Gondwana Research, V 5, No. I, pp. 23-33. 0 2002 International Association for Gondwana Research, Japan. ISSN: 1342-937x

The Purana Basins of Southern Cratonic Province of India - A Case for Mesoproterozoic Fossil Rifts

Asru K. Chaudhuri', Dilip Sahal, Gautam K. Deb', Sarbani Patranabis Deb1, Mrinal Kanti Mukherjee' and Gautam Ghosh2

* Geological Studies Unit, Indian Statistical Institute, 203 B. T. Road, Calcutta - 700 035, India, E-mail: dsaha@isical.ac.in Department of Geology, Presidency College, 86/1 College Street, Calcutta - 700 073, lndia

(Manuscript received June 4,2001; accepted July 6,2001)

Abstract

Since its cratonization in the Palaeoproterozoic, southern peninsular India witnessed the development of a number of large intracratonic sedimentary basins, traditionally referred to as Purana basins, spanning in age from late Palaeoproterozoic through Neoproterozoic. The localization of these intracratonic basins, namely Pranhita-Godavari (PG), Chattisgarh and Cuddapah basins, is apparently controlled by pre-existing sutures and/or weak zones. The PG basin and the Cuddapah basin host unconformity-bound, thick, sediment-dominated successions attesting to several cycles of fluvial-shallow marine to shelf-slope-basin sedimentation. Deposition was punctuated by block uplifts resulting in local hiatuses and/or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites. Basin-margin deep faults apparently played a role in the facies distribution in these basins. Based on these features we propose that these basins initiated as continental rifts which, however, never opened up into a full- fledged ocean basin, but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the Chattisgarh succession. Spatial distribution of facies and sediment thickness in the Cuddapah and PG basins suggest that an open seaway existed to the east of the south Indian cratonic province during the Mesoproterozoic, while similar criteria point to the existence of an open seaway north of the Chattisgarh basin. Development history, including nature of inversion, suggest that the southern cratonic province of India existed as a single large continental mass since the Mesoproterozoic, in spite of episodes of supercontinent build- up and fragmentation involving India and East Gondwana during the Proterozoic.

Key words: Fossil rift, Mesoproterozoic, Purana basins, South Indian craton.

Introduction

The Archaean shield complexes of the Indian peninsula were variously assembled, sundered, and reassembled into two large cratonic provinces at ca. 2500 Ma (Acharyya, 1997; Pandey and Agrawal, 1999), each with a collage of stable crustal blocks and intervening weaker zones of suturing, or remnants of Archaean greenstone belts, tectonic zones and magmatic arcs. The above provinces were separated by an ENE-WSW trending narrow linear zone, known as Central Indian Mobile Belt (Acharyya, 1997) or Central Indian Tectonic Zone, CITZ (Chakraborty, 2000), which developed as an ensialic greenstone belt of rift setting (Jain et al., 1995) or an aborted rift with mantle activated volcanics and volcano-sedimentaries (Acharyya, 1997). The CITZ had complex history of basin opening, deformation, metamorphism, and emplacement of igneous rocks between 2.5 and 1.6 Ga (Jain et al., 1995).

The belt extends across the entire peninsula of India, and the Son-Narmada lineament, the northern margin of the CITZ, has been traced as far as to Madagascar (Crawford, 1978). The northern and southern cratonic provinces constituting the Indian peninsula and an undetermined amount of the crust subducted under the Himalayas was a part of the Gondwanaland (Rogers, 1986; Acharyya, 1997), and presumably also of an earlier Proterozoic supercontinent. However, the peninsula does not bear any signature of fragmentation and separation since its stabilization in the Palaeoproterozoic, notwithstanding the periodic events of remobilization, and multiple episodes of supercontinent assembly and fragmentation.

The Proterozoic Indian craton of continental proportion witnessed development of several cratonic basins designated as Purana basins (Holland, 1906) in the late Palaeoproterozoic, Mesoproterozoic and Neoproterozoic. The Purana basins are comparable with the Proterozoic -

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24 A.K. CHAUDHURI ET AL.

early Palaeozoic cratonic basins of North America, such as Williston, Michigan or Illinois basin (Sleep and Sloss, 1980; Miall, 1999), the basins on the Russian platform (Aleinikov et al., 1980), or the Australian basins in respect of the prolonged duration of the basin history, shape, size, sediment thickness and depositional systems. The basins covered several tens of thousands square kilometers of the cratonic area and accommodated several thousand meters of sediments. The preserved sedimentary basin- fill deposits are, in general, weakly metamorphosed and only locally strongly deformed. Stratigraphy, sedimentation and deformation of the Purana successions have been studied in details in several basins in last several decades (cf. Kale, 1991). However, regional aspects of Purana geology have rarely been addressed, and origin of the basins still remains a matter of conjecture.

The question of origin of cratonic basins has been debated for decades, and none of the proposed models adequately explain the major elements of the basins or their subsidence history (for discussion, see, Sloss, 1991; Miall, 1999). Nevertheless, a close relationship of craton- interior histories with plate tectonics and variations in the heat flow regime underneath the continental crust is now fairly well established (Anderson, 1982; Gurnis, 1988; Sloss, 1991). Periods of formation of many cratonic basins are coincident with the fragmentation of supercontinents (Hartley and Allen, 1994), indicating genetic linkage between the two processes.

In this paper, we intend to discuss the major stratigraphic, sedimentologic and deformational attributes of three Purana basins south of the Central Indian Tectonic Zone. We would also make an endeavor to evaluate the origin and inversion of the basins in the light of fragmentation and assembly of Proterozoic supercontinents.

The South Indian Cratonic Province

The southern cratonic province of peninsular India comprises the Dhanvar cratonic block (including western Dharwar, eastern Dharwar and Southern Granulite Terrain), the Bastar and Singhbhum cratonic nuclei (Fig. 1). The Dharwar and the Bastar blocks are juxtaposed along a NW-SE trending tectonic contact represented by a linear belt of granulites (> 2.6 Ga, Rajesham et al., 1993) and younger intrusive granite (ca. 2.5 Ga, Shirahata et al., 1994). The granulite (Godavari granulite) belt, exposed in two linear zones flanking the outer margins of the present Pranhita-Godavari Valley has an average width of about 150-170 km (Fig. 2).

The contact between the Bastar and Singhbhum nuclei is not adequately constrained, as the Mahanadi rift developed along the interface is covered by the Permo-

- 500 km

10"N \ h,$ ry] Mobile belt

A r c h e a n nuclei

73'E 78' 83" 88" I

Fig. 1. South Indian craton with major Archaean nuclei and Purana basins shown. Outline of the Central Indian Tectonic Zone (CITZ) is after Palaeoproterozoic disposition of the Son- Narmada-Tapti lineament zone by Jain et al. (1995).

Triassic Gondwana deposits, and does not have any proven Proterozoic activity (Rogers, 1986). However, a linear belt of granulite-facies rocks occur immediately to the south of the Sukhinda thrust (between the Sukhinda thrust and the Mahanadi rift), and the granulites show NW-SE fabrics parallel to the trend of the Mahanadi valley (Chatterjee et al., 1964) as well as of fabrics in the 'Godavari granulite belt'.

The Archaean crust beneath the cratonic nuclei differs in petrography, chemistry and seismic structure. However, seismic studies and gravity modeling indicate a general crustal thickness of about 35 km for the Indian peninsular region (Kaila et al., 1979; McCarthy et al., 1983; Verma and Subrahmanyam, 1984). The lithospheric thickness of the craton, on the other hand, varies from 65 to 148 km with an average of about 104 km, which is about half or less than that for many other ancient cratons (Pandey and Agrawal, 1999). The heat flow regime of the craton and its lithospheric thickness are quite unusual and strongly indicate deformation and shearing at the lithospheric mantle level since the Mesoproterozoic. The lithosphere is characterized by high average reduced heat

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PURANA BASINS OF SOUTHERN INDIA 25

Fig. 2. The Purana (Proterozoic) intracratonic basins of peninsular India, south of the Central Indian Tectonic Zone (CITZ). The Eastern Ghats granulite belt and granulites flanking the Pranhita-Godavari Valley (Karimnagar and Bhopalpatnam granulites) and some of the greenstone remnants are also shown. S-Sonakhan greenstone belt; K-Kadiri schist belt (greenstone).

flow of 35 mW/mz and high average Moho temperature of about 550 C (Pandey and Agrawal, 1999), much higher than that found in many other major shield areas (Chapman and Pollock 1974; Kutas, 1977; Jessop and Lewis, 1978; Sass and Lachenbruch, 1979). Higher heat flow is attributed to high enrichment of lithophilid radioactive elements in the crust and upper mantle, caused by intermittent remobilization, rise of the isotherms and intrusion of granitoids since the stabilization of the Indian craton at ca. 2500 Ma, (Rogers and Callahan, 1987).

Purana Basins

The Pranhita-Godavari (PG) VafZey

Stratigraphy

The Proterozoic sedimentary rocks in the PG valley occur in two NW-SE trending linear outcrop belts separated by a strip of Paleozoic-Mesozoic Gondwana rocks and flanked along their outer margins by two belts of granulites and Archaean gneissic rocks (Fig. 2). The

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26 A.K. CHAUDHURI ET AL.

Table 1. Stratigraphic succession, lithology and depositional setting of the Godavari Supergroup in central and southern parts of the PG Valley.

Mancherial-Ramgundam Mallampalli-Pakhal Lake (central part of the valley) (southern part of the valley)

Lithologic assemblage Depositional setting

G

0 D A V A R I

S

U P E R G R 0 U

Gondwana Supergroup __x_____-__- Unconformity -

- Sullavai Group Sullavai Group (1375 m)

Conglomerate, feldspathic and quartzose sands (red bed)

Quartzose sandstone and shale --- Unconformity - -- Albaka Group

(3255 m)

--- Unconformity --- Penganga Group

(1225 m)

Conglomerate and feldspathic sandstone in lower part, limestone and shale in upper part Rb-Sr date: 7755 30 Ma

790k 30 Ma ------------ Unconformity -----I ------- -

P A K H A L G R 0 u P -

Mulug Subgroup (945m) A

K H A L G

Subgroup 0 (390m) U

P

Mallampalli

. Mulug Subgroup (713 m) Conglomearte and feldspathic

sandstone in lower part,

Mallampalli Subgroup (1420 m)

dolomitic limestone, calcareous shale and subordinate chert in upper part

Dolomitic limestone and minor conglomerate in lower part, quartzose sandstone and calcareous shale in upper part

Fault-controlled, unstable: fan- fluvial complex and erg deposits

Wide stable shelf: shallow marine environments

Fault controlled, unstable: fan and fan delta; rapidly subsiding shelf, slope, base of slope

Fault-controlled unstable: coastal fan, fan delta and carbonate shelf

Wide stable shelf shallow marine environments

Unconformity K-Ar date: 13305 53 Ma Archaean Basement

Proterozoic outcrops extend for about 400 km. from Khammam (17O5'N, 80°81'E) in the southeast where they are juxtaposed against the Eastern Ghats, to Wardha (20 45 N':78 37' E) in the northwest where they are covered by the Tertiary Deccan volcanics. Geophysical studies indicate that the basin extends further northwestwards under cover of the volcanics for about 400 km, and meets the Son-Narmada rift (Biswas, 1999).

The sedimentaries are unconformably underlain by the Archaean basement rocks (Nageswara Rao, 1964; Basumallick, 1967; Chaudhuri, 1985), either the gneisses or granulites and the younger granites (Deb and Chaudhuri, under preparation), and overlain by the Gondwana rocks. The basin developed as a narrow, linear one, along the join between the Dharwar and Bastar cratonic blocks (also see Rogers, 1986). The unconformable contact between the basement and the cover sequences along the southwestern margin of the valley represents the linear trend of the basin margin. The disposition of the basin margin along the northeastern margin of the valley has been obliterated by the boundary fault.

Authigenic glauconitic minerals from the Mallampalli Subgroup about 200 m above the base of the sequence yield an age of 1330rt 53 Ma (Vinogradov et al., 1964), and those from the overlying Penganga Group yield ages of 775+ 30 and 790+ 30 Ma (Chaudhuri et al., 1989) (Table 1). Considering an authigenic origin of glauconitic minerals by replacing detrital feldspar grains (Chaudhuri et al., 1994), probable loss of Ar on deep burial, and position of the analysed samples in the sequence, an age between 1600 and 1500 Ma would apparently be a reasonable estimate for the initiation of sedimentation.

The Proterozoic succession of the Valley, defined as the Godavari Supergroup (Chaudhuri and Chanda, 1991), is characterized by wide variation in lithofacies, facies association and depositional environments in space and in time (Table 1). The correlation of the formations or groups on a regional scale has been attained through mapping the regional unconformities and by correlating the unconformity-bound groups (sequences) or subgroups (subsequences). Despite internal variations, each sequence is marked by a set of distinctive features that impart an individuality to each sequence, facilitating

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PURANA BASINS OF SOUTHERN INDIA 27

regional correlation (cf. Sloss, 1991). The stratigraphic successions from Mancherial-Ramgundam area, in the central part of the basin, and Mallampalli-Pakhal Lake area, in the southern part of the basin (Table 1) indicate that the Penganga and Albaka Groups occur only in the central and southern parts respectively, though in their respective areas of occurrences, both the groups unconformably overlie the Mulug subgroup. Contrasting stability regimes of the Albaka and Penganga Groups (Table 1) militate against their lateral correlation (Chaudhuri, submitted), and the stratigraphic relationship is attributed to uneven rate of tectonic movements in different parts of the basin and erosion in the uplifted blocks during the hiatuses. Tectonic uplift of fault blocks and erosion of 1000 metres or more in the uplifted blocks during the hiatuses separating different groups is a hallmark of the basin (Deb and Chaudhuri, under preparation). The unconformities punctuating the succession are manifestations of episodic tectonic uplift and/or depression of the basin floor. Occurrence of conglomerates and coarse arkosic sandstones deposited as fans, fan deltas, and braided fluvial deposits at the basal part of different sequences (Table 1; Chaudhuri and Howard, 1985; Deb and Chaudhuri, under preparation) also attests to fault-controlled uplift of the cratonic hinterland and sedimentation. The tectonic behaviour of the basin and the depositional mode characterize the PG basin as an oscillatory one (cf. Sloss, 1984) and a cratonic rift.

Felsic tuffs, including welded tuff, occur in different formations at different stratigraphic levels and geographic locales. It has been suggested that because of the behavior of the mature Proterozoic continental crust as a heat lens, extensive partial melting at lower crustal levels led to localized stretching and rifting (Patranabis Deb, in press). Origin of the PG basin as a rift is also supported by geophysical investigations (Qureshy et al., 1968; Biswas, 1999).

Despite local variations, thicknesses of unconformity bound sequences, as well as of individual formations increase towards southeast, indicating that the basin was deeper and wider towards the eastern sea board of the present craton. The Pakhal and Albaka rocks bear strong signatures of open marine circulation attesting to the presence of a major basin in the area occupied by the Eastern Ghats during the Meso- and early Neoproterozoic (Chaudhuri and Deb, 2001). The 1400 to 1500 Ma age of emplacement of sub-alkaline basalt in the Eastern Ghats (Shaw et al., 1997) may indicate a period of continental rifting, and development of the ocean basin.

Deformation structures

The deformation structures are essentially restricted

Gondwana Rrsearch, V. 5, No. 1, 2002

to the strata underlying the Neoproterozoic Sullavai Group, thus constraining a Mesoproterozoic age of deformation for the Pakhal and Somanpalli Groups. Pending proper resolution of the age of the Penganga Group, the timing of deformation of the Penganga succession is open to question.

Geometry of the structures in the southwestern belt is guided by the major faults, subparallel to the Valley margin and traceable on kilometre-scale, which transgress the lithologic contacts at places. Association of slope deposits as well as contractional structures with some of the faults indicates that these were reactivated, basin-opening extensional faults dipping broadly towards NE.

In the northern part of the belt, asymmetric repetition of the succession, basement gneiss overlain by the Penganga sequence, and abrupt facies change are interpreted as a set of NW-SE trending subparallel thrust sheets. Small-scale layer-parallel veins, shear fractures, folds and thrusts in the limestone beds and shale, particularly in the vicinity of major thrusts, represent internal deformation of the thrust sheets. The folds usually have NW-SE axial trends, and are asymmetric, SW-verging, and often associated with small-scale thrusts. Locally a set of ENE-WSW-trending small-scale thrust and related folds are developed in some thrust sheets. The vergence of the folds indicates that the senses of movement along the NW-SE and ENE-WSW trending thrusts are top to SW and top to SSE respectively (Deb, in press).

In the central part of the belt where the complete Proterozoic succession is preserved, detailed mapping reveals that the Pakhal and Penganga Groups were deformed into a fold-fault belt, while the overlying Sullavai Group remained undeformed. In the southern part of the belt, the Pakhal Group of rocks overlying unconformably above the basement were folded into a series of map-scale, NNW- to NNE-ly plunging synclines and anticlines (Ramamohana Rao, 1964,1971; Sengupta and Roychaudhuri, 1999). In the southeasternmost region close to the Eastern Ghats, the map-scale synclines show curved axes swinging from NW-SE to NE-SW, and thrusting along limbs with top-to-NW sense of movement (Ramamohana Rao, 1969).

The Proterozoic rock succession in the northeastern belt is bounded by NW-SE extensional faults against the basement rocks of the Bastar craton in the NE and the Gondwana sedimentaries in the SW. The contractional deformation is best recorded in the Somanpalli Group, which was affected by successive development of NW-SE trending large-scale folds, imbricate thrusts and related shear zones, and E-W strike-slip faults (Saha, 1990, 1992; Ghosh, 1997). NW-SE trending normal faults within the Somanpalli Group were reactivated as reverse faults, and

28 A.K. CHAUDHURI ET AL

they bound the large-scale folds in the adjacent formations (Saha, 1992). Major thrusts traceable for tens of kilometres show either southwesterly or northeasterly transport, and some imbricate thrusts are developed in piggy-back sequence with a northeasterly sense of transport (Saha, 1990). Axial planar cleavage developed with varied intensity indicating heterogeneous strain (Saha, 1990). Locally sharp change in fold axial trend from NW-SE to E-W across the strike-slip fault zones is observed within the Somanpalli and Albaka Groups (Srinivasa Rao et al., 1979; Saha, 1988).

The Chattisgarh Basin

Stratigraphy

The outcrops of the Chattisgarh Supergroup occur in an east-west elongated oval-shaped area and cover about 36000 sq. km of the Bastar cratonic block. South of the main Chattisgarh outcrops, there are several smaller occurrences of Proterozoic sedimentaries, lithologically similar to the Chattisgarh rocks, and these outcrops are often referred to as deposits of separate basins, such as, Khariar basin, Ampani basin, Indravati basin and Sukma basin (Fig. 2). However, there is a prevailing opinion (Ahmad, 1958; Ramakrishnan, 1987) that the isolated outcrops are parts of a more extensive basin-fill deposit that was fragmented and separated by post lithification faulting or doming up of the basement and erosion of structural highs. Occurrence of extensive blankets of shelf sandstone or thick deepwater shale in many of the outcrops, suggests a large continuous basin with open marine connection. The disposition of the outcrops (Fig. 2) indicates that the postulated large basin was

elongated in a north-south direction. The Chattisgarh succession unconformably overlies the

Archaean crystalline basement including the Sonakhan granite-greenstone belt and the Dongargarh-Kotri volcanics with a strong N-S trending structural grain. The age of the Chattisgarh sequence is very poorly constrained. WAr dates from glauconitic minerals from the lower part of the sequence yield an age of 700 - 750 Ma ( Kreuzer et al., 1977). Murti (1987) places the sequence at 1250 to 1300 Ma on the basis of palaeomagnetic studies, whereas assemblage of algal stromatolites in the carbonate horizons point to Middle to Upper Riphean age (Chatterjee et al., 1990).

The succession in the eastern part of the Chattisgarh outcrop belt consists of a conglomerate and coarse arkosic sandstone in a shale-dominated sequence, the Chandarpur Group, and a limestone-shale dominated sequence, the Raipur Group (Murti, 1987,1996; Patranabis Deb, 2000). Sequence stratigraphic analysis suggests that the Raipur assemblage developed as the distal facies equivalent of the Chandarpur assemblage (Table 2). The Chandarpur sequence comprises coalescing fan-fan delta-prodelta deposits, tide-storm dominated prograding-shelf deposits, and high-energy strandline to shoreface deposits, whereas the Raipur sequence was deposited in outer shelf, slope and basin environments (Patranabis Deb, 2000).

The proximal assemblage consists of three sandstone intervals separated by two mud-dominated sequences (Table 2). The basal sandy interval unconformably overlies the Archaean basement, and consists of arkosic clastics deposited as alluvial fans and fan deltas, attesting to fault- controlled sedimentation in a cratonic rift basin (Patranabis Deb and Chaudhuri, in press). The episodic deposition of thick sandstone horizons (Table 2) also

Table 2. Stratigraphic succession of the Chattisgarh Supergroup in the eastern part of the basin.

Proximal assemblage

Formation Kansapathar Formation (60 m)

Gomada Formation (525 m)

Lohardih Formation (150 m)

Lithology and environment Subarkosic to quartzose sandstones; Storm-tide influenced shoreface and shelf Mud-dominated sequence (250 m); Tide-dominated shelf

Daihan Sandstone Member (150 m); Subarkosic sandstones: storm-dominated foreshore to shorcface

Mud-dominated sequence (125 m); Storm-dominated prodelta, outer shelf to shoreface transition

. K-Ar date: 700-750 Ma

Conglomerate to coarse arkosic sandstones: fan-fan delta

-----1---11-1 Unconformity ---------I - - - I Archaean gneissic basement enclosing granite-greenstone belt

Distal assemblane ~~~ ~

Formation Gunderdehi Shale ( > 430 m)

Sarangarh Limestone (150 m)

Bijepur Shale (>lo0 m)

Lithology and environment Brown shale with sandy turbidites and seismoturbidites; Slope and basin

Lithographic limestone with intercalated sandstones at the lower part; Shelf and slope

Shale with minor sandstoncs; Outer shelf and slope

??

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PURANA BASINS OF SOUTHERN INDIA 29

points to episodic tectonic uplift of the cratonic hinterland. The shelf limestone in the Raipur sequence contains a large number of storm deposited sandstone layers, whereas slope and basinal deposits include large channels filled up with slumped limestone beds and limestone clasts, auto-lime-clast debris flow conglomerates, thin beds of sandy turbidites and seismoturbidites. Thus the tectonic perturbation of the basin is also corroborated by the distal assemblage. The sedimentological attributes of the sequence comprise an extensive carbonate platform, and thick horizons of sandstones with signatures of strong tides and storms indicate that the rift basin was exposed to open marine circulation and was connected with an ocean basin.

Both the assemblages contain felsic tuffs at different stratigraphic levels, and the Gunderdehi Shale, the uppermost formation in the distal assemblage (Table 2) contains thick horizons of ignimbrite with several beds of welded tuff. Deb and Patranabis Deb (submitted) analyzed the composition of magmatic garnet in the welded tuff

and inferred that the melt was generated at a depth of about 30 - 35 km through partial melting at the lower crust. The partial melting and upward migration of the magma was accompanied by localized stretching and rifting of the crust at the upper level.

Deformation structures

The inversion of the basin was initiated by transport with top-to-south/southeast sense of the distal assemblage over the deposits of the proximal assemblage across a major ENE-WSW trending thrust. The thrust was displaced by a set of subvertical N-S trending faults, several of which can be traced for more than 100 km. A few faults have a reverse sense of movement that has brought up thin slices of granitic basement and repeated the succession. The reverse faults are parallel to the trend of the rift and the structural grain of the underlying granite-greenstone terrane, and point to strong E-W shortening. It is interpreted that the reverse faults are rejuvenated normal faults that opened up the N-S trending rift basin.

Table 3. Stratigraphic succession in the Cuddapah basin, after Nagaraja Rao et al. (1987). Thickness values after Pascoe (1973)

Group Formation Lithology

Kuxnool Group (450 m)

Nandyal Shale Shale Koilkuntala Limestone Limestone Paniam Quartzite Quartzite Owk Shale Shales Narji Limestone Banaganapalle Formation Quartzite, diamondiferous conglomerate

Massive to flaggy limestone

C U D D A u

.I

8 I’ U A 2 €1 a g S

U 1’ E

G

0 U

u .I 8 L 0 K u U L K - $ 0 8 U

Red quartzite, minor shale

Nallamalai Group (1650 m)

Srisailam Formation --- Unconformity---

Cumbum Formation =Pullampet Shale

Bairenkonda Quartzite (=Nagari Quartzite)

(3200 m)

L w I’ L Papaghni Group

(1370 m)

Tadpatri Formation

Pulivendla Quartzite

Chelima lamproite (ca. 1400 Ma) Shale, slate Dolomite Sandstone-shale

Quartz arenite Sandstone-shale Quartzite Pebbly arkosic grit

Sandstone, minor shale

Shale, mafic flows and sills, ignimbrites Quartzite

mafic sills (ca.1800 Ma)

Shale, stromatolitic dolomite

Quartzite, conglomerate, minor Vempalle Formation

Gulcheru Quartzite heterolithic sandstone-shale ------------- Unconformity I I - - - I - - - - - - - - Granitic gneisses and schists with enclaves of menstone belts

Peninsular gneisses / Dhanvar schists

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30 A.K. CHAUDHURI ET AL. -

The Cuddapah Basin

Stratigraphic succession

The crescent shaped Cuddapah basin is the second largest of the Proterozoic basins in India in terms of areal extent (Kale, 1991). The present outcrop width of the succession, with a cumulative thickness of 12 km, is 150 km along lSo N latitude, while the distance between two apices of the crescent is 440 km. The convex-to-west crescent shape is the result of joining of the relatively older sub-elliptical Papaghni sub-basin in the west and the N-S elongated Nallamalai sub-basin in the east with an aspect ratio of 1:6.

The Cuddapah basin is a polyhistory basin hosting sedimentary successions ranging in age from the Paleoproterozoic through the Neoproterozoic with internal unconformities (Table 3). The basin as a whole is floored by granitic gneisses with enclaves of greenstone belts. The Gulcheru Quartzite, the lowest lithologic unit of the Cuddapah Supergroup rests unconformably over the gneisses with enclaves of mafic schists. The NNW trending Kadiri schist belt in southwest Cuddapah represents one of the prominent schist belts, the outcrop of which abuts against the arcuate boundary of the Gulcheru Quartzite in the Papaghni sub-basin (Fig. 2). An angular unconformity separates the Papaghni and Chitravati Groups from the Nallamalai Group. In the southern part of the basin the Nagari Quartzite (Bairenkonda Formation) rests directly on the basement gneisses, suggesting geometric shifts in depocenter with progressive evolution of the basin. The Neoproterozoic Kurnool Group occurs in two separate locales, namely the Kundair valley in the west and the Palnad area in the northeast. In the Kundair valley, the Kurnool Group unconformably overlies the basement gneisses and different members of the underlying Papaghni and Chitravati groups in different parts of the Kurnool basin. In the Palnad basin, the Kurnool Group unconformably overlies the basement and the Srisailam Formation (upper part of the Nallamalai Group).

The sedimentation in the Papaghni subbasin begins with deposition of fluvial quartzites and conglomerates in an arcuate belt along the western margin (fan delta) with minor intercalation of heterolithic sandstone-shale of peritidal origin. The succeeding Vempalle Formation comprises mainly stromatolitic dolomite, calcareous sandstone and shale representing a shallow shelf deposit. The Gulcheru-Vempalle transition is a transgressive event. The upper part of the Vempalle Formation is marked by doleritic dykes and sills intruding the dolomite and shales. The mafic igneous activity is dated at around 1800 Ma (Bhaskar Rao et al., 199.5). The commonly occurring mafic sills and dykes and silicic ash fall tuffs in the Tadpatri

Formation represent a second episode of Proterozoic crustal extension possibly heralding the onset of the Nallamalai subbasin. The Nallamalai Group consists of the lower sandstone-dominated Bairenkonda Formation and the upper Cumbum Formation, consisting mainly of shales punctuated by sandstones and dolomites. The basal part of the Bairenkonda Formation shows coarse to pebbly fluvial sandstones overlain by a hummocky cross stratified sandstone-siltstone of shallow shelf origin. The Bairenkonda-Cumbum transition also represents a major transgressive event. The presence of ash beds, mass-flow conglomerates with clasts of intrabasinal carbonates, slumped beds, etc., suggest synsedimentary faulting and volcanism during the deposition of the Cumbum Formation.

The folded Nallamalai Group is intruded along its eastern margin by 1573 Ma old granite (Crawford and Compston, 1973) and Chelima lamproite dated around 1400 Ma (Chalapathi Rao et al., 1999). The Kurnool Group with its diamondiferous conglomerates at the base and succeeding sandstone-shale and limestone represent a third and younger sedimentation cycle. The quartzite- conglomerates of the Banaganapalle Formation in the Kurnool subbasin were deposited in an alluvial setting (Lakshminarayana et al., 1999), but the succeeding Narji Limestone represents a basinal limestone with little input from the terrestrial sources.

Deep Structure

The deep structure of the Cuddapah basin as interpreted from the DSS profiles (Kaila et al., 1979; Kaila and Tewari, 1985) and gravity data (NGRI, 1975) show that the basin can be divided into several blocks separated by deep faults. The most prominent one of the N-S set of faults runs along the western boundary of the Nallamalai ranges (Fault #6 in profile I and # S in profile I1 of Kaila and Tewari, 1985). East of the Kundair valley, the rocks of the younger Kurnool Group are juxtaposed against the rocks of the Nallamalai Group along this steep, easterly dipping fault. The seismic profile indicates an apparent reverse movement on the fault, which is in agreement with the structural interpretation suggested by Saha (1994).

Another gently dipping fault (#3 of Kaila and Tewari, 1985) offsets the basement across the eastern margin of the Nallamalai fold belt. The Nallamalai sediments have been overthrust by the granitic basement with a displacement of about 5 km. The basement depth, hence the apparent thickness of the Nallamalai sediments, varies between 7-1 1 km from N to S. The thickness of sediments in the Kurnool basin is much lower, 200-590 m. The gravity profile across the basin also shows a negative

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PURANA BASINS OF SOUTHERN INDIA 31

Bouguer anomaly of -80 to -90 milligals beneath the Nallamalai ranges. The depth to Moho remains almost constant across the basin at 36-40 km.

The steeply dipping boundary fault in the west coupled with preponderance of coarse arenites (fan-delta) facies along the western margin of the Nallamalai fold belt, and the incidence of syn-depositional faulting indicate a rift- like setting for rocks of the NFB.

Main deformation features and basin evolution

The major fold-thrust structures in the Cuddapah basin are restricted to the eastern half of the basin, east of the deep N-S faults #5 and #6 interpreted from seismic sections of Kaila and Tewari (1985). Along the eastern margin of the basin the rocks of the Nellore schist belt are thrust over the Nallamalai Group of rocks. The structural trend varies from NW-SE in the southern part to NE-SW in the north. Overall E-W shortening across the Nallamalai belt is accompanied by westerly thrust transport (Saha, 1994; Mukherjee, 2001). Overprinting relationships of folds and cleavages suggest multiple episodes of deformation in the Nallamalai Group. The Nallamalai Group suffered an early cycle of Mesoproterozoic deformation (older than the 1400 Ma Chelima lamproite, Chalapathi Rao et al., 1999) prior to the deposition of the Neoproterozoic Kurnool Group. A late deformation affects both the Nallamalai Group and the Kurnool Group in the Palnad area.

Concluding Remarks

The south Indian cratonic province witnessed development of a number of Meso- to Neoproterozoic intracratonic basins of which the Pranhita-Godavari, Chattisgarh and Cuddapah basins are prominent ones, covering several tens of thousands of square kilometres of outcrop each and hosting several thousand metres of sedimentary successions each. Deep, basin-margin faults, apparent from geological maps, seismic sections and/or Bouguer anomaly data as in the case of Pranhita-Godavari and Cuddapah basin (Nallamalai belt), controlled the overall geometry and subsequent evolution of the basins. Features indicating fault-block-controlled differential uplifts, synsedimentation tectonic disturbance and limited volcanic outbursts are common in the successions of these in tracratonic basins. The successions, bounded by regional unconformities, with variation from fluvial-shallow marine to slope-basin depositional characters indicate oscillatory movements during evolution of these Proterozoic basins with prolonged history. Based on these signatures we suggest that these intracratonic basins developed in a rift setting. None of these rifts developed into a full- scale

oceanic rifts, hence are labeled as fossil rifts. In the three large Purana basins we examined, it seems possible that the initial rifts may have followed pre-existing lineaments defined by either Archaean greenstone belts or belts of crustal convergence. It also appears from the geometry of Purana basins developed in the southern cratonic province of India, that the province behaved as a large single cratonic entity since the fragmentation of a Me so pro t erozo ic supercontinent, which may have incIuded India and other East Gondwana fragments. However, marine transgressions recorded in the sedimentary svccessions of the PG Valley and Cuddapah basins suggest the existence of an open seaway east of the Mesoproterozoic cratonic province of south India. In the Chattisgarh basin the open seaway appears to have existed north of the cratonic block.

The south Indian cratonic province remained as one unit throughout the Mesoproterozoic and Neoproterozoic, but probable amalgamation with Antarctica (and possibly also N. America) during the Rodinia assembly left some imprints on the craton-interior (Mesoproterozoic) successions. However, early episodes of deformation in the Nallamalai fold belt may have originated from continental convergence leading to the development of a Mesoproterozoic (> 1400 Ma) supercontinent.

Acknowledgments

We are grateful to Prof. J. J. W. Rogers for suggesting to AKC and DS to contribute to this Gondwana Research Special Issue and showing enormous patience while we took time to finalize the manuscript. The paper draws heavily on published and unpublished data gathered over decades in connection with several field based projects, some still ongoing, of the Geological Studies Unit, Indian Statistical Institute. We acknowledge the support provided by the Institute. We have also benefited from discussion with colleagues in the Geological Survey of India and National Geophysical Research Institute, Hyderabad. However, we take the entire responsibility for the views expressed here.

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