Deformation of the Proterozoic successions in the Pranhita–Godavari basin, south India—a regional perspective

Journal of Asian Earth Sciences 21 (2003) 557±565

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Deformation of the Proterozoic successions in the Pranhita±Godavari basin, south IndiaÐa regional perspective Dilip Saha*, Asru K. Chaudhuri Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Kolkata 700 108, India Received 1 December 2000; revised 30 August 2001; accepted 17 January 2002

Abstract A number of Proterozoic intracratonic basins, namely Cuddapah, Pranhita±Godavari (PG), Abujhmar, Indravati, Ampani, Chattisgarh and Khariar basins, occur to the immediate west of the Eastern Ghats granulite belt (EGGB) in India. Of these, the PG basin hosts an axial outcrop of late Paleozoic±Mesozoic Gondwana rocks ¯anked on either side by Proterozoic sedimentary successions. Among the latter, the Somanpalli Group in the eastern part and the Penganga and Yellandlapad Groups in the western part show contractional folds, imbricate thrusts and cleavage. These deformed rock groups and the Pakhal Group in the western belt include shallow to deep marine sedimentary successions. Stratigraphic relations indicate that the major episode of deformation in the PG basin occurred prior to deposition of the unconformably overlying Sullavai Group. The latter is a Neoproterozic, undeformed ¯uvial sequence with some aeolian intercalation. Deformed successions also occur in other Proterozoic basins of the south Indian craton, notably those in the Nallamalai fold belt (NFB) along the eastern part of the Cuddapah basin. The major fold±fault structures in the arcuate NFB vary in trend from NNW±SSE in the southern part to NE±SW in the northern part. At least one phase of folding and thrusting predates the ,1000 Ma old event in the EGGB. Deformation associated with the Grenville and later Pan-African tectonic mobility has imprints on the Neoproterozic Kurnool Group in the Palnad sub-basin (northern NFB). The major structural trend in the PG basin is NW±SE, except for the Yellandlapad area in the southern part, where late superposed folds trend E±W. The deformation along the eastern margin of the Chattisgarh basin, its southern extension in Khariar, Ampani and Indravati basins may also be related with the main (Grenville) deformation in the EGGB. A regional, Mesoproterozic episode of basin closure and inversion is thus evident from the Proterozoic cratonic successions west of the Eastern Ghats. This occurred prior to a regional development of Neoproterozoic successions at these sites. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Contractional deformation; Proterozoic cratonic basins; South India

1. Introduction A number of Proterozoic intracratonic basins occur in proximity of, and to the west, of the main orographic features represented by the Eastern Ghats (Radhakrishna and Naqvi, 1986). South of the Son±Narmada lineament the major basins are the Chattisgarh basin, Khariar basin, Indravati basin, Pranhita±Godavari (PG) basin and Cuddapah basin (Fig. 1). The Eastern Ghats apparently had a prolonged history of tectonic and magmatic evolution starting from late Archean (Sarkar et al., 1981) through middle Proterozoic (Paul et al., 1990) to late Precambrian (Yoshida et al., 1996). A signi®cant tectonic mobilization and attendant magmatic activity took place during the middle Proterozoic as indicated by a cluster of U±Pb and 40Ar/ 39Ar dates obtained from the Eastern Ghats (Mezger and Cosca, 1999). * Corresponding author. E-mail address: [email protected] (D. Saha).

The Nallamalai fold belt (NFB) represents the deformed eastern half of the Cuddapah basin in the south (Narayanswami, 1966; Meijerink et al., 1984; Nagaraja Rao et al., 1987; Saha, 2000; Mukherjee, 2001). The cratonic successions along the eastern fringe of the Bastar craton (bordering the Eastern Ghats front) are also known to have suffered deformation in the Ampani and Khariar basins (Mishra et al., 1988; Balakrishnan and Mahesh Babu, 1987) and in the Abujhmar basin (Mishra et al., 1987). On the other hand, the Neoproterozoic successions of the Chattisgarh basin and Indravati basin are largely ¯at lying sedimentary successions. The putative understanding of the Proterozoic successions in the PG basin as undeformed sedimentary sequences is not in agreement with the classical description of the sheared rocks of the Sironcha country in the eastern belt and the deformed and metamorphosed succession from around Singareni (Yellandlapad) to Cherla at the southeastern extremity of the Proterozoic PG basin (King, 1881;

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Fig. 1. (a) The middle Proterozoic mobile belt (MPMB) of Radhakrishna and Naqvi (1986) and major Proterozoic basins in peninsular India. (b) The Proterozoic cratonic basins of south and central India adjoining the Eastern Ghats, after Geological Survey of India map, 1981. Hatches indicate deformed sector in these basins.

Pascoe, 1973; Ramamohana Rao, 1964, 1971). The Sullavai rocks unconformably overlie the deformed Proterozoic rocks at Rajul Gutta in the central part of the PG basin (Johnson, 1967). More recent works have demonstrated that, for a length of ,150 km in the eastern belt from Cherla in the southeast to around Allapalli in the northwest, the lower part of the Albaka succession and the Somanpalli Group show signs of contractional deformation (Srinivasa Rao et al., 1979; Saha, 1988, 1992a,b; Saha and Ghosh, 1998; Ghosh and Saha, 1999). The Penganga Group occurring west of the Wardha (Pranhita) valley in the western belt, also has contractional fold±fault structures (Sarkar, 1990; Mukhopadhyay, 1997; Deb, 1999). Given the widespread records of the contractional deformation in Proterozoic rocks of the PG basin (Fig. 2), how would one relate this deformation to the regional geology? Is it possible to connect this deformation to the widespread record of tectonic mobility in the Eastern Ghats granulite belt (EGGB) along the southeastern margin of Indian peninsula? These questions are addressed in the present work. 2. Proterozoic rock groups in the PG valley Two narrow belts of Proterozoic rocks, separated by a median outcrop of Late Paleozoic±Mesozoic Gondwana rocks, occur in the PG Valley (Fig. 2; King, 1881). Small inliers of Proterozoic rocks are reported from the central part of the basin around Chinnur (Rajal Gutta inlier, John-

son, 1967) and from around Yellandlapad (Ramamohana Rao, 1964, 1971). The above belts are ¯anked on the east and west by granite±gneisses of the Bastar craton and Eastern Dharwar craton, respectively (Rogers and Callahan, 1987). In central part of the western Proterozoic belt, the rede®ned Pakhal Group, the Albaka Group and the Sullavai Group represent unconformity-bounded sequences (Chaudhuri, 1999). The mixed siliciclastic±carbonate rocks of the Mesoproterozoic Pakhal Group unconformably overlie the granitoid basement. Originally the Albaka Sandstone, a thick succession of shelf sandstone, was thought to be restricted to the eastern belt (King, 1881; Sreenivasa Rao, 1987). However, on the basis of lithostratigraphy the `subAlbaka' unconformity is also traced in the western belt. The Mulug Orthoquartzite±Mulug Shale succession, which unconformably overlies the carbonate of the Mallampalli Sub-group (Basumallick, 1967; Chaudhuri, 1985), is assigned to a newly de®ned group, namely the Albaka Group which in the eastern belt is thought to be represented by the Somandevara Quartzite, Tippapuram Shale and Chalamala (Albaka) Sandstone (Table 1; Chaudhuri, 1999). The Somanpalli Group in the eastern belt is unconformably overlain by the Albaka Sandstone (Saha and Ghosh, 1998). The Neoproterozoic Sullavai Group, consisting of ¯uvial±aeolian sandstones (Chakraborty, 1994), unconformably overlie the Pakhal Group in the Ramgundam area of the western belt or the Penganga Group north of the Godavari river (Mukhopadhyay, 1997). In the eastern belt, the Albaka Sandstone is unconformably overlain by a conglomerate±sandstone succession correlated with the Sullavai Group (Sreenivasa Rao, 1987). The Penganga Group, consisting of sandstones, shale and limestones (shelf to deepwater; Mukhopadhyay and Chaudhuri, 1999), unconformably overlies the basement granitoids in the Mancheral area of the western belt, but its relationship with the Pakhal and/or Albaka Group is not yet clear. 3. Deformation in the PG valley In the older literature, there is passing reference to the deformed Proterozoic succession in the Yellandlapad area and the sheared rocks of the Sironcha country (King, 1881; Pascoe, 1959 (1973 reprint)); Ramamohana Rao, 1964, 1971). The Proterozoic inlier in the central part of the valley is also affected by folds and faults (Johnson, 1967). The Yellandlapad outcrop lies to the south of the type Pakhal outcrops of the Pakhal lake area (Basumallick, 1967). Both the classical description of King (1881) and later works by Basumallick (1967) and Chaudhuri (1985) lead us to generally accept Pakhal rocks in the western belt to be undeformed sedimentary successions. More recently, contractional deformation from within the Penganga Group is reported (Sarkar, 1990; Mukhopadhyay, 1997; Deb, 1999). In the Adilabad area, contractional folds

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559

Fig. 1. (continued)

and thrusts have a north±northwesterly trend in conformity with the overall regional strike. Around Yellandlapad, however, northwesterly trending early folds are overprinted by E±W fold structures (Table 2). The Somanpalli Group in the eastern belt, is highly deformed with development of large north±northwesterly folds, thrust imbricates and cleavage (Saha and Ghosh, 1998; Saha, 1990, 1992a,b; Ghosh and Saha, 1999). The Cherla Formation, representing Proterozoic rocks in the southern end of the Albaka belt, is also deformed (Saha, 1988). Thus a signi®cant part of the Proterozoic successions in the PG valley has suffered contractional deformation and low grade metamorphism (Table 2). 4. Timing of deformation in the PG valley In the Ramgundam area, the undeformed Sullavai rocks unconformably overlie the Pakhal Group (Chaudhuri, 1985;

Chakraborty, 1999). Similarly the Penganga Group of rocks is unconformably overlain by the undeformed Sullavai Group in the Mancheral area (Chakraborty, 1994). It may be recalled that the Penganga Group is deformed, and the Pakhals also locally bear signs of contractional deformation. The glauconites from the basal part of the Pakhal Group are dated at ,1330 Ma (Vinogradov et al., 1964). On the other hand, glauconites from the lower part of the Sullavai Group is dated at ,870 Ma (Chaudhuri and Howard, 1985). Glauconitic minerals from the Chanda Limestone, belonging to the Penganga Group, have been dated at 775 ^ 30 Ma (Chaudhuri et al., 1989). While the Penganga Group was deposited in shallow shelf to deep marine basin (Chaudhuri et al., 1989; Mukhopadhyay and Chaudhuri, 1999), the terrigenous Sullavai sediments were deposited in alluvial fans and a braided river, anastomosing ephemeral stream system or ergs (e.g. Venkatpur Sandstone; Chakraborty, 1994). The deformed Somanpalli Group and the Albaka Sandstone in the eastern belt is unconformably overlain by

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sandstone±conglomerate correlated with the Sullavai Group (Sreenivasa Rao, 1987). Thus on the whole, one major episode of deformation both in the eastern belt and the western belt occurred prior to the deposition of the continental, Neoproterozoic Sullavai Group. In view of the apparent con¯ict between the reported radiometric date from the Penganga Group (Chaudhuri et al., 1989) and the stratigraphic relationship espoused by Chakraborty (1994), the timing of deformation in the Penganga Group can be con®rmed only after well constrained dates and detailed stratigraphic relations based on ®eld work are available. At this stage, one cannot rule out the possibility of a Neoproterozoic deformation in the PG basin altogether. The stratigraphic status of the deformed Yellandlapad Group consisting of a succession of arkosic sandstones, phyllites and marbles is not yet resolved (Chaudhuri and Chanda, 1991). However, the common relict of stromatolite structures in the marbles of this succession indicates shallow marine deposition of the precursor sediments. Hence the group is unlikely to be correlated with the largely continental Sullavai Group and in all probability represents the

southern extension of the Pakhal Group of rocks described from the Pakhal lake area. 5. Deformation in other Proterozoic cratonic basins adjoining the EGGB A number of Proterozoic intracratonic basins occur in proximity to, and to the west of, the EGGB. They include the Cuddapah basin in the south, followed northwards by the PG basin, the Abujhmar basin and the Indravati and Chattisgarh basins of the Bastar craton and its supposed southeastern extension in the Nawapara±Khariar area. 5.1. Cuddapah basin The NFB, marking the eastern half of the Cuddapah basin, attests to deformation of the basin in®ll. The exact timing of this deformation is yet to be fully resolved. The available geologic constraints suggest that the major deformation in the NFB occurred earlier than the deposition of the Neoproterozoic Kurnool Group. The Kurnool Group is

Fig. 2. Geology of the PG Valley. Asterisks indicate localities from which contractional structures have been documented.

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Table 1 Rock groups in the eastern and western Proterozoic belts of the PG Valley and their tentative correlation. Curly lines indicate unconformity Western belt

Eastern belt

Age

Deccan Traps

Paleogene

Gondwana Supergroup

Gondwana Supergroup

Late Paleozoic and Mesozoic

Sullavai Group a,b (1000 m)

Sullavai Group c (1400 m)

Neoproterozoic

Penganga Group d (500 to ,1300 m) `Mulug Orthoquartzite±Mulug Shale' < Albaka Group a (,2600 m) e

Albaka Group a,c (,3000 m)

Pakhal Group a < 'lower Mulug 1 Mallampalle Sub-groups' (?Yellandlapad Group) f (6000 m)

Somanpalle Group g (3500 m)

Mesoproterozoic

Basement gneisses

Basement gneisses

?Archaean

a b c d e f g

Chaudhuri, 1999. Chakraborty, 1994. Srinivasa Rao et al., 1979; Sreenivasa Rao, 1987. Mukhopadhyay, 1997; Mukhopadhyay and Chaudhuri, 1999. Basumallick, 1967. Chaudhuri and Chanda, 1991. Saha and Ghosh, 1998.

in turn involved in a later deformation in the Palnad subbasin or in the western fringe of the NFB in the Kundair valley, suggesting multistage evolution of the NFB (Saha, 2000). It may be noted here that the regional strike in the NFB varies from NNW±SSE in the southern half (Matin and Guha, 1996; Mukherjee, 2001) through N±S in the central part (Saha, 1994) to NE±SW in the northern part (Narayanswami, 1966; Natarajan and Rajagopalan Nair, 1977; Saha, 2000). The sedimentation in the western part of the Cuddapah basin started at around ,1800 Ma (Bhaskar Rao et al., 1995). The Agnigundla granite cutting the folded Nallamalai rocks was emplaced at around ,1575 Ma (Crawford and Compston, 1973). Since the deformed Kurnool rocks are overthrust by the Cumbum rocks in the Paland tract, a later (Neoproterozoic) deformation must have affected the

NFB. Considering that the Kurnool Group is Neoproterozoic, the later deformation may be associated with the tectonic evolution of the Eastern Ghats linked to the amalgamation of India and Antarctica during the Grenville orogeny. 5.2. Abujhmar basin In the western part of Bastar craton, the Abujhmar Group unconformably overlies the BIF bearing Bailadila Group in the east and Dongargarh Granite on the west. The rock group consisting of sandstone, shale and basic rocks are correlated with the Cuddapah rocks on the basis of lithologic similarity and overall stratigraphic position (Mishra et al., 1987). The Abujhmar Group bears signs of deformation in the NW trending Bodhghat

Table 2 Structural trends and grade of metamorphism in different Proterozic rock groups of the PG Valley. Data sources: Johnson, 1967; Sreenivasa Rao, 1987; Saha, 1988, 1990, 1992a,b; Ghosh, 1997; Deb, 1999; Ghosh and Saha, 1999; Sengupta and Raychaudhuri, 1999; and other unpublished works of Geological Studies Unit, Indian Statistical Institute Rock units

Metamorphism

Major fold trends

Cleavage

Thrust transport

Somanpalli Group, Somanpalli Albaka Group, Albaka-Cherla

Low greenschist Anchizone to low greenschist

11 1

Majority ! NE; some ! SE ! ENE; locally ! S

Yellandlapad Group

Hornblende±hornfels/middle amphibolite

NW±SE; locally ESE±WNW N±S varying to NNW; locally E±W NNW varying to NE; E±W overprint

Not examined

Mulug Shale, Mulug Penganga Group, Adilabad

Anchizone Low anchizone

11More than one set; plus mylonitic foliation 1

Pakhals, Rajul Gutta inlier

2

2 NW±SE Local minor E±W folds NW±SE

2

2 Majority ! SW; some ! S 2

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Fig. 3. The Grenville belt of North America and its possible correlatives in other parts of the globe. In this reconstruction, the Eastern Ghats is connected to East Antarctica during the Neoproterozoic; after Moores, 1991.

outcrop which shows schistosity in metabasalt and phyllites and NW±SE trending minor folds. The Abujhmar Group is thought to be older than the generally undeformed sedimentary rocks of the Indravati basin. The upper part of the Indravati Group is Neoproterozoic, but the lower part may be Mesoproterozoic. 5.3. Chattisgarh basin In the eastern Chattishgarh basin, a succession of conglomerates, pebbly sandstones and coarse immature sandstones and shales deposited in coastal and shallow shelf environment unconformably overlies the basement gneisses and rocks of the granite±greenstone belt. A succession of turbiditic shale, pyroclastics and limestone are tectonically juxtaposed against the proximal siliciclastic succession. The western part of the basin, by contrast, is dominated by mature sandstones and stromatolite bearing dolomitic limestone. Although the southern part of the basin is largely undisturbed, folds and faults are common along

the eastern margin in the Barapahar hills of the Sambalpur district, Orissa. Large N±S trending faults are also mapped from the Sarangarh area. Along the eastern margin of the Khariar basin and occurring further south, a sedimentary succession correlated with the Chattisgarh rocks are juxtaposed against the granite± gneisses and granulites of the Eastern Ghats. The sedimentary rocks are folded and faulted with two phases of structuresÐone trending NW±SE and a later set trending N±S (Mishra et al., 1988). The Ampani basin occurs to the west of the Eastern Ghats domain in the northeastern segment of the Jeypore±Bastar craton. A succession consisting mainly of sandstone and shale unconformably overlies the basement gneisses and Iron ore Group of rocks, which has faulted boundary with the basement complex on the eastern side. Close to the boundary fault, N±S trending overturned folds affect the Ampani succession (Balakrishnan and Mahesh Babu, 1987). 6. Tectonic evolution of the Eastern Ghats granulite belt

Fig. 4. Bar chart showing the frequency of radiometric dates …N ˆ 48† obtained from the EGGB. Compiled from Vinogradov et al., 1964; Peeraju et al., 1979; Rao et al., 1980; Sarkar et al., 1981; Grew and Manton, 1986; Paul et al., 1990; Aftalion et al., 1988; Sarkar et al., 1981; Sarkar and Nanda, 1994; Shaw et al., 1997; Mezger and Cosca, 1999.

A swathe of high-grade gneisses, schists, granulites and intrusive granitoids running northward from Ongole in the south, through the Chilka area, up to the Mahanadi Graben constitute the Eastern Ghats belt. Although the overall trend of the EGGB is NE±SW, local strikes vary between NNW and ENE. The cratonic basins referred to in the earlier sections are spatially associated with the Eastern Ghats (Fig. 1), which is considered to be a deeply exhumed orogenic belt. In recent years there have been a tendency to connect the evolution of the EGGB with that of the Grenville orogen and its possible correlatives, particularly in view of the comparable geologic and thermal history obtained from the East Antarctica and parts of southern India (Fig. 3; Mezger and Cosca, 1999 and references therein). In this

D. Saha, A.K. Chaudhuri / Journal of Asian Earth Sciences 21 (2003) 557±565

scenario, the major deformation and metamorphism in the EGGB represent a Grenville episode. Some workers consider the high-grade belt in coastal East Antarctica to represent essentially a continuation of the EGGB (Yoshida et al., 1992; Rogers, 1996). It should therefore be kept in mind that any collisional suture representing closure of a Mesoproterozoic ocean in such reconstruction should lie further inland in Antarctica beneath the ice sheet. However, both the geologic and geochronologic data obtained mainly from the northern part of the EGGB suggest a greater antiquity of the belt (Paul et al., 1990; Bhattacharya et al., 1995). It has also been pointed out that the broad swathe of the EGGB can be sub-divided into distinct lithotectonic units, which may have a separate history (Ramakrishnan et al., 1998). The radiometric data obtained from the EGGB, as collated from published works show that there is not only a major cluster of dates coinciding with the Grenville event (,1000 Ma). A second cluster of dates also occurs at ,1500 Ma (Fig. 4). Still older and younger dates are also not uncommon. The exposed granulites are explained by basin inversion through continental collision, tectonic duplication of crustal material, high-grade metamorphism, anatexis and subsequent uplift and exhumation. The older granulite patches suggest more than one cycle of basin opening and closure in the EGGB. However, better geochronological constraints on regional granulite formation and other metamorphic events are necessary before a concrete tectonic model for the evolution of the polymetamorphic EGGB can be established (Dasgupta, 1998). 7. Discussion and conclusions There is a remarkable spatial association of the EGGB and the Proterozoic cratonic basins, namely the Chattisgarh, Khariar, Indravati, Abujhmar, PG Valley and Cuddapah basin. The presence of a major thrust zone along the western fringe of the EGGB suggests tectonic convergence from the east or southeast. As pointed out earlier, one cycle of deformation and basin closure occurred prior to deposition of a continental rift related ¯uvial to aeolian Sullavai Group in the PG Valley or deposition of a second cycle of marine deposits such as in the Kurnool Group in the Palnad or the main Kurnool basin in the Kundair valley. Both the Sullavai and Kurnool groups are Neoproterozoic in age (Chaudhuri, 1999). It may be pointed out here that the major structural trend in the PG valley, as well as in the southern Nallamalai, is NW±SE (Saha and Ghosh, 1998; Saha, 1990, 1992a,b; Matin and Guha, 1996). In the deformed outcrop of the Abujhmar Group, the trend of folds and foliation is also NW±SE and these are considered to be older than largely undeformed Indravati Group (Mishra et al., 1987). Radiometric dates are so far not available from the Indravari Group, but geologic evidence suggests that the upper part of the Indravati Group (Jagdalpur Formation) is Neoproterozoic in age.

563

NE trending tight folds with axial plane cleavage and boundary faults along the eastern margin of the Indravati basin is probably related to deformation in the adjoining EGGB (Ramakrishnan, 1987). The deformation along the eastern margin of the Chattisgarh basin in the Barapahar area, and those along the eastern margin of the Khariar basin, are in proximity of boundary faults marking the western margin of the EGGB. However, at Khariar early set of northwesterly folds are at an angle to the local EGGB trend. K±Ar dates from the Chapordih Formation, Chattisgarh basin (Kruezer et al., 1977) indicate that the Chattisgarh Supergroup may at least in part be Neoproterozoic. Proterozoic cratonic successions lying to the west of the EGGB record contractional deformation. The most prominent of the fold±thrust structures occur in the NFB (Cuddapah basin) and in the PG valley. Contrary to popular understanding, large-scale structures and penetrative deformation structures are recorded not only from the southern part (Yellandlapad, Albaka and Somanpalli), but also from the central and northern parts (Adilabad, Mancheral) of the PG Valley. The major structural trend in the PG valley is NW±SE, except for Yellandlapad area, where the late superposed folds trend E±W. In the NFB, at least one major episode of folding and thrusting predates the Grenville event (Saha, 2000). Thus one may relate an earlier episode of basin closure/inversion prior to the emplacement of the Neoproterozoic successions in these cratonic basins and by association, prior to the main Grenville event in the EGGB. Deformation associated with the Grenville and later Pan-African tectonic mobility have left some imprints on the cratonic successions of the Palnad basin (NFB), or in part, at Yellandlapad and Cherla area of the PG valley. The deformation along the eastern margin of the Chattisgarh basin, its southern extension in Khariar, Ampani and Indravati basins may also be related to the main (Grenville) deformation in the Eastern Ghats.

Acknowledgements The work is supported by Indian Statistical Institute. We gratefully acknowledge some improvements on an earlier version of the manuscript suggested by the reviewers.

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