The Age of the Pakhals of the Godavari Valley by T

THE AGE OF THE PAKHALS OF THE GODAVARI VALLEY BY T. IL~t~,MOHA~ R.~,O (Department of Geology, Andhra University, Waltair) Received March 12, 1964 (Communicated by Dr. C. S. Pichamuthu,F.A.SC.)

ABSTRACT

The Pakhals consist of conglomerate, gritty sandstone, quartzite, slate, phyllite and limestone, and extend due north-west from the Singareni Collie- ries in the Godavari valley. The age of the Paldaals, whether they corres- pond to the Dharwars or the Cuddapahs or younger than the Cuddapahs has been a subject of controversy. Recent studies by the author on the structure and stratigraphy of the area around the Singareni Collieries have shown that the Pakhals lie unconformably on the granite gneisses as well as on a sequence of metasedi- merits which consists of quartzite, phyllite, talc schists, chlorite schists, biotite schists, limestone, griJnerite gneiss and ferruginous quartzite. The metasediments occur as two narrow strips, one in the neighbourhood of Singareni village and the other near Mulakalapalli, and are described as the ' Singareni strip' and the ' Mulakalapalli strip' respectively. The rocks of these strips were considered previously to be a part of the Pakhals but are now differentiated by the author as SINGARENI SERIES. Based on the available evidence, the ' Shernavala strip' of metasediments occurring to the east of Khammamett is considered to belong to the Singareni series, and the occurrence of baryte in this strip is shown to be genetically dissimilar to the occurrence of baryte in the Cuddapahs of the type area. The stratigraphic position of the Pakhals is clarified and it is shown that the granitic intrusion and/or granitisation involves the rocks of the Singareni series and not the rocks of the Pakhals. The position of the present area in the tectonic framework of the region is referred to and the structural disturbance at the south-eastern end of the Pakhal belt is related to the Eastern Ghats orogeny. The author considers the Pakhals to be Purana in age and presumably the time-equivalents of the Cuddapahs since they are overlain by the Sullavais. The Pakhals, lithologically dissimilar to the Cuddapahs and characterised by a general N.W,-S.E. strike, may probably be Late Dharwar in age. 70 The Age of the Pakhals of the Godavari Valley 71

1. INTRODUCTION The Pakhals, extending due north-west from the Singareni Collieries (Yellandlapad) in the Godavari valley, were described by King (1881) who considered them to be the equi'~alents of the Cuddapzhs and overlying un- comormably the granites and gneisses. The 'Shernavala strip' of metasediments (Bruce Foote, 1885) m the Muneru valley, extends north-south to the east of Khamrnamett and is believed to be the connecting link between the Cuddapah basin and the Pakhzl belt. The study of the south-eastern part of the Pal~h~l belt in recent years has led to different interpretations regarding their strztigraphic position and age. Mahadevan (1949) considered the Pakhals to be intrudedby granite and opined that the Pakhals corresponded to the Middle Dharwars. Heron (1949) held the view that conglomerates occur at the granite-Pakhal junction and that the Pakhats belong to the Cuddapahs. Janardan Rao (1958) considered the Pakhals to be older than the granite which according to him is only a granitised part of the former. According to Sarma and AppaveJdhanulu (I963) the Pakhals are of Cuddepeh age and overlie the granites unconformably. Raychaudhuri (1963) ccnsidered the granite-Pakhal contact to be depositional and according to him, the Pakhals are younger than the Cuddapahs. An area of nearly 200 square miles around Yellandlapad has been geo- logically mapped by the author. The present study deals with the age or" the Pakhals. Figure 1 is a geological map of the south-e~stern part of the Pakhal belt modified after King (1881). Part of the Shernavala strip is represented in Fig. 1. The spellings of the x illages are according to the latest topo-sheets and an error in the longitude (2 ° 27' shift to the west, Glennie, 1951) which is found in King's map hes also been corrected.

2. STRUCTURE AND STRATIGILA~PHY The Pakhals are structurally disturbed at the south-eastern end of the belt and are considerably metamorphosed. In the Singareni Collieries area the Pakhals surround three 'bays' (since they close in the north and open in the south) of Peninsular gneiss. Heron (194.9) designated these from west to east as (a) Karepalli Railway Stetion bay (Singareni village), (b) Yellandlapad bay (Singareni Collieries) and (c) Beth0mpudi-Ansettipalle bay. Around the granite bays, the Pakhals are tightly folded into three synclines with connecting anticlines and these plunge towards north. The three syndines may be designated from west to east as (a) Navalapad, tb) Lachchaguda, and (c) Ravlapad. The Ravlapad syncline is essentially 72 T. RAMAMORA_~ RAO

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INDEX TO INSET INDEX TO MAP |. U, GONOWANAS 2. L. GONDWANAS ~ GONDWANAS SCALE IN MILES 0 8 16 3. SULLAVAIS ~ PAKHALS l I " I 4. KURNOOLS S. PAKHALS I I UNCLASSIFIED CRY.AL'I.ES 6. CUDDAPAHS '~ SINGARENI 7- EASTERN GHATS SERIES 8. UNCLASSIFIED CRYSTALLINES

RG. 1. Geological map of the south-eastern part of the Pakhal belt. (Modified aRcr W. King, 1881). The Age of the Pakhals of the Godavari Valley 73

an isoeline and the easternmost anticline, which is overturned to the west, appears in a hairpin bend of the Pakhal quartzites in the much disturbed Ulvanuru outlier. The trend of the fold axes is N.E.-S.W. (parallel to the trend of the Eastern Ghats), which is almost at right angles to the general strike of the Pakhals in the upper part of the belt. Structural disturbance in the Pakhal belt, away from the Singareni Collieries area has also been recorded by Sarma and Appavadhanulu (1963). According to these authors the central part of the Navalapad syncline (A 1144')might have been arched up and that the western margin of the Karepalli bay is probably faulted. The Pakhals are represented in the south by conglomerate, gritty sandstone, quartzite, slate, phyllite and limestone, which are more altered than those in the north, where they are relatively undisturbed. The metamorphic impress on the Pakhals around Singareni Collieries, as recorded by the development of garnet, staurolite, andalusite, tremolite, etc., in the appropriate rocks is due to the deformation they suffered. The contact between the granite and the Pakhals has been found to be depositional, marked by a conglomerate bed, which is sheared in places (see See. 3.1).

The present study has shown that the Pakhals lie unconformably on granite gneisses as well as on a sequence of metasediments, which occurs in two narrow strips, one in the neighbourhood of Singareni village and the other near Mulakalapalli. The Mulakalapalli strip has been recognised independently by Raychaudhuri (1963). The basal conglomerate of the Pakhals overlying the ' Singareni strip ' is made up of quartz and ferruginous quartzite pebbles in a ferruginous matrix and is well exposed 350 metres west of the Karepalli-Yellandlapad Railway line between 273 and 274 mile- posts. The basal conglomerate of the Pakhals overlying the Mulakalapalli strip is made up of quartz and chert pebbles in a siliceous matrix and is best seen in the neighbourbood of Mulakalapalli and Bod.

The metasediments of the two strips referred to above are represented by quartzite, phyllite, talc schist, chlorite schist, biotite schist, limestone, grunerite gneiss and ferruginous quartzite. These were previously considered to be a part of the Pakhals, and the author now designates them as Singareni Series, after the village of {he same name. The Singareni series are intruded by granite, pegmatite, amphibolite and dolerite around the hill A 980', north- east of Singareni village, it may be remarked here that enclaves of Phyllite with talcose material, biotite schist and phyllite capped by ferruginous quartzite, similar to the Singareni series, occur within the granite bays. The strike of the Singareni series is conformable with the foliation of the adjacent granite 74 T. ~O~NA I~o gneisses and changes from N.N.W.-S.S.E. then to N.-S. and finally to N.E.S.W. with steep easterly dips. The ' Shernavala strip' of the metasediments occurring to the east of Khammamett, extends from Komanpalle southward up to Chirnamala (17 ° 01' N. 80 ° 14' E.). It is highly disturbed with numerous en echelon folds though no repetition of the beds occurs, and is faulted in places as near Raghunathpalem. This strip is composed of quartzite, phyllite, limestone frequently cherty and with veins of baryte, and ferruginous quartzite. There is no conglomerate bed at its contact with the granite. The phyllite of this strip is in contact with the talc schists, chlorite schists, biotite schists (Archaean schists, in which granite, pegmatite, amphibolite and dolerite occur) to its east. The relationship between the phyllite of the Shernavala strip and the Archaean schists is conformable and is reflected in the increasing grade of metamorphism from west to east. The Shernavala strip of rocks may, therefore, be correlated with the Singareni series and considered to belong to the Archaeans. Sarma and Appavadhanulu (1963) considered the Shernavala strip of rocks to be of Pakhal age (Cuddapah) and attributed the absence of conglomerate at their contact with the granites on the west to their being "squeezed into the pink granite-composite granite junction" Raychaudhuri (1963) correlated the Shernavala strip with the Mulakalapalli strip (underlying the Pakhals), but could not come to any conclusion on its relationship with the granite gneiss. According to him the Shernavala strip of rocks may be of Cuddapah age because baryte occurs there. The occurrence of baryte or the supposed "squeezing into junction of granites" will not give a Cuddapah age to the Shernavala strip for the reasons stated below. Baryte occurs in the Khammamett and Krishna Districts in rocks of different lithological characters and physical properties, such as the incompetent slaty phyllites on the hill /~ 1144' in the Navalapad syncline (described as Cheruvupttram area), in the competent dolomitic limestones and quartiites in Rudrammakota area, and in the incompetent chlorite- biotite schists of the Vemalanarava area (16 ° 59' 30" N-80 ° 17' E.). In all the deposits of these three areas, baryte is associated with veins of white chert and quartz which traverse the host rocks. It occurs as veins mostly parallel to the bedding planes and is confined to the areas where there has been intense tolding or fracturing (Appavadhanulu, 1961). In the Cuddapahs of the type area, baryte is genetically associated with the basic dykes and sills which are intrusives into the Vempalli limestone and occurs in areas which are relatively undisturbed (Coulson, 1933). Thus, the occurrence of baryte in the rocks of the Khammamett and Krishna Districts differs in several res- The Age of the Pakhals of the Godavari Valley 75

pects from that in the rocks of the Cuddapah basin and hence the two cannot be correlated. It is probable that the rocks containing baryte in the Khamma- mett and Krishna Districts may actually belong to the Archaeans, for it is known that the chlorite-biotite schists of the Vemalanarava area belong undoubtedly to the Archaeans (Bruce Foote, I885). The dolomitic limestones and quartzites of the Rudrammakota area which form part of the Shernavala strip, and the slaty phyllites on A 1144' should therefore belong to the Archaeans, because the occurrence of baryte in these two areas is genetically similar to that of the Vemalanarava area. The slaty phyllites on the hill A 1144' are associated with granites which are capped by Pakhal sandstone, and the occurrence of these older rocks in the heart of the Pakhals is due to the fact that the central part of the Navalapad syncline is arched up (Sarma and Appavadhanulu, 1963). The Post-Pakhal disturbance has thus brought these Archaean rocks to the surface by arching up, squeezing, telescoping and ecoulement. The succession of the Precambriati formation of the Yellandlapad area is given in Table I. TABLE I Precambrian succession in the Yellandlapad area

Purana Pakhals Limestone, phyllite, slate, quartzite, gritty sandstone and conglomerate.

Unconformity Granite gneiss complex Granite gneiss, ttypersthene granulite, (Unclassified crystallines) amphibolite, dolerite and pegmatite. Archaean Singareni series (Dharwars) Ferruginous quartzite, limestone, biotite schist, chlorite schist, talc schist, phyllite and quartzite.

3. STRATIGRAPHICPOSITION OF THE PAKHALS 3.1. Conglomerate at the Base of the Pakhals The PakhMs lie unconformably on the granite gneisses as well as on the Singareni series with a basal conglomerate bed, which is impersistent and highly sheared in places. Good exposures of the conglomerate at the base of the Pakhals can be seen at the following places. 76 T. RA~OHANA RAO (i) Immediately to the east and two miles north-west of Dabripet around the Paldlal Lake. (ii) On the western slopes of the hill /~ 1905', north of Byaram and at several places from a mile west of Navalapad through Cheruvupuram to Regulagudem along the Muneru River. (iii) About six furlongs south-east of Nizampet, about a mile and a half to the north-eest of Pirpalle around the northern part of the Karepalli Bay. (iv) Immediately to the south-weft of Yell~.mbailu in the Kinars~,ni River, two miles north-east of the same village, and tc the north and east of Gorla- madagu in the Uivanuru outlire. (v) A furlong north of Manditog and a furlong south-east and south-west of Puballi along the northern part of the Yellendk pad Bay. (vi) Two furlongs west of the Karepalli-Yallandl~pad Railway line be ween 273 and 274 mile-posts end two furlongs north-east of the Mulakala- palli village and near the village Bod. (The above list includes some of the localities mentioned by King, 1881; Heron, 1949; Sarma, 1962; Appava- dhanulu and Subrahmanyam, 1962; Sarma and Appavadhanulu, 1963; and Raychaudhuri, 1963). The composition of the basal beds of the Pakhals varies widely from one place to another and is represented by the following: Conglomerate composed of pebbles of quartz and banded ferruginous quartzite in a ferruginous or siliceous matrix and quartz and chert pebbles in a siliceous matrix; breccia with angular fragments of quartz, pink feldspar and phyllite chips in a shaly matrix; sheared arkoses and gritty sandstone. According to Mahadevan (1949) the granites are intrusive into the Pakhals. Janardan Rao (1958) considered the granite in the bays to be the granitised Pakhals. Balakrishna and Janardan Rao (1960) contended that the age of the pakhals could not be determined from the supposed Archaean age of the granite, since there are granites as late as Tertiery. It may be remarked here that some of the sheared and reworked erkosic conglomerate at the base of the Pakhals appear on a cursory examination to be quartzo- feldspathic gneisses (see Section 3.2). 3.2. Granitic Intrusion and/or Granitisation King (1881) reported the occurrence of schistose rocks in intimate associ- ation with the granites and gneisses in the Singareni region, Yenkatapur promontory t.17° 27' 30" N.-80024 ' E.)and at the Yellambailu end in the south-eastern part of the Pakhal belt and expressed the doubt whether these schistose rocks belong to the Pakhals. Mahadevan (1949) recorded the The Age of the Pakhals of the Godavari Falley 77 occurrence of granitic, pegmatitie and basic intrusives into the schistose rocks around the Singareni village and Kotturugudem (17 ° 40' N.-80 ° 41' 45" E.) and took this as a criterion to relegate the whole Pakhal belt to the Middle Dharwars. Heron (1949) conceded the occurrences of acid and basic intrusives in the areas cited by Mahadevan, but explained them as due to the refusion of the granite as a consequence of deep folding in post-Pakhal time. According to Janardan Rao (1958, 1963) the granite bays represent the re- gionally metamorphosed, migmatised, and granitised complex of the Pakhals in the anticlinal cores of the folds, and Balakrishna and Janardan Rao (1960) believed that if the Pakhals are of Cuddapah age, the granites in the bays were not of Archaean age. Field evidence is reasonably clear that the Pakhals overlie unconformably the granite gneisses as well as the Singareni series in the Singareni and Mulakala- palli areas (Section 3.2). The elastic character of the basal conglomerate of the Pakhals is discernible even when it is sheared and reworked. The Pakhals literally wrap round the resistant masses of granites due td folding which make the granite appear on the geological map to be occupying the anticlinal cores of the folded Pakhals. The post-Pakhal intrusives are only quartz veins and there is no evidence of refusion of the granite in the bays. The migmatisation, granitisation and/or granitic intrusion belong to a period earlier than the Pakhal and is probably related to the Singareni series. It may be remarked here that the contact between the Singareni series and the granite is not sharp but gradational. It is often difficult to define the boundary between these two and to dec~de whether a particular rock in the contact zone belongs to the granite gneiss complex or to the Singareni series. The peripheral parts of the granite bays are characterised by schistose gneisses and feldspathised schists and on the geological map (Fig. 1) these rocks are included by the author into the bay complex. It is worth recalling here the references made by Fermor (1936, pp. 103--04) to the schistose rocks in the Khammamett District along the northern edge of the Cuddapah basin. He considered these Schistose strips as the extension of the Nellore schist belt underneath the Cuddapahs and that the minor development of these rocks is due to the invasion of the massive granite of Hyderabad (Province No. 1.0 of Fermor). These schistose strips may, therefore, be considered to represent unassimilated xenoliths or ungranifised relics of the parent schists. Fermor described these schistose outcrops on the south-western and the north-eastern sides of the Gondwanas of the Godavari valley, as the "north-east promontory" of the Dharwar- Mysore-Nellore province (his Province No. 1) which according to him is a 78 T. RAMAMOHANARAo

structural connecting link between Provinces 1 and 2 (Chanda-Bastar Province). 4. AcE OF THE PAKHALS Fermor (1936, 1940) made the first systematic attempt to correlate the ancient schistose formations of Peninsular India and adopted the "presence of either the granitic or pegmatitic intrusives" as a criterion to distinguish the Archaean from the Purana group of rocks (equivalent to the Algonkian). The applicability of this criterion to differentiate between the Archaean and the Purana in Southern India is more or less valid. The present study has shown that granitic intrusion and/or granitisation has not affected the Pakhals buta sequence of Archaean metasediments designated as ' Singareni series' by the author. The overlying Pakhals should, therefore, belong to the Puranas. Raychaudhuri (1963) opines that the Pakhals may be younger than the Cudda- pahs since they lie unconformably on the ' Mulakalapalli strip ', which probably belongs to the Cuddapahs because it is well aligned with the ' Sherna- vala strip' in which baryte occurs indicating an affinity with the Cuddapahs. As stated earlier (Section 2) the occurrence of baryte in the rocks of the Shernavala strip is genetically dissimilar to that of the rocks of the Cuddapah basin and hence the two cannot be correlated. On the other hand, it is shown that the Shernavala strip as well as the Singareni series of rocks belong to the Archaeans and may be considered to represent the unassimilated or ungranitised relics of the Dharwar schists (Pascoe, 1950, p. 78). Therefore, the Pakhals which lie unconformably on the Archaeans (Singareni series and the Granite gneisses) belong to the Puranas and probably are the time-equivalents of the Cuddapahs of the type area since they are unconformably overlain by the Sullavais. However, the Pakhals are lithologically dissimilar and cannot be correlated with the Cuddapahs of the type area on the stage-wise basis as admitted by King (1881). The general trend of the Pakhals is N.W.-S.E. with easterly dips, and is similar to the trend of the Dharwars. The Pakhals are structurally disturbed at the south-eastern end of the belt and are metamorphosed with the development of andalusite, garnet, staurolite, and tremolite in the appropriate lithological types. In the vicinity of the Singareni Collieries different structural trends meet (Krishnan, 1961, p. 9), and the effects of the younger orogeny are clearly felt on the older rocks. Fermor (1936, p. 104) has described the crystalline schists to the south-west and north-east of the Gondwanas of the Godavari valley as a structural connecting link between the Dharwar- Mysore-Nellore Province and the Chanda-Bastar Province, i.e., between the Dharwar structural province and the Mahanadi structural province. The Post-Pakhal disturbance is apparently related to the Eastern Ghats orogeny, The Age of the Pakhals of the Godavari Valley 79

since the N.E.-S.W. trend of the fold-axes of the Pakhals is similar to the trend of the Eastern Ghats. The Pakhals as well as the underlying Singareni series and the Granite gneisses exhibit similar trends which vary fzom N.N.W.-S.S.E. to N.-S. and to N.E.-S.W., thus adjusting to the trend of the Eastern Ghats. Similar changes of the trend in the Dharwars of the type area in the southern part of the schistose belt have been recorded and were supposed to be the effects of the Eastern Ghats orogeny. It is probable, therefore, that the distnrbance in the Pakhals is coeval with the Eastern Ghats orogeny (1600m.y., Holmes, 1955). The disturbance along the eastern margin of the Cuddapah basin may be due to the supposed uplift of the Eastern Ghats in Late Cuddapah times (Aswathanarayana, Personal commu- nication). Hence, the Pakhals affected by the Eastern Ghats orogeny will be older than the Eastern Ghats and probably Late Dharwar in age. 5. CONCLUSIONS In the area around Singareni Collieries, the Pakhals lie unconformably on the granite gneisses as well as a sequence of metasediments which are intruded (or subjected to granitisation) by granite and basic rocks. The metasediments were previously considered to be a part of the Pakhals and the author has designated them as Singareni series. Based on the available evidence, the Shernavala strip of the metasediments occurring to the east of Khammamett is considered to belong to the Singareni series and it is shown that the occurrence of baryte in this strip is genetically dissimilar to that of the Cuddapahs of the type area. The author considers the Pakhals to belong to the Purana age and presumably the time-equivalents of the Cuddapahs since the Pakhals are overlain by the Sullavais. The disturbance at the south-eastern end of the Pakhal belt is inferred to be related to the period of the Eastern Ghats orogeny. The Pakhals, lithologicaUy dissimilar to the Cuddapahs and characterised by a general N.W.-S.E. trend, may probably be Late Dharwar in age. 6. ACKNOWLEDGEMENTS This work was carried out under the general guidance of the late Pro- fessor C. Mahadevan and the author wishes to place on record his grateful thanks to him. It is a pleasure to thank Professor C. S. Pichamuthu for his kind interest in the author's work. The author is also thankful to Dr. R. Vaidyanadhan and Dr. M. Subba Rao for their help at different stages of the work. The financial assistance given by the Andhra University and the Uni- versity Grants Commission is gratefully aCknowledged. 80 T. RAMAMOHANARAO

7. REFERENCES

Appavadhanulu, K. •. "'Barytes deposits of paris of Khammam and Krishna Districts," Symposium on 'Minerals of India', 1961, Department ol Mines and Geol., Govt. of Andhra Pradesh, Hyderabafl and Subrahmanyam, P. "A short note on the occurrence of conglomerate at the base of Pakhals in Khammam District, Andhra Pradesh," Indian Minerals, 1962, 16 (1), 80-81. Balakrishna, S. and Janardan "The stratigraphic position of the Pakhals," Proc. 47th Ind. Rao, Y. Sci. Cong., 1960, Part III, p. 248. Bruce Foote, R. .. "Notes on the country between Singareni Coalfield and the Krishna River," Rec. GeoL Surv. India, 1885, 18, 12-25. Coulson, A. L. • . "Barytes in the Ceded Districts of the Madras Presidency with notes on its occurrences in other parts of India," Mem. Geol. Surv. Ind., 1933, Part I, 64, 1-142. Fen'nor, L. L. •. "An attempt at the correlation of the ancient schistose formations of the Peninsular India," Ibid., 1936 and 1940, 70, Parts I and II. Glennie, E. A. .. "The Cuddapah basin in India and crustal warping," Monthly Notices of the Royal Astronomical Society, Geophysical Supplement, 1951, 6 (3). Heron, A. M. .. "'Synopsis of the Purana Formations of Hyderabad, '~ Jour. .Hyd. Geol. Surv., 1949, 5, Part II, 1-129. Holmes, A. .. "'Dating the Precambrian of Peninsular India and Ceylon," Proe. Geol. Assoc. Canada, 1955, 7, Part lI, 81-106. Janardan Ran, Y. .. "Granite-Pakhal relationship (Yellandlapad)," Curt. Sci., 1958, 127, 490-91. • . "Regional metamorphism and granitisation in the plutonic rocks around Yellandiapad, Andhra Pradesh," Jour. Deccan Geol. and Mining Soc., 1963, 3, 61-71. King, W. • . "Geology of the Pranhita-Godavari valley," Mere. GeoL Surv. Ind., 1881, 18, Part HI, 151-311. Krishnan, M. S. .. "Structural and tectonic history of India," Ibid., 1961, 81, 1-109 (Revised and reprinted). Mahadevan, C. .. "A re-examination of some aspects of Puranas and Archaeans of South India," Pres. Addr. See. Geol. and Geogr., 36th Ind. Sci. Cong., Allahabad Session, 1949. Pascoe, E. H. .. Manual of Geology of India and Burma, Government of India Press, New Delhi (3rd Edition), 1950, Vols. 1 and 2. Raychaudhuri, B. .. "A revision of the Precambrian stratigraphy in the Godavari valley (Khammamett Dist.) and some related problems," Proc. 50th Ind. Sci. Congr., 1963, Part III, Abstracts, pp. 277-78. Sarma, S.R. .. "Conglomeratic breccia in the Pakhals near Pirpalle," Quart. Jour. Geol. Min. and Met. Soc. Ind., 1962, 34(1), 9-10. ------and Appavadhanulu, K. "Some aspects of the Pakhals at their south-eastern margin," Jour. Deccan GeoL and Min. Soc., 1963, 3, 80-87.

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