the similarities, between basins in the region. The study

also illustrates the lack of age data available for most of the basins. Here, in Figure 10, we’ve

modified and updated their correlation figure in light of the new data presented here, in

Bickford et al. (2013) and from the Pranhita-Godavari Basin (Amarasinghe et al., 2014).

The new data here support a strong temporal correlation between the Nallamalai Group in the

Cuddapah Basin and the Somanpalli Group in the Pranhita-Godavari Valley (Fig. 10). The latter

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group has been recently shown to be deposited at ~1620 Ma (Amarasinghe et al., 2014),

consistent with the age constraints for the Nallamalai Group (between 1659 ± 22 Ma and ~1590

Ma, Table 2). Both sequences were deposited directly after the Krishna Orogeny (Henderson et

al., 2014) and are dominated by detritus from that orogen. The Srisailam Formation and Pakhal

Group are also likely to correlate with these units. In contrast to the conclusions of Saha et al.

(2014), we suggest that these sequences were deposited in a syn-orogenic basin, forelandward

of the Krishna Orogen. This is in contrast to a new model proposed by Sarkar and co-workers

(Sarkar and Schenk, in press; Sarkar et al., in press), where, based on monazite chemical ages,

the Ongole Domain is interpreted to have collided with the Dharwar Craton at ~1540 Ma. The

implications of the Sarkar model would be that either the Nallamalai Group was not sourced

from the Krishna Orogen, or that the Nallamalai Group would be younger than the well-dated

Vinukonda Granite. The first alternative is unlikely as detrital zircons in both the Nallamalai

Group and metasedimentary rocks of the Ongole Domain share near-identical U-Pb age and Hf

isotopic values (Fig. 8b). In the second scenario, the apparently similar Vellaturu Granite, which

demonstrably cuts the Nallamalai Group (Saha and Chakraborty, 2003), would have to be

considerably younger than both its ~1575 Ma Rb-Sr model age (Crawford and Compston, 1973)

and the Vinukonda Granite. We suggest that these situations are unlikely and that the simplest

interpretation is that the Nallamalai Group is the foreland basin succession of the Krishna

Orogen and the monazite chemical dates from the Ongole Domain are discordant and/or reflect

post-tectonic reactivation.

The extensive Chhattisgarh Basin to the north (Fig. 1) did not form until the Mesoproterozoic

(Fig. 10), at a similar time as the smaller Khariar and Indravati Basins (Saha and Patranabis-Deb,

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2014). This may well be due to the fact that the Krishna Orogen does not occur along the

margin of the Bastar Craton, but is only found in the south (Fig. 1). All basins experienced

Neoproterozoic deposition, which whilst not appearing to form as proximal foreland basins,

may be the result of warping of the foreland due to the development of the Tonian Eastern

Ghats-Rayner Orogeny.

6. Conclusions

The ~46,000km2 Cuddapah Basin is a globally significant Proterozoic Basin that preserves

extensive clastic and carbonate sedimentary sequences that extend from the Palaeoproterozoic

to the Neoproterozoic. This extensive, stratigraphically-controlled, detrital zircon study that

coupled U-Pb age dating and Hf isotopic analysis with localized 40Ar/39Ar detrital muscovite

dating, revealed a broad tripartite subdivision of the sequences within the basin. An early-

middle Palaeoproterozoic sequence (Papaghni and Chitravati Group – excluding the Gandikota

Formation), interpreted as a rift to passive-margin sequence that was modified towards its top

by large-igneous province volcanism. A second latest Palaeoproterozoic (and possibly into

earliest Mesoproterozoic) syn-orogenic sequence (Nallamalai Group and Srisailam Formation),

which correlates with the Somanpalli Group of the Pranhita-Godavari Valley further north

(Amarasinghe et al., 2014), was largely derived from, and deposited in front of, the Krishna

Orogen. Finally, the latest Mesoproterozoic to Neoproterozoic Kurnool Group (and Gandikota

Formation) that mark a return to largely west/northwest derived detritus.

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Acknowledgements

This paper forms TRaX Record #304 and an output of Australia-India Strategic Research Fund

Project ST030046 and Australian Research Council grant FT120100340. John Terlet, Angus

Netting and Aoife McFadden are thanked for assistance and support with analytical work in

Adelaide Microscopy. Drs Talari Chetty and Bhaskar Rao from the National Geophysical

Research Institute, Hyderabad, are thanked for assistance with field logistics. A. Barker is

thanked for field assistance. C. Mayers and Z. Martelli are thanked for assistance with the

muscovite sample preparation. Profs Abhijit Basu and Marion Bickford are thanked for

constructive reviews that improved the manuscript.

Table Headings

Table 1

Sample details with locations and youngest near concordant detrital zircon data.

Table 2

Summary of depositional age constraints for all major units in the Cuddapah Basin.

Figure Headings

Figure 1

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Location of the Cuddapah Basin. Insert - map of the Proterozoic basins that overlie the southern

and eastern Dharwar and Bastar Cratons—locations of the Cuddapah, Chhattisgarh, Indravati

and Pranhita-Godavari Valley basins are indicated, along with the Krishna and Eastern Ghats

orogens of Dobmeier and Raith (2003). Main figure – Major tectonic units of the Krishna

Orogen (in green) and sedimentary units of the Cuddapah Basin in blue, pink , orange and

yellow (modified from Patranabis-Deb et al., 2012). Sub-basins of Nagaraja Rao et al. (1987) are

indicated in italic script. Locations of the Vellaturu Granite (Ve), Vinikondu Granite (Vi) and

Chelima lamproites (Ch) are indicated as are locations of the samples discussed in this paper.

The major tectonostratigraphic units of the Dharwar Craton are also indicated in grey-scale

(after Peucat et al., 2013).

Figure 2

Cuddapah Basin stratigraphy with existing interpreted igneous crystallization age constraints

(modified after Saha and Patranabis-Deb, 2014). Qtz. = Quartzite, Sh. = Shale, Lst. = limestone.

Figure 3

Field photographs of major units within the Cuddapah Basin. a) Nonconformable contact of the

Gulcheru formation (distant) over the Eastern Dharwar Craton (foreground), person for scale. b)

Meter-scale conglomerate and coarse lithic-arenites within the Gulcheru Formation (site of

sample GF01), B4 notebook for scale. c) Decimeter-scale tabular cross-bedded coarse arenite

bed within the predominantly carbonate Vempalle Formation, pen for scale (site of sample

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GF14) (N15˚20’50.7”, E78˚05’10.2”). d) Shales and interbedded m-scale beds of stromatolitic

limestone within the Tadpatri Formation at Lotu Vagu (N15˚20’50.7”, E78˚05’10.2”), pen for

scale. e) Lotu Vagu Gorge with the Kurnool Group (Banganapalle and Narji Formations)

unconformably overlying the Chitravati Group (Pulivendla and Tadpatri Formations), vegetation

for scale (N15˚21’14.2”, E78˚05’13.1”). f) Folded m-bedded arenites of the Nallamalai Group at

Diguvametta railway cutting (N15˚25’32.6”, E78 ˚45’43.9”) – close to the site of sample EA05. g)

The Krishna River Gorge directly southeast of Srisailam dam. Cliffs of the Srisailam Formation

quartz arenites well exposed. h) Panium Formation quartz arenites overlying limestones of the

Narji Formation near Betamcherla village (N15˚25’25.5”, E78 ˚09’49.5”).

Figure 4

U-Pb Concordia plots of 21 samples taken from throughout the Cuddapah Basin stratigraphy.

Error ellipses are at the 2 standard deviation level. Sample locations are illustrated in Fig. 1 and

located in Table 1.

Figure 5

Probability density (Ludwig, 2009)(black lines) and kernel distributions (Vermeesch, 2012)(grey

fills) of the ≤10% discordant detrital zircon ages from 21 samples taken from throughout the

Cuddapah Basin stratigraphy. ‘Tectonic’ = indicative tectonic contact, ‘unconformity’ implies an

unconformable relationship. n = number of analyses. Gp. = Group, Fm. = Formation. The

probability density distribution of zircon ages from the Pulivendla Formation are also shown

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(white) to illustrate their difference from those from the Gandikota Formation, higher in the

Chitravati Group. Approximate ages of the peak maxima are shown. Circles beneath the plots

indicate the spread of data.

Figure 6

U-Pb age versus epsilon Hf plots of detrital zircon data from 12 samples from throughout the

Cuddapah Basin stratigraphy. Colours refer to zircons from the different samples analysed. The

kernel density estimate distributions for the samples are plotted below the U-Pb age versus

epsilon Hf plots to provide a measure of the importance of each group of data.

Figure 7

Plot of 40Ar/39Ar total fusion ages from five detrital muscovite analyses of sample RG02 from

the Srisailam Formation. Individual results are plotted at the 2σ error level. Data are presented

in the supplementary publication.

Figure 8

U-Pb age versus epsilon Hf plot of published hafnium isotopic data from zircons from the a)

eastern and central Dharwar Craton (replotted from Glorie et al., 2014; Mohan et al., 2014;

Sarma et al., 2012) compared with the Gulcheru Formation and Kurnool Group Archaean data

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from this study, and b) from the metasedimentary rocks within the Ongole Domain (Krishna

Orogen)(Henderson et al., 2014) compared with the Nallamalai Group data from this study.

Figure 9

Palaeogeographic cartoons interpreting the tectonic situation of the three main stages of basin

formation. Initially, the Cuddapah Basin formed as a Palaeoproterozoic rift-passive margin on

the edge of the Eastern Dharwar Craton. By the end of the Palaeoproterozoic, the Ongole

Domain (and Enderby Land in Antarctica) collided with the passive margin, whence it evolved

into a foreland basin in front of the resulting Krishna Orogen. After a period of erosion, the

latest Mesoproterozoic to Neoproterozoic Kurnool Group reflects small-scale tectonic

movements cratonward of the Tonian Eastern Ghats Orogen and erosion of the Eastern

Dharwar Craton.

Figure 10

Revised correlation diagram of the Cuddapah Basin, the Pranhita-Godavari Valley basins and

the Chhattisgarh Basin; all Proterozoic basins lying unconformably over the Dharwar and Bastar

Cratons. Sh. =- shale, Lst. = limestone, Qtz. = quartzite, ss. = sandstone, MDA = Maximum

depositional age.

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Table 1

Stratigraphic

Unit

Sample Lithology Hf? Location Major

U-Pb

detrital

age

peaks

(Ma)

No. U-Pb

analyses

No >10%

conc. U-

Pb

analyses

Youngest

>10%

conc. U-

Pb

analysis

(Ma)*

Gulcheru Fm GF01 Polymict

conglomerate

N14˚35’05.3”

E77˚56’10.2”

~2520,

2975,

3420

96 27 2490 ± 19

Gandikota Fm GF06 Quartz arenite

N14˚45’03.4”

E78˚17’51.1”

~2500 96 27 1207 ± 22

Vempalle Fm GF14 Cross-bedded

quartz arenite

N14˚33’20.8”

E77˚58’32.2”

~2510 100 59 2422 ± 17

Gulcheru Fm CU10-

01

Granule

conglomerate

N15˚46’06.9”

E78˚03’23.4”

~2520,

2545

50 9 2502 ± 17

Banganapalle

Fm

CU10-

05

Coarse quartz

arenite

N15˚32’55.2”

E78˚16’53.2”

~2610,

3105,

3160

22 11 2516 ± 19

Banganapalle

Fm

CU10-

06

Coarse quartz

arenite

N15˚32’55.2”

E78˚16’53.2”

~2530,

2620,

3100,

3310

58 10 2542 ± 18

Nallamalai Gp CU10-

09

Laminated

quartz arenite

N15˚23’34.2”

E78˚39’40.4”

~1850,

2535

96 58 1774 ± 21

Nallamalai Gp CU10-

10

Coarse lithic

arenite

N15˚23’34.2”

E78˚39’40.4”

~1800,

2510

50

14 1659 ± 22

Pulivendla Fm CU10-

19

Coarse quartz

arenite

N15˚21’03.5”

E78˚05’08.7”

~1935,

2490

50 8 1899 ± 19

Panium Fm CU10-

21

Quartz arenite N15˚25’25.5”

E78˚09’49.5”

~1735 20 7 913 ± 11*

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Panium Fm CU10-

22

Quartz arenite

N15˚25’25.5”

E78˚09’49.5”

~2500 39

26 2061 ± 22

Nallamalai Gp EA01 Quartz arenite

N16˚05’41.1”

E79˚41’40.6”

~2535 107 63 1882 ± 22

Nallamalai Gp EA04 Muscovite-

rich

arenaceous

schist

N15˚34’41.9”

E79˚18’06.4”

~1850,

2045,

2340,

2480

109 87 1683 ± 25

Nallamalai Gp EA05 Quartz arenite

N15˚25’37.5”

E78˚45’42.1”

~1840,

2500

85 48 1661 ± 20

Nallamalai Gp EA06 Quartz arenite N15˚11’12.0”

E78˚38’01.8”

~2515 17 9 1843 ± 32

Nallamalai Gp EA07 Medium

grained quartz

arenite

N15˚41’25.0”

E79˚09’59.8”

~1855,

2470,

2670

79 65 1783 ± 71

Nallamalai Gp EA08 Medium

grained quartz

arenite

N15˚45’43.5”

E79˚12’29.2”

~1865,

2510,

2690

80 72 1836 ± 20

Srisailam Fm RG01 Quartz arenite

N16˚02’46.2’

E78˚54’24.5’

~2530 137 69 1787 ± 22

Srisailam Fm RG02 Quartz arenite N16˚18’23.2”

E78˚43’58.9”

~2485 46 4 2479 ± 17

Srisailam Fm RG04 Quartz arenite

N16˚05’30.4”

E78˚54’37.0”

~2500 61 18 2062 ± 20

Srisailam Fm RG15 Quartz arenite N16˚05’05.4”

E78˚54’07.2”

~2500 104 39 2374 ± 19

* = 207

Pb/206

Pb age quoted unless marked with an asterisks when 206

Pb/238

U age quoted. Errors quoted at 1 sigma

level.

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Table 2

Stratigraphic Unit Interpreted Maximum Depositional Age Interpreted Minimum

Depositional Age

Kurnool Group 913 ± 11 Ma [single zircon – next youngest =

1717 ± 20 Ma]

Recent

Srisailam Formation 1773 ± 18 Ma [mean of five muscovites] <913 ± 11 Ma [mda of Kurnool

Group]

Nallamalai Group 1659 ± 22 Ma [single zircon] ~1590 Ma [post-tectonic

intrusion]

Gandikota Formation

(Chitravati Group)

1181 ± 29 Ma [lower intercept of 5 zircons] Recent

Pulivendla Formation

(Chitravati Group)

1923 ± 22 Ma [upper intercept of 5 zircons] 1885 ± 3 Ma [age of sill in

Tadpatri Fm]

Vempalle Formation 2422 ± 17 Ma [single zircon] 1885 ± 3 Ma [age of sill in

Tadpatri Fm]

Gulcheru Formation 2524 ± 9 Ma [mean of 17 zircons] 1885 ± 3 Ma [age of sill in

Tadpatri Fm]

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Graphical abstract

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Highlights

Deposition of Cuddapah Basin constrained by 21 detrital zircon samples

Papaghni and lower Chitravati Gps Dharwar Craton formed a rift-passive margin

Nallamalai Gp deposited in foreland basin to Krishna Orogen at ~1620 Ma

Kurnool Gp formed intracontinental basin derived from Dharwar Carton

Detrital zircon U-Pb, Hf and muscovite 40Ar/39Ar unravel basin evolution